Error Suppression in Continuous-time Quantum Computing*

In the quantum optimisation setting, we build on [Young et al., PRA 88,062314, 2013] where logical qubits in multiple copies, constituting an Ising spin system, are linked together to increase the logical system's robustness to error. We introduce several refinements that improve the scheme significantly. First we note that only one copy needs to be correct by the end of computation, since solution quality can be checked efficiently. Second, we find that ferromagnetic links do not help in the "one correct copy" situation, but low strength anti-ferromagnetic links do help sometimes. Third, we developed a protocol based on local field and coupling strengths in the problem Hamiltonian, to decide whether logical qubits should be connected anti-ferromagnetically, or left disconnected.  We have tested our method on small instances of spin glasses from [Callison et al., NJP 21, 123022, 2019] and similar small spin chain instances, and we find improved error tolerance for three or more copies in configurations that include frustration.