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arXiv:1812.09532 · Optics Express

Influence of pump coherence on the generation of position-momentum entanglement in optical parametric down-conversion

Wuhong Zhang, Robert Fickler, Enno Giese, Lixiang Chen, Robert W. Boyd

Published Jul 10, 2019• 50 citations
arXivHTMLPDF

Abstract

We examine experimentally how the degree of position-momentum entanglement of photon pairs depends on the transverse coherence of the pump beam that excites them in a process of spontaneous parametric down-conversion. Using spatially incoherent light from a light-emitting diode, we obtain strong position correlation of the photons, but we find that transverse momentum correlation, and thus entanglement, is entirely absent. When we continuously vary the degree of spatial coherence on the pump beam, we observe the emergence of stronger momentum correlations and entanglement. We present theoretical arguments that explain our experimental results. Our results shed light on entanglement generation and can be applied to control entanglement for quantum information applications.

DOI
10.1364/oe.27.020745

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We examine experimentally how the degree of position-momentum entanglement of photon pairs depends on the transverse coherence of the pump beam that excites them in a process of spontaneous parametric down-conversion. Using spatially incoherent light from a light-emitting diode, we obtain strong position correlation of the photons, but we find that transverse momentum correlation, and thus entanglement, is entirely absent. When we continuously vary the degree of spatial coherence on the pump beam, we observe the emergence of stronger momentum correlations and entanglement. We present theoretical arguments that explain our experimental results. Our results shed light on entanglement generation and can be applied to control entanglement for quantum information applications.
The correlation properties of the pump field in spontaneous parametric down-conversion are crucial in determining the degree of entanglement of generated signal and idler photons. We find theoretically that continuous-variable entanglement of the transverse positions and momenta of these photons can be achieved only if the coherence of the pump beam is sufficiently high. The positions of signal and idler photons are found to be correlated, even for an incoherent pump. However, the momenta of the signal and idler photons are not anti-correlated, even though transverse momentum is conserved.
Abstract The correlation properties of the pump field in spontaneous parametric down-conversion are crucial in determining the degree of entanglement of generated signal and idler photons. We find theoretically that continuous-variable entanglement of the transverse positions and momenta of these photons can be achieved only if the coherence of the pump beam is sufficiently high. The positions of signal and idler photons are found to be correlated, even for an incoherent pump. However, the momenta of the signal and idler photons are not anti-correlated, even though transverse momentum is conserved.
We demonstrate experimental generation of spatially entangled photon pairs by spontaneous parametric down-conversion (SPDC) using a partial spatially coherent pump beam. By varying the spatial coherence of the pump, we show its influence on the down-converted photon's spatial correlations and on its degree of entanglement, in excellent agreement with theory. We then exploit this property to produce pairs of photons with a specific degree of entanglement by tailoring of the pump coherence length. This work thus unravels the fundamental transfer of coherence occurring in SPDC processes, and provides a simple experimental scheme to generate photon pairs with a well-defined degree of spatial entanglement, which may be useful for quantum communication and information processing.
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)-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.
The entanglement of optical angular position and orbital angular momentum has become one of the important degrees of freedom for quantum information science. Quantum control of the two conjugate variables may provide a gate to control the production of the entanglement. Here, we theoretically and experimentally show that by controlling the transverse spatial coherence of the pump beam, one can modulate the strength of the quantum correlations between the angular position and orbital angular momentum in down-converted photon pairs. By determining the relation of the Einstein, Podolsky, and Rosen correlations with the transverse spatial coherence of the pump, the boundary of the entanglement can be determined. Our work shows that the transverse spatial coherence of the pump can act as a switch to control the production of quantum entanglement both for discrete and continuous conjugate variables and may provide another platform for the quantum control research field.