Robust sparse IQP sampling in constant depth

TL;DR

This paper presents a fault-tolerant scheme for sparse IQP quantum circuits that achieves a provable superpolynomial quantum advantage with minimal error correction, constant depth, and efficient state preparation.

Contribution

It introduces the tetrahelix code enabling fault-tolerant, constant-depth implementation of sparse IQP circuits with minimal overhead and classical feed-forward.

Findings

Achieves superpolynomial quantum advantage under complexity conjectures.

Provides a fault-tolerant, constant-depth implementation of sparse IQP circuits.

Demonstrates efficient state preparation with classical feed-forward.

Abstract

Between NISQ (noisy intermediate scale quantum) approaches without any proof of robust quantum advantage and fully fault-tolerant quantum computation, we propose a scheme to achieve a provable superpolynomial quantum advantage (under some widely accepted complexity conjectures) that is robust to noise with minimal error correction requirements. We choose a class of sampling problems with commuting gates known as sparse IQP (Instantaneous Quantum Polynomial-time) circuits and we ensure its fault-tolerant implementation by introducing the tetrahelix code. This new code is obtained by merging several tetrahedral codes (3D color codes) and has the following properties: each sparse IQP gate admits a transversal implementation, and the depth of the logical circuit can be traded for its width. Combining those, we obtain a depth-1 implementation of any sparse IQP circuit up to the preparation of encoded states. This comes at the cost of a space overhead which is only polylogarithmic in the width of the original circuit. We furthermore show that the state preparation can also be performed in constant depth with a single step of feed-forward from classical computation. Our construction thus exhibits a robust superpolynomial quantum advantage for a sampling problem implemented on a constant depth circuit with a single round of measurement and feed-forward.