  Randomized benchmarking (RB) protocols are the most widely used methods for
assessing the performance of quantum gates. However, the existing RB methods
either do not scale to many qubits or cannot benchmark a universal gate set.
Here, we introduce and demonstrate a technique for scalable RB of many
universal and continuously parameterized gate sets, using a class of circuits
called randomized mirror circuits. Our technique can be applied to a gate set
containing an entangling Clifford gate and the set of arbitrary single-qubit
gates, as well as gate sets containing controlled rotations about the Pauli
axes. We use our technique to benchmark universal gate sets on four qubits of
the Advanced Quantum Testbed, including a gate set containing a controlled-S
gate and its inverse, and we investigate how the observed error rate is
impacted by the inclusion of non-Clifford gates. Finally, we demonstrate that
our technique scales to many qubits with experiments on a 27-qubit IBM Q
processor. We use our technique to quantify the impact of crosstalk on this
27-qubit device, and we find that it contributes approximately 2/3 of the total
error per gate in random many-qubit circuit layers.
