Influence of quasiparticles in the energy gap on the characteristics of superconducting devices

Loren D. Alegria, Charlotte G. L. Bøttcher, Andrew K. Saydjari +5 more

Roman-Pascal Riwar, Gianluigi Catelani

Quasiparticles represent an intrinsic source of perturbation for superconducting qubits, leading to both dissipation of the qubit energy and dephasing. Recently, it has been shown that normal-metal traps may efficiently reduce the quasiparticle population and improve the qubit lifetime, provided the trap surpasses a certain characteristic size. Moreover, while the trap itself introduces new relaxation mechanisms, they are not expected to harm state-of-the-art transmon qubits under the condition that the traps are not placed too close to extremal positions where electric fields are high. Here we study a different type of trap, realized through gap engineering. We find that gap-engineered traps relax the remaining constraints imposed on normal metal traps. First, the characteristic trap size, above which the trap is efficient, is reduced with respect to normal metal traps, such that here, strong traps are possible in smaller devices. Second, the losses caused by the trap are now greatly reduced, providing more flexibility in trap placement. The latter point is of particular importance, since for efficient protection from quasiparticles, the traps ideally should be placed close to the active parts of the qubit device, where electric fields are typically high.

Takeshi Kobayashi, Yoshio Miura, Masayoshi Tonouchi +1 more

New effects of quasiparticle injection on microwave irradiated Josephson triodes are demonstrated. The device comprises Pb-insulator (Nb2O5)-Nb SIS junction with direct contact of the Nb film to n+-GaAs gate substrate. In this device, the very thin Nb film (300 - 600 Å) as a base electrode of the SIS detector also serves as an acceptor of the quasiparticles injected from GaAs - Nb Sehottky barrier. The emphasis was placed on injections of hot quasiparticles with the energy in excess of 0.5 eV. The zeroth step height of microwave (10 GHz) irradiated Josephson current Iowas efficiently modulated by a sufficiently low density quasiparticle injection current Iinj(1 - 2 A/cm2), though no sign of change was found in the superconductive gap parameter and the Josephson critical current itself. These characteristic features became less pronounced as the energy of injected quasiparticles was lowered below 50 meV. Though the present experiment is in a preliminary stage, a current amplification (dIo/dIinj) of higher than two has been attained on a test specimen.

Takashi Noguchi, Masato Naruse, Masakazu Sekine +2 more

Tejas Guruswamy, Goldie, DJ, S. Withington

Understanding the detailed behaviour of superconducting pair breaking photon detectors such as Kinetic Inductance Detectors (KIDs) requires knowledge of the nonequilibrium quasiparticle energy distributions. We have previously calculated the steady state distributions resulting from uniform absorption of monochromatic sub gap and above gap frequency radiation by thin films. In this work, we use the same methods to calculate the effect of illumination by broadband sources, such as thermal radiation from astrophysical phenomena or from the readout system. Absorption of photons at multiple above gap frequencies is shown to leave unchanged the structure of the quasiparticle energy distribution close to the superconducting gap. Hence for typical absorbed powers, we find the effects of absorption of broadband pair breaking radiation can simply be considered as the sum of the effects of absorption of many monochromatic sources. Distribution averaged quantities, like quasiparticle generation efficiency η, match exactly a weighted average over the bandwidth of the source of calculations assuming a monochromatic source. For sub gap frequencies, however, distributing the absorbed power across multiple frequencies does change the low energy quasiparticle distribution. For moderate and high absorbed powers, this results in a significantly larger η–a higher number of excess quasiparticles for a broadband source compared to a monochromatic source of equal total absorbed power. Typically in KIDs the microwave power absorbed has a very narrow bandwidth, but in devices with broad resonance characteristics (low quality factors), this increase in η may be measurable.

Владимир Федорович Елесин