Polarization Entanglement from Parametric Down-Conversion with a LED Pump

TL;DR

This paper demonstrates that a commercial LED can serve as a pump for spontaneous parametric down-conversion to generate polarization-entangled photon pairs, challenging the conventional need for a coherent laser source.

Contribution

It introduces a novel method of using incoherent LED light as a pump in SPDC, enabling entanglement generation without a laser, with potential applications in space-based quantum communication.

Findings

Achieved Bell inequality violation with LED pump (S=2.33).

Successfully generated polarization entanglement from incoherent light.

Showed potential for electricity-free quantum communication using natural light sources.

Abstract

Spontaneous parametric down-conversion (SPDC) is a reliable platform for entanglement generation. Routinely, a coherent laser beam is an essential prerequisite for pumping the nonlinear crystal. Here we break this barrier to generate polarization entangled photon pairs by using a commercial light-emitting diode (LED) source to serve as the pump beam. This effect is counterintuitive, as the LED source is of extremely low spatial coherence, which is transferred during the down-conversion process to the biphoton wavefunction. However, the type-II phase-matching condition naturally filters the specific frequency and wavelength of LED light exclusively to participate in SPDC such that localized polarization Bell states can be generated, regardless of the global incoherence over the full transverse plane. In our experiment, we characterize the degree of LED light-induced polarization entanglement in the standard framework of the violation of Bell inequality. We have achieved the Bell value $S=2.33\pm 0.097$, obviously surpassing the classical bound $S=2$ and thus witnessing the quantum entanglement. Our work can be extended to prepare polarization entanglement by using other natural light sources, such as sunlight and bio-light, which holds promise for electricity-free quantum communications in outer space.