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Connolly T, Kurilovich PD, Diamond S, Nho H, Bøttcher CGL, Glazman LI, Fatemi V, Devoret MH. Coexistence of Nonequilibrium Density and Equilibrium Energy Distribution of Quasiparticles in a Superconducting Qubit. PHYSICAL REVIEW LETTERS 2024; 132:217001. [PMID: 38856268 DOI: 10.1103/physrevlett.132.217001] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/21/2023] [Revised: 08/10/2023] [Accepted: 03/21/2024] [Indexed: 06/11/2024]
Abstract
The density of quasiparticles typically observed in superconducting qubits exceeds the value expected in equilibrium by many orders of magnitude. Can this out-of-equilibrium quasiparticle density still possess an energy distribution in equilibrium with the phonon bath? Here, we answer this question affirmatively by measuring the thermal activation of charge-parity switching in a transmon qubit with a difference in superconducting gap on the two sides of the Josephson junction. We then demonstrate how the gap asymmetry of the device can be exploited to manipulate its parity.
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Affiliation(s)
- Thomas Connolly
- Departments of Applied Physics and Physics, Yale University, New Haven, Connecticut 06520, USA
| | - Pavel D Kurilovich
- Departments of Applied Physics and Physics, Yale University, New Haven, Connecticut 06520, USA
| | - Spencer Diamond
- Departments of Applied Physics and Physics, Yale University, New Haven, Connecticut 06520, USA
| | - Heekun Nho
- Departments of Applied Physics and Physics, Yale University, New Haven, Connecticut 06520, USA
| | - Charlotte G L Bøttcher
- Departments of Applied Physics and Physics, Yale University, New Haven, Connecticut 06520, USA
| | - Leonid I Glazman
- Departments of Applied Physics and Physics, Yale University, New Haven, Connecticut 06520, USA
| | - Valla Fatemi
- Departments of Applied Physics and Physics, Yale University, New Haven, Connecticut 06520, USA
- School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, USA
| | - Michel H Devoret
- Departments of Applied Physics and Physics, Yale University, New Haven, Connecticut 06520, USA
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Liu CH, Harrison DC, Patel S, Wilen CD, Rafferty O, Shearrow A, Ballard A, Iaia V, Ku J, Plourde BLT, McDermott R. Quasiparticle Poisoning of Superconducting Qubits from Resonant Absorption of Pair-Breaking Photons. PHYSICAL REVIEW LETTERS 2024; 132:017001. [PMID: 38242669 DOI: 10.1103/physrevlett.132.017001] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/07/2022] [Revised: 11/22/2023] [Accepted: 11/29/2023] [Indexed: 01/21/2024]
Abstract
The ideal superconductor provides a pristine environment for the delicate states of a quantum computer: because there is an energy gap to excitations, there are no spurious modes with which the qubits can interact, causing irreversible decay of the quantum state. As a practical matter, however, there exists a high density of excitations out of the superconducting ground state even at ultralow temperature; these are known as quasiparticles. Observed quasiparticle densities are of order 1 μm^{-3}, tens of orders of magnitude greater than the equilibrium density expected from theory. Nonequilibrium quasiparticles extract energy from the qubit mode and can induce dephasing. Here we show that a dominant mechanism for quasiparticle poisoning is direct absorption of high-energy photons at the qubit junction. We use a Josephson junction-based photon source to controllably dose qubit circuits with millimeter-wave radiation, and we use an interferometric quantum gate sequence to reconstruct the charge parity of the qubit. We find that the structure of the qubit itself acts as a resonant antenna for millimeter-wave radiation, providing an efficient path for photons to generate quasiparticles. A deep understanding of this physics will pave the way to realization of next-generation superconducting qubits that are robust against quasiparticle poisoning.
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Affiliation(s)
- C H Liu
- Department of Physics, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA
| | - D C Harrison
- Intelligence Community Postdoctoral Research Fellowship Program, Department of Physics, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA
| | - S Patel
- Department of Physics, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA
| | - C D Wilen
- Department of Physics, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA
| | - O Rafferty
- Department of Physics, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA
| | - A Shearrow
- Department of Physics, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA
| | - A Ballard
- Department of Physics, Syracuse University, Syracuse, New York 13244, USA
| | - V Iaia
- Department of Physics, Syracuse University, Syracuse, New York 13244, USA
| | - J Ku
- Department of Physics, Syracuse University, Syracuse, New York 13244, USA
| | - B L T Plourde
- Department of Physics, Syracuse University, Syracuse, New York 13244, USA
| | - R McDermott
- Department of Physics, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA
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