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Attractive photons in a quantum nonlinear medium


The fundamental properties of light derive from its constituent particles—massless quanta (photons) that do not interact with one another1. However, it has long been known that the realization of coherent interactions between individual photons, akin to those associated with conventional massive particles, could enable a wide variety of novel scientific and engineering applications2,3. Here we demonstrate a quantum nonlinear medium inside which individual photons travel as massive particles with strong mutual attraction, such that the propagation of photon pairs is dominated by a two-photon bound state4,5,6,7. We achieve this through dispersive coupling of light to strongly interacting atoms in highly excited Rydberg states. We measure the dynamical evolution of the two-photon wavefunction using time-resolved quantum state tomography, and demonstrate a conditional phase shift8 exceeding one radian, resulting in polarization-entangled photon pairs. Particular applications of this technique include all-optical switching, deterministic photonic quantum logic and the generation of strongly correlated states of light9.

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Figure 1: Photons with strong mutual attraction in a quantum nonlinear medium.
Figure 2: Propagation of interacting photon pairs.
Figure 3: Dependence of the photon–photon interaction on detuning.
Figure 4: Quantum coherence and entanglement.


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We thank H. P. Büchler, T. Pohl, J. Otterbach, P. Strack, M. Gullans and S. Choi for discussions. This work was supported by the NSF, the CUA, DARPA and the AFOSR Quantum Memories MURI and the Packard Foundations. O.F. acknowledges support from the HQOC. A.V.G. and M.D.L. thank KITP for hospitality. A.V.G. acknowledges funding from the Lee A. DuBridge Foundation and the IQIM, an NSF Physics Frontiers Center with support of the Gordon and Betty Moore Foundation.

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The experiment and analysis were carried out by O.F., T.P. and Q.-Y.L. The theoretical modelling was done by A.V.G. All experimental and theoretical work was supervised by M.D.L. and V.V. All authors discussed the results and contributed to the manuscript.

Corresponding authors

Correspondence to Mikhail D. Lukin or Vladan Vuletić.

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The authors declare no competing financial interests.

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

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