  We present a set of methods to generate less complex error channels by
quantum circuit parallelisation. The resulting errors are simplified as a
consequence of their symmetrisation and randomisation. Initially, the case of a
single error channel is analysed; these results are then generalised to
multiple error channels. Error simplification for each method is shown to be
either constant, linear, or exponential in terms of system size. Finally,
example applications are provided, along with experiments run on
superconducting quantum hardware and numerical simulation. These applications
are: (1) reducing the sample complexity of matrix-inversion measurement error
mitigation by error symmetrisation, (2) improving the effectiveness of
noise-estimation circuit error mitigation by error randomisation, and (3)
improving the predictability of noisy circuit performance by error
randomisation.
