Abstract

Strong photon-photon interactions could enable deterministic two-qubit logic gate [1, 2] which is still missing in the optical quantum information processing toolbox. Here we demonstrate that by illuminating a sample of atoms with a weak coherent state, but post-selecting on subsequent detection of a photon at the far side of the sample using a single-photon counting module (SPCM), we can observe the nonlinear effect of that one additional photon on a probe beam. Using electromagnetically induced transparency (EIT) in a gas of laser-cooled ⁸⁵Rb atoms, we report the observation of a cross-phase shift (XPS) of 18 microradians per photon, and demonstrate the contribution due to individual post-selected photons. This is the first direct measurement of the XPS due to a single photon.

© 2015 IEEE