Artificial coherent states of light from a single photon stream*

Coherent optical states consist of a large quantum superposition of different photon number (Fock) states, but because they do not form an orthogonal basis, no photon number states can be obtained from it by linear optics. Here we demonstrate the reverse, by manipulating a random continuous single photon stream using quantum interference in an optical Sagnac loop, we show that artificial quantum states of light can be obtained with tunable photon statistics, including approximately coherent states. We demonstrate this experimentally using a true single photon stream produced by a semiconductor quantum dot in an optical micro cavity, and show that we can obtain light with coherent photon statistics in agreement with our theory, which can only be explained by quantum interference of 3 or more photons. The produced artificial light states are, however, much more complex than coherent states, containing quantum entanglement of photons at different times, making them a resource for photonic quantum information.