We introduce a new entangling gate between two fixed-frequency qubits statically coupled via a microwave resonator bus which combines the following desirable qualities: all-microwavecontrol, appreciable qubit separation for reduction of crosstalk and leakage errors, and the ability to function as a two-qubit conditional-phase gate. A fixed, always-on interaction is explicitly designed between higher energy (non-computational) states of two transmon qubits, and then a conditional-phase gate is `activated‘ on the otherwise unperturbed qubit subspace via a microwave drive. We implement this microwave-activated conditional-phase gate with a fidelity from quantum process tomography of 87%.
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 describe the back action of microwave-photon detection via a Josephson
photomultiplier (JPM), a superconducting qubit coupled strongly to a
high-quality microwave cavity. The backaction operator depends qualitatively
on the duration of the measurement interval, resembling the regular photon
annihilation operator at short interaction times and approaching a variant of
the photon subtraction operator at long times. The optimal operating conditions
of the JPM differ from those considered optimal for processing and storing of
quantum information, in that a short $T_2$ of the JPM suppresses the cavity
dephasing incurred during measurement. Understanding this back action opens the
possibility to perform multiple JPM measurements on the same state, hence
performing efficient state tomography.
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.