Exponential Error Suppression for Near-Term Quantum Devices

Abstract

As quantum computers mature, quantum error correcting codes (QECs) will be adopted in order to suppress errors to any desired level $\\mathcal{E}$ at a cost in qubit-count $n$ that is merely poly-logarithmic in $\\mathcal{E}^{-1}$. However in the NISQ era, the complexity and scale required to adopt even the smallest QEC is prohibitive. Instead, error mitigation techniques have been employed; typically these do not require a significant increase in qubit-count but cannot provide exponential error suppression. Here we show that, for the crucial case of estimating expectation values of observables (key to almost all NISQ algorithms) one can indeed achieve an effective exponential suppression. At the cost of a qubit count increase by a factor of $n\\geq 2$ the error becomes $Q^n$ where $Q<1$ is a suppression factor that depends on the entropy of the error probabilities. The approach takes $n$ independently-prepared circuit outputs and applies a controlled derangement operator to create a state whose symmetries prevent error-burdened states from contributing bias to the expected value. The approach is therefore `NISQ-friendly as it is modular in the main computation and requires only a shallow circuit that bridges the $n$ copies immediately prior to measurement. Imperfections in our derangement circuit do degrade performance and therefore we propose an approach to efficiently mitigate this effect to arbitrary precision due to the remarkable properties of derangements. a) they decompose into a linear number of elementary gates -- limiting the impact of noise b) they are highly resilient to noise and the effect of imperfections on them is (almost) trivial. In numerical simulations validating our approach we confirm error suppression below $10^{-6}$ for circuits consisting of several hundred noisy gates (two-qubit gate error $0.5\\%$) using no more than $n=4$ circuit copies.