  Medium-scale quantum devices that integrate about hundreds of physical qubits
are likely to be developed in the near future. However, such devices will lack
the resources for realizing quantum fault tolerance. Therefore, the main
challenge of exploring the advantage of quantum computation is to minimize the
impact of device and control imperfections without encoding. Quantum error
mitigation is a solution satisfying the requirement. Here, we demonstrate an
error mitigation protocol based on gate set tomography and quasiprobability
decomposition. One- and two-qubit circuits are tested on a superconducting
device, and computation errors are successfully suppressed. Because this
protocol is universal for digital quantum computers and algorithms computing
expected values, our results suggest that error mitigation can be an essential
component of near-future quantum computation.
