diff --git a/src/thesis/chapters/3_fault_tolerant_qec.tex b/src/thesis/chapters/3_fault_tolerant_qec.tex index 2efaf62..c49b776 100644 --- a/src/thesis/chapters/3_fault_tolerant_qec.tex +++ b/src/thesis/chapters/3_fault_tolerant_qec.tex @@ -160,9 +160,22 @@ different error locations in the circuit. We will illustrate the most widely used types of error models on the example of the three-qubit repetition code for $X$ errors. -This code has stabilizers $Z_1Z_2$ and $Z_2Z_3$. -\autoref{fig:pure_syndrome_extraction} shows the respective -check matrix and syndrome extraction circuit. +This is a code with check matrix +\begin{align*} + \bm{H} = + \left[ + \begin{array}{ccc|ccc} + 0 & 0 & 0 & 0 & 0 & 0 \\ + 0 & 0 & 0 & 0 & 0 & 0 \\ + 0 & 0 & 0 & 1 & 1 & 0 \\ + 0 & 0 & 0 & 0 & 1 & 1 + \end{array} + \right] + . +\end{align*} +We can see that it has stabilizers $Z_1Z_2$ and $Z_2Z_3$. +\autoref{fig:pure_syndrome_extraction} shows the corresponding +syndrome extraction circuit. We refer to the qubits carrying the logical state $\ket{\psi}_\text{L}$ as \emph{data qubits}. Note that this is a concrete implementation using CNOT gates, as @@ -247,30 +260,15 @@ error locations. \begin{figure}[t] \centering - \begin{minipage}{0.5\textwidth} - \begin{align*} - \bm{H} = - \left[ - \begin{array}{ccc|ccc} - 0 & 0 & 0 & 0 & 0 & 0 \\ - 0 & 0 & 0 & 0 & 0 & 0 \\ - 0 & 0 & 0 & 1 & 1 & 0 \\ - 0 & 0 & 0 & 0 & 1 & 1 - \end{array} - \right] - \end{align*} - \end{minipage}% - \begin{minipage}{0.5\textwidth} - % tex-fmt: off - \begin{quantikz}%[row sep=4mm, column sep=4mm] - \lstick[3]{$\ket{\psi}_\text{L}$} & \ctrl{3} & & & & & \\ - & & \ctrl{2} & \ctrl{3} & & & \\ - & & & & \ctrl{2} & & \\ - \lstick{$\ket{0}_{\text{A}_1}$} & \targ{} & \targ{} & & & \meter{} & \setwiretype{c} \\ - \lstick{$\ket{0}_{\text{A}_2}$} & & & \targ{} & \targ{} & \meter{} & \setwiretype{c} - \end{quantikz} - % tex-fmt: on - \end{minipage}% + % tex-fmt: off + \begin{quantikz}%[row sep=4mm, column sep=4mm] + \lstick[3]{$\ket{\psi}_\text{L}$} & \ctrl{3} & & & & & \\ + & & \ctrl{2} & \ctrl{3} & & & \\ + & & & & \ctrl{2} & & \\ + \lstick{$\ket{0}_{\text{A}_1}$} & \targ{} & \targ{} & & & \meter{} & \setwiretype{c} \\ + \lstick{$\ket{0}_{\text{A}_2}$} & & & \targ{} & \targ{} & \meter{} & \setwiretype{c} + \end{quantikz} + % tex-fmt: on \caption{ Syndrome extraction circuit for the three-qubit repetition @@ -400,16 +398,29 @@ error locations. \section{Detector Error Models} \label{sec:Detector Error Models} -\emph{Detector error models} constitue a standardized framework for -passing information about the circuit used for \ac{qec} to a decoder. -They are also useful in the design of fault-tolerant \ldots such as -fault-tolerant quantum computing schemes \cite[Sec.~1]{derks_designing_2025}. -% While alternate ways of considering fault tolerance exist, detector -% error models -% benefit from the fact that -\content{Benefits of this approach \cite[Sec.~4.2]{derks_designing_2025}} +\emph{Detector error models} (\acsp{dem}) constitue a standardized 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}. -\content{Where they were introduced originally} +Other approaches of implementing fault tolerance exist, such as +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 +\cite[Sec.~4.2]{derks_designing_2025}: +\begin{itemize} + \item They distinguish between errors based on their effect on + the measurements, not based on their location in the circuit. + This allows for merging equivalent errors, which decreases + decoding complexity. + \item Errors on the data qubits and on the measurements are + 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. % Core idea