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arXiv:2210.16229 · Physical Review A

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

Cheng Li, Boris Braverman, Girish Ashok Kulkarni, Robert W. Boyd

Published Apr 10, 2023• 13 citations
arXivHTMLPDF

Abstract

The authors experimentally demonstrate the generation of polarization entanglement through spontaneous parametric down-conversion pumped by a spatiotemporally highly incoherent light beam. However, polarization entanglement from the spatiotemporally incoherent pump is susceptible to degradation. Theoretical analysis shows that the degradation stems from the coupling between the spatiotemporal and polarization degrees of freedom, which is introduced by the birefringence and dispersion of the nonlinear crystal.

DOI
10.1103/physreva.107.l041701

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The authors experimentally demonstrate the generation of polarization entanglement through spontaneous parametric down-conversion pumped by a spatiotemporally highly incoherent light beam. However, polarization entanglement from the spatiotemporally incoherent pump is susceptible to degradation. Theoretical analysis shows that the degradation stems from the coupling between the spatiotemporal and polarization degrees of freedom, which is introduced by the birefringence and dispersion of the nonlinear crystal.
Spontaneous parametric down conversion (SPDC)-based sources of photonic entangled states are usually generated using coherent pump fields. For temporal, spatial, and polarization degree of freedom, the down-converted light entanglement is limited by the coherence of the pump in the same degree of freedom. However, the effect of pump coherence on the entanglement in a different degree of freedom remains largely unexplored. Here we experimentally demonstrate the generation of polarization entanglement from SPDC pumped by a light emitting diode (LED), a spatiotemporally incoherent pump. The two-photon state is found to have a concurrence of 0.562 and purity of 0.647.
Spontaneous parametric down-conversion (SPDC) can produce entangled photons from a nonlinear medium driven by a pump beam, and has thus been a key resource in photonic quantum technologies. Historically, the low efficiency of SPDC prompted the use of lasers, which are high-intensity light sources, as pump sources. However, the practical implementation of laser-driven systems faces an imminent energy bottleneck. Lasers typically have suboptimal energy efficiencies due to lasing thresholds and the technical demands of thermal management. As a result, laser-pumped entangled photon sources suffer from significant power overhead, rendering quantum devices less sustainable. Over the past few decades, light-emitting diodes (LEDs) have emerged as inexpensive and energy-efficient alternatives for high-intensity light sources. Since 2010, studies have shown that LEDs can drive SPDC and generate photon pairs. These results suggests the potential to replace lasers with ubiquitous and natural light sources to build environmentally friendly quantum devices. However, replacing lasers with other light sources entails a lack of coherence in all degrees of freedom (DoFs) of the pump beam, including spatial, temporal-spectral, and polarization, which may influence the resulting entanglement generated from SPDC. While earlier studies established that pump coherence in a specific DoF sets an upper bound for entanglement in that same DoF, the cross-influence between different DoFs has not been fully investigated. In this thesis, we study the influence of the pump's spatiotemporal coherence on the polarization entanglement generated from SPDC. First, we experimentally demonstrate that polarization-entangled two-photon states can be generated from SPDC pumped by spatiotemporally incoherent light emitted from an LED. Although the resulting entanglement appears lower than that of laser-pumped SPDC, we theoretically show that spatiotemporal incoherence does not fundamentally constrain polarization entanglement; rather, the reduction arises from a coupling between spatiotemporal and polarization DoFs introduced by the experimental setup. We determined experimentally that this cross-influence acts as a deterministic mapping from the spatial components of the pump to the two-photon polarization state. Because this mapping depends on technical aspects of the setup, we envisioned that it could be either (i) engineered to structure entanglement in higher dimensions, or (ii) compensated to reduce spatial distinguishability. Building on the latter, we show that by placing the nonlinear medium in a phase-compensated interferometric setup, SPDC pumped by spatially incoherent light can produce high polarization entanglement that violates local realism, even without postselecting the SPDC field for a single spatial mode. Finally, as a proof-of-principle demonstration of a truly environmentally friendly quantum light source, we demonstrate the generation of polarization-entangled two-photon states from SPDC pumped by a natural source - sunlight.
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±0.097S=2.33\pm 0.097S=2.33±0.097, obviously surpassing the classical bound S=2S=2S=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.
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 wave function. 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's inequality. We achieve the Bell value $S=2.33\ifmmode\pm\else\textpm\fi{}0.097$, obviously surpassing the classical bound S=2S=2S=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 biolight, which holds promise for electricity-free quantum communications in outer space.
Entangled photons produced by spontaneous parametric down-conversion have been of paramount importance for our current understanding of quantum mechanics and advances in quantum information. In this process, the quantum correlations of the down-converted photons are governed by the optical properties of the pump beam illuminating the nonlinear crystal. Extensively, the pump beam has been modeled by either coherent beams or by the well-known Gaussian-Schell model, which leads to the natural conclusion that a high degree of optical coherence is required for the generation of highly entangled states. Here, we show that when a novel class of partially coherent Gaussian pump beams is considered, a distinct type of quantum state can be generated for which the amount of entanglement increases inversely with the degree of coherence of the pump beam. This leads to highly incoherent yet highly entangled multiphoton states, which should have interesting consequences for photonic quantum information science.