We experimentally study the behavior of a parametrically pumped nonlinear oscillator, which is based on a superconducting lambda /4 resonator, and is terminated by a flux-tunable SQUID.We extract parameters for two devices. In particular, we study the effect of the nonlinearities in the system and compare to theory. The Duffing nonlinearity, \alpha, is determined from the probe-power dependent frequency shift of the oscillator, and the nonlinearity, \beta, related to the parametric flux pumping, is determined from the pump amplitude for the onset of parametric oscillations. Both nonlinearities depend on the parameters of the device and can be tuned in-situ by the applied dc flux. We also suggest how to cancel the effect of \beta by adding a small dc flux and a pump tone at twice the pump frequency.
We present a new method for determining pulse imperfections and improving the
single-gate fidelity in a superconducting qubit. By applying consecutive
positive and negative $pi$ pulses,we amplify the qubit evolution due to
microwave pulse distortion, which causes the qubit state to rotate around an
axis perpendicular to the intended rotation axis. Measuring these rotations as
a function of pulse period allows us to reconstruct the shape of the microwave
pulse arriving at the sample. Using the extracted response to predistort the
input signal, we are able to improve the pulse shapes and to reach an average
single-qubit gate fidelity higher than 99.8%.
In the presence of time-reversal symmetry, quantum interference gives strong
corrections to the electric conductivity of disordered systems. The
self-interference of an electron wavefunctiontraveling time-reversed paths
leads to effects such as weak localization and universal conductance
fluctuations. Here, we investigate the effects of broken time-reversal symmetry
in a driven artificial two-level system. Using a superconducting flux qubit, we
implement scattering events as multiple Landau-Zener transitions by driving the
qubit periodically back and forth through an avoided crossing. Interference
between different qubit trajectories give rise to a speckle pattern in the
qubit transition rate, similar to the interference patterns created when
coherent light is scattered off a disordered potential. Since the scattering
events are imposed by the driving protocol, we can control the time-reversal
symmetry of the system by making the drive waveform symmetric or asymmetric in
time. We find that the fluctuations of the transition rate exhibit a sharp peak
when the drive is time-symmetric, similar to universal conductance fluctuations
in electronic transport through mesoscopic systems.
We implement dynamical decoupling techniques to mitigate noise and enhance
the lifetime of an entangled state that is formed in a superconducting flux
qubit coupled to a microscopictwo-level system. By rapidly changing the
qubit’s transition frequency relative to the two-level system, we realize a
refocusing pulse that reduces dephasing due to fluctuations in the transition
frequencies, thereby improving the coherence time of the entangled state. The
coupling coherence is further enhanced when applying multiple refocusing
pulses, in agreement with our $1/f$ noise model. The results are applicable to
any two-qubit system with transverse coupling, and they highlight the potential
of decoupling techniques for improving two-qubit gate fidelities, an essential
prerequisite for implementing fault-tolerant quantum computing.