QDev Seminar: Ioan Pop

Coherent Suppression of Quasiparticle Dissipation

We demonstrate immunity to quasiparticle dissipation for the central Josephson junction of a superconducting quantum bit (qubit).
At the foundation of this protection rests a prediction by Brian Josephson from fifty years ago: the particle-hole interference
of superconducting quasiparticles when tunneling across a Josephson junction [1]. The junction under study is the central element
of a fluxonium qubit, which we place in an extremely low loss environment and measure using radio-frequency dispersive
techniques [2]. Furthermore, by using a quantum limited amplifier (a Josephson Parametric Converter) we can observe quantum jumps
between the 0 and 1 states of the qubit in thermal equilibrium with the environment. The distribution of the times in-between the
quantum jumps reveals quantitative information about the population and dynamics of quasiparticles [3]. The data is entirely
consistent with the hypothesis that our system is sensitive to single quasiparticle excitations while being immune to charge offsets.
The ability to count quasiparticles opens new perspectives for single-quasiparticle mesoscopic devices, such as hybrid
superconducting/semiconducting circuits or ultra-sensitive THz radiation detectors.

[1] B. D. Josephson, Physics Letters 1, 251 (1962)
[2] I. M. Pop et al., Nature 508 (2014)
[3] U. Vool, I. M. Pop et al., PRL 113 (2014)