  We provide and experimentally demonstrate an accreditation protocol that
upper-bounds the variation distance between noisy and noiseless probability
distributions of the outputs of arbitrary quantum computations. We accredit the
outputs of twenty-four quantum circuits executed on programmable
superconducting hardware, ranging from depth nine circuits on ten qubits to
depth twenty-one circuits on four qubits. Our protocol requires implementing
the "target" quantum circuit along with a number of random Clifford circuits
and subsequently post-processing the outputs of these Clifford circuits.
Importantly, the number of Clifford circuits is chosen to obtain the bound with
the desired confidence and accuracy, and is independent of the size and nature
of the target circuit. We thus demonstrate a practical and scalable method of
ascertaining the correctness of the outputs of arbitrary-sized noisy quantum
computers--the ultimate arbiter of the utility of the computer itself.
