  We introduce a quantum error mitigation technique based on probabilistic
error cancellation to eliminate errors which have accumulated during the
application of a quantum circuit. Our approach is based on applying an optimal
"denoiser" after the action of a noisy circuit and can be performed with an
arbitrary number of extra gates. The denoiser is given by an ensemble of
circuits distributed with a quasiprobability distribution. For a simple noise
model, we show that efficient, local denoisers can be found, and we demonstrate
their effectiveness for the digital quantum simulation of the time evolution of
simple spin chains.
