On fault tolerant single-shot logical state preparation and robust long-range entanglement

TL;DR

This paper demonstrates that single-shot logical state preparation is possible for arbitrary quantum LDPC codes using constant depth circuits, enabling robust long-range entanglement with minimal overhead.

Contribution

It generalizes the single-shot preparation method from surface codes to all quantum LDPC codes, proving its feasibility and robustness.

Findings

Single-shot preparation of logical states for quantum LDPC codes.

Proof of robust long-range entanglement generation.

Application to preparing encoded GHZ states.

Abstract

Preparing encoded logical states is the first step in a fault-tolerant quantum computation. Standard approaches based on concatenation or repeated measurement incur a significant time overhead. The Raussendorf-Bravyi-Harrington cluster state offers an alternative: a single-shot preparation of encoded states of the surface code, by means of a constant depth quantum circuit, followed by a single round of measurement and classical feedforward. In this work we generalize this approach and prove that single-shot logical state preparation can be achieved for arbitrary quantum LDPC codes. Our proof relies on a minimum-weight decoder and is based on a generalization of Gottesman's clustering-of-errors argument. As an application, we also prove single-shot preparation of the encoded GHZ state in arbitrary quantum LDPC codes. This shows that adaptive noisy constant depth quantum circuits are capable of generating generic robust long-range entanglement.