L. de Santis, C Anton, B. Reznychenko, N. Somaschi, G. Coppola, J. Senellart, C. Gomez, A. Lemaitre, I. Sagnes, A. G. White, L. Lanco, A. Auffeves, P. Senellart.
A strong limitation of linear optical quantum computing is the probabilistic operation of two-quantum bit gates  based on the coalescence of indistinguishable photons. A route to deterministic operation is to exploit the single-photon nonlinearity of an atomic transition. Through engineering of the atom-photon interaction, phase shifters, photon filters and photon-photon gates have been demonstrated with natural atoms. Proofs of concept have been reportedwith semiconductor quantum dots, yet limited by ine_cient atom-photon interfaces and dephasing. We have fabricated a highly efficient single-photon filter based on a large optical non-linearity at the single photon level, in a near-optimal quantum-dot cavity interface. When probed with coherent light wavepackets, the device shows a record nonlinearity threshold around 0.3 incident photons. We demonstrate that directly reected pulses consist of 80% single-photon Fock state and that the two- and three-photon components are strongly suppressed compared to the single-photon one.
Figure : (a) Measured and (c) calculated reflectivity of a 125 ps coherent pulse (resonant at the QD transition) as function of the incident average photon-number. The straight lines in panel (a) are guide to the eyes showingthe nonlinear threshold. (b) Measured and (d) calculated time-integrated second-order correlation function, g(2)(0), asa function of average photon number. (e) Fraction of single photon (black symbols) and of coherent light (open symbols) in the reflected beam.