Interactive quantum advantage with noisy, shallow Clifford circuits

TL;DR

This paper demonstrates that noisy, shallow quantum circuits can outperform classical circuits in interactive tasks, establishing new separations and robustness results by leveraging average-case hardness and cryptographic techniques.

Contribution

It introduces a general method to add noise tolerance to quantum-classical circuit separations and proves average-case hardness for classical simulation of noisy quantum interactive tasks.

Findings

Unconditional separation between noisy QNC^0 and AC^0[p] circuits.

Conditional separation between noisy QNC^0 and log-space classical machines.

Classical simulation remains hard even with constant error probability.

Abstract

Recent work by Bravyi et al. constructs a relation problem that a noisy constant-depth quantum circuit (QNC$^0$) can solve with near certainty (probability $1 - o(1)$), but that any bounded fan-in constant-depth classical circuit (NC$^0$) fails with some constant probability. We show that this robustness to noise can be achieved in the other low-depth quantum/classical circuit separations in this area. In particular, we show a general strategy for adding noise tolerance to the interactive protocols of Grier and Schaeffer. As a consequence, we obtain an unconditional separation between noisy QNC$^0$ circuits and AC$^0[p]$ circuits for all primes $p \geq 2$, and a conditional separation between noisy QNC$^0$ circuits and log-space classical machines under a plausible complexity-theoretic conjecture. A key component of this reduction is showing average-case hardness for the classical simulation tasks -- that is, showing that a classical simulation of the quantum interactive task is still powerful even if it is allowed to err with constant probability over a uniformly random input. We show that is true even for quantum tasks which are $\oplus$L-hard to simulate. To do this, we borrow techniques from randomized encodings used in cryptography.