Non-Poissonian Quantum Jumps of a Fluxonium Qubit due to Quasiparticle Excitations

TL;DR

This study observes non-Poissonian quantum jumps in a fluxonium qubit caused by quasiparticle excitations, revealing large fluctuations in quasiparticle populations and their controllable dynamics, impacting qubit coherence.

Contribution

It provides direct, high-resolution measurements of quasiparticle-induced quantum jumps, demonstrating non-Poissonian statistics and controllable quasiparticle dynamics in superconducting qubits.

Findings

Quantum jumps switch between Poissonian and non-Poissonian statistics.

Large fluctuations in quasiparticle populations occur over seconds to hours.

Quasiparticle dynamics can be controlled by injection and magnetic vortices.

Abstract

As the energy relaxation time of superconducting qubits steadily improves, non-equilibrium quasiparticle excitations above the superconducting gap emerge as an increasingly relevant limit for qubit coherence. We measure fluctuations in the number of quasiparticle excitations by continuously monitoring the spontaneous quantum jumps between the states of a fluxonium qubit, in conditions where relaxation is dominated by quasiparticle loss. Resolution on the scale of a single quasiparticle is obtained by performing quantum non-demolition projective measurements within a time interval much shorter than $T_1$, using a quantum limited amplifier (Josephson Parametric Converter). The quantum jumps statistics switches between the expected Poisson distribution and a non-Poissonian one, indicating large relative fluctuations in the quasiparticle population, on time scales varying from seconds to hours. This dynamics can be modified controllably by injecting quasiparticles or by seeding quasiparticle-trapping vortices by cooling down in magnetic field.