Dynamical error suppression techniques are commonly used to improve coherence in quantum systems. They reduce dephasing errors by applying control pulses designed to reverse erroneouscoherent evolution driven by environmental noise. However, such methods cannot correct for irreversible processes such as energy relaxation. In this work, we investigate a complementary, stochastic approach to reducing errors: instead of deterministically reversing the unwanted qubit evolution, we use control pulses to shape the noise environment dynamically. In the context of superconducting qubits, we implement a pumping sequence to reduce the number of unpaired electrons (quasiparticles) in close proximity to the device. We report a 70% reduction in the quasiparticle density, resulting in a threefold enhancement in qubit relaxation times, and a comparable reduction in coherence variability.
A novel mechanism of asymmetric frequency conversion is investigated in nonlinear dispersive devices driven parametrically with a biharmonic pump. When the relative phase between thefirst and second harmonics combined in a two-tone pump is appropriately tuned, nonreciprocal frequency conversion, either upward or downward, can occur. Full directionality and efficiency of the conversion process is possible, provided that the distribution of pump power over the harmonics is set correctly. While this asymmetric conversion effect is generic, we describe its practical realization in a model system consisting of a current-biased, resistively-shunted Josephson junction. Here, the multiharmonic Josephson oscillations, generated internally from the static current bias, provide the pump drive.
We present a new theoretical framework to analyze microwave amplifiers based
on the dc SQUID. Our analysis applies input-output theory generalized for
Josephson junction devices biasedin the running state. Using this approach we
express the high frequency dynamics of the SQUID as a scattering between the
participating modes. This enables us to elucidate the inherently nonreciprocal
nature of gain as a function of bias current and input frequency. This method
can, in principle, accommodate an arbitrary number of Josephson harmonics
generated in the running state of the junction. We report detailed calculations
taking into account the first few harmonics that provide simple
semi-quantitative results showing a degradation of gain, directionality and
noise of the device as a function of increasing signal frequency. We also
discuss the fundamental limits on device performance and applications of this
formalism to real devices.
We demonstrate high-fidelity, quantum nondemolition, single-shot readout of a
superconducting flux qubit in which the pointer state distributions can be
resolved to below one part in1000. In the weak excitation regime, continuous
measurement permits the use of heralding to ensure initialization to a fiducial
state, such as the ground state. This procedure boosts readout fidelity to
93.9% by suppressing errors due to spurious thermal population. Furthermore,
heralding potentially enables a simple, fast qubit reset protocol without
changing the system parameters to induce Purcell relaxation.