Symmetry-protected collisions between strongly interacting photons


Realizing robust quantum phenomena in strongly interacting systems is one of the central challenges in modern physical science. Approaches ranging from topological protection to quantum error correction are currently being explored across many different experimental platforms, including electrons in condensed-matter systems1, trapped atoms2 and photons3. Although photon–photon interactions are typically negligible in conventional optical media, strong interactions between individual photons have recently been engineered in several systems4,5,6,7,8,9,10. Here, using coherent coupling between light and Rydberg excitations in an ultracold atomic gas, we demonstrate a controlled and coherent exchange collision between two photons that is accompanied by a π/2 phase shift. The effect is robust in that the value of the phase shift is determined by the interaction symmetry rather than the precise experimental parameters7,10,11,12,13, and in that it occurs under conditions where photon absorption is minimal. The measured phase shift of 0.48(3)π is in excellent agreement with a theoretical model. These observations open a route to realizing robust single-photon switches and all-optical quantum logic gates, and to exploring novel quantum many-body phenomena with strongly interacting photons.

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Figure 1: Photon collisions mediated by long-range exchange interactions.
Figure 2: Observation of photon collisions.
Figure 3: Density dependence and robustness of the scattering phase.
Figure 4: Polariton exchange between separated transverse modes.


  1. 1

    Stern, A. & Lindner, N. H. Topological quantum computation—from basic concepts to first experiments. Science 339, 1179–1184 (2013)

    CAS  ADS  Article  Google Scholar 

  2. 2

    Monroe, C. & Kim, J. Scaling the ion trap quantum processor. Science 339, 1164–1169 (2013)

    CAS  ADS  Article  Google Scholar 

  3. 3

    Lu, L., Joannopoulos, J. D. & Soljačić, M. Topological photonics. Nat. Photon. 8, 821–829 (2014)

    CAS  ADS  Article  Google Scholar 

  4. 4

    Birnbaum, K. et al. Photon blockade in an optical cavity with one trapped atom. Nature 436, 87–90 (2005)

    CAS  ADS  Article  Google Scholar 

  5. 5

    Chang, D. E., Vuletic, V. & Lukin, M. D. Quantum nonlinear optics — photon by photon. Nat. Photon. 8, 685–694 (2014)

    CAS  ADS  Article  Google Scholar 

  6. 6

    Dudin, Y. O. & Kuzmich, A. Strongly interacting Rydberg excitations of a cold atomic gas. Science 336, 887–889 (2012)

    CAS  ADS  Article  Google Scholar 

  7. 7

    Hacker, B., Welte, S., Rempe, G. & Ritter, S. A photon–photon quantum gate based on a single atom in an optical resonator. Nature 536, 193–196 (2016)

    CAS  ADS  Article  Google Scholar 

  8. 8

    Peyronel, T. et al. Quantum nonlinear optics with single photons enabled by strongly interacting atoms. Nature 488, 57–60 (2012)

    CAS  ADS  Article  Google Scholar 

  9. 9

    Pritchard, J. D. et al. Cooperative atom-light interaction in a blockaded Rydberg ensemble. Phys. Rev. Lett. 105, 193603 (2010)

    CAS  ADS  Article  Google Scholar 

  10. 10

    Beck, K., Hosseini, M., Duan, Y. & Vuletić?, V. Large conditional single-photon cross-phase modulation. Proc. Natl Acad. Sci. USA 113, 9740–9744 (2016)

    CAS  ADS  Article  Google Scholar 

  11. 11

    Gorniaczyk, H., Tresp, C., Schmidt, J., Fedder, H. & Hofferberth, S. Single photon transistor mediated by Rydberg interaction. Phys. Rev. Lett. 113, 053601 (2014)

    CAS  ADS  Article  Google Scholar 

  12. 12

    Tiarks, D., Baur, S., Schneider, K., Durr, S. & Rempe, G. Single-photon transistor using a Förster resonance. Phys. Rev. Lett. 113, 053602 (2014)

    ADS  Article  Google Scholar 

  13. 13

    Tiarks, D., Schmidt, S., Rempe, G. & Dürr, S. Optical π phase shift created with a single-photon pulse. Sci. Adv. 2, e1600036 (2016)

    ADS  Article  Google Scholar 

  14. 14

    Schuster, I. et al. Nonlinear spectroscopy of photons bound to one atom. Nat. Phys. 4, 382–385 (2008)

    CAS  Article  Google Scholar 

  15. 15

    Lukin, M. et al. Dipole blockade and quantum information processing in mesoscopic atomic ensembles. Phys. Rev. Lett. 87, 037901 (2001)

    CAS  ADS  Article  Google Scholar 

  16. 16

    Friedler, I., Petrosyan, D., Fleischhauer, M. & Kurizki, G. Long-range interactions and entanglement of slow single-photon pulses. Phys. Rev. A 72, 043803 (2005)

    ADS  Article  Google Scholar 

  17. 17

    Gorshkov, A. V., Otterbach, J., Fleischhauer, M., Pohl, T. & Lukin, M. D. Photon-photon interactions via Rydberg blockade. Phys. Rev. Lett. 107, 133602 (2011)

    ADS  Article  Google Scholar 

  18. 18

    Petrosyan, D., Otterbach, J. & Fleischhauer, M. Electromagnetically induced transparency with Rydberg atoms. Phys. Rev. Lett. 107, 213601 (2011)

    ADS  Article  Google Scholar 

  19. 19

    Firstenberg, O. et al. Attractive photons in a quantum nonlinear medium. Nature 502, 71–75 (2013)

    CAS  ADS  Article  Google Scholar 

  20. 20

    Maxwell, D. et al. Storage and control of optical photons using Rydberg polaritons. Phys. Rev. Lett. 110, 103001 (2013)

    CAS  ADS  Article  Google Scholar 

  21. 21

    Ravets, S. et al. Coherent dipole–dipole coupling between two single Rydberg atoms at an electrically-tuned Forster resonance. Nat. Phys. 10, 914–917 (2014)

    CAS  Article  Google Scholar 

  22. 22

    Saffman, M., Walker, T. G. & Molmer, K. Quantum information with Rydberg atoms. Rev. Mod. Phys. 82, 2313–2363 (2010)

    CAS  ADS  Article  Google Scholar 

  23. 23

    Gorniaczyk, H. et al. Enhancement of Rydberg-mediated single-photon nonlinearities by electrically tuned Förster resonances. Nat. Commun. 7, 12480 (2016)

    CAS  ADS  Article  Google Scholar 

  24. 24

    Murray, C. R., Gorshkov, A. V. & Pohl, T. Many-body decoherence dynamics and optimized operation of a single-photon switch. New J. Phys. 18, 092001 (2016)

    ADS  Article  Google Scholar 

  25. 25

    Beenakker, C. W. J. Random-matrix theory of Majorana fermions and topological superconductors. Rev. Mod. Phys. 87, 1037–1066 (2015)

    CAS  ADS  MathSciNet  Article  Google Scholar 

  26. 26

    Schnyder, A. P., Ryu, S., Furusaki, A. & Ludwig, A. W. W. Classification of topological insulators and superconductors in three spatial dimensions. Phys. Rev. B 78, 195125 (2008)

    ADS  Article  Google Scholar 

  27. 27

    Kitaev, A. Y. Unpaired Majorana fermions in quantum wires. Phys. Uspekhi 44 (Suppl.), 131–136 (2001)

    ADS  Article  Google Scholar 

  28. 28

    Mourik, V. et al. Signatures of Majorana fermions in hybrid superconductor-semiconductor nanowire devices. Science 336, 1003–1007 (2012)

    CAS  ADS  Article  Google Scholar 

  29. 29

    Maghrebi, M. F. et al. Fractional quantum Hall states of Rydberg polaritons. Phys. Rev. A 91, 033838 (2015)

    ADS  MathSciNet  Article  Google Scholar 

  30. 30

    Sommer, A., Büchler, H. P. & Simon, J. Quantum crystals and Laughlin droplets of cavity Rydberg polaritons. Preprint at (2015)

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We acknowledge conversations with A. V. Gorshkov, M. J. Gullans, O. Firstenberg, S. Hofferberth and R. Johne. Funding was provided by the NSF, the Center for Ultracold Atoms, DARPA, ARO, ARO MURI, AFOSR MURI, the ARL, U-FET grant number 512862 (HAIRS), the H2020-FETPROACT-2014 grant number 640378 (RYSQ), the DFG (SPP 1929) and the Kwanjeong Educational Foundation.

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The experiment and analysis were carried out by J.D.T., T.L.N., Q.-Y.L., S.H.C., A.V.V. and I.A.F. Theoretical modelling was done by S.C., D.V. and T.P. All work was supervised by M.D.L. and V.V. All authors discussed the results and contributed to the manuscript.

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Correspondence to Mikhail D. Lukin or Vladan Vuletić.

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The authors declare no competing financial interests.

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Reviewer InformationNature thanks M. Saffman and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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This file contains Supplementary Text and Data 1-8, Supplementary Figures 1 -10, Supplementary Table 1 and additional references. (PDF 4597 kb)

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Thompson, J., Nicholson, T., Liang, Q. et al. Symmetry-protected collisions between strongly interacting photons. Nature 542, 206–209 (2017).

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