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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.

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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, QY. et al. Symmetry-protected collisions between strongly interacting photons. Nature 542, 206–209 (2017).

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