Nonlinear π phase shift for single fibre-guided photons interacting with a single resonator-enhanced atom

Abstract

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 Ï €, 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 Ï €. 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.