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We report the experimental demonstration of electromagnetically-induced-transparency-based cross-phase-modulation at attojoule or, equivalently, few-hundred-photon levels. A phase shift of 0.005 rad of a probe pulse modulated by a signal pulse with an energy of $~100$ aJ, equivalent to $~400$ photons, was observed in a four-level system of cold ⁸⁷Rb atoms.
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Lo et al. (2011) studied this question.
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