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10Gb/s Entanglement Assisted Communication over Free-Space Optical Link with Phase Conjugation on Idler Photons and Improvements from Adaptive Optics

Vijay Nafria, Ivan B. Djordjević

Published Jan 1, 2024• 2 citations
Publisher

Abstract

We demonstrate record 10 Gb/s entanglement assisted communication over 1.5 km long turbulent free-space optical link in which optical phase-conjugation is performed on bright idler photons. To further improve the system performance adaptive optics is used.
DOI
10.1364/cleo_si.2024.sth3q.5

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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.
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 entanglement assisted communications and sensing the optical phase-conjugation (OPC), required before homodyne detection takes place, is typically performed on signal photons on receive side. Unfortunately, in turbulent free-space optical channels, the signal photons can be scattered or absorbed by the channel, and if they reach the receive side they are very weak. Here we propose an entanglement assisted radar technique in which the OPC is performed on bright idler photons instead. By performing the OPC on idler photons on transmit side, while employing adaptive optics and classical homodyne detection on receive side, we experimentally demonstrate that the target detection probability of the proposed entanglement assisted radar over turbulent free-space optical channel is significantly better than that of corresponding classical detection scheme.