We propose and experimentally verify a scheme to engineer arbitrary states of traveling light field up to the two-photon level. The desired state is remotely prepared in the signal channel of spontaneous parametric down-conversion by means of conditional measurements on the idler channel. The measurement consists of bringing the idler field into interference with two ancilla coherent states, followed by two single-photon detectors, which, in coincidence, herald the preparation event. By varying the amplitudes and phases of the ancillae, we can prepare any arbitrary superposition of zero- one- and two-photon states.
@article{arxiv.0908.4113,
title = {Quantum-optical state engineering up to the two-photon level},
author = {Erwan Bimbard and Nitin Jain and Andrew MacRae and A. I. Lvovsky},
journal= {arXiv preprint arXiv:0908.4113},
year = {2015}
}