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OSA Advanced Photonics Congress

Entanglement Assisted Communication over Turbulent Free-Space Optical (FSO) Link with Phase Conjugation on Idler Photons and Adaptive Optics

Vijay Nafria, Ivan B. Djordjević

Published Jan 1, 2023• 2 citations
Publisher

Abstract

To improve the reliability of entanglement-assisted (EA) communication over turbulent FSO channels we propose to perform the phase conjugation on idler photons instead of turbulence affected signal photons and use adaptive optics. We experimentally demonstrate that reliable EA communication over outdoor 1.45km FSO link, established at University of Arizona, is possible.

DOI
10.1364/sppcom.2023.spth2d.2

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To improve the reliability of entanglement-assisted (EA) communication over turbulent FSO channels we propose to perform the phase conjugation on idler photons instead of turbulence affected signal photons and use adaptive optics. We experimentally demonstrate that reliable EA communication over outdoor 1.45km FSO link, established at University of Arizona, is possible.
Optical communications over terrestrial free-space optical (FSO) channels are affected by various effects including diffraction, absorption, scattering, and turbulence effects. In several recent papers we have shown that entanglement assisted (EA) communication systems significantly outperform corresponding classical counterparts in strong turbulence regime. In traditional EA communications the phase-conjugation, required before the homodyne detection takes place, is performed on signal photons. However, given that many photons get lost due to diffraction, scattering, absorption, and turbulence effects, it is extremely difficult to perform the phase-conjugation on received signal photons when average number of received photons is smaller than 1. To solve this problem, we recently have proposed to perform phase-conjugation on bright idler photons instead. To operate the EA communication system over turbulent FSO links it is required to generate many bright entangled photons so that signal photons can survive beyond strong turbulence regime. To solve this problem, we have proposed to properly design periodically poled lithium niobate waveguides so that we can use telecom available lasers operated in S-, C-, and L-bands as the pump lasers instead of high-cost 780 nm pump lasers. In this invited paper, we will review our recent activities in EA communion studies over terrestrial FSO links and present recent experimental results demonstrating that EA communication systems performing the phase-conjugation on idler photons significantly outperforms both classical counterpart and the EA communications with phase-conjugated receiver in strong turbulence regime.
In this paper, we are presenting results that go against the common belief that entanglement is destroyed by the amplification using an EDFA. Here we demonstrate the quantum advantage of entanglement-assisted communication at 10Gb/s, employing LDPC-coded BPSK, over classical laser communication even after the amplification of signal photons is performed by the EDFA in order to improve the reliability of entanglement-assisted (EA) communication operating in turbulent 1.5 km terrestrial FSO channels. To make the EA system more robust against various atmospheric effects such as scattering, absorption, and turbulence effects we perform the optical phase-conjugation on idler photons rather than turbulence-affected signal photons and use adaptive optics to make additional improvements in terms of the bit-error rate.
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.
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.