Optical spin squeezing: bright beams as high-flux entangled photon sources

TL;DR

This paper develops inequalities for classical optical fields, demonstrating that polarization-squeezed light from an OPO can produce bright, robust entangled photon pairs, surpassing existing sources in brightness and efficiency.

Contribution

It introduces polarization-squeezing inequalities for classical fields and shows their violation indicates entanglement in bright, high-flux optical beams, enabling improved entangled photon sources.

Findings

Polarization entanglement occurs for photon pairs within the OPO coherence time.

Entanglement is robust against losses.

Brightness and efficiency exceed existing ultra-bright pair sources.

Abstract

In analogy with the spin-squeezing inequality of Wang and Sanders [Physical Review A 68, 012101 (2003)], we find inequalities describing macroscopic polarization correlations that are obeyed by all classical fields, and whose violation implies entanglement of the photons that make up the optical beam. We consider a realistic and exactly-solvable experimental scenario employing polarization- squeezed light from an optical parametric oscillator (OPO) and find that any two photons separated by less than the OPO coherence time are polarization entangled. The polarization entanglement is robust against losses and extremely bright: efficiency can exceed that of existing "ultra-bright" pair sources by at least an order of magnitude. This translation of spin-squeezing inequalities to the optical domain will enable direct tests of the squeezing-entanglement relationship.