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International Conference on Transparent Optical Networks

Entanglement Assisted Communications with Phase-Conjugation on Idler Photons Outperforming Entanglement Assisted Systems with Phase-Conjugated Receivers

Ivan B. Djordjević

Published Jul 6, 2025• 0 citations
Publisher

Abstract

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.

DOI
10.1109/icton67126.2025.11125378

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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.
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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.
Entanglement based (EB) communication, networking, and sensing represent excellent alternatives to corresponding classical counterparts in particular in either lowbrightness or high-attenuation regime and when the signal photons are buried in noise. In EB applications idler photons are stored in memory while the signal photons, carrying information, are transmitted over lossy, noisy, and for communication/networking over free-space optical (FSO) channels signal photons are also affected by the atmospheric turbulence and scattering effects. In optical phase conjugation (OPC)-based EB receiver, the OPC operation is performed on signal photons, which are severely affected by FSO channel impairments, with many signal photons being either absorbed or scattered by the FSO channel. Here we propose to perform the OPC operation on idler photons instead. The OPC operation is performed using low-cost, C-band components including tunable laser, periodically polled lithium niobate (PPLN) waveguides, tunable filter, and WDM demultiplexer; and this implementation is suitable for the photonic integrated circuit (PIC) realization. A similar approach is applied to implement the C-band devicesbased entanglement source. Given that the idler photons are brighter compared to the signal photons transmitted over either FSO channel or fiber-optics channel, the OPC operation is much more efficient. In the OPC-based receiver for FSO communications, we need to couple the signal photons to the single-mode fiber (SMF) before the OPC takes place, making the OPC process sensitive to the FSO-to-SMF coupling inefficiency. By performing the OPC on the idler photons instead, the OPC process is insensitive to poor coupling efficiency. In addition to being superior in terms of spectral efficiency compared to classical counterparts, in particular in highly lossy and noisy environment, the proposed EB scheme with the phaseconjugation on idler photons is superior to the corresponding scheme performing the phase-conjugation on signal photons in terms of receiver sensitivity making it an attractive option for radar applications. To validate the proposed concept at the University of Arizona campus we have developed a terrestrial FSO testbed. For detection probability of 0.99 the proposed EB radar outperforms the classical counterpart by 6.7 dB. We experimentally demonstrate that the target detection probability of the proposed entanglement based radar over turbulent FSO channel is significantly better than that of corresponding classical detection scheme.
We propose a degenerate entanglement-assisted (EA) free-space optical (FSO) communication system employing spatial light modulator (SLM)-based beam steering and a novel dual-pump configuration. The system utilizes a type-0 periodically poled lithium niobate (PPLN) waveguide to generate signal and idler photon pairs at 1570 nm, using two independent low-cost C- and L-band pump lasers. To enable homodyne detection, unmodulated idler photons are transmitted alongside modulated signal photons thus avoiding the need for phase conjugation and quantum memories. We demonstrate 1 Gb/s degenerate entanglement assisted transmission over a 1.5 km terrestrial FSO link at the University of Arizona campus, operating in the L-band. Experimental results show that the proposed degenerate EA system significantly outperforms its classical counterpart, particularly under strong atmospheric turbulence conditions.