24 Commits

Author SHA1 Message Date
4dfb3a7c35 Last changes 2026-05-06 01:58:49 +02:00
10d791fe04 Final readthrough corrections of quantum fundamentals 2026-05-04 23:04:28 +02:00
06852b8e62 Final readthrough corrections of classical fundamentals 2026-05-04 21:07:25 +02:00
400dc47df0 Incorporate Jonathan's corrections to classical fundamentals 2026-05-04 20:56:35 +02:00
ece8fc1715 Center error marker 2026-05-04 20:24:27 +02:00
56e3a0e5ca Consistently capitalize character after semicolon 2026-05-04 20:21:21 +02:00
8d6df8a79d Final readthrough corrections for fault tolerance chapter 2026-05-04 20:06:18 +02:00
c41ac9f61f Incorporate Jonathan's corrections to Fault Tolerance Chapter 2026-05-04 19:45:15 +02:00
a41e0b05fe Add Lia as supervisor 2026-05-04 19:20:08 +02:00
1edc3f301a Final readthrough corrections for decoding chapter 2026-05-04 18:42:39 +02:00
a977860ddb Incorporate Jonathan's correction to sliding-window decoding sections 2026-05-04 17:35:33 +02:00
7bf1b2f8d7 Incorporate Jonathan's corrections to numerical results section 2026-05-04 17:07:41 +02:00
72acea0321 Incorporate Jonathan's corrections to the introduction 2026-05-04 16:31:31 +02:00
f1a5aaf3f8 Make ToC be on one page 2026-05-04 16:20:37 +02:00
23828b671a Minor changes to conclusion 2026-05-04 16:08:56 +02:00
09893d527e Incorporate Jonathan's corrections to Abstract 2026-05-04 15:36:15 +02:00
25789a6bd3 Incorporate Jonathans's corrections to Conclusion 2026-05-04 15:28:11 +02:00
001ca614bb Fix bibliography 2026-05-04 15:17:05 +02:00
9e5eaaf985 Incorporate Lia's corrections to fault tolerance 2026-05-04 14:59:49 +02:00
17191382cf Incorporate Lia's corrections to QM and QEC fundamentals 2026-05-04 13:01:54 +02:00
aa907ef4a3 Incorporate Lia's corrections to classical fundamentals 2026-05-04 12:12:10 +02:00
12036caa91 Fix bibliography titlecase (in clean_bibliography.sh) and a few things in the bibliography itself 2026-05-04 10:53:50 +02:00
4c206ae9c4 Rephrase first sentence of abstract 2026-05-04 10:34:53 +02:00
01a754e5da Reset acronyms after abstract 2026-05-04 10:31:54 +02:00
9 changed files with 775 additions and 667 deletions

View File

@@ -7,14 +7,14 @@
language = {en},
number = {3},
journal = {Mathematical Proceedings of the Cambridge Philosophical Society},
author = {Dirac, P. a. M.},
author = {Dirac, P. A. M.},
month = jul,
year = {1939},
pages = {416--418},
}
@misc{huang_improved_2023,
title = {Improved {Noisy} {Syndrome} {Decoding} of {Quantum} {LDPC} {Codes} with {Sliding} {Window}},
title = {Improved Noisy Syndrome Decoding of Quantum {LDPC} Codes with Sliding Window},
doi = {10.48550/arXiv.2311.03307},
publisher = {arXiv},
author = {Huang, Shilin and Puri, Shruti},
@@ -47,7 +47,7 @@
}
@article{gidney_stability_2022,
title = {Stability {Experiments}: {The} {Overlooked} {Dual} of {Memory} {Experiments}},
title = {Stability Experiments: The Overlooked Dual of Memory Experiments},
volume = {6},
issn = {2521-327X},
shorttitle = {Stability {Experiments}},
@@ -60,7 +60,7 @@
}
@misc{koutsioumpas_colour_2025,
title = {Colour {Codes} {Reach} {Surface} {Code} {Performance} using {Vibe} {Decoding}},
title = {Colour Codes Reach Surface Code Performance using Vibe Decoding},
doi = {10.48550/arXiv.2508.15743},
publisher = {arXiv},
author = {Koutsioumpas, Stergios and Noszko, Tamas and Sayginel, Hasan and Webster, Mark and Roffe, Joschka},
@@ -69,16 +69,17 @@
howpublished = {arXiv:2508.15743},
}
@article{koutsioumpas_automorphism_2025,
title = {Automorphism {Ensemble} {Decoding} of {Quantum} {LDPC} {Codes}},
@misc{koutsioumpas_automorphism_2025,
title = {Automorphism Ensemble Decoding of Quantum {LDPC} Codes},
language = {en},
author = {Koutsioumpas, Stergios and Sayginel, Hasan and Webster, Mark and Browne, Dan E},
month = mar,
year = {2025},
howpublished = {arXiv:2503.01738},
}
@misc{gottesman_heisenberg_1998,
title = {The {Heisenberg} {Representation} of {Quantum} {Computers}},
title = {The Heisenberg Representation of Quantum Computers},
doi = {10.48550/arXiv.quant-ph/9807006},
publisher = {arXiv},
author = {Gottesman, Daniel},
@@ -102,8 +103,8 @@
}
@phdthesis{higgott_practical_2024,
type = {Doctoral},
title = {Practical and {Efficient} {Quantum} {Error} {Correction}},
type = {Ph.D. {Thesis}},
title = {Practical and Efficient Quantum Error Correction},
copyright = {open},
language = {eng},
school = {UCL (University College London)},
@@ -121,16 +122,17 @@
}
@misc{gong_toward_2024,
title = {Toward {Low}-latency {Iterative} {Decoding} of {QLDPC} {Codes} {Under} {Circuit}-{Level} {Noise}},
title = {Toward Low-latency Iterative Decoding of {QLDPC} Codes Under Circuit-Level Noise},
language = {en},
journal = {arXiv.org},
author = {Gong, Anqi and Cammerer, Sebastian and Renes, Joseph M.},
month = mar,
howpublished = {arXiv:2403.18901},
year = {2024},
}
@article{miao_quaternary_2025,
title = {Quaternary {Neural} {Belief} {Propagation} {Decoding} of {Quantum} {LDPC} {Codes} with {Overcomplete} {Check} {Matrices}},
title = {Quaternary Neural Belief Propagation Decoding of Quantum {LDPC} Codes with Overcomplete Check Matrices},
volume = {13},
issn = {2169-3536},
doi = {10.1109/ACCESS.2025.3539475},
@@ -142,7 +144,7 @@
}
@misc{tsouchlos_ccam_2024,
title = {{CCAM} {Summary}},
title = {{CCAM} Summary},
author = {Tsouchlos, Andreas},
month = oct,
year = {2024},
@@ -162,7 +164,7 @@
}
@book{griffiths_introduction_1995,
title = {Introduction to {Quantum} {Mechanics}},
title = {Introduction to Quantum Mechanics},
isbn = {0-13-124405-1},
language = {en},
publisher = {Prentice Hall},
@@ -171,7 +173,7 @@
}
@misc{bradley_tensor_2018,
title = {The {Tensor} {Product}, {Demystified}},
title = {The Tensor Product, Demystified},
author = {Bradley, Tai-Danae},
month = nov,
year = {2018},
@@ -179,7 +181,7 @@
@book{nielsen_quantum_2010,
address = {Cambridge},
title = {Quantum {Computation} and {Quantum} {Information}: 10th {Anniversary} {Edition}},
title = {Quantum Computation and Quantum Information: 10th Anniversary Edition},
isbn = {978-0-511-97666-7},
shorttitle = {Quantum {Computation} and {Quantum} {Information}},
doi = {10.1017/CBO9780511976667},
@@ -191,7 +193,7 @@
}
@article{geiselhart_automorphism_2021,
title = {Automorphism {Ensemble} {Decoding} of {Reed}{Muller} {Codes}},
title = {Automorphism Ensemble Decoding of ReedMuller Codes},
volume = {69},
issn = {1558-0857},
doi = {10.1109/TCOMM.2021.3098798},
@@ -216,7 +218,7 @@
@phdthesis{klinke_neural_2025,
address = {Karlsruhe},
type = {Bachelor's {Thesis}},
title = {Neural {Belief} {Propagation} {Ensemble} {Decoding} of {Quantum} {LDPC} {Codes}},
title = {Neural Belief Propagation Ensemble Decoding of Quantum {LDPC} Codes},
language = {English},
school = {KIT},
author = {Klinke, Jeremi},
@@ -225,7 +227,7 @@
}
@misc{camps-moreno_toward_2024,
title = {Toward {Quantum} {CSS}-{T} {Codes} from {Sparse} {Matrices}},
title = {Toward Quantum {CSS}-{T} Codes from Sparse Matrices},
doi = {10.48550/arXiv.2406.00425},
abstract = {CSS-T codes were recently introduced as quantum error-correcting codes that respect a transversal gate. A CSS-T code depends on a pair \$(C\_1, C\_2)\$ of binary linear codes \$C\_1\$ and \$C\_2\$ that satisfy certain conditions. We prove that \$C\_1\$ and \$C\_2\$ form a CSS-T pair if and only if \$C\_2 {\textbackslash}subset {\textbackslash}operatorname\{Hull\}(C\_1) {\textbackslash}cap {\textbackslash}operatorname\{Hull\}(C\_1{\textasciicircum}2)\$, where the hull of a code is the intersection of the code with its dual. We show that if \$(C\_1,C\_2)\$ is a CSS-T pair, and the code \$C\_2\$ is degenerated on \${\textbackslash}\{i{\textbackslash}\}\$, meaning that the \$i{\textasciicircum}\{th\}\$-entry is zero for all the elements in \$C\_2\$, then the pair of punctured codes \$(C\_1{\textbar}\_i,C\_2{\textbar}\_i)\$ is also a CSS-T pair. Finally, we provide Magma code based on our results and quasi-cyclic codes as a step toward finding quantum LDPC or LDGM CSS-T codes computationally.},
publisher = {arXiv},
@@ -251,7 +253,7 @@
}
@misc{gottesman_introduction_2009,
title = {An {Introduction} to {Quantum} {Error} {Correction} and {Fault}-{Tolerant} {Quantum} {Computation}},
title = {An Introduction to Quantum Error Correction and Fault-Tolerant Quantum Computation},
doi = {10.48550/arXiv.0904.2557},
publisher = {arXiv},
author = {Gottesman, Daniel},
@@ -274,7 +276,7 @@
}
@misc{calderbank_quantum_1997,
title = {Quantum {Error} {Correction} via {Codes} over {GF}(4)},
title = {Quantum Error Correction via Codes over {GF}(4)},
doi = {10.48550/arXiv.quant-ph/9608006},
publisher = {arXiv},
author = {Calderbank, A. R. and Rains, E. M. and Shor, P. W. and Sloane, N. J. A.},
@@ -284,13 +286,13 @@
}
@misc{gottesman_stabilizer_1997,
title = {Stabilizer {Codes} and {Quantum} {Error} {Correction}},
title = {Stabilizer Codes and Quantum Error Correction},
doi = {10.48550/arXiv.quant-ph/9705052},
publisher = {arXiv},
author = {Gottesman, Daniel},
month = may,
year = {1997},
howpublished = {arXiv:quant-ph/9705052},
howpublished = {Ph.D. {Thesis}, arXiv:quant-ph/9705052},
}
@misc{shor_fault-tolerant_1997,
@@ -304,7 +306,7 @@
}
@article{divincenzo_fault-tolerant_1996,
title = {Fault-{Tolerant} {Error} {Correction} with {Efficient} {Quantum} {Codes}},
title = {Fault-Tolerant Error Correction with Efficient Quantum Codes},
volume = {77},
issn = {0031-9007, 1079-7114},
doi = {10.1103/PhysRevLett.77.3260},
@@ -345,7 +347,7 @@
}
@article{terhal_quantum_2015,
title = {Quantum {Error} {Correction} for {Quantum} {Memories}},
title = {Quantum Error Correction for Quantum Memories},
volume = {87},
issn = {0034-6861, 1539-0756},
doi = {10.1103/RevModPhys.87.307},
@@ -363,7 +365,7 @@
title = {Guidelines for snowballing in systematic literature studies and a replication in software engineering},
isbn = {978-1-4503-2476-2},
doi = {10.1145/2601248.2601268},
booktitle = {Proceedings of the 18th {International} {Conference} on {Evaluation} and {Assessment} in {Software} {Engineering}},
booktitle = {Proceedings of the 18th International Conference on Evaluation and Assessment in Software Engineering},
publisher = {Association for Computing Machinery},
author = {Wohlin, Claes},
month = may,
@@ -395,10 +397,10 @@
}
@inproceedings{chatterjee_quantum_2023,
title = {Quantum {Error} {Correction} {For} {Dummies}},
title = {Quantum Error Correction For Dummies},
volume = {01},
doi = {10.1109/QCE57702.2023.00017},
booktitle = {2023 {IEEE} {International} {Conference} on {Quantum} {Computing} and {Engineering} ({QCE})},
booktitle = {2023 {IEEE} International Conference on Quantum Computing and Engineering ({QCE})},
author = {Chatterjee, Avimita and Phalak, Koustubh and Ghosh, Swaroop},
month = sep,
year = {2023},
@@ -406,7 +408,7 @@
}
@inproceedings{petersen_systematic_2008,
title = {Systematic {Mapping} {Studies} in {Software} {Engineering}},
title = {Systematic Mapping Studies in Software Engineering},
doi = {10.14236/ewic/EASE2008.8},
language = {en},
publisher = {BCS Learning \& Development},
@@ -416,7 +418,7 @@
}
@article{postler_demonstration_2024,
title = {Demonstration of {Fault}-{Tolerant} {Steane} {Quantum} {Error} {Correction}},
title = {Demonstration of Fault-Tolerant Steane Quantum Error Correction},
volume = {5},
doi = {10.1103/PRXQuantum.5.030326},
number = {3},
@@ -429,7 +431,7 @@
}
@article{cao_exact_2025,
title = {Exact {Decoding} of {Quantum} {Error}-{Correcting} {Codes}},
title = {Exact Decoding of Quantum Error-Correcting Codes},
volume = {134},
doi = {10.1103/PhysRevLett.134.190603},
number = {19},
@@ -442,7 +444,7 @@
}
@misc{beni_tesseract_2025,
title = {Tesseract: {A} {Search}-{Based} {Decoder} for {Quantum} {Error} {Correction}},
title = {Tesseract: {A} Search-Based Decoder for Quantum Error Correction},
shorttitle = {Tesseract},
doi = {10.48550/arXiv.2503.10988},
publisher = {arXiv},
@@ -469,7 +471,7 @@
}
@misc{bhardwaj_adaptive_2025,
title = {Adaptive {Estimation} of {Drifting} {Noise} in {Quantum} {Error} {Correction}},
title = {Adaptive Estimation of Drifting Noise in Quantum Error Correction},
doi = {10.48550/arXiv.2511.09491},
publisher = {arXiv},
author = {Bhardwaj, Devansh and Takou, Evangelia and Lin, Yingjia and Brown, Kenneth R.},
@@ -505,7 +507,7 @@
}
@article{bausch_learning_2024-1,
title = {Learning to {Decode} the {Surface} {Code} with a {Recurrent}, {Transformer}-{Based} {Neural} {Network}},
title = {Learning to Decode the Surface Code with a Recurrent, Transformer-Based Neural Network},
volume = {635},
issn = {0028-0836, 1476-4687},
doi = {10.1038/s41586-024-08148-8},
@@ -528,7 +530,7 @@
}
@misc{fan_accelerating_2025,
title = {Accelerating {BP}-{OSD} {Decoder} for {QLDPC} {Codes} with {Local} {Syndrome}-{Based} {Preprocessing}},
title = {Accelerating {BP}-{OSD} Decoder for {QLDPC} Codes with Local Syndrome-Based Preprocessing},
doi = {10.48550/arXiv.2509.01892},
publisher = {arXiv},
author = {Fan, Wenxuan and Suzuki, Yasunari and Ravi, Gokul Subramanian and Ueno, Yosuke and Inoue, Koji and Tanimoto, Teruo},
@@ -548,11 +550,12 @@
}
@misc{wang_fully_2025,
title = {Fully {Parallelized} {BP} {Decoding} for {Quantum} {LDPC} {Codes} {Can} {Outperform} {BP}-{OSD}},
title = {Fully Parallelized {BP} Decoding for Quantum {LDPC} Codes Can Outperform {BP}-{OSD}},
language = {en},
author = {Wang, Ming and Li, Ang and Mueller, Frank},
month = jun,
year = {2025},
howpublished = {arXiv:2507.00254},
}
@misc{ye_beam_2025,
@@ -581,7 +584,7 @@
}
@article{higgott_improved_2023,
title = {Improved {Decoding} of {Circuit} {Noise} and {Fragile} {Boundaries} of {Tailored} {Surface} {Codes}},
title = {Improved Decoding of Circuit Noise and Fragile Boundaries of Tailored Surface Codes},
volume = {13},
doi = {10.1103/PhysRevX.13.031007},
number = {3},
@@ -594,7 +597,7 @@
}
@misc{tsubouchi_degeneracy_2025,
title = {Degeneracy {Cutting}: {A} {Local} and {Efficient} {Post}-{Processing} for {Belief} {Propagation} {Decoding} of {Quantum} {Low}-{Density} {Parity}-{Check} {Codes}},
title = {Degeneracy Cutting: {A} Local and Efficient Post-Processing for Belief Propagation Decoding of Quantum Low-Density Parity-Check Codes},
shorttitle = {Degeneracy {Cutting}},
doi = {10.48550/arXiv.2510.08695},
publisher = {arXiv},
@@ -605,7 +608,7 @@
}
@misc{lee_scalable_2025,
title = {Scalable {Neural} {Decoders} for {Practical} {Real}-{Time} {Quantum} {Error} {Correction}},
title = {Scalable Neural Decoders for Practical Real-Time Quantum Error Correction},
doi = {10.48550/arXiv.2510.22724},
publisher = {arXiv},
author = {Lee, Changwon and Hur, Tak and Park, Daniel K.},
@@ -615,7 +618,7 @@
}
@misc{maan_decoding_2025,
title = {Decoding {Correlated} {Errors} in {Quantum} {LDPC} {Codes}},
title = {Decoding Correlated Errors in Quantum {LDPC} Codes},
doi = {10.48550/arXiv.2510.14060},
publisher = {arXiv},
author = {Maan, Arshpreet Singh and Herrero, Francisco-Garcia and Paler, Alexandru and Savin, Valentin},
@@ -641,7 +644,7 @@
}
@article{higgott_sparse_2025,
title = {Sparse {Blossom}: correcting a million errors per core second with minimum-weight matching},
title = {Sparse Blossom: correcting a million errors per core second with minimum-weight matching},
volume = {9},
shorttitle = {Sparse {Blossom}},
doi = {10.22331/q-2025-01-20-1600},
@@ -655,7 +658,7 @@
}
@article{breuckmann_quantum_2021,
title = {Quantum {Low}-{Density} {Parity}-{Check} {Codes}},
title = {Quantum Low-Density Parity-Check Codes},
volume = {2},
doi = {10.1103/PRXQuantum.2.040101},
number = {4},
@@ -668,10 +671,10 @@
}
@inproceedings{gokduman_erasure_2024,
title = {Erasure {Decoding} for {Quantum} {LDPC} {Codes} via {Belief} {Propagation} with {Guided} {Decimation}},
title = {Erasure Decoding for Quantum {LDPC} Codes via Belief Propagation with Guided Decimation},
issn = {2836-4503},
doi = {10.1109/Allerton63246.2024.10735275},
booktitle = {2024 60th {Annual} {Allerton} {Conference} on {Communication}, {Control}, and {Computing}},
booktitle = {2024 60th Annual Allerton Conference on Communication, Control, and Computing},
author = {Gökduman, Mert and Yao, Hanwen and Pfister, Henry D.},
month = sep,
year = {2024},
@@ -679,7 +682,7 @@
}
@misc{swierkowska_eccentric_2025,
title = {{ECCentric}: {An} {Empirical} {Analysis} of {Quantum} {Error} {Correction} {Codes}},
title = {ECCentric: An Empirical Analysis of Quantum Error Correction Codes},
shorttitle = {{ECCentric}},
doi = {10.48550/arXiv.2511.01062},
publisher = {arXiv},
@@ -713,7 +716,7 @@
}
@article{tan_scalable_2023,
title = {Scalable {Surface}-{Code} {Decoders} with {Parallelization} in {Time}},
title = {Scalable Surface-Code Decoders with Parallelization in Time},
volume = {4},
doi = {10.1103/PRXQuantum.4.040344},
number = {4},
@@ -755,7 +758,7 @@
}
@misc{kuo_fault-tolerant_2024,
title = {Fault-{Tolerant} {Belief} {Propagation} for {Practical} {Quantum} {Memory}},
title = {Fault-Tolerant Belief Propagation for Practical Quantum Memory},
doi = {10.48550/arXiv.2409.18689},
publisher = {arXiv},
author = {Kuo, Kao-Yueh and Lai, Ching-Yi},
@@ -765,7 +768,7 @@
}
@misc{poor_ultra_2025,
title = {Ultra {Low} {Overhead} {Syndrome} {Extraction} for the {Steane} code},
title = {Ultra Low Overhead Syndrome Extraction for the Steane code},
doi = {10.48550/arXiv.2511.13700},
publisher = {arXiv},
author = {Poór, Boldizsár and Rodatz, Benjamin and Kissinger, Aleks},
@@ -792,15 +795,16 @@
title = {Algorithms for quantum computation: discrete logarithms and factoring},
shorttitle = {Algorithms for quantum computation},
doi = {10.1109/SFCS.1994.365700},
booktitle = {Proceedings 35th {Annual} {Symposium} on {Foundations} of {Computer} {Science}},
booktitle = {Proc. Annual Symposium on Foundations of Computer Science},
author = {Shor, P.W.},
address = {Santa Fe},
month = nov,
year = {1994},
pages = {124--134},
}
@article{preskill_quantum_2018,
title = {Quantum {Computing} in the {NISQ} era and beyond},
title = {Quantum Computing in the {NISQ} era and beyond},
volume = {2},
doi = {10.22331/q-2018-08-06-79},
language = {en-GB},
@@ -813,7 +817,7 @@
}
@misc{google_quantum_ai_quantum_nodate,
title = {Quantum {Computing} {Roadmap}},
title = {Quantum Computing Roadmap},
language = {en},
journal = {Google Quantum AI},
author = {{Google Quantum AI}},
@@ -833,7 +837,7 @@
}
@article{zhang_classical_2023,
title = {A {Classical} {Architecture} for {Digital} {Quantum} {Computers}},
title = {A Classical Architecture for Digital Quantum Computers},
volume = {5},
doi = {10.1145/3626199},
number = {1},
@@ -865,11 +869,11 @@
}
@misc{noauthor_reproducing_nodate,
title = {Reproducing repetition and {Shor} code simulations using stim},
title = {Reproducing repetition and Shor code simulations using stim},
}
@misc{noauthor_tutorial_nodate,
title = {Tutorial - {Estimating} the {Surface} {Code} {Threshold}{NordIQuEst} {Application} {Library}},
title = {Tutorial - Estimating the Surface Code Threshold — NordIQuEst Application Library},
}
@misc{noauthor_simulating_nodate,
@@ -877,7 +881,7 @@
}
@article{ryan-anderson_realization_2021,
title = {Realization of {Real}-{Time} {Fault}-{Tolerant} {Quantum} {Error} {Correction}},
title = {Realization of Real-Time Fault-Tolerant Quantum Error Correction},
volume = {11},
doi = {10.1103/PhysRevX.11.041058},
number = {4},
@@ -904,11 +908,11 @@
}
@misc{noauthor_tutorial_nodate-1,
title = {Tutorial - {Fault}-{Tolerant} {Quantum} {Computing} with {CSS} codes},
title = {Tutorial - Fault-Tolerant Quantum Computing with {CSS} codes},
}
@article{panteleev_degenerate_2021,
title = {Degenerate {Quantum} {LDPC} {Codes} {With} {Good} {Finite} {Length} {Performance}},
title = {Degenerate Quantum {LDPC} Codes With Good Finite Length Performance},
volume = {5},
doi = {10.22331/q-2021-11-22-585},
language = {en-GB},
@@ -921,18 +925,19 @@
}
@article{babar_fifteen_2015,
title = {Fifteen {Years} of {Quantum} {LDPC} {Coding} and {Improved} {Decoding} {Strategies}},
title = {Fifteen Years of Quantum {LDPC} Coding and Improved Decoding Strategies},
volume = {3},
issn = {2169-3536},
doi = {10.1109/ACCESS.2015.2503267},
journal = {IEEE Access},
author = {Babar, Zunaira and Botsinis, Panagiotis and Alanis, Dimitrios and Ng, Soon Xin and Hanzo, Lajos},
month = nov,
year = {2015},
pages = {2492--2519},
}
@misc{yao_belief_2024,
title = {Belief {Propagation} {Decoding} of {Quantum} {LDPC} {Codes} with {Guided} {Decimation}},
title = {Belief Propagation Decoding of Quantum {LDPC} Codes with Guided Decimation},
doi = {10.48550/arXiv.2312.10950},
publisher = {arXiv},
author = {Yao, Hanwen and Laban, Waleed Abu and Häger, Christian and Amat, Alexandre Graell i and Pfister, Henry D.},
@@ -942,7 +947,7 @@
}
@article{sharon_efficient_2007,
title = {Efficient {Serial} {Message}-{Passing} {Schedules} for {LDPC} {Decoding}},
title = {Efficient Serial Message-Passing Schedules for {LDPC} Decoding},
volume = {53},
issn = {1557-9654},
doi = {10.1109/TIT.2007.907507},
@@ -968,7 +973,7 @@
}
@book{ryan_channel_2009,
title = {Channel {Codes}: {Classical} and {Modern}},
title = {Channel Codes: Classical and Modern},
isbn = {978-1-139-48301-8},
shorttitle = {Channel {Codes}},
language = {en},
@@ -979,7 +984,7 @@
}
@book{macwilliams_theory_1977,
title = {The {Theory} of {Error}-correcting {Codes}},
title = {The Theory of Error-correcting Codes},
isbn = {978-0-444-85010-2},
language = {en},
publisher = {Elsevier},
@@ -989,7 +994,7 @@
@book{richardson_modern_2008,
address = {Cambridge},
title = {Modern {Coding} {Theory}},
title = {Modern Coding Theory},
isbn = {978-0-521-85229-6},
doi = {10.1017/CBO9780511791338},
publisher = {Cambridge University Press},
@@ -998,7 +1003,7 @@
}
@phdthesis{gallager_low_1960,
type = {Thesis},
type = {Ph.D. {Thesis}},
title = {Low density parity check codes},
copyright = {M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.},
language = {eng},
@@ -1011,11 +1016,11 @@
title = {Fully parallel window decoder architecture for spatially-coupled {LDPC} codes},
issn = {1938-1883},
doi = {10.1109/ICC.2016.7511553},
booktitle = {2016 {IEEE} {International} {Conference} on {Communications} ({ICC})},
booktitle = {Proc. {IEEE} International Conference on Communications ({ICC})},
author = {Hassan, Najeeb Ul and Schlüter, Martin and Fettweis, Gerhard P.},
address = {Kuala Lumpur},
month = may,
year = {2016},
pages = {1--6},
}
@article{costello_spatially_2014,
@@ -1044,7 +1049,7 @@
}
@article{kang_quits_2025,
title = {{QUITS}: {A} modular {Qldpc} code {circUIT} {Simulator}},
title = {{QUITS}: {A} modular Qldpc code circUIT Simulator},
volume = {9},
issn = {2521-327X},
shorttitle = {{QUITS}},
@@ -1058,7 +1063,7 @@
@book{griffiths_consistent_2001,
address = {Cambridge},
title = {Consistent {Quantum} {Theory}},
title = {Consistent Quantum Theory},
isbn = {978-0-521-53929-6},
doi = {10.1017/CBO9780511606052},
publisher = {Cambridge University Press},
@@ -1067,7 +1072,7 @@
}
@misc{gottesman_fault-tolerant_2014,
title = {Fault-{Tolerant} {Quantum} {Computation} with {Constant} {Overhead}},
title = {Fault-Tolerant Quantum Computation with Constant Overhead},
doi = {10.48550/arXiv.1310.2984},
publisher = {arXiv},
author = {Gottesman, Daniel},
@@ -1087,7 +1092,7 @@
}
@article{gidney_fault-tolerant_2021,
title = {A {Fault}-{Tolerant} {Honeycomb} {Memory}},
title = {A Fault-Tolerant Honeycomb Memory},
volume = {5},
issn = {2521-327X},
doi = {10.22331/q-2021-12-20-605},
@@ -1151,7 +1156,7 @@
}
@misc{leverrier_decoding_2022,
title = {Decoding quantum {Tanner} codes},
title = {Decoding quantum Tanner codes},
doi = {10.48550/arXiv.2208.05537},
publisher = {arXiv},
author = {Leverrier, Anthony and Z{\'e}mor, Gilles},

View File

@@ -1,11 +1,13 @@
\chapter{Introduction}
\label{ch:Introduction}
\acresetall
% Intro to quantum computing
In 1982, Richard Feynman, motivated by the difficulty of simulating
quantum-mechanical systems on classical hardware, put forward the
idea of building computers from quantum hardware themselves
idea of building computers that are themselves quantum mechanical
\cite{feynman_simulating_1982}.
The use of such quantum computers has since been shown to offer promising
prospects not only with regard to simulating quantum systems but also
@@ -15,7 +17,7 @@ factorization \cite{shor_algorithms_1994}.
Similar to the way classical computers are built from bits and gates,
quantum computers are built from \emph{qubits} and \emph{quantum gates}.
Because of quantum entanglement, it is not enough to consider the
Because of quantum entanglement, it does not suffice to consider the
qubits individually, we also have to consider correlations between them.
For a system of $n$ qubits, this makes the state space grow with
$2^n$ instead of linearly with $n$, as would be the case for a classical system
@@ -28,12 +30,11 @@ what provides them with their power \cite[Sec.~2.1]{roffe_decoding_2020}.
Realizing algorithms that leverage these quantum-mechanical effects
requires hardware that can execute long quantum computations reliably.
This poses a problem, because the qubits making up current devices
are difficult to sufficiently isolate from their environment
\cite[Sec.~1]{roffe_quantum_2019}.
Their interaction with the environment acts as a continuous small-scale
measurement, an effect we call \emph{decoherence} of the stored quantum
state.
Decoherence is the reason large systems don't exhibit visible quantum
consistently interact with their environment \cite[Sec.~1]{roffe_quantum_2019}.
This interaction acts as a continuous small-scale measurement, an
effect we call \emph{decoherence} of the stored quantum state, which
results in errors on the qubits.
Decoherence is the reason large systems do not exhibit visible quantum
properties at human scales \cite[Sec.~1]{gottesman_stabilizer_1997}.
% Intro to QEC
@@ -43,8 +44,8 @@ It addresses the issue by encoding the information of $k$
\emph{logical qubits} into a larger number $n>k$ of \emph{physical
qubits}, in close analogy to classical channel coding
\cite[Sec.~1]{roffe_quantum_2019}.
The redundancy introduced this way can then be used to restore
the quantum state, should it be disturbed.
The redundancy introduced this way can then be used to detect and
correct a corrupted the quantum state.
The quantum setting imposes some important constraints that do not exist in the
classical case, however \cite[Sec.~2.4]{roffe_quantum_2019}:
\begin{itemize}
@@ -52,7 +53,7 @@ classical case, however \cite[Sec.~2.4]{roffe_quantum_2019}:
\item In addition to the bit-flip errors we know from the
classical setting, qubits are subject to \emph{phase-flips}.
\item We are not allowed to directly measure the encoded qubits,
as that would disturb their quantum states.
as that would collapse their quantum states.
\end{itemize}
We can deal with the first constraint by not duplicating information, instead
spreading the quantum state across the physical qubits
@@ -72,8 +73,8 @@ subsequent decoding process on the measured syndrome.
Another difference between \ac{qec} and classical channel coding is
the resource constraints.
For \ac{qec}, low latency matters more than low overall computational
complexity, due to the backlog problem
For \ac{qec}, achieving low latency matters more than having a low
overall computational complexity, due to the backlog problem
\cite[Sec.~II.G.3.]{terhal_quantum_2015}: Certain gates turn
single-qubit errors into multi-qubit ones, so errors must be
corrected beforehand.
@@ -81,7 +82,7 @@ A \ac{qec} system that is too slow accumulates a backlog at these points,
causing exponential slowdown.
Several code constructions have been proposed for \ac{qec} codes over the years.
Topological codes such as surface codes have been the industry
Topological codes, such as surface codes, have been the industry
standard for experimental applications for a long time
\cite[Sec.~I]{koutsioumpas_colour_2025}, due to their
reliance on only local connections between qubits
@@ -114,15 +115,15 @@ focusing only on the relationship between possible errors
and their effects on the syndrome \cite[Sec.~1.4.3]{higgott_practical_2024}.
A \emph{detector error matrix} is generated from the circuit, which is
used for decoding instead of the original check matrix.
Decoding under a \ac{dem} poses a challenge with respect to the
latency constraint.
This is because the detector error matrix is much larger than the
The detector error matrix is much larger than the
check matrix of the underlying code, since it needs to represent many
more error locations.
For example, in our experiments using the $\llbracket 144,12,12
\rrbracket$ \ac{bb} code with $12$ syndrome measurement rounds, the
number of \acp{vn} grew from $144$ to $9504$ and the number of
\acp{cn} grew from $72$ to $1008$.
Therefore, decoding under a \ac{dem} poses a challenge with respect to the
latency constraint.
To keep the latency of \ac{dem} decoding manageable, one approach is
\emph{sliding-window decoding}.
@@ -152,7 +153,7 @@ We propose \emph{warm-start sliding-window decoding}, in which the
\ac{bp} messages from the overlap region of the previous window are
reused to initialize \ac{bp} in the current window in place of the
standard cold-start initialization.
We formulate the warm start first for plain \ac{bp} and then for
We formulate the warm start for standard \ac{bp} and for
\ac{bpgd}, a variant of \ac{bp} with better convergence properties
for \ac{qec} codes.
The decoders are evaluated by Monte Carlo simulation on the
@@ -164,6 +165,7 @@ low-latency operation.
% Outline of the Thesis
This thesis is structured as follows:
\Cref{ch:Fundamentals} reviews the fundamentals of classical and
quantum error correction.
On the classical side, it covers binary linear block codes,

File diff suppressed because it is too large Load Diff

View File

@@ -16,17 +16,19 @@ using qubits.
While the use of error correcting codes may facilitate this, it also
introduces two new challenges \cite[Sec.~4]{gottesman_introduction_2009}:
\begin{itemize}
\item We must be able to perform operations on the encoded state
in such a way that we do not lose the protection against errors.
\item \ac{qec} systems are themselves partially implemented in
quantum hardware. In addition to the errors we have
originally introduced them for, these systems must
be able to account for the fact they are implemented on noisy
hardware themselves.
\item To realize a quantum algorithm, we must be able to
perform operations on the encoded state in such a way that we
do not lose the protection against errors.
\item \ac{qec} systems, in particular the syndrome extraction
circuit, are themselves partially implemented in
quantum hardware.
In addition to the errors we have originally introduced them
for, these systems must therefore be able to account for the
fact they are implemented on noisy hardware themselves.
\end{itemize}
In the literature, both of these points are viewed under the umbrella
of \emph{fault tolerance}.
We focus only on the second aspect in this work.
of \emph{fault-tolerant} quantum computing.
In this thesis, we focus on the second aspect.
It was recognized early on as a challenge of \ac{qec} that the correction
machinery itself may introduce new faults \cite[Sec.~III]{shor_scheme_1995}.
@@ -43,16 +45,16 @@ address both.
We model the possible occurrence of errors during any processing
stage as different \emph{error locations} $E_i,~i\in [1:N]$
in the circuit.
$N \in \mathbb{N}$ is the total number of considered error locations.
The parameter $N \in \mathbb{N}$ is the total number of considered
error locations.
The \emph{circuit error vector} $\bm{e} \in \{0,1\}^N$ is a vector
indicating which errors occurred, with
\begin{align*}
e_i :=
\begin{cases}
1, & \text{Error $E_i$ occurred} \\
0, & \text{otherwise}
1, & \text{error $E_i$ occurred}, \\
0, & \text{otherwise}.
\end{cases}
.%
\end{align*}
\Cref{fig:fault_tolerance_overview} illustrates the flow of errors.
Specifically for \ac{css} codes, a \ac{qec} procedure is deemed
@@ -72,12 +74,14 @@ fault-tolerant, if \cite[Def.~4.2]{derks_designing_2025}
where $t = \lfloor (d_\text{min} -1)/2 \rfloor$ is the number of
errors the code is able to correct.
The vectors $\bm{e}_{\text{output},X}$ and $\bm{e}_{\text{output},Z}$
denote only $X$ and $Z$ errors respectively.
denote only $X$ and $Z$ errors, respectively.
% TODO: Properly introduce d_min for QEC, specifically for CSS codes
In order to deal with internal errors that flip syndrome bits,
multiple rounds of syndrome measurements must be performed.
Typically, the number of syndrome extraction rounds is chosen as $d_\text{min}$.
multiple rounds of syndrome measurements are performed.
Typically, the number of syndrome extraction rounds is chosen as
$d_\text{min}$, e.g., \cite{gong_toward_2024}
\cite{koutsioumpas_automorphism_2025}.
% % This is the definition of a fault-tolerant QEC gadget
% A \ac{qec} procedure is deemed fault tolerant if
@@ -150,7 +154,7 @@ Typically, the number of syndrome extraction rounds is chosen as $d_\text{min}$.
% Intro
We collect the probabilities of error at each location in the
\emph{noise model}, a vector $\bm{p} \in [0,1]^N$.
\emph{noise model}, represented by a vector $\bm{p} \in [0,1]^N$.
There are different types of noise models, each allowing for
different error locations in the circuit.
@@ -178,8 +182,7 @@ $\ket{\psi}_\text{L}$ as \emph{data qubits}.
Note that this is a concrete implementation using CNOT gates, as
opposed to the system-level view introduced in
\Cref{subsec:Stabilizer Codes}.
We visualize the different types of noise models in
\Cref{fig:noise_model_types}.
\Cref{fig:noise_model_types} visualizes the different types of noise models.
%%%%%%%%%%%%%%%%
\subsection{Bit-Flip Noise}
@@ -188,9 +191,11 @@ We visualize the different types of noise models in
The simplest type of noise model is \emph{bit-flip} noise.
This corresponds to the classical \ac{bsc}, i.e., only $X$ errors on the
data qubits are possible \cite[Appendix~A]{gidney_new_2023}.
This type of noise model is shown in \Cref{subfig:bit_flip}.
The occurrence of bit-flip errors is modeled as a Bernoulli process
$\text{Bern}(p)$.
\Cref{subfig:bit_flip} shows this type of noise model.
Note that we cannot use bit-flip noise to develop fault-tolerant
Note that bit-flip noise is not suitable for developing fault-tolerant
systems, as it does not account for errors during the syndrome extraction.
%%%%%%%%%%%%%%%%
@@ -221,7 +226,7 @@ Here, we consider multiple rounds of syndrome measurements with a
depolarizing channel before each round.
Additionally, we allow for measurement errors by having $X$ error
locations right before each measurement \cite[Appendix~A]{gidney_new_2023}.
Note that it is enough to only consider $X$ errors at these points,
Note that it is enough to only consider $X$ errors before measuring,
since that is the only type of error directly affecting the
measurement outcomes.
This model is depicted in \Cref{subfig:phenomenological}.
@@ -243,15 +248,15 @@ Here we not only consider noise between syndrome extraction rounds
and at the measurements, but at each gate.
Specifically, we allow arbitrary $n$-qubit Pauli errors after each
$n$-qubit gate \cite[Def.~2.5]{derks_designing_2025}.
An $n$-qubit Pauli error is simply a series of correlated Pauli
An $n$-qubit Pauli error can be written as a series of correlated Pauli
errors on each related individual qubit.
This type of noise model is shown in \Cref{subfig:circuit_level}.
While phenomenological noise is useful for some design aspects of
fault tolerant circuitry, for simulations, circuit-level noise should
fault-tolerant circuitry, for simulations, circuit-level noise should
always be used \cite[Sec.~4.2]{derks_designing_2025}.
Note that this introduces new challenges during the decoding process,
as the decoding complexity is increased considerably due to the many
as the decoding complexity is considerably increased due to the many
error locations.
\begin{figure}[t]
@@ -282,11 +287,11 @@ error locations.
framework for
passing information about a circuit used for \ac{qec} to a decoder.
They are also useful as a theoretical tool to aid in the design of
fault-tolerant \ac{qec} schemes.
E.g., they can be used to easily determine whether a measurement
schedule is fault-tolerant \cite[Example~12]{derks_designing_2025}.
fault-tolerant \ac{qec} schemes, e.g., they can be used to easily
determine whether a measurement schedule is fault-tolerant
\cite[Example~12]{derks_designing_2025}.
Other approaches of implementing fault tolerance exist, such as
Other approaches of implementing fault-tolerance circuits exist, e.g.,
flag error correction, which uses additional ancilla qubits to detect
potentially damaging high-weight errors \cite[Sec.~1]{chamberland_flag_2018}.
However, \acp{dem} offer some unique advantages
@@ -300,8 +305,7 @@ However, \acp{dem} offer some unique advantages
treated in a unified manner. This leads to a more powerful
description of the overall circuit.
\end{itemize}
In this work, we only consider the process of decoding under the
\ac{dem} framework.
In this work, we consider the process of decoding under the \ac{dem} framework.
% Core idea
@@ -309,7 +313,7 @@ To achieve fault tolerance, the goal we strive towards is to
consider the internal errors in addition to the input errors during
the decoding process.
The core idea behind detector error models is to do this by defining
a new \emph{circuit code} that describes the circuit.
a new \emph{circuit code} describing the whole circuit.
Each \ac{vn} of this new code corresponds to an error location in the
circuit and each \ac{cn} corresponds to a syndrome measurement.
% This circuit code, combined with the prior probabilities of error
@@ -445,12 +449,11 @@ matrix} $\bm{\Omega} \in \mathbb{F}_2^{M\times N}$, with
\begin{align*}
\Omega_{\ell,i} =
\begin{cases}
1, & \text{Error $i$ flips measurement $\ell$}\\
0, & \text{otherwise}
1, & \text{error $i$ flips measurement $\ell$},\\
0, & \text{otherwise},
\end{cases}
,%
\end{align*}
where $M \in \mathbb{N}$ is the number of measurements.
where $M \in \mathbb{N}$ is the number of performed syndrome measurements.
To obtain $\bm{\Omega}$, we must propagate Pauli errors through the
circuit, tracking which measurements they affect
\cite[Sec.~2.4]{derks_designing_2025}.
@@ -459,15 +462,22 @@ circuit, tracking which measurements they affect
We turn to our example of the three-qubit repetition code to
illustrate the construction of the syndrome measurement matrix.
We begin by extending our check matrix in \Cref{eq:rep_code_H}
to represent three rounds of syndrome extraction.
We begin by extending our check matrix $\bm{H}_Z$ in
\Cref{eq:rep_code_H} to represent three rounds of syndrome extraction.
Each round yields an additional set of syndrome bits,
and we combine them by stacking them in a new vector
$\bm{s} \in \mathbb{F}_2^{R(n-k)}$.
We thus have to replicate the rows of $\bm{\Omega}$, once for each
additional syndrome measurement, to obtain
$\bm{s} \in \mathbb{F}_2^{R(n-k)}$, where $R \in \mathbb{N}$ is the
number of syndrome measurement rounds.
Thus, we have to replicate the rows of $\bm{H}_Z$, once for each
additional syndrome measurement, and obtain
\begin{align*}
\bm{\Omega} =
\bm{\Omega}_0 =
\begin{pmatrix}
\bm{H}_Z \\
\bm{H}_Z \\
\bm{H}_Z
\end{pmatrix}
=
\begin{pmatrix}
1 & 1 & 0 \\
0 & 1 & 1 \\
@@ -482,31 +492,31 @@ additional syndrome measurement, to obtain
depicts the corresponding circuit.
Note that we have not yet introduced error locations in the syndrome
extraction circuitry, so we still consider only bit flip noise at this stage.
Recall that $\bm{\Omega}$ describes which \ac{vn} is connected to
Recall that $\bm{\Omega}_0$ describes which \ac{vn} is connected to
which parity check and the syndrome indicates which parity checks
are violated.
This means that if an error exists at only a single \ac{vn}, we can
read off the syndrome in the corresponding column.
Therefore, if an error occurs that corresponds to a single \ac{vn},
the measured syndrome is the corresponding column.
If errors occur at multiple locations, the resulting syndrome will be
the linear combination of the respective columns.
We thus have
Thus, we have
\begin{align*}
\bm{s} \in \text{span} \{\bm{\Omega}\}
\bm{s} \in \text{span} \{\bm{\Omega}_0\}
.%
\end{align*}
% Expand to phenomenological
We now wish to expand the error model to phenomenological noise, though
Next, we expand the error model to phenomenological noise, though
only considering $X$ errors in this case.
We introduce new error locations at the appropriate positions,
arriving at the circuit depicted in
resulting in the circuit depicted in
\Cref{fig:rep_code_multiple_rounds_phenomenological}.
For each additional error location, we extend $\bm{\Omega}$ by
appending the corresponding syndrome vector as a column.
For each additional error location, we extend $\bm{\Omega}_0$ by
appending the corresponding syndrome vector as a column, yielding
\begin{gather}
\label{eq:syndrome_matrix_ex}
\bm{\Omega} =
\bm{\Omega}_1 =
\left(
\begin{array}{ccccccccccccccc}
1 & 1 & 0 & 1 & 0 & 0 & 0 & 0 & 0 & 0
@@ -523,24 +533,25 @@ appending the corresponding syndrome vector as a column.
& 0 & 1 & 1 & 0 & 1
\end{array}
\right) . \\[-6mm]
\hspace*{-58.7mm}
\hspace*{-56.7mm}
\underbrace{
\phantom{
\begin{array}{ccc}
0 & 0 & 0
\end{array}
}
}_\text{Original matrix}
}_{\bm{\Omega}_0} \nonumber
\end{gather}
Notice that the first three columns correspond to the original
measurement syndrome matrix, as these columns correspond to the error
locations on the data qubits.
measurement syndrome matrix $\bm{\Omega}_0$, as these columns
correspond to the error locations on the data qubits.
In this example, all measurements we considered were syndrome measurements.
Assuming no errors, the results of those measurements were
deterministic, irrespective of the actual logical state
$\ket{\psi}_\text{L}$, as they only depend on whether
$\ket{\psi}_\text{L} \in \mathcal{C}$, not on the concrete state.
Assuming no errors, the results of those measurements are
deterministic: They are not subject to any probabilistic behavior
despite the quantum mechanical nature of the underlying system.
They only depend on whether $\ket{\psi}_\text{L} \in \mathcal{C}$,
not on the concrete state.
It is, in general, possible to also consider non-deterministic measurements.
As an example, it is usual to consider a round of noiseless
measurements of the actual data qubit states after the last syndrome
@@ -557,7 +568,7 @@ extraction round.
\centering
\begin{tikzpicture}
\node{$%
\bm{\Omega} =
\bm{\Omega}_0 =
\begin{pmatrix}
1 & 1 & 0 \\
0 & 1 & 1 \\
@@ -659,7 +670,7 @@ extraction round.
\begin{figure}[t]
\begin{gather*}
\hspace*{-33.3mm}%
\hspace*{-31.8mm}%
\begin{array}{c}
E_6 \\
\downarrow
@@ -667,7 +678,7 @@ extraction round.
\end{gather*}
\vspace*{-8mm}
\begin{gather*}
\bm{\Omega} =
\bm{\Omega}_1 =
\left(
\begin{array}{
cccccc%
@@ -761,10 +772,10 @@ Instead of using stabilizer measurement results directly, we
generalize the notion of what constitutes a parity check slightly.
We formally define a \emph{detector} as a deterministic parity constraint on
a set of measurement outcomes \cite[Def.~2.1]{derks_designing_2025}.
It can be seen that we will have as many linearly
independent detectors as there are separate deterministic measurements.
In the most straightforward case, we may simply use the stabilizer
measurements as detectors.
We immediately recognize that we will have as many linearly
independent detectors as there are separate deterministic measurements.
We generally aim to utilize the maximum number of linearly
independent detectors \cite[Sec.~2.2]{derks_designing_2025}.
@@ -775,23 +786,22 @@ the \emph{detector matrix} $\bm{D} \in \mathbb{F}_2^{D\times M}$
\cite[Def.~2.2]{derks_designing_2025}, with $~D\in \mathbb{N}$
denoting the number of detectors.
Similar to the way a \ac{pcm} associates bits with parity checks, the
detector matrix links measurements and detectors.
Each column corresponds to a measurement, while each rows corresponds
detector matrix links measurement outcomes and detectors.
Each column corresponds to a measurement, while each row corresponds
to a detector.
We should note at this point that the combination of measurements
into detectors has no bearing on the actual construction of the
syndrome extraction circuitry.
It is something that happens ``virtually'' after the fact and only
affects the decoder.
It is something that happens ``virtually'' and only affects the decoder.
Note that we can use the detector matrix $\bm{D}$ to describe the set
of possible measurement outcomes under the absence of noise.
The same way we use a \ac{pcm} to describe the code space as
\begin{align*}
Similar to the we use a \ac{pcm} to describe the code space as
\begin{equation*}
\mathcal{C}
= \{ \bm{x} \in \mathbb{F}_2^{n} : \bm{H}\bm{x}^\text{T} = \bm{0} \}
= \{ \bm{x} \in \mathbb{F}_2^{n} : \bm{H}\bm{x}^\mathsf{T} = \bm{0} \}
,%
\end{align*}
\end{equation*}
the set of possible measurement outcomes is simply $\text{kern}\{\bm{D}\}$
\cite[Sec.~2.2]{derks_designing_2025}.
@@ -806,7 +816,7 @@ affect the measurements (through $\bm{\Omega}$), and we know how the
measurements relate to the detectors (through $\bm{D}$).
For decoding, we are interested in the effect of the errors on the
detectors directly.
We thus construct the \emph{detector error matrix} $\bm{H} \in
Thus, we construct the \emph{detector error matrix} $\bm{H} \in
\mathbb{F}_2^{D\times N}$ \cite[Def.~2.9]{derks_designing_2025} as
\begin{align*}
\bm{H} := \bm{D}\bm{\Omega}
@@ -834,10 +844,10 @@ violate the same set of detectors, i.e.,
\begin{align*}
\hspace{-15mm}
% tex-fmt: off
&& \bm{H} \bm{e}_1^\text{T} & \neq \bm{H} \bm{e}_2^\text{T} \\
\iff \hspace{-33mm} && \bm{H} \left( \bm{e}_1 - \bm{e}_2 \right)^\text{T} & \neq 0 \\
\iff \hspace{-33mm} && \bm{D} \bm{\Omega} \left( \bm{e}_1 - \bm{e}_2 \right)^\text{T} & \neq 0 \\
\iff \hspace{-33mm} && \bm{\Omega} \left( \bm{e}_1 - \bm{e}_2 \right)^\text{T} & \notin \text{kern} \{\bm{D}\}
&& \bm{H} \bm{e}_1^\mathsf{T} & \neq \bm{H} \bm{e}_2^\mathsf{T} \\
\iff \hspace{-33mm} && \bm{H} \left( \bm{e}_1 - \bm{e}_2 \right)^\mathsf{T} & \neq 0 \\
\iff \hspace{-33mm} && \bm{D} \bm{\Omega} \left( \bm{e}_1 - \bm{e}_2 \right)^\mathsf{T} & \neq 0 \\
\iff \hspace{-33mm} && \bm{\Omega} \left( \bm{e}_1 - \bm{e}_2 \right)^\mathsf{T} & \notin \text{kern} \{\bm{D}\}
% tex-fmt: on
.%
\end{align*}
@@ -850,7 +860,7 @@ It may, however, change the decoding performance when using a practical decoder.
What constitutes a good set of detectors is difficult to assess
without performing explicit decoding simulations, since it ultimately
depends on the decoder employed.
depends on the employed decoder.
For iterative decoders, high sparsity is generally beneficial, but
finding detectors that maximize sparsity is an NP-complete problem
\cite[Sec.~2.6]{derks_designing_2025}.
@@ -859,7 +869,7 @@ at a later stage.
To the measurement results from each syndrome extraction round we
can add the results from the previous round, as illustrated in
\Cref{fig:detectors_from_measurements_general}.
We thus have $D=n-k$.
Thus, we have $D=n-k$.
Concretely, we denote the outcome of
measurement $\ell \in [1:n-k]$ in round $r \in [1:R]$ by
$m_\ell^{(r)} \in \mathbb{F}_2$
@@ -915,7 +925,8 @@ with $\bm{m}^{(0)} = \bm{0}$.
We again turn our attention to the three-qubit repetition code.
In \Cref{fig:rep_code_multiple_rounds_phenomenological} we can see
that $E_6$ has occurred and has subsequently tripped the last four measurements.
that $E_6$ has occurred and has subsequently triggered the last four
measurements.
We now take those measurements and combine them according to
\Cref{eq:measurement_combination}.
We can see this process graphically in
@@ -923,19 +934,20 @@ We can see this process graphically in
To understand why this way of defining the detectors is useful, we
note that the error $E_6$ in
\Cref{fig:rep_code_multiple_rounds_phenomenological} has not only
tripped the measurements in the syndrome extraction round immediately
triggered the measurements in the syndrome extraction round immediately
afterwards, but all subsequent ones as well.
To only see errors in the rounds immediately following them, we
consider our newly defined detectors instead of the measurements,
that effectively compute the difference between the measurements.
To only see the effect of errors in the syndrome measurement round
immediately following them, we consider our newly defined detectors
instead of the measurements.
These effectively compute the difference between the measurements.
Each error can only trip syndrome bits that follow it.
Each error can only trigger syndrome bits that follow it.
This is reflected in the triangular structure of $\bm{\Omega}$ in
\Cref{eq:syndrome_matrix_ex}.
Combining the measurements into detectors according to
\Cref{eq:measurement_combination}, we are effectively performing
row additions in such a way as to clear the bottom left of the matrix.
The detector error matrix
The resulting detector error matrix
\begin{align*}
\bm{H} =
\left(
@@ -949,7 +961,7 @@ The detector error matrix
\end{array}
\right)
\end{align*}
we obtain this way has a block-diagonal structure.
has a block-diagonal structure.
Note that we exploit the fact that each syndrome measurement round is
identical to obtain this structure.
@@ -998,9 +1010,8 @@ error matrix $\bm{H}$ and the noise model $\bm{p}$.
\cite[Sec.~6]{derks_designing_2025}.
It serves as an abstract representation of a circuit and can be used
both to transfer information to a decoder but also to aid in the
design of fault-tolerant systems.
E.g., it can be used to investigate the properties of a circuit with
respect to fault tolerance.
design of fault-tolerant systems, e.g., it can be used to investigate
the properties of a circuit with respect to fault tolerance.
It contains all information necessary for the decoding process.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
@@ -1030,11 +1041,11 @@ measurements) models \cite[Sec.~2.1]{gidney_fault-tolerant_2021}.
These differ in the way they compute individual error probabilities
from the physical error rate.
In this work we only consider \emph{standard circuit-based depolarizing
noise}, as this is the standard approach in the literature.
We thus set the error probabilities of all error locations in the
circuit-level noise model to the same value, the physical error rate
$p_\text{phys}$.
In this work we consider the \emph{standard circuit-based depolarizing
noise} variant of circuit-level noise, as this is the standard
approach in the literature:
We set the error probabilities of all error locations to the same
value, the physical error rate $p_\text{phys}$.
%%%%%%%%%%%%%%%%
\subsection{Per-Round Logical Error Rate}
@@ -1042,7 +1053,7 @@ $p_\text{phys}$.
% Per-round LER
Another aspect that is important to consider is the meaning of the
Another important aspect to consider is the meaning of the
\ac{ler} in the context of a \ac{qec} system with multiple
rounds of syndrome measurements.
In order to facilitate the comparability of results obtained from
@@ -1053,7 +1064,7 @@ The simplest way of calculating the per-round \ac{ler} is by modeling
each round as an independent experiment.
For each experiment, an error might occur with a certain probability
$p_\text{e,round}$.
The overall probability of error is then
Then the overall probability of error is
\begin{align}
\hspace{-12mm}
p_\text{e,total} &= 1 - (1 - p_\text{e,round})^{R} \nonumber\\
@@ -1063,15 +1074,15 @@ The overall probability of error is then
.%
\hspace{12mm}
\end{align}
We approximate $p_\text{e,total}$ using a Monte Carlo simulation and
compute the per-round-\ac{ler} using \Cref{eq:per_round_ler}.
This is a common approach taken in the literature
\cite{gong_toward_2024}\cite{wang_fully_2025}.
To this end, we approximate $p_\text{e,total}$ using a Monte Carlo
simulation and
compute the per-round-\ac{ler} according to \Cref{eq:per_round_ler}.
This is the approach taken in \cite{gong_toward_2024}\cite{wang_fully_2025}.
Another common approach \cite{chen_exponential_2021}%
Another approach \cite{chen_exponential_2021}%
\cite{bausch_learning_2024}\cite{beni_tesseract_2025} is to assume an
exponential decay for the decoder's \emph{logical fidelity}
\cite[Eq.~2]{bausch_learning_2024}
exponential decay for the \emph{logical fidelity} of the decoder
\cite[Eq.~(2)]{bausch_learning_2024}
\begin{align*}
F_\text{total} = (F_\text{round})^{R}
.%
@@ -1079,7 +1090,7 @@ exponential decay for the decoder's \emph{logical fidelity}
The logical fidelity is a measure of the quality of a logical state
\cite[Appendix~E]{postler_demonstration_2024}.
As it is related to the error rate through $F = 1 - 2p$, we obtain
\cite[Eq.~4]{bausch_learning_2024}
\cite[Eq.~(4)]{bausch_learning_2024}
\begin{align}
(1 - 2p_\text{e,total}) &= (1 - 2p_\text{e,round})^{R} \nonumber\\
\implies \hspace{15mm} p_\text{e,round} &= \frac{1}{2}
@@ -1095,10 +1106,10 @@ topic to our own work.
\subsection{Stim}
\label{subsec:Stim}
It is not immediately apparent how the \ac{dem} will look from looking
at a code's \ac{pcm}, because it heavily depends on the exact circuit
construction and choice of noise model.
As we noted in \Cref{subsec:Measurement Syndrome Matrix}, we can
It is not immediately apparent how the \ac{dem} will look from
considering the \ac{pcm} of a code, because it heavily depends on the
exact circuit construction and choice of noise model.
As we noted in \Cref{subsec:Measurement Syndrome Matrix}, we
obtain a measurement syndrome matrix by propagating Pauli frames
through the circuit.
The standard choice of simulation tool used for this purpose is
@@ -1109,16 +1120,16 @@ pypi package.
In fact, it was in this tool that the concept of the \ac{dem} was
first introduced.
One capability of stim, and \acp{dem} in general, that we didn't go
into detail about in this chapter is the merging of error mechanisms.
One capability of stim, and \acp{dem} in general, that we did not
explain in detail in this chapter, is the merging of error mechanisms.
Since \acp{dem} differentiate errors based on their effect on the
measurements and not on their Pauli type and location
\cite[Sec.~1.4.3]{higgott_practical_2024}, it is natural to group
errors that have the same effect.
errors that have the same effect, i.e., syndrome.
This slightly lowers the computational complexity of decoding, as the
number of resulting \acp{vn} is reduced.
While stim is a useful tool for circuit simulation, it doesn't
While stim is a useful tool for circuit simulation, it does not
include many utilities for building syndrome extraction circuitry automatically.
The user has to define most, if not all, of the circuit manually,
depending on the code in question.

View File

@@ -2,35 +2,34 @@
\chapter{Decoding under Detector Error Models}
\label{ch:Decoding}
In \Cref{ch:Fundamentals} we introduced the fundamentals of classical
error correction, before moving on to quantum information science and
In \Cref{ch:Fundamentals}, we introduced the fundamentals of classical
error correction, before turning to quantum information science and
finally combining the two in \acf{qec}.
In \Cref{ch:Fault tolerance} we then turned to fault-tolerance, with
In \Cref{ch:Fault tolerance}, we then considered fault-tolerance, with
a focus on a specific way of implementing it, called \acfp{dem}.
In this chapter, we move on from the fundamental concepts and examine
how to apply them in practice.
Specifically, we concern ourselves with the practical aspects of decoding
under \acp{dem}.
Specifically, we consider the practical aspects of decoding under \acp{dem}.
We investigate decoding \acf{qldpc} codes under \acp{dem} in particular.
In particular, we investigate decoding \acf{qldpc} codes under \acp{dem}.
We focus on \ac{qldpc} codes, as they have emerged as leading
candidates for practical quantum error correction, offering
comparable thresholds with substantially improved encoding rates
good thresholds with substantially improved encoding rates
\cite[Sec.~1]{bravyi_high-threshold_2024}.
Because of this, the decoding algorithms we consider will all be
related to \acf{bp} in some way.
based on \acf{bp}.
Our aim is to build a fault-tolerant \ac{qec} system that works well
even in the presence of circuit-level noise.
We must overcome two main challenges to achieve this.
First, recall the problems related to degeneracy, which is inherent
to quantum codes.
Because multiple minimum-weight codewords exist, the \ac{bp}
algorithm becomes uncertain of the direction to proceed in.
Because multiple minimum-weight solutions to the decoding problem may
exist, the \ac{bp} algorithm becomes uncertain of the direction to proceed in.
Additionally, the commutativity conditions of the stabilizers
necessitate the existence of short cycles.
Together, these two aspects lead to substantial convergence problems
of \ac{bp} for quantum codes, when it is used on its own.
of \ac{bp} for quantum codes, when employed on its own.
Second, the consideration of circuit-level noise introduces many more
error locations into the circuit.
@@ -40,28 +39,28 @@ We also perform multiple rounds of syndrome measurements,
exacerbating the problem.
This leads to a massively increased computational complexity and
latency of the decoding process.
In our experiments using the $\llbracket 144,12,12 \rrbracket$
\acf{bb} code with $12$ syndrome measurement rounds, for example, the
number of \acp{vn} grew from $144$ to $9504$, and the
number of \acfp{cn} grew from $72$ to $1008$.
For example, in our experiments using the $\llbracket 144,12,12
\rrbracket$ \acf{bb} code with $12$ syndrome measurement rounds, the
number of \acp{vn} grew from $144$ to $9504$, and the number of
\acfp{cn} grew from $72$ to $1008$.
The first problem is not inherent to \acp{dem} or fault-tolerance,
but rather quantum codes in general.
Many different approaches to solving it exist, usually centered
around somehow modifying \ac{bp}.
The most popular approach is combining a few initial
iterations of \ac{bp} with a second decoding algorithm, \ac{osd}
around modifying \ac{bp}.
The most popular approach is combining a few initial iterations of
\ac{bp} with a second decoding algorithm, \ac{osd}
\cite{roffe_decoding_2020}.
Other approaches exist, such as \ac{aed}
\cite{koutsioumpas_automorphism_2025}, where multiple variations of
the code are decoded simultaneously to increase the chances of convergence.
the syndrome, based on graph and code symmetries, are decoded
simultaneously to increase the chances of convergence.
Here, we will focus on the \acf{bpgd} algorithm
\cite{yao_belief_2024} we already introduced in \Cref{ch:Fundamentals},
for reasons that will become clear later in the chapter.
\cite{yao_belief_2024} introduced in \Cref{ch:Fundamentals}.
The second problem is inherent to decoding using \acp{dem}.
This is an area that has received less attention.
As we saw in \Cref{sec:Quantum Error Correction}, for \ac{qec},
This is an area that has so far received less attention in the literature.
As discerned in \Cref{sec:Quantum Error Correction}, for \ac{qec},
latency is the main constraint, not raw computational complexity.
The main way this is addressed in the literature is \emph{sliding
window decoding}, which attempts to divide the overall decoding
@@ -70,7 +69,7 @@ problem into many smaller ones that can be solved more efficiently.
% TODO: This could potentially be a bit more text (e.g., go into
% SC-LDPC like structure that serves as the inspiration for the
% warm-start decoding. Or just go into warm-start decoding)
Our own work will focus mostly on the the solution of the second
In this thesis, we will focus mostly on the the solution of the second
problem using sliding-window decoding.
We will start by briefly reviewing the existing work related to
sliding-window decoding,
@@ -200,7 +199,7 @@ Each of these windows is then decoded separately.
% Some general notes
\Cref{fig:literature} gives an overview over the existing body of work
\Cref{fig:literature} gives an overview over the existing works
related to sliding-window decoding.
The papers \cite{huang_improved_2023} and \cite{huang_increasing_2024} are
lumped together, as they share the same content;
@@ -217,9 +216,9 @@ software freely available online%
\footnote{
\url{https://github.com/gongaa/SlidingWindowDecoder}
}.
A final thing to note is that \cite{dennis_topological_2002} never
explicitly mentions sliding windows; the authors call their scheme
``overlapping recovery''.
Finally, note that \cite{dennis_topological_2002} never explicitly
mentions sliding windows; the authors call their scheme ``overlapping
recovery''.
% Topological vs QLDPC
@@ -244,7 +243,7 @@ Finally, \cite{gong_toward_2024} explores \ac{bb} codes.
% Sequential vs parallel
After having divided the whole circuit into separate windows, the question
arises of how exactly to realize the decoding.
arises of how to make use of the window-like structure for decoding.
There are two main approaches, with differing mechanisms of reducing
the latency.
Some papers decode the sliding windows in a parallel fashion.
@@ -252,7 +251,8 @@ The benefit in this case is
is that classical hardware can be utilized more effectively.
Others choose a sequential approach.
Here, decoding can start earlier, as there is no need to wait for the
syndrome measurements of all windows before beginning with the decoding.
syndrome measurements of subsequent windows before beginning with the
decoding of earlier windows.
With the exception of \cite{dennis_topological_2002}, literature
treating topological codes has mostly focused on parallel decoding
while literature treating \ac{qldpc} codes has wholly considered
@@ -261,7 +261,7 @@ sequential decoding.
% Deep-dive into QLDPC methods
For this work, the publications treating \ac{qldpc} codes are
especially interesting.
particularly interesting.
The experimental conditions for these are summarized in
\Cref{table:experimental_conditions}.
As we noted above, \ac{hgp} and \ac{lp} codes are considered in
@@ -274,7 +274,7 @@ The employed noise models also differ;
Finally, in \cite{gong_toward_2024} the authors introduce their own variation of
\ac{bpgd}, \ac{bp} with \ac{gdg}, while \cite{huang_increasing_2024}
and \cite{kang_quits_2025} use \ac{bp} + \ac{osd}.
We would additionally like to note that only in
We would additionally like to note that only
\cite{gong_toward_2024} and \cite{kang_quits_2025}
explicitly work with the \ac{dem} formalism.
@@ -286,12 +286,12 @@ explicitly work with the \ac{dem} formalism.
sliding-window decoding for \ac{qldpc} codes.}
\vspace*{3mm}
\label{table:experimental_conditions}
\begin{tabular}{l|ccc}
\begin{tabular}{lccc}\toprule
% tex-fmt: off
Publication & Code & Noise Model & Decoder \\ \hline
\hspace{-2.5mm}\cite{huang_improved_2023},\cite{huang_increasing_2024} & \acs{hgp}, \acs{lp} & Phenomenological noise & \acs{bp} + \acs{osd} \\
\hspace{-2.5mm}\cite{gong_toward_2024} & \acs{bb} & Circuit-level noise & \acs{bp} + \acs{gdg} \\
\hspace{-2.5mm}\cite{kang_quits_2025} & \acs{hgp}, \acs{lp}, \acs{bpc} & Circuit-level noise & \acs{bp} + \ac{osd}
Publication & Code & Noise Model & Decoder \\ \midrule
\hspace{-2.5mm}\cite{huang_improved_2023},\cite{huang_increasing_2024} & \acs{hgp}, \acs{lp} & Phenomenological noise & \acs{bp} + \acs{osd} \\
\hspace{-2.5mm}\cite{gong_toward_2024} & \acs{bb} & Circuit-level noise & \acs{bp} + \acs{gdg} \\
\hspace{-2.5mm}\cite{kang_quits_2025} & \acs{hgp}, \acs{lp}, \acs{bpc} & Circuit-level noise & \acs{bp} + \acs{osd} \\ \bottomrule
% tex-fmt: on
\end{tabular}
\end{table}
@@ -382,7 +382,7 @@ explicitly work with the \ac{dem} formalism.
\subsection{Window Splitting and Sequential Sliding-Window Decoding}
\label{subsec:Window Splitting and Sequential Sliding-Window Decoding}
In this section, we will examine the methodology by which a detector
In this section, we examine the methodology by which a detector
error matrix is divided into overlapping windows.
The algorithm detailed here follows \cite{kang_quits_2025}, which
is in turn based on \cite{huang_increasing_2024}.
@@ -392,7 +392,7 @@ is in turn based on \cite{huang_increasing_2024}.
Sliding-window decoding is made possible by the time-like structure
of the syndrome extraction circuitry.
This is especially clearly visible under the \ac{dem} formalism, where
this manifests as a block-diagonal structure of the detector
it manifests as a block-diagonal structure of the detector
error matrix $\bm{H}$.
Note that this presupposes a choice of detectors as seen in
\Cref{subsec:Detector Error Matrix}.
@@ -411,11 +411,10 @@ After decoding a window, there is a subset of \acp{cn} that
no longer contribute to decoding, since none of their
neighboring \acp{vn} appear in subsequent windows.
We call the set of \acp{vn} connected to those \acp{cn} the
\emph{commit region} and we wish to commit them before moving to the
next window, i.e., fix the values we estimate for the corresponding bits.
As mentioned above, the benefit of this sequential sliding-window
decoding approach
is that the decoding process can begin as soon as the syndrome
\emph{commit region} and we commit them before moving to the
next window, i.e., we fix the values we estimate for the corresponding bits.
The benefit of this sequential sliding-window decoding approach is
that the decoding process can begin as soon as the syndrome
measurements for the first window are complete.
% W and F and why we look at rows, not columns
@@ -425,15 +424,15 @@ The \emph{window size} $W \in \mathbb{N}$ represents the number of
syndrome extraction rounds lumped into one window, while
the \emph{step size} $F \in \mathbb{N}$ represents the number of
syndrome extraction rounds skipped before starting the next window.
$W$ controls the size of the windows while $F$ controls the overlap
between them.
The parameter $W$ controls the size of the windows while $F$ controls
the overlap between them.
As illustrated in \Cref{fig:windowing_pcm}, $W$ and $F$ control the
window dimensions and locations by defining the related \acp{cn},
not the \acp{vn}.
This is because while the number of overall \acp{cn} is only affected
This is because the number of overall \acp{cn} is only affected
by the choice of the underlying code and the number of syndrome
measurement rounds, the number of \acp{vn} depends on the noise model
and is difficult to predict beforehand.
measurement rounds, while the number of \acp{vn} depends on the noise
model and is difficult to predict beforehand.
\begin{figure}[t]
\centering
@@ -469,18 +468,16 @@ and is difficult to predict beforehand.
matrix generated from the $\llbracket 72, 6, 6 \rrbracket$
BB code under circuit-level noise.
The block-diagonal structure reflects the time-like locality
of the syndrome extraction circuit., with each block
of the syndrome extraction circuit, with each block
corresponding to one syndrome measurement round.
Two consecutive windows are highlighted: the window size $W$
controls the number of syndrome rounds included in each
window, while the step size $F$ controls how many rounds
separate the start of one window from the next.
Two consecutive windows are highlighted: The window size $W
\in \mathbb{N}$ controls the number of syndrome rounds
included in each window, while the step size $F \in
\mathbb{N}$ controls how many rounds separate the start of
one window from the next.
The bracketed region indicates the commit
region of the first window, i.e., the \acp{vn} that are committed
before moving to the second window.
% Visualization of the windowing process on a detector
% error matrix generated from the $\llbracket 72, 6, 6
% \rrbracket$ BB code.
before moving to the decoding of the second window.
}
\label{fig:windowing_pcm}
\end{figure}
@@ -493,52 +490,53 @@ We use the variables $n,m \in \mathbb{N}$ to describe the number of
We index the \acp{vn} using the variable $i \in \mathcal{I} :=
[0:n-1]$ and the \acp{cn} using the variable $j \in \mathcal{J} := [ 0 : m-1]$.
Finally, we call $\mathcal{N}_\text{V}(i) = \left\{ j\in \mathcal{J}:
\bm{H}_{j,i} = 1 \right\}$ and $\mathcal{N}_\text{C}(j) := \left\{ i
\in \mathcal{I} : \bm{H}_{j,i} = 1 \right\}$ the neighborhoods of the
corresponding nodes.
H_{j,i} = 1 \right\}$ and $\mathcal{N}_\text{C}(j) := \left\{ i
\in \mathcal{I} : H_{j,i} = 1 \right\}$ the neighborhoods of the
respective nodes.
In this case, we take $\bm{H} \in \mathbb{F}_2^{m\times n}$ to be the
check matrix of the underlying code, from which the \ac{dem} was generated.
We use $m_\text{DEM}, \mathcal{I}_\text{DEM}$, and $\mathcal{J}_\text{DEM}$
to refer to the respective values defined from the detector error matrix.
to refer to the respective values defined for the detector error matrix.
% How we get the corresponding rows
We begin by describing the sets of \acp{cn} relevant to each window.
First, we describe the sets of \acp{cn} relevant to each window.
For indexing, we use the variable $\ell \in [0:n_\text{win} - 1]$,
where $n_\text{win} \in \mathbb{N}$ is the number of windows.
Because we defined the step size $F$ as the number of syndrome
extraction rounds to skip, the first \ac{cn} of window $\ell$ should have index
Since we define the step size $F$ as the number of syndrome
extraction rounds to skip, the first \ac{cn} of window $\ell$ has index
$\ell F m$.
Similarly, because of the way we defined the window size $W$, the
number of \acp{cn} should be $Wm$ for all but the last window.
Similarly, due to the definition of the window size $W$, the
number of \acp{cn} per window is $Wm$ for all but the last window.
The number of \acp{cn} in the last window may differ if there are
not enough \acp{cn} left to completely fill it.
We thus define
Thus, we define
\begin{align*}
\mathcal{J}_\text{win}^{(\ell)} &:= \left\{ j\in \mathcal{J}_\text{DEM}:~
\ell F m \le j < \min \left\{m_\text{DEM}, (\ell F + W) m \right\}
\right\} \\[2mm]
& \hspace{30mm} \text{and} \\[2mm]
& \hspace{37mm} \text{and} \\[2mm]
\mathcal{J}_\text{commit}^{(\ell)} &:= \left\{ j\in \mathcal{J}_\text{DEM}:~
\ell F m \le j < \min \left\{m_\text{DEM}, (\ell + 1) F m \right\}
\right\}
.%
,%
\end{align*}
$\mathcal{J}_\text{win}^{(\ell)}$ is the set of all \acp{cn} in the
window while $\mathcal{J}_\text{commit}^{(\ell)}$ is the set of \acp{cn}
where $\mathcal{J}_\text{win}^{(\ell)}$ is the set of all \acp{cn} in the
window and $\mathcal{J}_\text{commit}^{(\ell)}$ is the set of \acp{cn}
that do not contribute to the next window and whose neighboring
\acp{vn} will thus be committed.
We can additionally define the set of \acp{cn} that are shared between windows
Additionally, we can define the set of \acp{cn} that are shared between windows
$\ell$ and $\ell + 1$ as $\mathcal{J}_\text{overlap}^{(\ell)} :=
\mathcal{J}_\text{win}^{(\ell)}\setminus \mathcal{J}_\text{commit}^{(\ell)}$.
% How we get the corresponding columns
We can now turn our attention to defining the sets of \acp{vn} relevant
We now turn our attention to defining the sets of \acp{vn} relevant
to each window.
We first introduce a helper function $i_\text{max} :
\mathcal{P}(\mathbb{N}) \to \mathbb{N}$, which takes a set of
\ac{cn} indices and returns the largest neighboring \ac{vn} index.
\mathcal{P}(\mathbb{N}) \to \mathbb{N}$, which takes a set
$\mathcal{S} \in \mathcal{P}(\mathbb{N})$ of \ac{cn} indices and
returns the largest neighboring \ac{vn} index.
We define
\begin{align*}
i_\text{max}\left( \mathcal{S} \right) := \max \left\{ i\in
@@ -552,13 +550,13 @@ where we set $i_\text{max} (\emptyset) = -1$ by convention%
and $\mathcal{I}_\text{win}^{(\ell)}$ appropriately.
}%
.
The commit region of window $\ell$ should include all of the \acp{vn}
The commit region of window $\ell$ includes all of the \acp{vn}
neighboring any of the \acp{cn} in $\mathcal{J}_\text{commit}^{(\ell)}$.
Consequently, the maximum index of the \acp{vn} we consider should be
Consequently, the maximum index of the \acp{vn} we consider is
$i_\text{max}(\mathcal{J}_\text{commit}^{(\ell)})$.
Additionally, the set of \acp{vn} committed in the next window should
start immediately afterwards.
We thus define
contain the next largest index.
Thus we define
\begin{align*}
\mathcal{I}_\text{commit}^{(\ell)}
&:= \left\{i \in \mathcal{I}_\text{DEM} :~
@@ -680,7 +678,7 @@ and after decoding all windows we will therefore have committed all \acp{vn}.
% Syndrome update
\Cref{fig:vis_rep} illustrates the meaning of the various sets of nodes.
\Cref{fig:vis_rep} illustrates the the various sets of nodes.
We can also see a subtlety we must handle carefully when
moving on to decode the next window.
While the \acp{vn} in $\mathcal{J}_\text{commit}^{(\ell)}$ have no
@@ -690,9 +688,12 @@ This is the case because these \acp{vn} have neighboring \acp{cn} in
the next window.
The part of the detector error matrix $\bm{H}_\text{DEM}$ describing
these connections is
$\bm{H}_\text{overlap}^{(\ell)} =
\left(\bm{H}_\text{DEM}\right)_{\mathcal{J}_\text{overlap}^{(\ell)},
\mathcal{I}_\text{commit}^{(\ell)}}$.
\begin{align*}
\bm{H}_\text{overlap}^{(\ell)} :=
\left(\bm{H}_\text{DEM}\right)_{\mathcal{J}_\text{overlap}^{(\ell)},
\mathcal{I}_\text{commit}^{(\ell)}}
.%
\end{align*}
We have to account for this fact by updating the syndrome $\bm{s}$
based on the committed bit values.
Specifically, if $\hat{\bm{e}}_\text{commit}^{(\ell)}$ describes the error
@@ -700,7 +701,7 @@ estimates committed after decoding window $\ell$, we have to set
\begin{align*}
\left(\bm{s}\right)_{\mathcal{J}_\text{overlap}^{(\ell)}} =
\bm{H}_\text{overlap}^{(\ell)}
\left( \hat{\bm{e}}_\text{commit}^{(\ell)} \right)^\text{T}
\left( \hat{\bm{e}}_\text{commit}^{(\ell)} \right)^\mathsf{T}
.%
\end{align*}
@@ -711,7 +712,7 @@ estimates committed after decoding window $\ell$, we have to set
% Intro: Problem with above procedure
The sliding-window structure visible in \Cref{fig:windowing_pcm} is
highly reminiscent of windowed decoding for \ac{sc}-\ac{ldpc} codes.
reminiscent of windowed decoding for \ac{sc}-\ac{ldpc} codes.
Switching our viewpoint to the Tanner graph depicted in
\Cref{fig:messages_decimation_tanner}, however, we can see an important
difference between \ac{sc}-\ac{ldpc} decoding and the
@@ -719,7 +720,7 @@ sliding-window decoding procedure detailed above.
While the windowing process is similar, the algorithm above
reinitializes the decoder to start from a clean state when moving to
the next window.
It therefore does not make use of the integral property of
Therefore, it does not make use of the integral property of
windowed \ac{sc}-\ac{ldpc} decoding of exploiting the spatially coupled
structure by passing soft information from earlier to later spatial positions.
@@ -731,8 +732,9 @@ still relevant to the decoding of the next.
This may somewhat limit the variety of \emph{inner decoders}, i.e.,
the decoders decoding the individual windows, the warm-start
initialization can be used with.
E.g., \ac{bp}+\ac{osd} does not immediately seem suitable, though
this remains to be investigated.
For instance, \ac{bp}+\ac{osd} does not immediately seem suitable, as
it performs a hard decision on the \acp{vn}, though this remains to
be investigated.
We chose to investigate first plain \ac{bp} due to its simplicity and
then \ac{bpgd} because of the availability of recently computed messages.
@@ -900,7 +902,8 @@ To see how we realize this in practice, we reiterate the steps of the
\right) \\[3mm]
\text{\ac{cn} Update (Min-Sum): }&
\displaystyle L_{i \leftarrow j} = (-1)^{s_j}\cdot \prod_{i'
\in \mathcal{N}_\text{C}(j)\setminus \{i\}} \sign \left( L_{i' \rightarrow j}
\in \mathcal{N}_\text{C}(j)\setminus \{i\}} \sign \left( L_{i'
\rightarrow j}
\right) \cdot \min_{i' \in \mathcal{N}_\text{C}(j)\setminus \{i\}} \lvert
L_{i'\rightarrow j} \rvert \\[3mm]
\label{eq:vn_update}
@@ -943,7 +946,7 @@ We can then continue decoding the next window as usual.
We can further simplify the algorithm.
Looking carefully at \Cref{eq:vn_update} we notice that when the
\ac{cn} to \ac{vn} messages $L_{i\leftarrow j}$ have been zero-initialized,
\ac{cn} to \ac{vn} messages $L_{i\leftarrow j}$ have been initialized to zero,
the \ac{vn} update degenerates to
\begin{align*}
\displaystyle L_{i \rightarrow j} =
@@ -971,7 +974,7 @@ Note that the decoding procedure performed on the individual windows
\label{alg:warm_start_bp}
\begin{algorithmic}[1]
\State \textbf{Initialize:} $\hat{\bm{e}}^\text{total} \leftarrow \bm{0}$
\State \textbf{Initialize:} $L_{i\leftarrow j} = 0
\State \textbf{Initialize:} $L_{i\leftarrow j} = 0,
~\forall~ i\in \mathcal{I}, j\in \mathcal{J}$
\For{$\ell = 0, \ldots, n_\text{win}-1$}
\For{$\nu = 0, \ldots, n_\text{iter}-1$}
@@ -983,7 +986,7 @@ Note that the decoding procedure performed on the individual windows
\State $\displaystyle\left(\hat{\bm{e}}^\text{total}\right)_{\mathcal{I}^{(\ell)}_\text{commit}} \leftarrow \hat{\bm{e}}^{(\ell)}_\text{commit}$
\State $\displaystyle\left(\bm{s}\right)_{\mathcal{J}_\text{overlap}^{(\ell)}}
\leftarrow \bm{H}_\text{overlap}^{(\ell)}
\left( \hat{\bm{e}}_\text{commit}^{(\ell)} \right)^\text{T}$
\left( \hat{\bm{e}}_\text{commit}^{(\ell)} \right)^\mathsf{T}$
\If{$\ell < n_\text{win} - 1$}
\State $L^{(\ell+1)}_{i\leftarrow j} \leftarrow
L^{(\ell)}_{i\leftarrow j}
@@ -1010,8 +1013,8 @@ the most reliable \ac{vn}, meaning we perform a hard decision and
remove it from the following decoding process.
This means that when moving from one window to the next, we now have
more information available: not just the \ac{bp} messages but also the
information about what \acp{vn} were decimated and to what values.
more information available: Not just the \ac{bp} messages but also the
Information about what \acp{vn} were decimated and to what values.
We call this \emph{decimation information} in the following.
We can extend \Cref{alg:warm_start_bp} by additionally passing the
decimation information after initializing the \ac{cn} to \ac{vn} messages.
@@ -1181,7 +1184,7 @@ decimation information after initializing the \ac{cn} to \ac{vn} messages.
% \State $\displaystyle\left(\hat{\bm{e}}^\text{total}\right)_{\mathcal{I}^{(\ell)}_\text{commit}} \leftarrow \hat{\bm{e}}^{(\ell)}_\text{commit}$
% \State $\displaystyle\left(\bm{s}\right)_{\mathcal{J}_\text{overlap}^{(\ell)}}
% \leftarrow \bm{H}_\text{overlap}^{(\ell)}
% \left( \hat{\bm{e}}_\text{commit}^{(\ell)} \right)^\text{T}$
% \left( \hat{\bm{e}}_\text{commit}^{(\ell)} \right)^\mathsf{T}$
% \If{$\ell < n_\text{win} - 1$}
% \State $L^{(\ell+1)}_{i\leftarrow j} \leftarrow
% L^{(\ell)}_{i\leftarrow j}
@@ -1227,7 +1230,7 @@ model, both of which depend on the code and noise model in question.
% Software stack: Layer 3
Even further up, given an already constructed syndrome extraction
circuit and the resulting \acf{dem}, we must split the detector error
circuit and the resulting \acf{dem}, we split the detector error
matrix into separate windows and manage the interplay between the
inner decoders acting on those individual windows.
@@ -1246,12 +1249,11 @@ For the circuit generation, we employed utilities from QUITS
\cite{kang_quits_2025}, which provides syndrome extraction circuitry
generation for a number of different \ac{qldpc} codes.
We initially created a Python implementation, which used QUITS for the window
splitting and subsequent sliding-window decoding as well.
The \ac{bp} and \ac{bpgd} decoders were also initially implemented in Python.
After a preliminary investigation, we opted for a complete
reimplementation in Rust to achieve higher simulation speeds leveraging
the compiled nature of the language.
We reimplemented both the window splitting and the decoders.
splitting and subsequent sliding-window decoding as well, before
reimplementing in Rust.
The \ac{bp} and \ac{bpgd} are implemented in Rust to achieve
higher simulation speeds leveraging the compiled nature of the
language.
% Global experimental setup
@@ -1267,7 +1269,7 @@ For the generation of the \ac{dem} we set the number of syndrome
extraction rounds to $12$, similarly to \cite{gong_toward_2024}, and
we defined our detectors as in the example in
\Cref{subsec:Detector Error Matrix}.
We employed circuit-lose noise as described in
We employed circuit-level noise as described in
\Cref{subsec:Choice of Noise Model} as our noise model, specifically standard
ciruit-based depolarizing noise \cite[Sec.~VIII]{fowler_high-threshold_2009},
i.e., all error locations in the circuit get assigned the same
@@ -1282,21 +1284,22 @@ generated by simulating at least $200$ logical error events.
% Local experimental setup
We began our investigation by using \ac{bp} with no further
We begin our investigation by using \ac{bp} with no further
modifications as the inner decoder.
We chose the min-sum variant of \ac{bp} due to its low computational complexity.
We choose the min-sum variant of \ac{bp} due to its low computational
complexity.
% [Thread] Get impression for max gain
We initially wanted to gain an impression for the performance gain we could
We initially want to gain an impression for the performance gain we could
expect from a modification to the sliding-window decoding procedure.
To this end, we began by analyzing the decoding performance of the
To this end, we begin by analyzing the decoding performance of the
original process, without our warm-start modification.
We will call this \emph{cold-start} decoding in the following.
Because we expected more global decoding to work better (the inner
decoder then has access to a larger portion of the long-range
Because we expect more global decoding to work better (the inner
decoder has access to a larger portion of the long-range
correlations encoded in the detector error matrix before any commit
is made) we initially decided to use decoding on the whole detector
is made) we initially decide to use decoding on the whole detector
error matrix as a proxy for the attainable decoding performance.
\begin{figure}[t]
@@ -1400,8 +1403,8 @@ this trend and, as expected, achieves the strongest performance.
The fact that the $W = 5$ curve is already very close to the
whole-block decoder indicates that the marginal benefit of enlarging
the window saturates after a certain point.
From a practical standpoint, the choice of $W$ thus represents a
trade-off between decoding latency and accuracy: larger windows
Thus, from a practical standpoint, the choice of $W$ represents a
trade-off between decoding latency and accuracy: Larger windows
delay the start of decoding by requiring more syndrome extraction
rounds to be collected upfront, while the diminishing returns above
$W = 4$ suggest that growing the window much further yields little
@@ -1409,7 +1412,7 @@ additional accuracy in return.
% [Thread] First comparison with warm start
Next, we additionally generated error rate curves for warm-start
Next, we additionally simulate error rate curves for warm-start
sliding-window decoding to assess how much of the gap between
cold-start and whole-block decoding can be recovered by our modification.
We chose the same window sizes as before, so that the warm- and
@@ -1508,7 +1511,7 @@ The dashed colored curves reproduce the cold-start results from
corresponding warm-start runs for the same window sizes
$W \in \{3, 4, 5\}$.
The remaining experimental parameters are unchanged:
the step size is fixed to $F = 1$,
The step size is fixed to $F = 1$,
the inner \ac{bp} decoder is allowed up to $200$ iterations per
window invocation, the black curve again gives the whole-block
reference, and the physical error rate is swept from $p = 0.001$ to
@@ -1537,16 +1540,15 @@ consecutive windows spans $W - F = W - 1$ syndrome rounds, so larger
$W$ implies that more messages are carried over and a larger fraction
of the next window starts in a warm state.
% TODO: Possibly insert explanation for higher gain at lowre error rates
A perhaps surprising observation is that the warm-start curve for
$W = 5$ actually lies below the whole-block reference across the
A perhaps surprising observation is that the warm-start for
$W = 5$ outperforms the whole-block reference across the
entire range of physical error rates, even though warm-start
sliding-window decoding is, by construction, more local than
whole-block decoding.
A possible explanation for this effect is discussed in the following.
% [Thread] Warm start is better than whole due to more effective iterations
A possible explanation for this surprising behavior lies in the
A possible explanation for this behavior lies in the
number of \ac{bp} iterations effectively spent on the \acp{vn}
inside the overlap region.
Each \ac{vn} in such an overlap is processed by multiple consecutive
@@ -1705,7 +1707,7 @@ $n_\text{iter} \in [32, 512]$.
All curves decrease monotonically with the iteration budget, but
contrary to our expectation, none of them appears to fully saturate
within the swept range: even at $n_\text{iter} = 4096$, every curve
within the swept range: Even at $n_\text{iter} = 4096$, every curve
still exhibits a noticeable downward slope.
At $n_\text{iter} = 32$, the whole-block curve lies below both the
$W=4$ and $W=5$ sliding-window curves.
@@ -1727,9 +1729,9 @@ mirroring the behavior already observed in \Cref{fig:whole_vs_cold_vs_warm}.
These observations are largely consistent with the effective-iterations
hypothesis put forward above.
The whole-block decoder eventually overtaking every windowed scheme
matches the prediction made there: with a sufficiently large
matches the prediction made there: With a sufficiently large
iteration budget, the whole-block decoder reaches an error rate
that nonone of the windowed schemes can beat, because of the more global
that none of the windowed schemes can beat, because of the more global
nature of the considered constraints.
Furthermore, the pronounced advantage of warm- over cold-start decoding at low
numbers of iterations makes sense if we consider the overall trend of the plots.
@@ -1742,15 +1744,15 @@ initialization diminishes, and the curves approach each other.
The fact that no curve clearly saturates within the swept range is
itself worth noting.
We know that \ac{bp} on \ac{qldpc} codes suffers from poor
convergence due to the short cycles in the underlying Tanner graph,
so even after several thousand iterations the
decoder may continue to slowly refine its message estimates rather
than settle into a stable fixed point.
convergence due to degeneracy and short cycles in the underlying
Tanner graph, so even after several thousand iterations the decoder
may continue to slowly refine its message estimates rather than
settle into a stable fixed point.
This is one of the core motivations for moving from plain \ac{bp} to
the guided-decimation variant studied in
\Cref{subsec:Belief Propagation with Guided Decimation}.
Another thing to note is that setting the per-invocation iteration
Furthermore, note that setting the per-invocation iteration
budget of the inner decoder equal to the iteration budget of the
whole-block decoder is not a fair comparison in terms of total
computational effort.
@@ -1762,14 +1764,14 @@ sliding-window approach is still at an advantage.
% [Thread] Exploration of the effect of the step size
Having examined the effect of the window size $W$, we next turned to
Having examined the effect of the window size $W$, we next turn to
the second windowing parameter, the step size $F$.
We carried out an investigation analogous to the one above:
we first compared warm- and cold-start decoding across the full range
We carry out an investigation analogous to the one above:
We first compare warm- and cold-start decoding across the full range
of physical error rates at a fixed iteration budget, and then we
examined the dependence on the iteration budget at a fixed physical
examine the dependence on the iteration budget at a fixed physical
error rate.
The window size was held fixed at $W = 5$ throughout, the value at
The window size is fixed at $W = 5$ throughout, the value at
which the warm-start variant produced the strongest performance in the
previous experiments.
@@ -1968,9 +1970,9 @@ previous experiments.
% [Experimental parameters] Figure 4.9
\Cref{fig:bp_f} summarizes the results of this investigation.
In both panels the dashed colored curves correspond to cold-start
sliding-window decoding for $F \in \{1, 2, 3\}$ and the solid colored
curves to the corresponding warm-start runs.
In both panels, the dashed curves correspond to cold-start
sliding-window decoding for $F \in \{1, 2, 3\}$ and the solid
curves to warm-start decoding.
The window size is fixed to $W = 5$ throughout.
\Cref{fig:bp_f_over_p} sweeps the physical error rate over
$p \in [0.001, 0.004]$ in steps of $0.0005$ at a fixed maximum of
@@ -1990,9 +1992,9 @@ monotonic increase of the per-round \ac{ler} with the physical
error rate.
At fixed $F$, the warm-start approach lies below
cold-start across the entire sweep, and at fixed
warm- or cold-start, smaller $F$ produces a lower \ac{ler}.
warm or cold start, smaller $F$ produces a lower \ac{ler}.
Both gaps grow as the physical error rate decreases:
the curves at $F = 1$ separate further from those at $F = 2$ and $F = 3$,
The curves at $F = 1$ separate further from those at $F = 2$ and $F = 3$,
and the warm-start curves separate further from the cold-start ones.
In \Cref{fig:bp_f_over_iter}, all six curves again decrease
monotonically with the iteration budget, with no clear saturation
@@ -2014,7 +2016,7 @@ With $W$ held fixed, decreasing $F$ enlarges the overlap between
consecutive windows from $W - F$ to $W - F + 1$ syndrome measurement rounds, so
a smaller step size is beneficial for the same reason that a larger
window size is:
each \ac{vn} in an overlap region participates in more window
Each \ac{vn} in an overlap region participates in more window
invocations, and the warm-start modification effectively accumulates
iterations on it across these invocations.
The widening of the warm/cold gap towards low iteration counts and
@@ -2032,7 +2034,7 @@ Similarly, assuming the decoder is fast enough to keep up with the
incoming syndrome measurements corresponding to the \acp{cn} of
subsequent windows, the time at which decoding is complete depends only
on the amount of time spent on decoding the very last window.
A smaller $F$ thus only costs additional total compute and not
Thus, smaller $F$ only costs additional total compute and not
additional latency, which is favorable for a warm-start
sliding-window implementation.
This is especially favorable for our warm-start modification, as it
@@ -2062,8 +2064,8 @@ both schemes process the same windows for the same number of
iterations and differ only in the initialization of the \ac{bp}
messages of each new window.
We also observed that plain \ac{bp} did not saturate even at $4096$
iterations, which we attribute to the short cycles in the underlying
Tanner graph.
iterations, which we attribute to the degeneracy and short cycles in
the underlying Tanner graph.
This motivates the next subsection, in which we replace the inner
\ac{bp} decoder by its guided-decimation variant.
@@ -2261,7 +2263,7 @@ that can occur before every \ac{vn} in the window has been decimated.
A preliminary investigation showed that \ac{bpgd} only delivers its
intended performance gain once most \acp{vn} have actually been decimated,
which motivated this choice.
The physical error rate was swept from $p = 0.001$ to $p = 0.004$
The physical error rate is swept from $p = 0.001$ to $p = 0.004$
in steps of $0.0005$.
\Cref{fig:bpgd_w} sweeps over the window size with
$W \in \{3, 4, 5\}$ at fixed step size $F = 1$, and
@@ -2279,7 +2281,7 @@ This is the opposite of what we observed for plain \ac{bp}, where
warm-start improved upon cold-start at every parameter setting.
The gap between the warm- and cold-start curves additionally widens
as the physical error rate decreases:
at the lowest sampled rate $p = 0.001$, the per-round \ac{ler} of the
At the lowest sampled rate $p = 0.001$, the per-round \ac{ler} of the
warm-start runs is more than two orders of magnitude above that of
the corresponding cold-start runs.
In \Cref{fig:bpgd_w}, larger window sizes yield lower per-round
@@ -2298,13 +2300,13 @@ than its cold-start counterpart is surprising in light of the results
for plain \ac{bp}, where the warm-start modification was uniformly beneficial.
The dependence on the window size in \Cref{fig:bpgd_w} is, on its own,
consistent with the same explanation that we gave for
\Cref{fig:whole_vs_cold}: larger windows expose the inner decoder to
\Cref{fig:whole_vs_cold}: Larger windows expose the inner decoder to
a larger fraction of the constraints encoded in the detector error
matrix at the time of decoding, and this benefits both warm- and
cold-start decoding.
The dependence on the step size in \Cref{fig:bpgd_f}, however, is the
opposite of the corresponding dependence under plain \ac{bp}
(\Cref{fig:bp_f_over_p}): for warm-start, smaller $F$ now hurts
(\Cref{fig:bp_f_over_p}): For warm-start, smaller $F$ now degrades performance
rather than helps, even though smaller $F$ implies a larger overlap
in both cases.
@@ -2314,18 +2316,18 @@ Recall from
that the warm start for \ac{bpgd} carries over not only the \ac{bp}
messages on the edges of the overlap region but also the decimation
information.
Because we run with an iteration budget large enough to decimate
Because we decode with an iteration budget large enough to decimate
every \ac{vn} in a window, by the time window $\ell$ ends, all
of its \acp{vn} have already been hard-decided.
For the \acp{vn} that lie in the overlap region with window $\ell + 1$
this hard decision is then carried into the next window through the
warm-start initialization, and the next window thus begins decoding
with a substantial fraction of its \acp{vn} already frozen, before
warm-start initialization, and the next window begins decoding
with a substantial fraction of its \acp{vn} already fixed, before
its own parity checks have had any chance to influence the
corresponding bit estimates.
This identifies one of two competing effects on the warm-start performance.
The larger the overlap, the more such prematurely frozen \acp{vn} the
next window inherits, which hurts performance.
The larger the overlap, the more such prematurely fixed \acp{vn} the
next window inherits, which degrades performance.
On the other hand, a larger window still exposes the inner decoder to
a larger set of constraints, which helps performance.
The two effects together are consistent with what we observe in
@@ -2346,7 +2348,7 @@ $n_\text{iter}$ should reduce the maximum number of \acp{vn} that can
be decimated before window $\ell$ commits, and the warm-start
performance should approach that of warm-start under plain \ac{bp} as
$n_\text{iter}$ is lowered.
We therefore now vary $n_\text{iter}$ at fixed window parameters and
Therefore, we vary $n_\text{iter}$ at fixed window parameters and
fixed physical error rate.
\begin{figure}[t]
@@ -2515,10 +2517,10 @@ fixed physical error rate.
\Cref{fig:bpgd_iter} shows the per-round \ac{ler} of \ac{bpgd}
sliding-window decoding as a function of the maximum number of inner
\ac{bp} iterations $n_\text{iter}$.
The dashed colored curves correspond to cold-start sliding-window
decoding and the solid colored curves to warm-start, again carrying
over both the \ac{bp} messages and the channel \acp{llr} on the
overlap region.
The dashed curves correspond to cold-start sliding-window
decoding and the solid curves to warm-start, which again
retains both the \ac{bp} messages and the decimaiton information on
the overlap region.
The physical error rate is fixed at $p = 0.0025$ and the iteration
budget is swept over $n_\text{iter} \in \{32, 128, 256, 512, 1024,
1536, 2048, 2560, 3072, 3584, 4096\}$.
@@ -2533,7 +2535,7 @@ For low iteration budgets, all curves in both panels behave similarly
to the plain-\ac{bp} curves in
\Cref{fig:bp_w_over_iter,fig:bp_f_over_iter}.
The per-round \ac{ler} decreases gradually with $n_\text{iter}$, and
the warm-start curves lie below their cold-start counterparts at
the warm-start configurations now outperform their cold-start counterparts at
matching window parameters.
As $n_\text{iter}$ continues to grow, however, the cold-start curves
undergo a sharp drop, after which they lie roughly an order of
@@ -2562,7 +2564,7 @@ the warm-start curves now show a clear reordering as $n_\text{iter}$
grows.
At low iteration budgets the warm-start ordering matches the
cold-start ordering, with $F = 1$ best and $F = 3$ worst, but at the
largest iteration budget this ordering is fully inverted: warm-start
largest iteration budget this ordering is fully inverted: Warm-start
$F = 1$ is now the worst and $F = 3$ the best.
% [Interpretation] Figure 4.11
@@ -2594,7 +2596,7 @@ decoding performance.
The same mechanism explains the inversion of the step-size ordering
in \Cref{fig:bpgd_iter_F}.
At low iteration budgets, the ordering is set by the same overlap
argument as for plain \ac{bp}: smaller $F$ implies a larger overlap
argument as for plain \ac{bp}: Smaller $F$ implies a larger overlap
between consecutive windows, more shared messages, and therefore
better warm-start performance.
At large iteration budgets, the ordering is set by the premature hard
@@ -2607,8 +2609,8 @@ of the warm-start curves and limit ourselves to noting it.
The natural consequence of the previous diagnosis is to drop the
problematic part of the warm-start initialization for \ac{bpgd} and
to carry over only the \ac{bp} messages on the edges of the overlap
region, as in \Cref{fig:messages_tanner}, while leaving the channel
\acp{llr} of the next window in their original cold-start state.
region, as in \Cref{fig:messages_tanner}, while leaving the
decimation information of the next window in its original cold-start state.
Note that some information about the previous window's decimation
state is still implicitly carried over through the \ac{bp} messages,
since the decimation decisions were made based on the messages themselves.
@@ -2775,7 +2777,7 @@ since the decimation decisions were made based on the messages themselves.
\Cref{fig:bpgd_msg} repeats the experiment of \Cref{fig:bpgd_wf}
with the modified warm-start procedure that carries over only the
\ac{bp} messages.
All other experimental parameters are unchanged: the maximum number
All other experimental parameters are unchanged: The maximum number
of inner \ac{bp} iterations is $n_\text{iter} = 5000$, and the
physical error rate is swept from $p = 0.001$ to $p = 0.004$ in steps
of $0.0005$.
@@ -2803,12 +2805,12 @@ as $F$ grows.
% [Description] Interpretation 4.12
Removing the channel \acp{llr} from the warm-start initialization lifts
Removing the decimation information from the warm-start initialization lifts
the warm-start regression observed in \Cref{fig:bpgd_wf},
and warm-start now consistently outperforms cold-start.
The dependence on the window size and the step size also recovers
the qualitative behavior we observed for plain \ac{bp} in
\Cref{fig:whole_vs_cold_vs_warm,fig:bp_f_over_p}: a larger overlap
\Cref{fig:whole_vs_cold_vs_warm,fig:bp_f_over_p}: A larger overlap
between consecutive windows, achieved either by enlarging $W$ or by
decreasing $F$, both improves the absolute decoding performance and
increases the warm-start advantage over cold-start.
@@ -2992,7 +2994,7 @@ cold-start curves across the entire range of $n_\text{iter}$ available to us.
\Cref{fig:bpgd_msg_iter} repeats the experiment of
\Cref{fig:bpgd_iter} with the modified warm-start procedure that
carries over only the \ac{bp} messages.
All other experimental parameters are unchanged: the physical error
All other experimental parameters are unchanged: The physical error
rate is fixed at $p = 0.0025$ and the iteration budget is swept over
$n_\text{iter} \in \{32, 128, 256, 512, 1024, 1536, 2048, 2560,
3072, 3584, 4096\}$.
@@ -3020,11 +3022,11 @@ and at $F = 1$, respectively.
These observations match our expectations.
With only the \ac{bp} messages carried over, the warm-start
initialization no longer freezes any \acp{vn} in the next window
initialization no longer freezes any \acp{vn} in the next window.
The dependence of this benefit on $W$ and $F$ also recovers the
pattern observed for plain \ac{bp} in
\Cref{fig:whole_vs_cold_vs_warm,fig:bp_f_over_p}:
larger overlap, achieved by larger $W$ or smaller $F$, yields more
Larger overlap, achieved by larger $W$ or smaller $F$, yields more
effective extra iterations and therefore a larger warm-start gain.
% BPGD conclusion
@@ -3034,9 +3036,9 @@ sliding-window decoding under \ac{bpgd} by summarizing our findings.
Warm-starting the inner decoder still provides a consistent
performance gain when the inner decoder is upgraded from plain
\ac{bp} to its guided-decimation variant, but only if some care is
taken in choosing what to carry over.
taken in choosing what to information carry over.
Passing the channel \acp{llr} along with the \ac{bp} messages,
as suggested by naively carrying over the warm-start idea to \ac{bpgd},
as suggested by naively transferring the warm-start idea to \ac{bpgd},
leads to premature hard decisions on \acp{vn} in the overlap region.
This leads to warm-start initialization actually worsening the
performance compared to cold-start initialization.
@@ -3046,6 +3048,20 @@ cold-start that follows the same behavior as for plain \ac{bp} with
regard to overlap.
A second observation specific to \ac{bpgd} is that its iteration
requirements are substantially larger than those of plain \ac{bp}:
the per-round \ac{ler} drops sharply only once the iteration budget
The per-round \ac{ler} drops sharply only once the iteration budget
is on the order of the number of \acp{vn} in each window.
Future work could include a softer treatment of the decimation state
in \ac{bpgd}.
Rather than discarding the decimation information of the previous
window entirely, as in the message-only warm start used here, one
could encode the decimation decisions as strong but finite biases on
the channel \acp{llr} of the next window, allowing the new window's parity
checks to override them if the syndrome calls for it.
This would interpolate between the two warm-start variants studied here and
might combine the benefits of both.
A related question is whether the decimation schedule itself should
be aware of the window structure, for instance by deferring
decimation of \acp{vn} in the overlap region until they have been
visited by the next window.

View File

@@ -3,24 +3,24 @@
% Recap of motivation
This thesis investigated decoding under \acp{dem} for fault-tolerant
This thesis investigates decoding under \acp{dem} for fault-tolerant
\ac{qec}, with a focus on low-latency decoding methods for \ac{qldpc} codes.
The repetition of the syndrome measurements, especially under
consideration of circuit-level noise, leads to a significant increase
in decoding complexity: in our experiments on the $\llbracket
in decoding complexity: In our experiments on the $\llbracket
144,12,12 \rrbracket$ \ac{bb} code with $12$ syndrome extraction
rounds, the check matrix grew from 144 \acp{vn} and 72
rounds, the check matrix grows from 144 \acp{vn} and 72
\acp{cn} to 9504 \acp{vn} and 1008 \acp{cn}.
% Recap of research gap and own work
Sliding-window decoding addresses the latency constraint by
exploiting the time-like locality of the syndrome extraction circuit,
which manifests as a block-diagonal structure in the detector error
exploiting the time-like locality of the syndrome extraction circuit.
This manifests as a block-diagonal structure in the detector error
matrix when detectors are defined as the difference of consecutive
syndrome measurement rounds.
We drew a comparison to windowed decoding for \ac{sc}-\ac{ldpc}
codes, but noted that the existing realizations of sliding-window
We draw a comparison to windowed decoding for \ac{sc}-\ac{ldpc}
codes, but note that the existing realizations of sliding-window
decoding discard the soft information produced inside one window
before moving to the next.
Building on this observation, we proposed warm-start sliding-window
@@ -29,34 +29,35 @@ the overlap region of the previous window are reused to initialise
the corresponding messages of the next window in place of the
standard cold-start initialisation.
We formulated the warm start first for plain \ac{bp} and then for
\ac{bpgd}, the latter being attractive as an inner decoder because it
We formulate the warm start for standard \ac{bp} and for
\ac{bpgd}.
The latter is particularly attractive as an inner decoder because it
addresses the convergence problems caused by short cycles and
degeneracy in \ac{qldpc} Tanner graphs.
The decoders were evaluated by Monte Carlo simulation on the
The decoders are evaluated by conducting Monte Carlo simulations on the
$\llbracket 144,12,12 \rrbracket$ \ac{bb} code over $12$ syndrome
extraction rounds under standard circuit-based depolarizing noise.
We focused on a qualitative analysis, refraining from further
We focus on a qualitative analysis, refraining from further
optimizations such as introducing a normalization parameter for the
min-sum algorithm.
% Recap of experimental conclusions
For plain min-sum \ac{bp}, the warm start was consistently beneficial
across the parameter ranges we examined. The size of the gain depended
on the overlap between consecutive windows: enlarging $W$ or
shrinking $F$, both of which enlarge the overlap, raised the
warm-start performance increase.
We argued that the underlying mechanism is an effective increase in
For standard min-sum \ac{bp}, the warm start is consistently
beneficial to the cold start, across the considered parameter ranges.
The size of the gain depends on the overlap between consecutive
windows: Enlarging $W$ or shrinking $F$, both of which enlarge the
overlap, result in larger gains of the warm-start.
We observe that the underlying mechanism is an effective increase in
the number of \ac{bp} iterations spent on the \acp{vn} in the overlap
region: each such \ac{vn} is processed by multiple consecutive window
region: Each such \ac{vn} is processed by multiple consecutive window
invocations, and the warm start lets these invocations accumulate
iterations on the same \acp{vn} rather than restarting from scratch.
The gain was most pronounced at low numbers of maximum iterations, where
every additional iteration carries proportionally more information.
For \ac{bpgd}, we noted that more information is available in the
overlap region of a window: in addition to the \ac{bp} messages,
For \ac{bpgd}, we note that more information is available in the
overlap region of a window: In addition to the \ac{bp} messages,
there is information about which \acp{vn} were decimated and to what value.
Passing this decimation information to the next window in addition to
the messages turned out to worsen the performance considerably, which
@@ -65,14 +66,14 @@ overlap region.
Restricting the warm start to the \ac{bp} messages alone, removed this effect.
The resulting message-only warm start recovered a consistent
improvement over cold-start that followed the same qualitative
behaviour as for plain \ac{bp}: larger overlap, achieved by larger
behaviour as for standard \ac{bp}: Larger overlap, achieved by larger
$W$ or smaller $F$, yielded a larger gain, and the
performance difference was most pronounced at low numbers of maximum iterations.
performance difference is most pronounced at low numbers of maximum iterations.
% Implications from experimental results
These observations imply that the warm-start modification to
sliding-window decoding provides a consistent improvement, as long as
sliding-window decoding can provide a consistent improvement, as long as
some care is taken with specifying the information to be passed to
the subsequent window.
Note that this comes at no additional cost to the decoding complexity,
@@ -94,25 +95,10 @@ underlying mechanism is structural rather than code-specific, but
quantifying the gain across code families and noise models is left to
future work.
A second direction is a systematic study of inner decoders under the
warm-start framework.
We considered plain min-sum \ac{bp} and \ac{bpgd}, but other
algorithms used for \ac{qldpc} decoding, such as automorphism
ensemble decoding \cite{koutsioumpas_automorphism_2025} or neural
\ac{bp} \cite{miao_quaternary_2025} may admit warm-start variants of their own.
A third direction is a softer treatment of the decimation state in \ac{bpgd}.
Rather than discarding the decimation information of the previous
window entirely, as in the message-only warm start used here, one
could encode the decimation decisions as strong but finite biases on
the channel \acp{llr} of the next window, allowing the new window's parity
checks to override them if the syndrome calls for it.
This would interpolate between the two warm-start variants studied here and
might combine the benefits of both.
A related question is whether the decimation schedule itself should
be aware of the window structure, for instance by deferring
decimation of \acp{vn} in the overlap region until they have been
visited by the next window.
A second direction is a systematic study of other inner decoders under the
warm-start framework, such as automorphism ensemble decoding
\cite{koutsioumpas_automorphism_2025} or neural \ac{bp}
\cite{miao_quaternary_2025}.
A final direction is suggested by the structural similarity between
sliding-window decoding for \acp{dem} and windowed decoding for

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@@ -4,6 +4,8 @@
\Ac{qec} protects fragile quantum states against decoherence by
encoding logical information into a larger number of physical qubits.
To obtain parity information on an encoded state without disturbing it, a
syndrome extraction is performed.
Because the syndrome extraction circuitry is itself implemented on
noisy quantum hardware, practical \ac{qec} must be fault-tolerant,
accounting for errors introduced by the correction procedure itself.
@@ -19,35 +21,35 @@ can be decoded.
Together, these factors pose a serious challenge for practical decoders.
Sliding-window decoding addresses this challenge by exploiting the
repeated structure of the syndrome extraction circuitry, partitioning
the \ac{dem}'s check matrix into overlapping windows that can be
the check matrix of the \ac{dem} into overlapping windows that can be
decoded sequentially.
This allows for an earlier start to the decoding process, before all
syndrome measurements have been completed, thereby lowering the latency.
Therefore, decoding can begin as soon as the syndrome components
associated with the first window have been measured.
% Our work: Identify research gap
In this thesis, we perform a review of the existing literature on
sliding-window decoding and draw an analogy to windowed
decoding for classical spatially-coupled low-density parity-check
decoding of classical spatially-coupled low-density parity-check
(\acs{sc}-\acs{ldpc}) codes.
We recognize that in contrast to the latter, existing realizations
of sliding-window decoding for \ac{qec} discard the soft information
produced inside one window before moving to the next.
produced inside one window before moving to the subsequent window.
% Our work: Warm-start
% TODO: Quantify improvement. Also for conclusion
We propose warm-start sliding-window decoding, in which the
\ac{bp} messages on the edges crossing into the overlap region of the previous
window are reused to initialize the corresponding messages of the
next window.
The warm start is formulated first for plain \ac{bp} and then extended to
To take this information into account, we propose warm-start
sliding-window decoding, in which the \ac{bp} messages on the edges
crossing into the overlap region of the previous window are reused to
initialize the corresponding messages of the next window.
The warm start is formulated first for standard \ac{bp} and then extended to
\ac{bp} with guided decimation (\acs{bpgd}).
For both plain min-sum \ac{bp} and \ac{bpgd} decoding, the warm-start
For both standard \ac{bp} and \ac{bpgd} decoding, the warm-start
initialization provides a consistent improvement across all examined
parameter settings.
We attribute this to an effective increase in \ac{bp} iterations on
variable nodes in the overlap regions: each such VN is processed by
variable nodes in the overlap regions: Each such VN is processed by
multiple consecutive windows, and warm-starting lets these
invocations accumulate iterations rather than restart from scratch.
Crucially, the warm-start modification incurs no additional

View File

@@ -19,6 +19,8 @@ with open(path) as f:
doi_re = re.compile(r"doi\s*=\s*\{10\.48550/arXiv\.([^}]+)\}")
type_re = re.compile(r"^@([A-Za-z]+)\{", re.MULTILINE)
howpublished_re = re.compile(r"^\s*howpublished\s*=\s*\{", re.MULTILINE)
title_field_re = re.compile(r"\b(title|booktitle)\s*=\s*\{", re.IGNORECASE)
inner_brace_re = re.compile(r"\{([A-Za-z0-9]+)\}")
# Split into entries by scanning for top-level "@type{...}" blocks. We walk
# brace depth so that the closing "}" of the entry is matched correctly even
@@ -47,7 +49,7 @@ def split_entries(s):
i = j
return out
def transform(entry):
def normalize_arxiv(entry):
doi_m = doi_re.search(entry)
if not doi_m:
return entry
@@ -64,6 +66,43 @@ def transform(entry):
)
return entry
# Strip protective braces around words inside title/booktitle values.
# BibTeX uses "{Word}" inside titles to preserve case against the bibliography
# style's title-casing rules. We keep that protection only when every character
# inside the braces is non-lowercase (e.g. acronyms like {NASA}); for ordinary
# words like {Quantum} we drop the braces so the style's casing applies.
def strip_title_braces(entry):
out, i, n = [], 0, len(entry)
while True:
m = title_field_re.search(entry, i)
if not m:
out.append(entry[i:])
break
out.append(entry[i:m.end()])
depth, j = 1, m.end()
while j < n and depth > 0:
c = entry[j]
if c == "{":
depth += 1
elif c == "}":
depth -= 1
if depth == 0:
break
j += 1
value = entry[m.end():j]
cleaned = inner_brace_re.sub(
lambda mm: mm.group(1) if any(c.islower() for c in mm.group(1)) else mm.group(0),
value,
)
out.append(cleaned)
if j < n:
out.append(entry[j])
i = j + 1
return "".join(out)
def transform(entry):
return strip_title_braces(normalize_arxiv(entry))
parts = split_entries(text)
new_text = "".join(transform(p) if kind == "entry" else p for kind, p in parts)

View File

@@ -29,6 +29,7 @@
\usepackage{colortbl}
\usepackage{cleveref}
\usepackage{lipsum}
\usepackage{booktabs}
\usetikzlibrary{calc, positioning, arrows, fit}
\usetikzlibrary{external}
@@ -42,7 +43,7 @@
\Crefname{equation}{}{}
\Crefname{section}{Section}{Sections}
\Crefname{subsection}{Subsection}{Subsections}
\Crefname{subsection}{Section}{Sections}
\Crefname{figure}{Figure}{Figures}
%
@@ -89,10 +90,10 @@
% \thesisHeadOfInstitute{Prof. Dr.-Ing. Peter Rost}
%\thesisHeadOfInstitute{Prof. Dr.-Ing. Peter Rost\\Prof. Dr.-Ing.
% Laurent Schmalen}
\thesisSupervisor{Jonathan Mandelbaum}
\thesisStartDate{01.11.2025}
\thesisEndDate{04.05.2026}
\thesisSignatureDate{04.05.2026}
\thesisSupervisor{Dr.-Ing. Hedongliang Liu\\ && M.Sc. Jonathan Mandelbaum}
\thesisStartDate{Nov. 1st, 2025}
\thesisEndDate{May 4th, 2026}
\thesisSignatureDate{May 4th, 2026}
\thesisSignature{res/Unterschrift_AT_blue.png}
\thesisSignatureHeight{2.4cm}
\thesisLanguage{english}
@@ -108,8 +109,11 @@
\cleardoublepage
\pagenumbering{arabic}
\tableofcontents
\newgeometry{a4paper,left=3cm,right=3cm,top=2cm,bottom=2.5cm}
\addtocontents{toc}{\protect\vspace*{-9mm}}
\tableofcontents
\cleardoublepage
\restoregeometry
\input{chapters/1_introduction.tex}
\input{chapters/2_fundamentals.tex}
@@ -122,6 +126,11 @@
% \listoftables
% \include{abbreviations}
\cleardoublepage
\phantomsection
\addcontentsline{toc}{chapter}{List of Abbreviations}
\printacronyms
\bibliography{lib/cel-thesis/IEEEabrv,src/thesis/bibliography}
\end{document}