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Journal of Experimental and Theoretical Physics Letters

On the Improvement of the Degree of Polarization Entanglement in the Double-Crystal Scheme of Spontaneous Parametric Down-Conversion

Dmitry N. Frolovtsev, Sergey A Magnitskiy

Published Jan 1, 2024• 1 citation
Publisher

Abstract

A method has been proposed to improve the degree of polarization entanglement of biphotons obtained by means of a double-crystal scheme. The effect of deviation of the polarization of spontaneous parametric down-converted radiation on the degree of polarization entanglement of biphotons has been examined theoretically and experimentally. The dependence of the concurrence as a measure of entanglement on the difference between the pump phases in the nonlinear crystals of the double-crystal scheme has been studied experimentally, and the improvement of the concurrence by a factor of 2.7 has been achieved at the optimal phase difference.

DOI
10.1134/s0021364023603500

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A method has been proposed to improve the degree of polarization entanglement of biphotons obtained by means of a double-crystal scheme. The effect of deviation of the polarization of spontaneous parametric down-converted radiation on the degree of polarization entanglement of biphotons has been examined theoretically and experimentally. The dependence of the concurrence as a measure of entanglement on the difference between the pump phases in the nonlinear crystals of the double-crystal scheme has been studied experimentally, and the improvement of the concurrence by a factor of 2.7 has been achieved at the optimal phase difference.
We consider a new method for the generation of polarization-frequency entangled states of photon pairs. We use a frequency-nondegenerate regime of spontaneous parametric down conversion where the photon pairs (biphotons) are produced with identical polarizations, propagate mostly in the same direction, but differ in frequency. Entanglement is achieved by a coherent superposition of pairs emitted from two nonlinear crystals, with the polarization of the biphotons from the first crystal being changed by a transformer placed between the two crystals. We show that this scheme allows the degree of entanglement to be controlled by the choice of biphoton frequencies.
We theoretically study the polarization entanglement and the non-local correlations manifested by biphotons using measures as concurrence and CHSH Bell test, generated in the spontaneous parametric down-conversion (SPDC) process for partially spatially coherent pump, affected by the spatial walk-off effects of the extraordinary beam. The study has been carried out for a well-known scheme for generating polarization entanglement utilizing non-collinear, frequency degenerate SPDC process in paired type-I uniaxial crystals. It is found that the polarization entanglement changes drastically with variation in spatial coherence of the pump, offering a possibility to realize a tunable source of polarization entanglement. The study reveals the overriding role of aperture size in compensation of the effect of spatial coherence of pump on the polarization entanglement and non-local correlations, especially in the conservation of polarization entanglement for incoherent and partially coherent pump beams. Free-space quantum secure communication utilizing these quantum states possessing maximal entanglement and partially spatially coherent properties would essentially offer robustness to the losses on propagation through atmospheric turbulence, resulting in a significant enhancement in the key rate.
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.
We report a new method for generation of polarization-entangled photon pairs in maximally entangled polarization states. Our approach is based on quantum interference between two polarization states which are non-orthogonal. We present necessary and sufficient conditions for generation the photon pair in a maximally entangled state. The effect of dependence of entanglement on the phase between the interfering states of photons is investigated theoretically and demonstrated experimentally with quantum states produced via spontaneous parametric down-conversion in the double-crystal scheme.