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Quantum enhanced metrology in the search for fundamental physical phenomena

by K. W. Lehnert

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Submission summary

Authors (as registered SciPost users): Konrad Lehnert
Submission information
Preprint Link:  (pdf)
Date accepted: 2021-12-08
Date submitted: 2021-11-30 05:01
Submitted by: Lehnert, Konrad
Submitted to: SciPost Physics Lecture Notes
Ontological classification
Academic field: Physics
  • Gravitation, Cosmology and Astroparticle Physics
  • Quantum Physics
Approaches: Theoretical, Experimental


These notes summarize lectures given at the 2019 Les Houches summer school on Quantum Information Machines. They describe and review an application of quantum metrology concepts to searches for ultralight dark matter. In particular, for ultralight dark matter that couples as a weak classical force to a laboratory harmonic oscillator, quantum squeezing benefits experiments in which the mass of the dark matter particle is unknown. This benefit is present even if the oscillatory dark matter signal is much more coherent than the harmonic oscillator that it couples to, as is the case for microwave frequency searches for dark matter axion particles.

Author comments upon resubmission

Thank you to the referee for his or her thoughtful comments. Changes have been made essentially as requested, with the exception of referee point 2. I have added an "abstract" paragraph in the new version, but at the end of section 2 rather in than in section 3.1. I thought that this addition was consistent with the intent of the referee's comment, but it seemed better organized to include the paragraph even earlier.

List of changes

1. Figure 2a now includes a \kappa annotation.

2. An abstract-like paragraph has been added to the end of section 2 to summarize the dark matter haloscope experiment for readers familiar with circuit quantum electrodynamics.

A. When first introduced in section 3.1, \kappa is now described as the total science cavity decay rate.

B. Paragraph 4 in section 3.1 has been rewritten to have a clearer topic sentence. The first sentence now reads "In this approximation, the quantum fluctuations of the dark matter field have no influence on the cavity’s evolution."

3. Figure 3 has been corrected so that the mean value of the two X distributions coincide, as intended.

4. On page 9, when G is first introduced it is now described as the "squeezing factor" rather than the "squeezer gain," but with a parenthetical stating that it is also known as the "squeezer gain."

5.n page 10, when introducing the notion of an ancilla cavity the ettesxt txt iads "Imagine that I introduce a second oscillatory system (a second cavity in our example) whose quantum state can be manipulated, but can be otherwise much simpler than the science cavity as it need not couple to the dark matter field." Hopefully, this phrasing provides better emphasis.

.6I nIthe end of section 4.2, I mention that in this application two-mode squeezing, though conceptually appealing, provides no benefit to an axion search over single-mode squeezing.

7. The arxiv link for (new manuscript) reference 41 is now correct. Citations to more recent JPA development papers are given when introducing Josephson parametric amplifiers in section 6, rather than just the 1989 Yurke paper.

8.) In section 6.1 the reader is reminded of the expected ratio of the axion to science cavity linewidth in a parenthetical after equation 10.

Other small changes:

A short footnote is added to the end of the first paragraph of section 6.2, where I mention the ability to squeeze thermal noise. In the footnote, I mention that a quantum limited amplifier alone, without squeezing, can overcome large thermal noise. In the context of dark matter searches, this case is considered in detail in the manuscript, arxiv:1803.01627, I reference I now include.

Minor typographical errors have also been corrected.

Published as SciPost Phys. Lect. Notes 40 (2022)

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