Coherent suppression of electromagnetic dissipation due to superconducting quasiparticles

Abstract

Owing to the low-loss propagation of electromagnetic signals in superconductors, Josephson junctions constitute ideal building blocks for quantum memories, amplifiers, detectors and high-speed processing units, operating over a wide band of microwave frequencies. Nevertheless, although transport in superconducting wires is perfectly lossless for direct current, transport of radio-frequency signals can be dissipative in the presence of quasiparticle excitations above the superconducting gap. Moreover, the exact mechanism of this dissipation in Josephson junctions has never been fully resolved experimentally. In particular, Josephson's key theoretical prediction that quasiparticle dissipation should vanish in transport through a junction when the phase difference across the junction is π (ref. 2) has never been observed. This subtle effect can be understood as resulting from the destructive interference of two separate dissipative channels involving electron-like and hole-like quasiparticles. Here we report the experimental observation of this quantum coherent suppression of quasiparticle dissipation across a Josephson junction. As the average phase bias across the junction is swept through π, we measure an increase of more than one order of magnitude in the energy relaxation time of a superconducting artificial atom. This striking suppression of dissipation, despite the presence of lossy quasiparticle excitations above the superconducting gap, provides a powerful tool for minimizing decoherence in quantum electronic systems and could be directly exploited in quantum information experiments with superconducting quantum bits.

Collective modes in the fluxonium qubit

“…2). Note that even at half-flux quantum, where ω 10 has a minimum [10,11], the correction is at most a few percent of ω 10 .…”

“…To realize in practice a sufficiently large inductance, arrays with many Josephson junctions are used. Even in this more complicated circuit, long relaxation times have been achieved, so long, in fact, to make possible the accurate measurement of the phase dependence of the quasiparticle dissipation through a Josephson junction [11]. Experimentally, the fluxonium coherence time is much shorter than the limit imposed by relaxation; in this paper, we theoretically investigate whether the complexity of the circuit, arising from the use of junction arrays, contributes to this shortness.…”

“…In the next section, we focus on the even sector. The analysis of the odd-sector modes, which are decoupled from the qubit, is in Appendix C. Throughout the paper, we will present examples calculated with the two-parameter set given in Table I; as explained in Appendix B, the parameters are chosen as to reflect realistic experimental values [9,11].…”

Effects of nonequilibrium quasiparticles in a thin-film superconducting microwave resonator under optical illumination

“…Quasiparticle generation from ionizing radiation or photons provides the physical basis for superconducting radiation detectors 5,6,10,14 , and is also relevant to some proposed hybrid quantum systems in which a superconducting device must function in close proximity to optically trapped atoms 16,17 . There has also been renewed interest in the role of quasiparticles as a loss and decoherence mechanism in superconducting qubits [18][19][20] . Although qubits operate at the singlephoton level, there is also interest in applying high rf powers to qubits and their readout resonators for both readout 21 and gate coupling 22 .…”

Measurement and control of quasiparticle dynamics in a superconducting qubit

“…Operating at 20 mK or lower temperature, superconducting aluminium in thermal equilibrium should have no more than one QP for the volume of the Earth. However, a substantial background of QPs has been observed in various devices from single electron 7 or Cooper-pair transistors 8 , kinetic inductance detectors 9,10 to superconducting qubits [11][12][13] . A detailed understanding of the generation mechanism and dominant relaxation processes will eventually be necessary to suppress this small background of QPs and continue the improvement of these devices.…”