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Non-equilibrium quasiparticles in superconducting circuits: photons vs. phonons

by G. Catelani, D. M. Basko

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Submission summary

Authors (as registered SciPost users): Denis M. Basko · Gianluigi Catelani
Submission information
Preprint Link: scipost_201811_00008v1  (pdf)
Date submitted: Nov. 8, 2018, 1 a.m.
Submitted by: Denis M. Basko
Submitted to: SciPost Physics
Ontological classification
Academic field: Physics
Specialties:
  • Condensed Matter Physics - Theory
  • Quantum Physics
Approach: Theoretical

Abstract

We study the effect of non-equilibrium quasiparticles on the operation of a superconducting device (a qubit or a resonator), including heating of the quasiparticles by the device operation. Focusing on the competition between heating via low-frequency photon absorption and cooling via photon and phonon emission, we obtain a remarkably simple non-thermal stationary solution of the kinetic equation for the quasiparticle distribution function. We estimate the influence of quasiparticles on relaxation and excitation rates for transmon qubits, and relate our findings to recent experiments.

Current status:
Has been resubmitted

Reports on this Submission

Report #1 by Anonymous (Referee 1) on 2018-12-8 (Invited Report)

  • Cite as: Anonymous, Report on arXiv:scipost_201811_00008v1, delivered 2018-12-08, doi: 10.21468/SciPost.Report.719

Strengths

1- detailed analytical and numerical study of an intricate non-equilibrium kinetic problem in the presence of two baths.

2- clear discussion on the relevance of the studied model to recent experiments on the out-of-equilibrium quasiparticle distribution in superconducting qubits.

Weaknesses

1- difficulty to grasps the main findings due to the large number of studied regimes (see requested changes)

Report

The authors study theoretically the energy dependence of the distribution function of superconducting quasiparticles coupled with a phonon bath and a low-energy (on the scale of the superconducting gap) driven qubit (or resonator mode). They find a variety of behaviors, depending on the qubit (or resonator) frequency and its effective temperature, the temperature of the phonon bath, and the ratio between the parameters that characterize the coupling of the quasiparticles to phonons and to the qubit (or resonator). They apply their results to the estimation of the quasiparticle contribution to the qubit relaxation and excitation, which have been recently measured in transmon qubits. They conclude that a qubit by itself cannot overheat quasiparticles. This points out to the necessity of adding additional mechanisms to the ones considered in the present study if one aims at understanding some recent experiments.


Overall, I find that the results are interesting and timely, and I recommend the manuscript to be published in Sci-Post, provided the author consider the following requested changes.

Requested changes

1- Plots indicating qualitatively the energy dependence of the distribution function in various energy windows, both in the so-called cold and hot quasiparticle regimes, would be helpful for the reader to grasp the main findings;

2- In Sec. 3.2, it is not particularly clear that the results are first summarized before they are derived (which looks like a different choice from the one made for the presentation of the results in Sec. 3.1);

3- It is not particularly obvious where the equation given in the footnote 1 (especially the numerical factors $\sqrt{3}$ and $\sqrt{128}$ inside it) comes from.

4- Eq. (7) seems to assume that the first term in the rhs of the non-numbered equation below Eq. (6) can be neglected in front of the second one. Is it obvious, or should it be checked self-consistently after a solution is found?

  • validity: top
  • significance: high
  • originality: high
  • clarity: good
  • formatting: excellent
  • grammar: excellent

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