title: Non-Poissonian quantum jumps of a fluxonium qubit due to quasiparticle excitations.
abstract: As the energy relaxation time of superconducting qubits steadily improves, nonequilibrium 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 nondemolition projective measurements within a time interval much shorter than T₁, using a quantum-limited amplifier (Josephson parametric converter). The quantum jump 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 a magnetic field.
authors.affiliation: Department of Applied Physics and Physics, Yale University, New Haven, Connecticut 06520, USA.
authors.forename: U
authors.initials: U
authors.lastname: Vool
authors.affiliation: Department of Applied Physics and Physics, Yale University, New Haven, Connecticut 06520, USA.
authors.forename: I M
authors.initials: IM
authors.lastname: Pop
authors.affiliation: Department of Applied Physics and Physics, Yale University, New Haven, Connecticut 06520, USA.
authors.forename: K
authors.initials: K
authors.lastname: Sliwa
authors.affiliation: Department of Applied Physics and Physics, Yale University, New Haven, Connecticut 06520, USA.
authors.forename: B
authors.initials: B
authors.lastname: Abdo
authors.affiliation: Department of Applied Physics and Physics, Yale University, New Haven, Connecticut 06520, USA.
authors.forename: C
authors.initials: C
authors.lastname: Wang
authors.affiliation: Department of Applied Physics and Physics, Yale University, New Haven, Connecticut 06520, USA.
authors.forename: T
authors.initials: T
authors.lastname: Brecht
authors.affiliation: Department of Applied Physics and Physics, Yale University, New Haven, Connecticut 06520, USA.
authors.forename: Y Y
authors.initials: YY
authors.lastname: Gao
authors.affiliation: Department of Applied Physics and Physics, Yale University, New Haven, Connecticut 06520, USA.
authors.forename: S
authors.initials: S
authors.lastname: Shankar
authors.affiliation: Department of Applied Physics and Physics, Yale University, New Haven, Connecticut 06520, USA.
authors.forename: M
authors.initials: M
authors.lastname: Hatridge
authors.affiliation: Peter Grünberg Institut (PGI-2), Forschungszentrum Jülich, 52425 Jülich, Germany.
authors.forename: G
authors.initials: G
authors.lastname: Catelani
authors.affiliation: Department of Applied Physics and Physics, Yale University, New Haven, Connecticut 06520, USA and INRIA Paris-Rocquencourt, Domaine de Voluceau, BP105, 78153 Le Chesnay cedex, France.
authors.forename: M
authors.initials: M
authors.lastname: Mirrahimi
authors.affiliation: Department of Applied Physics and Physics, Yale University, New Haven, Connecticut 06520, USA.
authors.forename: L
authors.initials: L
authors.lastname: Frunzio
authors.affiliation: Department of Applied Physics and Physics, Yale University, New Haven, Connecticut 06520, USA.
authors.forename: R J
authors.initials: RJ
authors.lastname: Schoelkopf
authors.affiliation: Department of Applied Physics and Physics, Yale University, New Haven, Connecticut 06520, USA.
authors.forename: L I
authors.initials: LI
authors.lastname: Glazman
authors.affiliation: Department of Applied Physics and Physics, Yale University, New Haven, Connecticut 06520, USA.
authors.forename: M H
authors.initials: MH
authors.lastname: Devoret