# Lightweight Detection of a Small Number of Large Errors in a Quantum Circuit

Noah Linden1 and Ronald de Wolf2

1School of Mathematics, University of Bristol
2QuSoft, CWI and University of Amsterdam, the Netherlands

### Abstract

Suppose we want to implement a unitary $U$, for instance a circuit for some quantum algorithm. Suppose our actual implementation is a unitary $\tilde{U}$, which we can only apply as a black-box. In general it is an exponentially-hard task to decide whether $\tilde{U}$ equals the intended $U$, or is significantly different in a worst-case norm. In this paper we consider two special cases where relatively efficient and lightweight procedures exist for this task.
First, we give an efficient procedure under the assumption that $U$ and $\tilde{U}$ (both of which we can now apply as a black-box) are either equal, or differ significantly in only one $k$-qubit gate, where $k=O(1)$ (the $k$ qubits need not be contiguous). Second, we give an even more lightweight procedure under the assumption that $U$ and $\tilde{U}$ are $\textit{Clifford}$ circuits which are either equal, or different in arbitrary ways (the specification of $U$ is now classically given while $\tilde{U}$ can still only be applied as a black-box). Both procedures only need to run $\tilde{U}$ a constant number of times to detect a constant error in a worst-case norm. We note that the Clifford result also follows from earlier work of Flammia and Liu, and da Silva, Landon-Cardinal, and Poulin.
In the Clifford case, our error-detection procedure also allows us efficiently to learn (and hence correct) $\tilde{U}$ if we have a small list of possible errors that could have happened to $U$; for example if we know that only $O(1)$ of the gates of $\tilde{U}$ are wrong, this list will be polynomially small and we can test each possible erroneous version of $U$ for equality with $\tilde{U}$.

Suppose we want to implement a unitary $U$, for instance a circuit for some quantum algorithm. Suppose our actual implementation is a unitary $\tilde{U}$, which we can only apply as a black-box (i.e. we only have access to the input and output). In general it is an exponentially-hard task to decide whether the output state from $\tilde{U}$ is close to that for the intended $U$ for all input states, or whether there are input states for which the output state from $\tilde{U}$ is significantly different from that from $U$. In this paper we consider two special cases where we can in fact provide relatively efficient and lightweight procedures for this discrimination task: (1) when $U$ and $\tilde{U}$ can use arbitrary gates but only differ in one gate on a constant number of (not necessarily neighbouring) qubits, and (2) when $U$ and $\tilde{U}$ are Clifford circuits.

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### Cited by

[1] Noah Linden and Ronald de Wolf, "Average-Case Verification of the Quantum Fourier Transform Enables Worst-Case Phase Estimation", arXiv:2109.10215.

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