Quantum process tomography is a necessary tool for verifying quantum gates
and diagnosing faults in architectures and gate design. We show that the
standard approach of process tomographyis grossly inaccurate in the case where
the states and measurement operators used to interrogate the system are
generated by gates that have some systematic error, a situation all but
unavoidable in any practical setting. These errors in tomography can not be
fully corrected through oversampling or by performing a larger set of
experiments. We present an alternative method for tomography to reconstruct an
entire library of gates in a self-consistent manner. The essential ingredient
is to define a likelihood function that assumes nothing about the gates used
for preparation and measurement. In order to make the resulting optimization
tractable we linearize about the target, a reasonable approximation when
benchmarking a quantum computer as opposed to probing a black-box function.
We implement a complete randomized benchmarking protocol on a system of two
superconducting qubits. The protocol consists of randomizing over gates in the
Clifford group, which experimentallyare generated via an improved two-qubit
cross-resonance gate implementation and single-qubit unitaries. From this we
extract an optimal average error per Clifford of 0.0936. We also perform an
interleaved experiment, alternating our optimal two-qubit gate with random
two-qubit Clifford gates, to obtain a two-qubit gate error of 0.0653. We
compare these values with a two-qubit gate error of ~0.12 obtained from quantum
process tomography, which is likely limited by state preparation and
measurement errors.
The control and handling of errors arising from cross-talk and unwanted
interactions in multi-qubit systems is an important issue in quantum
information processing architectures. Weintroduce a benchmarking protocol that
provides information about the amount of addressability present in the system
and implement it on coupled superconducting qubits. The protocol consists of
randomized benchmarking each qubit individually and then simultaneously, and
the amount of addressability is related to the difference of the average gate
fidelities of those experiments. We present the results on two similar samples
with different amounts of cross-talk and unwanted interactions, which agree
with predictions based on simple models for the amount of residual coupling.
We report a superconducting artificial atom with an observed quantum
coherence time of T2*=95us and energy relaxation time T1=70us. The system
consists of a single Josephson junctiontransmon qubit embedded in an otherwise
empty copper waveguide cavity whose lowest eigenmode is dispersively coupled to
the qubit transition. We attribute the factor of four increase in the coherence
quality factor relative to previous reports to device modifications aimed at
reducing qubit dephasing from residual cavity photons. This simple device holds
great promise as a robust and easily produced artificial quantum system whose
intrinsic coherence properties are sufficient to allow tests of quantum error
correction.