Realizing a strong interaction between individual photons is an important objective of research in quantum science and technology. It requires an optical medium in which light experiences a phase shift that depends nonlinearly on the photon number. Once the additional two-photon phase shift reaches π, such an ultra-strong nonlinearity could enable the implementation of high-fidelity quantum logic operations. However, the nonlinear response of standard optical media is orders of magnitude too weak. Here, we demonstrate a fibre-based nonlinearity that realizes an additional two-photon phase shift close to the ideal value of π. We employ a whispering-gallery-mode resonator, interfaced by an optical nanofibre, where the presence of a single rubidium atom in the resonator mode results in a strongly nonlinear response. We show that this results in entanglement of initially uncorrelated incident photons. This demonstration of a fibre-integrated, ultra-strong nonlinearity is a decisive step towards photon-based scalable quantum logics.
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The authors acknowledge financial support from the European Science Foundation (EURYI Award), the Volkswagen Foundation (Lichtenberg Professorship), the Austrian Science Fund (FWF; SFB FoQuS project no. F 4017 and DK CoQuS project no. W 1210-N16) and the European Commission (IP SIQS, No. 600645). J.V. acknowledges support from the European Commission (Marie Curie IEF grant 300392) and C.J. acknowledges support from the German National Academic Foundation. The authors thank the groups of P. Walther and J. Schmiedmayer for the loan of single photon-counting modules.
The authors declare no competing financial interests.
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Volz, J., Scheucher, M., Junge, C. et al. Nonlinear π phase shift for single fibre-guided photons interacting with a single resonator-enhanced atom. Nature Photon 8, 965–970 (2014). https://doi.org/10.1038/nphoton.2014.253
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