Error Mitigation with a Few Ancillas

Achieving high-fidelity quantum computation in the era of noisy intermediate scale quantum technologies requires the use of error mitigation. We introduce a new error mitigation technique that uses a small number of ancillas to mitigate errors by exploiting the symmetries preserved by the computation. Fidelity improvements are achieved by performing a pair of controlled unitary checks at two locations in the circuit. The method is a generalization of symmetry verification since the two controlled operations can be different, which relaxes the condition of commutation required in symmetry verification. Our approach is applicable to general circuits. We evaluate the performance with extensive numerical simulations using different error models, circuits, and unitary check placement. We also explore efficient techniques for finding suitable unitary checks. In this work we restrict these checks to the Pauli group.