Experimental generation of polarization entanglement from spontaneous parametric down-conversion pumped by spatiotemporally highly incoherent light

TL;DR

This study demonstrates that polarization entanglement can be generated via SPDC using a highly incoherent LED pump, revealing the effects of pump coherence on entanglement quality and potential applications in quantum communication.

Contribution

It provides the first experimental analysis of polarization entanglement generated with a spatiotemporally highly incoherent light source in SPDC, supported by theoretical predictions.

Findings

Entanglement with a concurrence of ~0.53 was achieved using an incoherent LED pump.

The generated two-qubit state had a purity of approximately 0.65.

The results confirm that STHI pumps can produce viable entangled states for quantum communication.

Abstract

The influence of pump coherence on the entanglement produced in spontaneous parametric down-conversion (SPDC) is important to understand, both from a fundamental perspective, and from a practical standpoint for controlled generation of entangled states. In this context, it is known that in the absence of postselection, the pump coherence in a given degree of freedom (DOF) imposes an upper limit on the generated entanglement in the same DOF. However, the cross-influence of the pump coherence on the generated entanglement in a different DOF is not well-understood. Here, we experimentally investigate the effect of a spatiotemporally highly-incoherent (STHI) light-emitting diode (LED) pump on the polarization entanglement generated in SPDC. Our quantum state tomography measurements using multimode collection fibers to reduce the influence of postselection yield a two-qubit state with a concurrence of 0.531+/-0.006 and a purity of 0.647+/-0.005, in excellent agreement with our theoretically predicted concurrence of 0.552 and purity of 0.652. Therefore, while the use of an STHI pump causes reduction in the entanglement and purity of the output polarization two-qubit state, the viability of SPDC with STHI pumps is nevertheless important for two reasons: (i) STHI sources are ubiquitous and available at a wider range of wavelengths than lasers, and (ii) the generated STHI polarization-entangled two-photon states could potentially be useful in long-distance quantum communication schemes due to their robustness to scattering.