Error mitigation by symmetry verification on a variational quantum eigensolver*

Current efforts to increase the accuracy of quantum algorithms focus on the design and
implementation of low-cost techniques to mitigate the impact of errors while avoiding fully-
fledged error correction schemes. Both simple and comprehensive, variational quantum
eigensolvers provide an excellent platform for exploring error and accuracy trade-offs. We
present the implementation of our own low-cost error detection scheme, named symmetry
verification (SV). We demonstrate an order of magnitude improvement in accuracy while
solving the hydrogen molecule in a transmon quantum processor. Furthermore, by matching
measurements to simulations, we report a breakdown of noise sources with which we
quantify the error mitigating effects of SV and its limitations.