Nonclassicality of induced coherence without induced emission

TL;DR

This paper demonstrates that quantum coherence between photon sources can be induced without stimulated emission, using single-photon pump sources, challenging classical explanations and suggesting broader applicability including fermions.

Contribution

It shows that induced coherence occurs without stimulated emission when using single-photon Fock states, providing new insights into the quantum nature of the phenomenon.

Findings

Coherence induced without stimulated emission with single-photon pumping

Joint detection rate becomes zero and independent of pump power

Classical or semi-classical explanations are ruled out

Abstract

Interference of two beams produced at separate biphoton sources was first observed more than two decades ago. The phenomenon, often called "induced coherence without induced emission", has recently gained attention after its applications to imaging, spectroscopy, and measuring biphoton correlations have been discovered. The sources used in the corresponding experiments are nonlinear crystals pumped by laser light. The use of a laser pump makes the occurrence of induced (stimulated) emission unavoidable and the effect of stimulated emission can be observed in the joint detection rate of the two beams. This fact raises the question whether the stimulated emission also lays a role in inducing the coherence. Here we investigate a case in which the crystals are pumped with a single-photon Fock state. We find that coherence is induced even though the possibility of stimulated emission is now fully ruled out. Furthermore, the joint detection rate of the two beams becomes ideally zero and does no longer change with the pump power. We illustrate our results by numerical simulations and by comparisons with experimental findings. Our results rule out any classical or semi-classical explanation of the phenomenon and also imply that similar experiments can be performed with fermions, for which stimulated emission is strictly forbidden.