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Nonequilibrium quasiparticle distribution in superconducting resonators: effect of pair-breaking photons
by Paul B. Fischer, Gianluigi Catelani
Submission summary
| Authors (as registered SciPost users): | Gianluigi Catelani |
| Submission information | |
|---|---|
| Preprint Link: | https://arxiv.org/abs/2401.12607v1 (pdf) |
| Date submitted: | 2024-01-31 09:03 |
| Submitted by: | Catelani, Gianluigi |
| Submitted to: | SciPost Physics |
| Ontological classification | |
|---|---|
| Academic field: | Physics |
| Specialties: |
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| Approach: | Theoretical |
Abstract
Many superconducting devices rely on the finite gap in the excitation spectrum of a superconductor: thanks to this gap, at temperatures much smaller than the critical one the number of excitations (quasiparticles) that can impact the device's behavior is exponentially small. Nevertheless, experiments at low temperature usually find a finite, non-negligible density of quasiparticles whose origin has been attributed to various non-equilibrium phenomena. Here, we investigate the role of photons with energy exceeding the pair-breaking threshold $2\Delta$ as a possible source for these quasiparticles in superconducting resonators. Modeling the interacting system of quasiparticles, phonons, sub-gap and pair-breaking photons using a kinetic equation approach, we find analytical expressions for the quasiparticles' density and their energy distribution. Applying our theory to measurements of quality factor as function of temperature and for various readout powers, we find they could be explained by assuming a small number of photons above the pair-breaking threshold. We also show that frequency shift data can give evidence of quasiparticle heating.
Current status:
Reports on this Submission
Strengths
1. The authors theoretically solve an important problem, which can potentially be useful for improving the quality of low temperature detectors
2. The authors rely on well established formalism and do not introduce additional fit parameters, untested assumptions etc.
Weaknesses
1. The paper is rather technical and, therefore, it is difficult to read.
2. The paper is the follow up of Ref. [17]. Therefore, the authors should clearly state what is the difference between the current manuscript and Ref. [17]
Report
The authors theoretically investigate the effect of high frequency
pair-breaking photons on the quality factor and on the frequency of
a superconducting resonator. They extend their previous work [17] on
this subject, and obtain approximate analytical expressions
for the quasiparticle distribution function in presence of both
pair breaking and of low frequency photons. They show that this
function may strongly deviate from equilibrium. Finally, they
use this result to study the dependence of the quality factor
and of the frequency of a superconducting resonator on power
and on temperature. The results reasonably well agree with
the experiment. Thus, the authors provide theoretical support
for the mechanism of non-equilibrium quasiparticle generation
by high frequency photons.
I am sure that the reported results are correct because
they are based on the well established kinetic equation formalism.
Nevertheless, in my opinion, the presentation of these results can be
improved. Indeed, the paper is full of complicated expressions,
which are difficult to follow. I understand that one cannot fully
avoid this and that the kinetic equations naturally lead to that.
However, it would be beneficial for the reader if the authors
would explain in simple terms the physical meaning of obtained
results, and would discuss the general system setup.
For example, the following issues can be additionally clarified:
1. The set of peaks in the distribution function plotted in Fig. 5
are located at energies m\omega_0. Can one interpret this as a result
of multi-photon processes, at least at the qualitative level?
The same question applies to Fig. 6.
2. Another unclear issue is related to the very low values of the distribution
function in Figs. 5 and 6. Namely, does it matter if this function
has a peak with the height 10^{-18}? What are the minimal values
of f(E), which are relevant for the experiment?
Do the peaks in Figs. 5 and 6 affect the quality factor in Figs. 8 and 9?
3. Why do the authors consider only one high frequency mode with the
frequency \omega_0 > 2\Delta? Does it correspond to the experimental
setup? I think assume that, for example, a high quality factor niobium cavity
of a centimeter size should have dense spectrum around 100 GHz frequency,
which corresponds to 2\Delta.
Finally, the authors should also clearly state the difference
between the current manuscript and Ref. [17], which has a lot
of similarities with the manuscript.
In conclusion, I recommend accepting the paper after minor
changes in the presentation, which have been mentioned above.
Requested changes
The requested changes are also mentioned in the report.
1. Clearly state what is new as compared to Ref. [17]
2. Add a bit more qualitative discussion of the results