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Photonics North (PN)

Entanglement Distribution over Free-Space Optical Turbulent Channels

Vijay Nafria, Ivan B. Djordjević

Published Jun 12, 2023• 0 citations
Publisher

Abstract

We demonstrate successful entanglement distribution over free-space optical link that we established at the University of Arizona in strong turbulence regime. To generate entangled photons, we use a low-cost solution based on a C-band laser diode and an EDFA as well as a PPLN waveguide in which wavelength conversion followed by spontaneous parametric downconversion takes place.

DOI
10.1109/pn58661.2023.10223066

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We demonstrate successful entanglement distribution over free-space optical link that we established at the University of Arizona in strong turbulence regime. To generate entangled photons, we use a low-cost solution based on a C-band laser diode and an EDFA as well as a PPLN waveguide in which wavelength conversion followed by spontaneous parametric downconversion takes place.
In this paper, we present our experiment results demonstrating multi-wavelength entanglement distribution over the turbulent outdoor free-space optical (FSO) link we established at the University of Arizona. We generated with the help of PPLN waveguide, C-band laser diode, and an EDFA entangled photon pairs, transmitted them over the turbulent FSO link, separated different wavelength channels with coarse WDM demultiplexers, and measured the coincidence counts of four wavelength channels with the help of correlated photon detection system. To improve the system tolerance to turbulence effects we use adaptive optics (AO) and demonstrate that the AO can be used to increase the coincidence counts.
In this invited paper we review our recent research activities on experimental demonstration of entanglement based (EB) radars, operated over strong atmospheric turbulence channels. In conventional EB communications, sensing, and radars the phase-conjugation, required before homodyne detection takes place, is performed on received signal photons. In atmospheric turbulent channels, the signal photons are affected by diffraction, absorption, scattering, and atmospheric turbulence effects so that only limited number of weak target probe returned signal photons reach the receiver side in EB radars. Moreover, it is extremely difficult to perform any phase-conjugation on weak signal photons when the average number of received photons is <<1. To solve this problem, we have recently proposed to perform phase-conjugation on bright idler photons instead. Namely, we perform the wavelength conversion by the PPLN waveguide on bright idler photons, so that the idler photons will have the same wavelength as the signal photons, and after that we use a classical homodyne balanced detector as an entanglement assisted detector. To generate entangled photon pairs, we use C-/L-band tunable laser, EDFA, the PPLN waveguide, and WDM demultiplexers. To demonstrate the high-potential of the proposed EB radar concept, we developed an experimental outdoor free-space optical (FSO) testbed at the University of Arizona campus. Using this FSO testbed we experimentally demonstrate that the proposed EB radar significantly outperforms the corresponding classical counterpart and can operate in strong turbulence regime. To improve the detection probabilities further, we use deformable mirror-based adaptive optics.
A high-speed entanglement assisted communication that operates at 10 Gb/s is proposed, which performs a highly efficient, PPLN-waveguide-based, entanglement generation by making the simultaneous use of S- and L-band pumps. The two-pump-based entanglement generation source satisfies the quasi-phase-matching-condition over the entire C-band. To improve the system reliability, our system performs the phase-conjugation on idler photons in contrast to conventional ways of performing the phase-conjugation on signal photons. To study the performance of the proposed entanglement-assisted system we have developed the 1.5 km long outdoor free-space optical (FSO) link at the University of Arizona campus. Experimental results indicate that the proposed entanglement-assisted system significantly outperforms the classical counterpart at 10 Gb/s, operated in strong turbulence regime. We also demonstrate that the traditional entanglement-assisted system performing the optical phase conjugation on signal photons at the receiver side is not operational at all in strong turbulence regime given that it is extremely difficult to perform the phase-conjugation on weak signal photons when the number of received photons is low. To improve the system performance the adaptive optics is performed on signal photons.
Scattering by atmospheric turbulence is one of the main challenges in creating long free-space optical links, and specifically links of entangled photons. Classical compensation methods are hard to apply to entangled photons, due to inherently low signal to noise ratios and the fragility of entanglement. We have recently shown that we can use a bright laser beam that pumps spontaneous parametric down conversion to control the spatial correlations between entangled photons for compensating their scattering. In this work, we apply the pump-shaping technique to compensate for the scrambling of correlations between entangled photons that scatter by emulated atmospheric turbulence. We use a spatial light modulator and Kolmogorov’s turbulence model to emulate atmospheric turbulence in the lab, and enhance the entangled photons’ signal by a factor of fifteen using pump optimization. We show this for both a static and dynamic emulated atmosphere, and also demonstrate the compensation of the scattering of a higher-order mode. Our results can open the door towards realizing free-space quantum links with entangled photons, used in applications such as quantum key distribution.
Contrary to the common belief that EDFA can destroy the entanglement, we demonstrate experimentally that in entanglement assisted communication systems dominated by attenuation effects rather than noise, the signal photons can be amplified by the EDFA without destroying the entanglement. We perform experiments over terrestrial free-space optical testbed developed at the University of Arizona, composed of the FSO link that is 1.5 km in length. We demonstrate that the amplified entanglement assisted communication system at 10 Gb/s10 \mathrm{~Gb} / \mathrm{s}10 Gb/s, operated in strong turbulence regime, significantly outperforms the corresponding classical communication system operated in medium turbulence regime, clearly representing the quantum advantage. To reduce the system cost, the entanglement generation source is implemented by employing only the telecom C-band based devices.