Energy Decay in Superconducting Josephson-Junction Qubits from Nonequilibrium Quasiparticle Excitations

Abstract

We calculate the energy decay rate of Josephson qubits and superconducting resonators from nonequilibrium quasiparticles. The decay rates from experiments are shown to be consistent with predictions based on a prior measurement of the quasiparticle density n(qp) = 10/microm(3), which suggests that nonequilibrium quasiparticles are an important decoherence source for Josephson qubits. Calculations of the energy-decay and diffusion of quasiparticles also indicate that prior engineered gap and trap structures, which reduce the density of quasiparticles, should be redesigned to improve their efficacy. This model also explains a striking feature in Josephson qubits and resonators-a small reduction in decay rate with increasing temperature.

Generation-Recombination Noise: The Fundamental Sensitivity Limit for Kinetic Inductance Detectors

“…The saturation in the quasiparticle lifetime (Fig. 2a) is consistent with the saturation in N qp , from which we conclude that the saturation in τ r , observed earlier [13,14], is here due to a saturation in N qp , consistent with calculations for superconducting qubits [15].…”

Measurement and control of quasiparticle dynamics in a superconducting qubit

“…The effective trapping rate s depends not only on the property and density of the trapping sites, but also their geometric distribution and associated diffusion timescale. The constant term g describes QP generation rate by pair-breaking stray radiation or other unidentified sources 36 . If trapping is dominant (s4 4rx qp for most of the measured range of x qp ), then decay of x qp follows an exponential function.…”

Readout-power heating and hysteretic switching between thermal quasiparticle states in kinetic inductance detectors

“…Measurements show, however, the internal quality factor saturates at values of around 10 5 -10 6 at T / T c Ϸ 0.2 for our resonators, which means that an additional dissipative mechanism is present. The source of this loss may be due to excess quasiparticles, 28 loss at the surface of the superconductor, or in the dielectric material but the exact origin is not known. 29,30 To make an improved estimate of the dissipated power, we take this saturation into account by modifying the internal quality factor in Eq.…”