All-optical sensing of the number of photons in one light field with another light field is a longstanding goal with intriguing prospects for various quantum applications1. A suitable system must be capable of strongly coupling individual photons of the two fields2. Here we report on the realization of such a system for two fields at wavelengths 780 nm and 795 nm. These fields drive two modes of an optical cavity, each strongly coupled to separate transitions of a single rubidium atom. An additional control laser addresses the atom and induces a tunable coupling between the modes, resulting in a doubly nonlinear energy-level structure of the photon–photon–atom system3,4,5,6. We observe strong correlations between the light fields, with photons either mutually blocking each other or transiting the system conjunctly, and demonstrate all-optical switching at the single-photon level as a first application. In this new setting of strongly coupled light fields, nondestructive counting of photons with photons7 and heralded n-photon sources8 might be within reach.
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We thank V. Paulisch and A. González-Tudela for discussions on the theoretical framework, P. Altin and H. Chibani for support and discussions at an early stage of the experiment, and G. Li and B. Wang for discussions on the manuscript and results. We acknowledge support from the Deutsche Forschungsgemeinschaft via the excellence cluster Nanosystems Initiative Munich (NIM).
The authors declare no competing interests.
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Hamsen, C., Tolazzi, K.N., Wilk, T. et al. Strong coupling between photons of two light fields mediated by one atom. Nature Phys 14, 885–889 (2018). https://doi.org/10.1038/s41567-018-0181-1
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