  Quantum processors can already execute tasks beyond the reach of classical
simulation, albeit for artificial problems. At this point, it is essential to
design error metrics that test the experimental accuracy of quantum algorithms
with potential for a practical quantum advantage. The distinction between
coherent errors and incoherent errors is crucial, as they often involve
different error suppression tools. The first class encompasses miscalibrations
of control signals and crosstalk, while the latter is usually related to
stochastic events and unwanted interactions with the environment. We introduce
the incoherent infidelity as a measure of incoherent errors and present a
scalable method for measuring it. This method is applicable to generic quantum
evolutions subjected to time-dependent Markovian noise. Moreover, it provides
an error quantifier for the target circuit, rather than an error averaged over
many circuits or quantum gates. The estimation of the incoherent infidelity is
suitable to assess circuits with sufficiently low error rates, regardless of
the circuit size, which is a natural requirement to run useful computations.
