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arxiv: 1907.09044 · v1 · pith:J7GDODQTnew · submitted 2019-07-21 · 🪐 quant-ph · physics.optics

Correlated photon-pair generation in a liquid-filled microcavity

Felix R\"onchen , Thorsten F. Langerfeld , Michael K\"ohl This is my paper

Pith reviewed 2026-05-24 18:21 UTC · model grok-4.3

classification 🪐 quant-ph physics.optics
keywords photon-pair generationspontaneous four-wave mixingliquid-filled microcavitynonlinear opticscorrelated photonstunable wavelengthquantum optics
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0 comments X

The pith

Liquid-filled microcavity boosts photon-pair generation rate by over 1000 times.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper demonstrates photon-pair generation in a microcavity completely filled with a liquid that acts as the nonlinear medium. Spontaneous four-wave mixing in this setup produces pairs with a bandwidth of about 300 MHz, tunable from 770 to 800 nm. Compared to previous measurements, the setup yields more than a 1000-fold increase in the pair correlation rate per unit pump power and a 1.7-fold improvement in the coincidence-to-accidental ratio. A reader would care because brighter and cleaner photon-pair sources support quantum technologies that use entangled or correlated light. The gains are attributed to the liquid filling the entire cavity volume.

Core claim

By employing a liquid as the nonlinear optical medium completely filling the microcavity, photon-pair generation by spontaneous four-wave mixing yields more than a factor of 10^3 increase of the pair correlation rate per unit pump power and a factor of 1.7 improvement in the coincidence/accidental ratio as compared to previous measurements, with a photon bandwidth of ~300 MHz and tunable emission wavelength between 770 and 800 nm.

What carries the argument

A microcavity completely filled with a liquid nonlinear optical medium for spontaneous four-wave mixing.

If this is right

  • The pair correlation rate per unit pump power increases by more than a factor of 1000.
  • The coincidence to accidental ratio improves by a factor of 1.7.
  • Emission wavelength tunes continuously between 770 and 800 nm.
  • The bandwidth of emitted photons is approximately 300 MHz.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Liquids could simplify uniform filling of microcavities compared to solid nonlinear materials.
  • The higher generation efficiency might allow operation at lower pump powers in quantum optics setups.
  • The approach may extend to other nonlinear processes such as second-harmonic generation in similar cavities.

Load-bearing premise

The observed performance gains result specifically from the liquid completely filling the microcavity and serving as the nonlinear medium.

What would settle it

Repeating the experiment with the same cavity and pump but without the liquid fill or with a non-liquid medium to test whether the 10^3 rate increase and 1.7 ratio improvement remain.

Figures

Figures reproduced from arXiv: 1907.09044 by Felix R\"onchen, Michael K\"ohl, Thorsten F. Langerfeld.

Figure 1
Figure 1. Figure 1: Schematic setup of the experiment. 2. Experiment We have constructed a Fabry-Perot microcavity composed of a micromachined and coated endfacet of an optical fiber as one mirror [15, 16, 17, 18, 19, 20, 21, 22, 11] and a conventional planar mirror with identical coating as the second mirror (see [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: (a) Bistability of the lineshape function of a non-linear optical cavity [see equation (1)]. (b) Measured bistable cavity lineshape. The lineshift β is determined by the endpoints of the interval {x|A(x) > 0.1Amax}, where A is the amplitude with its maximum value Amax. To calibrate the time-axis into frequency units the resonance is measured and fitted for the lowest possible power, for which the lineshape… view at source ↗
Figure 3
Figure 3. Figure 3: (a) Normalized coincidence signal of second order (n = 2) for a measurement time of 20 min. The shifted peak-position at -12 ns stems from a length difference of the detector cables. The solid line shows a Lorentzian fit to the data. (b) Power dependence of the coincidence signal. The solid line is a quadratic fit yielding a curvature of Γexp = 1.12 ± 0.05W−2 s −1 . (c) CAR-values corresponding to the data… view at source ↗
read the original abstract

We report on the realization of a liquid-filled optical microcavity and demonstrate photon-pair generation by spontaneous four-wave mixing. The bandwidth of the emitted photons is $\sim 300$ MHz and we demonstrate tuning of the emission wavelength between 770 and 800 nm. Moreover, by employing a liquid as the nonlinear optical medium completely filling the microcavity, we observe more than a factor $10^3$ increase of the pair correlation rate per unit pump power and a factor of 1.7 improvement in the coincidence/accidental ratio as compared to our previous measurements.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 1 minor

Summary. The manuscript reports the experimental realization of a liquid-filled optical microcavity for generating correlated photon pairs via spontaneous four-wave mixing. It claims a photon bandwidth of ~300 MHz, wavelength tuning between 770 and 800 nm, and—by using a liquid as the nonlinear medium completely filling the cavity—a factor of more than 10^3 increase in pair correlation rate per unit pump power together with a 1.7-fold improvement in coincidence-to-accidental ratio relative to the authors' prior measurements.

Significance. If the reported performance gains are robustly attributable to the liquid-filled geometry and the measurements are fully characterized, the work would constitute a useful experimental advance in compact, tunable sources of correlated photons for quantum optics. The approach could enable higher pair rates in microscale devices while retaining narrow bandwidths.

major comments (2)
  1. [Abstract] Abstract: the headline claim of a >10^3 increase in pair correlation rate per unit pump power (and 1.7 CAR improvement) is presented solely as a comparison to 'our previous measurements' without any table, figure, or explicit statement confirming that cavity length, mirror reflectivity, pump focusing, collection solid angle, and detection efficiency were held fixed; absent such controls the normalization per unit pump power cannot be validated.
  2. [Abstract] Abstract: the reported bandwidth of ~300 MHz, tuning range, and numerical improvement factors are stated without error bars, uncertainties, raw count rates, or statistical details on data acquisition, rendering it impossible to assess whether the central performance claims are robust.
minor comments (1)
  1. A dedicated methods or experimental-details section is needed to describe the liquid-filling procedure, cavity alignment protocol, and how the liquid is confirmed to be the sole nonlinear medium.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their careful reading and constructive comments. We address the two major comments on the abstract below and will revise the manuscript to improve clarity and robustness of the presented claims.

read point-by-point responses

  1. Referee: [Abstract] Abstract: the headline claim of a >10^3 increase in pair correlation rate per unit pump power (and 1.7 CAR improvement) is presented solely as a comparison to 'our previous measurements' without any table, figure, or explicit statement confirming that cavity length, mirror reflectivity, pump focusing, collection solid angle, and detection efficiency were held fixed; absent such controls the normalization per unit pump power cannot be validated.

    Authors: We acknowledge that the abstract does not explicitly confirm the fixed parameters. The main text describes the setup and states that the comparison uses the same microcavity geometry and detection system as our prior work, with the only change being the liquid filling the cavity. To address the concern directly, we will add an explicit statement in a revised abstract (or a short methods note) confirming that cavity length, mirror reflectivity, pump focusing, collection solid angle, and detection efficiency were identical, thereby validating the per-unit-pump-power normalization. revision: yes

  2. Referee: [Abstract] Abstract: the reported bandwidth of ~300 MHz, tuning range, and numerical improvement factors are stated without error bars, uncertainties, raw count rates, or statistical details on data acquisition, rendering it impossible to assess whether the central performance claims are robust.

    Authors: The abstract uses approximate values for brevity. The full manuscript already reports the bandwidth with supporting spectra, tuning data, raw coincidence rates, acquisition times, and statistical details in the results and methods sections. We will revise the abstract to include approximate uncertainties (e.g., bandwidth of ~300 MHz with typical variation noted) and add a parenthetical reference to the main text for full error analysis and raw data, making the claims more robust at the abstract level. revision: partial

Circularity Check

0 steps flagged

No circularity: experimental report with no derivations or self-referential reductions

full rationale

This is a pure experimental paper reporting measured photon-pair generation rates, bandwidths, tuning ranges, and ratios in a liquid-filled microcavity. The central claims rest on direct observations and a comparison to prior experimental data points; there are no equations, first-principles derivations, fitted parameters renamed as predictions, ansatzes, or uniqueness theorems that reduce to the paper's own inputs by construction. The self-citation to previous measurements serves only as a baseline for reported improvement and does not bear the load of any mathematical result.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

The paper is an experimental demonstration. The abstract contains no free parameters, mathematical axioms, or newly postulated entities.

pith-pipeline@v0.9.0 · 5624 in / 1151 out tokens · 33609 ms · 2026-05-24T18:21:57.407215+00:00 · methodology

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Reference graph

Works this paper leans on

34 extracted references · 34 canonical work pages

  1. [1]

    One particular challenge when coupling different quantum systems is to match their wavelengths [4, 5] or to bridge wavelength gaps

    Introduction Nonclassical states of light, such as correlated photon pairs [1, 2] or anti-bunched photons, have contributed significantly to the tests of fundamental quantum mechanics and to interconnect remote quantum systems [3]. One particular challenge when coupling different quantum systems is to match their wavelengths [4, 5] or to bridge wavelength g...

    work page

  2. [2]

    Experiment We have constructed a Fabry-Perot microcavity composed of a micromachined and coated endfacet of an optical fiber as one mirror [15, 16, 17, 18, 19, 20, 21, 22, 11] and a conventional planar mirror with identical coating as the second mirror (see Figure 1). The length of the cavity is L = 38.4 µm and the finesse is F = π/(T +L) = 12500± 500 with ...

    work page

  3. [3]

    The correlation signal shows a coincidence-to-accidental ratio (CAR) of 3.4± 0.5 and thus clearly signals the nonclassical nature of the emitted photon pairs [27]

    Results In Figure 2a we show a typical two-photon correlation measurement of ordern = 2, i.e., with photon frequencies ofω2,− = 2π×377.155 THz andω2,+ = 2π×387.155 THz for an intracavity power of 0.58 W. The correlation signal shows a coincidence-to-accidental ratio (CAR) of 3.4± 0.5 and thus clearly signals the nonclassical nature of the emitted photon p...

    work page

  4. [4]

    by the mechanisms discussed in the introduction. In principle, the integral over the Kerr coupling would even include the non-linear effects arising from the mirror coating [30, 11], however, they are approximately two orders of magnitude smaller and can be neglected. Comparing the theoretically expected flux Γ to the experimentally determined rate coefficien...

    work page 2004

  5. [5]

    C. K. Hong, Z. Y. Ou, and L. Mandel. Measurement of subpicosecond time intervals between two photons by interference. Phys. Rev. Lett., 59:2044–2046, Nov 1987

    work page 2044

  6. [6]

    Experimental quantum teleportation

    Dik Bouwmeester, Jian-Wei Pan, Klaus Mattle, Manfred Eibl, Harald Weinfurter, and Anton Zeilinger. Experimental quantum teleportation. Nature, 390:575–579, 1997

    work page 1997

  7. [7]

    H. J. Kimble. The quantum internet. Nature, 453:1023–1030, 2008

    work page 2008

  8. [8]

    Molecular photons interfaced with alkali atoms

    Petr Siyushev, Guilherme Stein, J¨ org Wrachtrup, and Ilja Gerhardt. Molecular photons interfaced with alkali atoms. Nature, 509:66, Apr 2014

    work page 2014

  9. [9]

    H. M. Meyer, R. Stockill, M. Steiner, C. Le Gall, C. Matthiesen, E. Clarke, A. Ludwig, J. Reichel, M. Atat¨ ure, and M. K¨ ohl. Direct photonic coupling of a semiconductor quantum dot and a trapped ion. Phys. Rev. Lett., 114:123001, Mar 2015

    work page 2015

  10. [10]

    Fiorentino, P.L

    M. Fiorentino, P.L. Voss, J.E. Sharping, and P. Kumar. All-fiber photon-pair source for quantum communications. IEEE Photonics Technology Letters, 14(7):983–985, jul 2002

    work page 2002

  11. [11]

    J. Fan, A. Migdall, and L. J. Wang. Efficient generation of correlated photon pairs in a microstructure fiber. Opt. Lett., 30(24):3368–3370, Dec 2005

    work page 2005

  12. [12]

    Photon pair generation from compact silicon microring resonators using microwatt-level pump powers

    Marc Savanier, Ranjeet Kumar, and Shayan Mookherjea. Photon pair generation from compact silicon microring resonators using microwatt-level pump powers. Opt. Express, 24(4):3313–3328, Feb 2016

    work page 2016

  13. [13]

    Spontaneous four-wave mixing in liquid-core fibers: towards fibered raman-free correlated photon sources

    M Barbier, I Zaquine, and P Delaye. Spontaneous four-wave mixing in liquid-core fibers: towards fibered raman-free correlated photon sources. New Journal of Physics , 17(5):053031, may 2015

    work page 2015

  14. [14]

    Sommer, E

    A. Sommer, E. M. Bothschafter, S. A. Sato, C. Jakubeit, T. Latka, O. Razskazovskaya, H. Fattahi, M. Jobst, W. Schweinberger, V. Shirvanyan, V. S. Yakovlev, R. Kienberger, K. Yabana, N. Karpowicz, M. Schultze, and F. Krausz. Attosecond nonlinear polarization and light-matter energy transfer in solids. Nature, 534:86, May 2016

    work page 2016

  15. [15]

    Langerfeld, Hendrik M

    Thorsten F. Langerfeld, Hendrik M. Meyer, and Michael K¨ ohl. Correlated-photon-pair emission from a cw-pumped fabry-perot microcavity. Phys. Rev. A , 97:023822, Feb 2018

    work page 2018

  16. [16]

    C. J. F. B¨ ottcher and P. Bordewijk. Theory of electric polarization, Vol. II . Elsevier, 1996

    work page 1996

  17. [17]

    Ultrafast optical kerr effect in liquids and solids

    Roberto Righini. Ultrafast optical kerr effect in liquids and solids. Science, 262(5138):1386–1390, 1993

    work page 1993

  18. [18]

    Hagan, and Eric W

    Peng Zhao, Matthew Reichert, Sepehr Benis, David J. Hagan, and Eric W. Van Stryland. Temporal and polarization dependence of the nonlinear optical response of solvents. Optica, 5(5):583–594, May 2018

    work page 2018

  19. [19]

    A fiber fabryperot cavity with high finesse

    D Hunger, T Steinmetz, Y Colombe, C Deutsch, T W Hnsch, and J Reichel. A fiber fabryperot cavity with high finesse. New Journal of Physics , 12(6):065038, 2010

    work page 2010

  20. [20]

    Flagg, John R

    Andreas Muller, Edward B. Flagg, John R. Lawall, and Glenn S. Solomon. Ultrahigh-finesse, low-mode-volume fabry–perot microcavity. Opt. Lett., 35(13):2293–2295, Jul 2010

    work page 2010

  21. [21]

    Meyer, Christian Deutsch, Jakob Reichel, and Michael K¨ ohl

    Matthias Steiner, Hendrik M. Meyer, Christian Deutsch, Jakob Reichel, and Michael K¨ ohl. Single ion coupled to an optical fiber cavity. Phys. Rev. Lett., 110:043003, Jan 2013

    work page 2013

  22. [22]

    Brandst¨ atter, A

    B. Brandst¨ atter, A. McClung, K. Sch¨ uppert, B. Casabone, K. Friebe, A. Stute, P. O. Schmidt, C. Deutsch, J. Reichel, R. Blatt, and T. E. Northup. Integrated fiber-mirror ion trap for strong ion-cavity coupling. Review of Scientific Instruments , 84(12):123104, 2013

    work page 2013

  23. [23]

    Novel laser machining of optical fibers for long cavities with low birefringence

    Hiroki Takahashi, Jack Morphew, Fedja Oruˇ cevi´ c, Atsushi Noguchi, Ezra Kassa, and Matthias Keller. Novel laser machining of optical fibers for long cavities with low birefringence. Opt. Express, 22(25):31317–31328, Dec 2014

    work page 2014

  24. [24]

    Frequency splitting of polarization eigenmodes in microscopic fabryperot cavities

    Manuel Uphoff, Manuel Brekenfeld, Gerhard Rempe, and Stephan Ritter. Frequency splitting of polarization eigenmodes in microscopic fabryperot cavities. New Journal of Physics , 17(1):013053, 2015

    work page 2015

  25. [25]

    Gallego, S

    J. Gallego, S. Ghosh, S. K. Alavi, W. Alt, M. Martinez-Dorantes, D. Meschede, and Correlated photon-pair generation in a liquid-filled microcavity 9 L. Ratschbacher. High-finesse fiber Fabry– Perot cavities: stabilization and mode matching analysis. Applied Physics B , 122(3):47, Mar 2016

    work page 2016

  26. [26]

    High mechanical bandwidth fiber-coupled fabry-perot cavity

    Erika Janitz, Maximilian Ruf, Yannik Fontana, Jack Sankey, and Lilian Childress. High mechanical bandwidth fiber-coupled fabry-perot cavity. Opt. Express, 25(17):20932–20943, Aug 2017

    work page 2017

  27. [27]

    https://www.gelest.com/product/phenylmethylsiloxane-oligomer/

    Gelest Inc. https://www.gelest.com/product/phenylmethylsiloxane-oligomer/. Accessed: 18-07- 2019

    work page 2019

  28. [28]

    An et al

    K. An et al. Optical bistability indiced by mirror absorption: measurement of absorption coefficients at the sub-ppm level. OPTICS LETTERS, 22(18):1433, 1997

    work page 1997

  29. [29]

    Dub´ e, L.-S

    P. Dub´ e, L.-S. Ma, J. Ye, P. Jungner, and J. L. Hall. Thermally induced self-locking of an optical cavity by overtone absorption in acetylene gas. J. Opt. Soc. Am. B , 13(9):2041–2054, Sep 1996

    work page 2041

  30. [30]

    T. L. Van Raalte. Measurement of the refractive index of silicone oils for the u.k.a.e.a. J. Opt. Soc. Am., 50(1):86, Jan 1960

    work page 1960

  31. [31]

    Zou, L.J

    X.Y. Zou, L.J. Wang, and L. Mandel ¸. Violation of classical probability in parametric down- conversion. Optics Communications, 84(5-6):351–354, aug 1991

    work page 1991

  32. [32]

    Motion induced radiation from a vibrating cavity

    Astrid Lambrecht, Marc-Thierry Jaekel, and Serge Reynaud. Motion induced radiation from a vibrating cavity. Phys. Rev. Lett., 77:615–618, Jul 1996

    work page 1996

  33. [33]

    Kerr constants of some mineral and silicone oils

    R Ledzion, K Bondarczuk, P Gorski, and W Kucharczyk. Kerr constants of some mineral and silicone oils. Quantum Electronics, 29(8):739–741, 1999

    work page 1999

  34. [34]

    Kerr effect in multilayer dielectric coatings

    Elena Fedulova, Michael Trubetskov, Tatiana Amotchkina, Kilian Fritsch, Peter Baum, Oleg Pronin, and Vladimir Pervak. Kerr effect in multilayer dielectric coatings. Opt. Express , 24(19):21802–21817, Sep 2016

    work page 2016