  The performance of quantum gates is often assessed using some form of
randomized benchmarking. However, the existing methods become infeasible for
more than approximately five qubits. Here we show how to use a simple and
customizable class of circuits -- randomized mirror circuits -- to perform
scalable, robust, and flexible randomized benchmarking of Clifford gates. We
show that this technique approximately estimates the infidelity of an average
many-qubit logic layer, and we use simulations of up to 225 qubits with
physically realistic error rates in the range 0.1-1% to demonstrate its
scalability. We then use up to 16 physical qubits of a cloud quantum computing
platform to demonstrate that our technique can reveal and quantify crosstalk
errors in many-qubit circuits.
