Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Universal pair polaritons in a strongly interacting Fermi gas


Cavity quantum electrodynamics (QED) manipulates the coupling of light with matter, and allows several emitters to couple coherently with one light mode1. However, even in a many-body system, the light–matter coupling mechanism has so far been restricted to one-body processes. Leveraging cavity QED for the quantum simulation of complex, many-body systems has thus far relied on multi-photon processes, scaling down the light–matter interaction to the low energy and slow time scales of the many-body problem2,3,4,5. Here we report cavity QED experiments using molecular transitions in a strongly interacting Fermi gas, directly coupling cavity photons to pairs of atoms. The interplay of strong light–matter and strong interparticle interactions leads to well-resolved pair polaritons—hybrid excitations coherently mixing photons, atom pairs and molecules. The dependence of the pair-polariton spectrum on interatomic interactions is universal, independent of the transition used, demonstrating a direct mapping between pair correlations in the ground state and the optical spectrum. This represents a magnification of many-body effects by two orders of magnitude in energy. In the dispersive regime, it enables fast, minimally destructive measurements of pair correlations, and opens the way to their measurement at the quantum limit and their coherent manipulation using dynamical, quantized optical fields.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Figure 1: Concept of the experiment.
Figure 2: Strong coupling on PA transitions.
Figure 3: Interaction-dependent photon–pair coupling.
Figure 4: Single-shot, repeated measurement of pair correlations.

Data availability

All data files are available from the corresponding author upon request. Accompanying data, including those for figures, are available from the Zenodo repository (


  1. 1.

    Haroche, S. & Raimond, J.-M. Exploring the Quantum: Atoms, Cavities, and Photons (Oxford Univ. Press, 2006).

  2. 2.

    Klinder, J., Keßler, H., Bakhtiari, M. R., Thorwart, M. & Hemmerich, A. Observation of a superradiant mott insulator in the dicke-hubbard model. Phys. Rev. Lett. 115, 230403 (2015).

    ADS  CAS  PubMed  Article  PubMed Central  Google Scholar 

  3. 3.

    Landig, R. et al. Quantum phases from competing short- and long-range interactions in an optical lattice. Nature 532, 476–479 (2016).

    ADS  CAS  PubMed  Article  PubMed Central  Google Scholar 

  4. 4.

    Vaidya, V. D. et al. Tunable-range, photon-mediated atomic interactions in multimode cavity qed. Phys. Rev. X 8, 011002 (2018).

    CAS  Google Scholar 

  5. 5.

    Norcia, M. A. et al. Cavity-mediated collective spin-exchange interactions in a strontium superradiant laser. Science 361, 259–262 (2018).

    ADS  CAS  PubMed  Article  PubMed Central  Google Scholar 

  6. 6.

    Tanji-Suzuki, H. et al. in Advances in Atomic, Molecular, and Optical Physics Vol. 60 (eds Arimondo, E., P.R. Berman, P. R. & Lin, C. C.) Ch. 4 (Elsevier, 2011).

  7. 7.

    Reiserer, A. & Rempe, G. Cavity-based quantum networks with single atoms and optical photons. Rev. Mod. Phys. 87, 1379–1418 (2015).

    ADS  CAS  Article  Google Scholar 

  8. 8.

    Krantz, P. et al. A quantum engineer’s guide to superconducting qubits. Appl. Phys. Rev. 6, 021318 (2019).

    ADS  Article  CAS  Google Scholar 

  9. 9.

    Mivehvar, F., Piazza, F., Donner, T. & Ritsch, H. Cavity qed with quantum gases: new paradigms in many-body physics. Preprint at (2021).

  10. 10.

    Münstermann, P., Fischer, T., Maunz, P., Pinkse, P. W. H. & Rempe, G. Observation of cavity-mediated long-range light forces between strongly coupled atoms. Phys. Rev. Lett. 84, 4068–4071 (2000).

    ADS  PubMed  Article  Google Scholar 

  11. 11.

    Mottl, R. et al. Roton-type mode softening in a quantum gas with cavity-mediated long-range interactions. Science 336, 1570–1573 (2012).

    ADS  CAS  PubMed  Article  Google Scholar 

  12. 12.

    Ritsch, H., Domokos, P., Brennecke, F. & Esslinger, T. Cold atoms in cavity-generated dynamical optical potentials. Rev. Mod. Phys. 85, 553–601 (2013).

    ADS  CAS  Article  Google Scholar 

  13. 13.

    Dogra, N. et al. Dissipation-induced structural instability and chiral dynamics in a quantum gas. Science 366, 1496–1499 (2019).

    ADS  CAS  PubMed  Article  Google Scholar 

  14. 14.

    Mekhov, I. B., Maschler, C. & Ritsch, H. Probing quantum phases of ultracold atoms in optical lattices by transmission spectra in cavity quantum electrodynamics. Nat. Phys. 3, 319–323 (2007).

    CAS  Article  Google Scholar 

  15. 15.

    Jones, K. M., Tiesinga, E., Lett, P. D. & Julienne, P. S. Ultracold photoassociation spectroscopy: long-range molecules and atomic scattering. Rev. Mod. Phys. 78, 483–535 (2006).

    ADS  CAS  Article  Google Scholar 

  16. 16.

    Partridge, G. B., Strecker, K. E., Kamar, R. I., Jack, M. W. & Hulet, R. G. Molecular probe of pairing in the bec-bcs crossover. Phys. Rev. Lett. 95, 020404 (2005).

    ADS  CAS  PubMed  Article  Google Scholar 

  17. 17.

    Kinoshita, T., Wenger, T. & Weiss, D. S. Local pair correlations in one-dimensional bose gases. Phys. Rev. Lett. 95, 190406 (2005).

    ADS  PubMed  Article  CAS  Google Scholar 

  18. 18.

    Semczuk, M., Gunton, W., Bowden, W. & Madison, K. W. Anomalous behavior of dark states in quantum gases of 6Li. Phys. Rev. Lett. 113, 055302 (2014).

    ADS  CAS  PubMed  Article  Google Scholar 

  19. 19.

    Liu, X.-P. et al. Observation of the density effect on the closed-channel fraction in a 6Li superfluid. Preprint at (2019).

  20. 20.

    Paintner, T. et al. Pair fraction in a finite-temperature fermi gas on the bec side of the bcs-bec crossover. Phys. Rev. A 99, 053617 (2019).

    ADS  CAS  Article  Google Scholar 

  21. 21.

    Yan, M., DeSalvo, B. J., Huang, Y., Naidon, P. & Killian, T. C. Rabi oscillations between atomic and molecular condensates driven with coherent one-color photoassociation. Phys. Rev. Lett. 111, 150402 (2013).

    ADS  PubMed  Article  CAS  PubMed Central  Google Scholar 

  22. 22.

    Taie, S., Watanabe, S., Ichinose, T. & Takahashi, Y. Feshbach-resonance-enhanced coherent atom-molecule conversion with ultranarrow photoassociation resonance. Phys. Rev. Lett. 116, 043202 (2016).

    ADS  PubMed  Article  CAS  PubMed Central  Google Scholar 

  23. 23.

    Roux, K., Konishi, H., Helson, V. & Brantut, J.-P. Strongly correlated fermions strongly coupled to light. Nature Communications 11, 2974 (2020).

    ADS  CAS  PubMed  PubMed Central  Article  Google Scholar 

  24. 24.

    Giorgini, S., Pitaevskii, L. P. & Stringari, S. Theory of ultracold atomic fermi gases. Rev. Mod. Phys. 80, 1215–1274 (2008).

    ADS  CAS  Article  Google Scholar 

  25. 25.

    Zwerger, W. (ed.) The BCS-BEC Crossover and the Unitary Fermi Gas Vol. 836 (Springer, 2012).

  26. 26.

    Stamper-Kurn, D. M. Cavity Optomechanics with Cold Atoms, 283–325 (Springer, 2014).

  27. 27.

    Abraham, E. R. I., Ritchie, N. W. M., McAlexander, W. I. & Hulet, R. G. Photoassociative spectroscopy of long range states of ultracold 6li2 and 7li2. J. Chem. Phys. 103, 7773–7778 (1995).

    ADS  CAS  Article  Google Scholar 

  28. 28.

    Tan, S. Energetics of a strongly correlated fermi gas. Ann. Phys. 323, 2952–2970 (2008).

    ADS  MathSciNet  CAS  MATH  Article  Google Scholar 

  29. 29.

    Hu, H., Liu, X.-J. & Drummond, P. D. Universal contact of strongly interacting fermions at finite temperatures. N. J. Phys. 13, 035007 (2011).

    Article  CAS  Google Scholar 

  30. 30.

    Hoinka, S. et al. Precise determination of the structure factor and contact in a unitary fermi gas. Phys. Rev. Lett. 110, 055305 (2013).

    ADS  PubMed  Article  CAS  PubMed Central  Google Scholar 

  31. 31.

    Kuhnle, E. D. et al. Universal behavior of pair correlations in a strongly interacting fermi gas. Phys. Rev. Lett. 105, 070402 (2010).

    ADS  CAS  PubMed  Article  PubMed Central  Google Scholar 

  32. 32.

    Sagi, Y., Drake, T. E., Paudel, R. & Jin, D. S. Measurement of the homogeneous contact of a unitary fermi gas. Phys. Rev. Lett. 109, 220402 (2012).

    ADS  PubMed  Article  CAS  Google Scholar 

  33. 33.

    Chang, R. et al. Momentum-resolved observation of thermal and quantum depletion in a bose gas. Phys. Rev. Lett. 117, 235303 (2016).

    ADS  CAS  PubMed  Article  Google Scholar 

  34. 34.

    Laurent, S. et al. Connecting few-body inelastic decay to quantum correlations in a many-body system: a weakly coupled impurity in a resonant fermi gas. Phys. Rev. Lett. 118, 103403 (2017).

    ADS  PubMed  Article  Google Scholar 

  35. 35.

    Carcy, C. et al. Contact and sum rules in a near-uniform fermi gas at unitarity. Phys. Rev. Lett. 122, 203401 (2019).

    ADS  CAS  PubMed  Article  Google Scholar 

  36. 36.

    Mukherjee, B. et al. Spectral response and contact of the unitary fermi gas. Phys. Rev. Lett. 122, 203402 (2019).

    ADS  CAS  PubMed  Article  Google Scholar 

  37. 37.

    Roux, K., Helson, V., Konishi, H. & Brantut, J. P. Cavity-assisted preparation and detection of a unitary Fermi gas. New J. Phys. 23, 043029 (2021).

    ADS  CAS  Article  Google Scholar 

  38. 38.

    Uchino, S., Ueda, M. & Brantut, J.-P. Universal noise in continuous transport measurements of interacting fermions. Phys. Rev. A 98, 063619 (2018).

    ADS  CAS  Article  Google Scholar 

  39. 39.

    Yamazaki, R., Taie, S., Sugawa, S. & Takahashi, Y. Submicron spatial modulation of an interatomic interaction in a bose-einstein condensate. Phys. Rev. Lett. 105, 050405 (2010).

    ADS  PubMed  Article  CAS  Google Scholar 

  40. 40.

    Enss, T. & Thywissen, J. H. Universal spin transport and quantum bounds for unitary fermions. Annu. Rev. Condens. Matter Phys. 10, 85–106 (2019).

    ADS  CAS  Article  Google Scholar 

  41. 41.

    Amico, A. et al. Time-resolved observation of competing attractive and repulsive short-range correlations in strongly interacting fermi gases. Phys. Rev. Lett. 121, 253602 (2018).

    ADS  CAS  PubMed  Article  PubMed Central  Google Scholar 

  42. 42.

    Krinner, S., Esslinger, T. & Brantut, J.-P. Two-terminal transport measurements with cold atoms. J. Phys. Condens. Matter 29, 343003 (2017).

    PubMed  Article  PubMed Central  Google Scholar 

  43. 43.

    Zeiher, J., Wolf, J., Isaacs, J. A., Kohler, J. & Stamper-Kurn, D. M. Tracking evaporative cooling of a mesoscopic atomic quantum gas in real time. Preprint at (2020).

  44. 44.

    Eisele, M., Maier, R. A. W. & Zimmermann, C. Fast in situ observation of atomic feshbach resonances by photoassociative ionization. Phys. Rev. Lett. 124, 123401 (2020).

    ADS  CAS  PubMed  Article  PubMed Central  Google Scholar 

  45. 45.

    Bohn, J. L., Rey, A. M. & Ye, J. Cold molecules: progress in quantum engineering of chemistry and quantum matter. Science 357, 1002–1010 (2017).

    ADS  MathSciNet  CAS  PubMed  MATH  Article  PubMed Central  Google Scholar 

  46. 46.

    Pérez-Ros, J., Kim, M. E. & Hung, C.-L. Ultracold molecule assembly with photonic crystals. N. J. Phys. 19, 123035 (2017).

    Article  CAS  Google Scholar 

  47. 47.

    Kampschulte, T. & Denschlag, J. H. Cavity-controlled formation of ultracold molecules. N. J. Phys. 20, 123015 (2018).

    CAS  Article  Google Scholar 

  48. 48.

    Wellnitz, D., Schütz, S., Whitlock, S., Schachenmayer, J. & Pupillo, G. Collective dissipative molecule formation in a cavity. Phys. Rev. Lett. 125, 193201 (2020).

    ADS  Article  Google Scholar 

  49. 49.

    Côté, R., Dalgarno, A., Sun, Y. & Hulet, R. G. Photoabsorption by ultracold atoms and the scattering length. Phys. Rev. Lett. 74, 3581–3584 (1995).

    ADS  PubMed  Article  PubMed Central  Google Scholar 

Download references


We thank T. Donner for discussions and careful reading of the manuscript; R. Hulet, P. Julienne and J. Hutson for discussions; C. Vale, H. Hu and J. Drut for providing the contact data; and T. Zwettler for experimental assistance. We acknowledge funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 714309), the Swiss National Science Foundation (grant no. 184654), the Sandoz Family Foundation-Monique de Meuron program for Academic Promotion and EPFL.

Author information




H.K., K.R. and V.H. performed the experiments and processed the data; H.K., K.R. and J.-P.B. wrote the paper; J.-P.B. planned and supervised the project.

Corresponding author

Correspondence to Jean-Philippe Brantut.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Peer review information Nature thanks Hui Zhai, Florian Schreck and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data figures and tables

Extended Data Figure 1 Fit to the spectrum.

a, Spectrum of PA4 at 832 G averaged over three realizations. b, Spectrum reconstructed by equation (2) using the fit results. The solid and dashed lines indicate the fitted positions of the PA resonance and the dispersively shifted cavity resonance. The colour scale is identical to that in the main text.

Extended Data Figure 2 Magnetic field dependence of the binding energies.

Positions of PA1–PA4 (green open circles, purple filled diamonds, orange open squares and light blue crosses, respectively) as a function of magnetic fields. The value at 730 G is subtracted for clarity. Linear fits presented by the solid lines yield 0.31, 0.67, 0.89 and −0.83 MHz G−1 for the four PA resonances, respectively.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Konishi, H., Roux, K., Helson, V. et al. Universal pair polaritons in a strongly interacting Fermi gas. Nature 596, 509–513 (2021).

Download citation


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing