  We apply quantum error mitigation techniques to a variety of benchmark
problems and quantum computers to evaluate the performance of quantum error
mitigation in practice. To do so, we define an empirically motivated,
resource-normalized metric of the improvement of error mitigation which we call
the improvement factor, and calculate this metric for each experiment we
perform. The experiments we perform consist of zero-noise extrapolation and
probabilistic error cancellation applied to two benchmark problems run on IBM,
IonQ, and Rigetti quantum computers, as well as noisy quantum computer
simulators. Our results show that error mitigation is on average more
beneficial than no error mitigation - even when normalized by the additional
resources used - but also emphasize that the performance of quantum error
mitigation depends on the underlying computer.
