  Recent work has demonstrated that high-threshold quantum error correction is
possible for biased-noise qubits, provided one can implement a controlled-not
(CX) gate that preserves the bias. Bias-preserving CX gates have been proposed
for several biased-noise qubit platforms, most notably Kerr cats. However,
experimentally measuring the noise bias is challenging as it requires
accurately estimating certain low-probability Pauli errors in the presence of
much larger state preparation and measurement (SPAM) errors. In this paper, we
introduce bias randomized benchmarking (BRB) as a technique for measuring bias
in quantum gates. BRB, like all RB protocols, is highly accurate and immune to
SPAM errors. Our first protocol, CX-dihedral BRB, is a straightforward method
to measure the bias of the entire CX-dihedral group. Our second protocol,
interleaved bias randomized benchmarking (IBRB), is a generalization of
interleaved RB tailored to the experimental constraints biased-noise qubits;
this is a more involved procedure that directly targets the bias of the CX gate
alone. Our BRB procedures occupy a middle ground between classic RB protocols
that only estimate the average fidelity, and tomographic RB protocols that
provide more detailed characterization of noise but require more measurements
as well as experimental capabilities that are not necessarily available in
biased-noise qubits.
