Using copies to improve precision in continuous-time quantum computing

TL;DR

This paper enhances a quantum error correction scheme using multiple copies of problem encodings, demonstrating improved error tolerance and increased effective precision, advancing towards fault-tolerant quantum annealing.

Contribution

It introduces novel modifications to the scheme, showing anti-ferromagnetic links improve robustness and effective precision, with numerical validation on spin glass instances.

Findings

Anti-ferromagnetic links suppress common errors.

Three or more copies increase error tolerance.

Effective precision is increased by several bits.

Abstract

In the quantum optimisation setting, we build on a scheme introduced by Young et al [PRA 88, 062314, 2013], where physical qubits in multiple copies of a problem encoded into an Ising spin Hamiltonian are linked together to increase the logical system's robustness to error. We introduce several innovations that improve this scheme significantly. First, we note that only one copy needs to be correct by the end of the computation, since solution quality can be checked efficiently. Second, we find that ferromagnetic links do not generally help in this "one correct copy" setting, but anti-ferromagnetic links do help on average, by suppressing the chance of the same error being present on all of the copies. Third, we find that minimum-strength anti-ferromagnetic links perform best, by counteracting the spin-flips induced by the errors. We have numerically tested our innovations on small instances of spin glasses from Callison et al [NJP 21, 123022, 2019], and we find improved error tolerance for three or more copies in configurations that include frustration. Interpreted as an effective precision increase, we obtain several extra bits of precision for three copies connected in a triangle. This provides proof-of-concept of a method for scaling quantum annealing beyond the precision limits of hardware, a step towards fault tolerance in this setting.