Proximity effect in normal-metal quasiparticle traps

Abstract

In many superconducting devices, including qubits, quasiparticle excitations are detrimental. A normal metal (N ) in contact with a superconductor (S) can trap these excitations; therefore, such a trap can potentially improve the device's performances. The two materials influence each other, a phenomenon known as proximity effect which has drawn attention since the 1960's. Here, we study whether this mutual influence places a limitation on the possible performance improvement in superconducting qubits. We first revisit the proximity effect in uniform NS bilayers; we show that the density of states is of the Dynes type above the minigap. We then extend our results to describe a nonuniform system in the vicinity of a trap edge. Using these results together with a phenomenological model for the suppression of the quasiparticle density due to the trap, we find in a transmon qubit an optimum trap-junction distance at which the qubit relaxation rate is minimized. This optimum distance, of the order of 4 to 20 coherence lengths, originates from the competition between proximity effect and quasiparticle density suppression. We conclude that the harmful influence of the proximity effect can be avoided so long as the trap is farther away from the junction than this optimum.

Efficient quasiparticle traps with low dissipation through gap engineering

“…which can easily be identified as the respective RC times in the main text, see Eqs. (30) and (31). As already pointed out in the main text, the quasiclassical approximation fails with certainty, if the quasiparticle distribution is hot, ω < T qp .…”

“…In fact, with Y and Y in the limit of cold quasiparticles, it is possible to show that the RC times in Eqs. (30) and (31) follow directly from the full quantum mechanical calculation, shown in Appendix B. Recent measurements of transmon transition rates [46] have been interpreted in terms of hot non-equilibrium quasiparticles, although a more likely explanation is in terms of photon-assisted Cooper pair breaking [47].…”

“…On the other hand, normal metal traps themselves introduce new processes that could limit the quality factor of the qubit [29,30]. The two most important processes are the dissipation in the normal metal bulk due to the ac charge redistribution in the circuit, and the losses due to arXiv:1907.04781v1 [cond-mat.supr-con] 10 Jul 2019 photo-assisted tunneling at the SN interface.…”

“…For sufficiently good interfaces, the proximity effect will introduce a spatially dependent gap in the circuit (see e.g. [30,31] and references therein) and provide a region where the quasiparticles can be trapped. Studying the quasiparticle diffusion dynamics, we predict that the minimal trap size to reach the strong trapping limit is reduced with respect to normal metal traps, making efficient traps possible in devices of smaller sizes.…”

Tunable refrigerator for nonlinear quantum electric circuits

“…Consequently, one should try to maximize the on/off ratio of the QCR. Note that if the Dynes broadening is caused by the weak proximity effect at the normal-metal-insulatorsuperconductor junction, we have γ D ∝ g T [52], and hence we can reduce γ D by fabricating more opaque tunnel barriers. As discussed below, a lower Dynes parameter yields a higher on/off ratio in general.…”