ma-thesis/src/midterm_presentation/main.tex

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\documentclass[overviewatsection, showsubsectionsatfirstoverview]{CELbeamer}
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\groupname{Communications Engineering Lab (CEL)}
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\usepackage{amsmath}
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\usepackage{acro}
\usepackage{braket}
\usepackage{qcircuit}
\title{Fault Tolerant Quantum Error Correction}
\subtitle{Master's Thesis Midterm Presentation}
\author[Tsouchlos]{Andreas Tsouchlos}
\date[]{February 5th, 2026}
\DeclareFieldFormat{note}{}
\DeclareFieldFormat{issn}{}
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\DeclareFieldFormat{doi}{}
\DeclareFieldFormat[article,book,inproceedings]{urldate}{}
\addbibresource{MA.bib}
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\newcommand{\red}[1]{\textcolor{red}{#1}}
\newcommand{\res}{src/midterm_presentation/res}
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% Acronyms
%
%
\DeclareAcronym{qec}{
short=QEC,
long=quantum error correction
}
\DeclareAcronym{css}{
short=CSS,
long=Calderbank Shor Steane
}
%
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% Document body
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\begin{document}
\begin{frame}[title white vertical, picture=images/IMG_7801-cut]
\titlepage
\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Introduction to Quantum Error Correction}
\label{sec:Introduction to Quantum Error Correction}
%%%%%%%%%%%%%%%%
\subsection{Motivation}
\label{subsec:Motivation}
\begin{frame}
\frametitle{Quantum Computing}
% Related interesting stuff
% - Gidney estimates we need 1399 (?) logical qubits to factor a 2048
% bit RSA integer
% - He goes on to estimate that to factor such an integer in less
% than a week would require around a million physical qubits
% [How to factor 2048 bit RSA integers with less than a million
% noisy qubits]
\vspace*{-15mm}
\begin{itemize}
\item Simulating quantum systems on classical hardware
is exponentially complex \\
$\rightarrow$ Can't we use quantum hardware to simulate
quantum systems? \citereference{feynman_simulating_1982}
\item Some problems that are ``hard'' to solve on classical
computers we can ``easily'' solve on quantum computers
\citereference{preskill_quantum_2018}
\end{itemize}
\vspace*{-5mm}
\begin{figure}[H]
\centering
\includegraphics[scale=0.43]{res/google_roadmap.png}
\vspace*{-3mm}
\caption{
Google Quantum AI's quantum computing roadmap
\citereference{google_quantum_ai_quantum_nodate}.
}
\end{figure}
\vspace*{3mm}
\addreferences
{feynman_simulating_1982}
{preskill_quantum_2018}
{google_quantum_ai_quantum_nodate}
\stopreferences
\end{frame}
% TODO: Where should I quote Preskill? There are multiple bullet
% points with info taken from his work
\begin{frame}
\frametitle{The Need for Quantum Error Correction}
\vspace*{-17mm}
% Related interesting stuff
% - Qubits differ from bits in that they can be in superpositions
% and be entangled with one another
% - Quantum computers derive their strenght from the exponential
% scaling of the state-space because of the way the information is
% encoded
% - Note that while a physical error rate of 10^{-3} may seem ok,
% we need a couple trillion operations (~ 10^{13}) to factor a
% 2048 bit RSA integer
% [How to factor 2048 bit RSA integers with less than a million
% noisy qubits]
% - The "physical error rate" is really the value all error rates
% in the system are set to for circuit level noise simulations
% [High-threshold universal quantum computation on the surface code]
% - The backlog problem is the fact that an increasing backlog of
% syndrome data will lead to an exponential slowdown during the
% computation
\begin{itemize}
\item Quantum computers represent information through
correlations of qubits, not their values \\
directly \citereference{preskill_quantum_2018}
\item We want to not disturb the quantum state but need to
interact with the system $\rightarrow$ Protect the state
with \ac{qec}
\item We employ more physical qubits to introduce
redundancy and use the resulting \emph{physical} state to
represent the \emph{logical} state
\citereference{roffe_quantum_2019}
\vspace*{8mm}
\item Typical scales
\begin{itemize}
\item IBM recently introduced a scheme encoding $12$ logical
qubits in $288$ physical ones
\citereference{bravyi_high-threshold_2024}
\item The physical error rate is typically assumed to
be $10^{-3}$ for
simulations (e.g.,
\citereference{bravyi_high-threshold_2024})
\item Decoding has to happen with ultra-low latency to avoid
the backlog problem (about $\SI{1}{us}$ per data
extraction round) \citereference{caune_demonstrating_2024}
% \citereference{terhal_quantum_2015}
\end{itemize}
\end{itemize}
\vspace*{7mm}
\addreferences
% {terhal_quantum_2015}
{preskill_quantum_2018}
{roffe_quantum_2019}
{bravyi_high-threshold_2024}
{caune_demonstrating_2024}
\stopreferences
\end{frame}
%%%%%%%%%%%%%%%%
\subsection{Fundamentals of Quantum Error Correction}
\label{subsec:Fundamentals of Quantum Error Correction}
% TODO: Is this all of this really necessary?
\begin{frame}
\frametitle{Peculiarities of the Quantum Setting}
\vspace*{-5mm}
% Related interesting stuff
% - No cloning theorem -> Not replication of state, protection
% through further entanglement
% - States are superpositions -> We theoretically need to be able
% to correct infinitely many different types of errors. Luckily,
% it turns out that in actual fact we only really need to correct
% two [Gottesman's Thesis]
% - Mention that kets are just vectors, used here to represent the state
% - There are actually infinitely many different errors that can
% happen, but we can digitize them and only need to consider X and Z
% - Not only do we only care about the coset, we specifically
% don't want to know more than the syndrome can tell us because
% that would mean that "we collapse the quantum mechanical state too
% much"
\begin{itemize}
\item As mentioned earlier, \ac{qec} is actually able to
protect the quantum state with all its correlations
\item We have to consider phase flip errors in addition to
bit flip errors \citereference{roffe_quantum_2019}
\vspace*{-10mm}
\begin{figure}[H]
\centering
\begin{subfigure}{0.5\textwidth}
\centering
\begin{align*}
\ket{0} &\rightarrow \ket{1} \\
\ket{1} &\rightarrow \ket{0}
\end{align*}
\caption{Bit flip (X) error}
\end{subfigure}%
\begin{subfigure}{0.5\textwidth}
\centering
\begin{align*}
\ket{0} &\rightarrow \phantom{-}\ket{0} \\
\ket{1} &\rightarrow -\ket{1}
\end{align*}
\caption{Phase flip (Z) error}
\end{subfigure}
\end{figure}
\item Measuring the qubits directly destroys superpositions
and entanglement \\
$\rightarrow$ We generally only work with the syndrome,
which we can measure \citereference{nielsen_quantum_2010}
\item We don't care about restoring the specific codeword,
only finding the coset it's in
\end{itemize}
\vspace*{15mm}
\addreferences
{nielsen_quantum_2010}
{roffe_quantum_2019}
\stopreferences
\end{frame}
\begin{frame}
\frametitle{Stabilizer and Calderbank Shor Steane Codes}
\vspace*{-5mm}
% Related interesting stuff
% - Using stabilizers to describe quantum codes is a bit like
% using parity check equations to describe classical codes
% -> stabilizer codes are the quantum analog of binary linear codes
% - For CSS codes, "the parity checks for the X errors and the
% parity checks for the Z errors can be represented independently
% of one another"
\begin{itemize}
\item Stabilizer codes \citereference{nielsen_quantum_2010}
\begin{itemize}
\item The code space can implicitly be defined using
\emph{stabilizer generators}
\item We can represent them using parity
check matrices
\item Quantum analog of linear codes
\end{itemize}
\vspace*{10mm}
\item \Ac{css} codes \citereference{nielsen_quantum_2010}
\begin{itemize}
\item Subset of stabilizer codes
\item Can correct X and Z errors independently
\item Described using two separate parity check
matrices $\bm{H}_\text{X}$ and $\bm{H}_\text{Z}$
\item Can be constructed from two binary linear codes
$\mathcal{C}_1 \left[ n, k_1 \right]$ and
$\mathcal{C}_2 \left[ n, k_2 \right]$ with
$\mathcal{C}_2 \subset \mathcal{C}_1$
\end{itemize}
\vspace*{10mm}
\item \red{Do I need to go more in depth for either
stabilizer codes or CSS codes?}
\end{itemize}
\vspace*{10mm}
\addreferences
{nielsen_quantum_2010}
\stopreferences
\end{frame}
% TODO: Do I need to show what the syndrome extraction circuitry for
% Z errors looks like?
\begin{frame}
\frametitle{Syndrome Extraction Circuits}
\vspace*{-16mm}
\begin{itemize}
\item We entangle the state with \emph{ancilla qubits} to
perform syndrome measurements \citereference{nielsen_quantum_2010}
\item Example: The 3-qubit repetition code%
\footnote {
Note that, for simplicity, this chosen example is a
code that is not only able to correct X errors (bit flips)
} %
\red{Do I need to show what the syndrome extraction
circuitry for Z errors looks like?}
\end{itemize}
\vspace*{-10mm}
\begin{align*}
\bm{H} =
\begin{pmatrix}
1 & 1 & 0 \\
0 & 1 & 1
\end{pmatrix}
\end{align*}
\vspace*{5mm}
\begin{figure}[H]
\centering
\mbox{
% tex-fmt: off
\Qcircuit @C=1em @R=.7em {
& & \ctrl{3} & \qw & \qw & \qw & \qw & \qw \\
\ket{\psi}_\text{L} & & \qw & \ctrl{2} & \ctrl{3} & \qw & \qw & \qw \\
& & \qw & \qw & \qw & \ctrl{2} & \qw & \qw \\
\ket{0}_{\text{A}_1} & & \targ & \targ & \qw & \qw & \meter & \\
\ket{0}_{\text{A}_2} & & \qw & \qw & \targ & \targ & \meter &
}
% tex-fmt: on
}
\vspace*{5mm}
\caption{Syndrome extraction circuit for the 3-qubit repetition code}
\end{figure}
% \vspace*{5mm}
\vspace*{-2mm}
\addreferences
{nielsen_quantum_2010}
\stopreferences
\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Fault Tolerance and Detector Error Models}
\label{sec:Fault Tolerance and Detector Error Models}
%%%%%%%%%%%%%%%%
\subsection{Fault Tolerance}
\label{subsec:Fault Tolerance}
\begin{frame}
\frametitle{Fault Tolerance}
\begin{itemize}
\item Quantum gates are faulty $\rightarrow$ we need QEC \\
But we do QEC with faulty gates $\rightarrow$ we need
fault tolerant QEC %
\footnote{
Designing fault-tolerant circuits using detector
error models - Gong et al, Section 4.1
}
\item We generally do multiple rounds of syndrome extraction
\item The Threshold theorem
\setcounter{footnote}{0}
\item Definition of fault tolerance \footnotemark
\item \red{Different approaches to fault tolerance?}
\end{itemize}
\end{frame}
% TODO: Where to we introduces the different kinds of noise models?
\begin{frame}
\frametitle{Noise models}
\begin{itemize}
\item The depolarizing channel
\item Phenomenological noise
\item Circuit-level noise (we generally have all error
probabilities equal the same value \\
for simulations \citereference{fowler_high-threshold_2009})
\end{itemize}
\vspace*{15mm}
\addreferences
{fowler_high-threshold_2009}
\stopreferences
\end{frame}
%%%%%%%%%%%%%%%%
\subsection{Detector Error Models}
\label{subsec:Detector Error Models}
\begin{frame}
\frametitle{Detector Error Models}
\begin{itemize}
\item Idea: Go "one layer of abstraction higher" \\
$\rightarrow$ Redefine syndrome and create new PC matrix from that
\end{itemize}
\end{frame}
\begin{frame}
\frametitle{Example: 3-Qubit Repetition Code Detector\\ Error
Model for Circuit Level Noise}
\begin{itemize}
\item New Syndrome Extraction Circuitry \red{Is a
repetition of the old circuitry needed?}
\item New parity check matrix
\item Highlighting of the SC-LDPC-code-like structure
\end{itemize}
\end{frame}
\begin{frame}
\frametitle{Challenges}
\begin{itemize}
\item \red{Multiple different errors are summarized
$\rightarrow$ short cycles \& degeneracy}
\footnote{
\texttt{
\red{https://www.math.cit.tum.de/fileadmin/w00ccg/math/\_my\_direct\_uploads/Dan\_Browne.pdf}
}
}
\\
\red{$\rightarrow$ We generally don't use "normal BP" (BP
+ OSD, BPGD, etc.)}
\end{itemize}
\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{State of the Art and Research Gap}
\label{sec:State of the Art and Research Gap}
%%%%%%%%%%%%%%%%
\subsection{State of the Art}
\label{subsec:State of the Art}
\begin{frame}
\frametitle{Sliding Window Decoding}
\begin{itemize}
% TODO: Do I have to explain BP?
\item \red{Do I have to explain BP}
\item Give overview of existing research
\item Explain exactly what they do in the main paper I am
basing my work on
\end{itemize}
\end{frame}
\begin{frame}
\frametitle{Guided Decimation Guessing Decoding}
\begin{itemize}
\item Update equations
\item Key ideas
\item Syndrome Based BP
\end{itemize}
\end{frame}
\begin{frame}
\frametitle{Memory experiments}
\begin{itemize}
\item What is a memory experiment?
\item Communications Engineering view (what are my inputs and
outpus? What do I expect?)
\end{itemize}
\end{frame}
\begin{frame}
\frametitle{Systemic overview}
\begin{itemize}
\item Top level overview of entire system: X and Z syndrome
extraction, logical operator measurement, where decoding
takes place, etc. (fig. 3 of \citereference{derks_designing_2025})
\end{itemize}
\vspace*{15mm}
\addreferences
{derks_designing_2025}
\stopreferences
\end{frame}
\begin{frame}
\frametitle{Research Gap}
\begin{itemize}
\item Use soft information for sliding window decoding
$\rightarrow$ Treat as spacially coupled LDPC code
\item Current work considers X and Z errors separately
(probably for latency reasons) $\rightarrow$ See how
decoding the jointly works out
\end{itemize}
\end{frame}
%%%%%%%%%%%%%%%%
\subsection{What we simulate}
\label{subsec:What we simulate}
\begin{frame}
\frametitle{The lack of a Standard Evaluation System}
\begin{itemize}
\item \red{Look into ECCentric}
\item There is not even a standard figure of merit (e.g.,
FER/BER over SNR in classical case) $\rightarrow$
Multiple different kinds of plots (e.g., footprint)
\item Overview of variables
\end{itemize}
\end{frame}
\begin{frame}
\frametitle{Proposed Evaluation Pipeline}
\begin{itemize}
\item To what values I will fix the parameters and why
\item What figure of merit I will use and why
\end{itemize}
\end{frame}
\begin{frame}[t]
\frametitle{Questions}
\begin{minipage}[c]{0.65\textwidth}
\centering
\LARGE Thank you for your attention!\\ Any questions?
\end{minipage}%
\begin{minipage}[c]{0.35\textwidth}
\centering
\begin{figure}[H]
\centering
\begin{tikzpicture}[every node/.style={scale=10}]
\node at (0, 0)
{\textcolor{kit-blue}{{\fontfamily{phv}\selectfont ?}}};
\end{tikzpicture}
\end{figure}
\end{minipage}
\end{frame}
\appendix
\beginbackup
% TODO: Move arrow into syndrome extraction lower (branch from other
% arrow) and change caption to "modified from [MSLS25]"
\begin{frame}
\frametitle{System Level Overview}
\vspace*{-15mm}
\begin{figure}[H]
\centering
\begin{subfigure}[t]{0.5\textwidth}
\centering
\includegraphics[scale=1.1]{res/architecture}
\vspace*{5mm}
\caption{Schematic workflow of surface code quantum
computation \citereference{zhang_classical_2023}.}
\end{subfigure}%
\begin{subfigure}[t]{0.5\textwidth}
\centering
\tikzset{
block/.style={
draw, rectangle,
fill = kit-blue!25,
minimum width=75mm, minimum height=15mm,
}
}
\scalebox{0.7}{
\begin{tikzpicture}[node distance=15mm and 20mm]
\node[block] (encoding) {Encoding};
\node[block, below=of encoding] (channel) {Quantum Channel};
\node[block, below=of channel] (reverse-op)
{Reverse Operation};
\node[block, right=of channel] (syn-extr)
{Syndrome Extraction};
\node[block, below=of syn-extr] (syn-dec)
{Syndrome Decoder};
\node[above=of encoding] (input) {$\ket{\phi}$};
\node[below=of reverse-op] (output)
{$\hat{\mathcal{E}}\mathcal{E}\ket{\psi}$};
\draw [-{Latex}] (encoding) -- (channel) node[midway,
right] {$\ket{\psi}$};
\draw [-{Latex}] (channel) -- (reverse-op)
node[midway, right] {$\mathcal{E}\ket{\psi}$};
\draw [-{Latex}] (channel) -- (syn-extr)
node[midway, above] {$\mathcal{E}\ket{\psi}$};
\draw [-{Latex}] (syn-extr) -- (syn-dec)
node[midway, right] {$z$};
\draw [-{Latex}] (syn-dec) -- (reverse-op)
node[midway, above] {$\hat{\mathcal{E}}$};
\draw [-{Latex}] (input) -- (encoding);
\draw [-{Latex}] (reverse-op) -- (output);
\end{tikzpicture}
}
\vspace*{5mm}
\caption{Block diagram of QEC using stabilizer codes
\citereference{miao_quaternary_2025}.}
\end{subfigure}
\end{figure}
% \vspace*{-2mm}
\addreferences
{zhang_classical_2023}
{miao_quaternary_2025}
\stopreferences
\end{frame}
% TODO: Is this really necessary?
\begin{frame}
\frametitle{The Quantum Error Correction Landscape}
\begin{itemize}
\item Give basic overview of most promising avenues of
research (as in ECCentric paper)
\end{itemize}
\end{frame}
\backupend
\end{document}