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Frontiers in Optics

Two-Pumps-Based Degenerate Entanglement Assisted Communication with SLM-Based Beam Pointing

Ivan B. Djordjević, Ignacio A. Rojas

Published Jan 1, 2025• 0 citations
Publisher

Abstract

We successfully demonstrate two-pumps-based degenerate entanglement assisted free-space optical communication system at 1 Gb/s operating in L-band with SLM-based beam pointing by employing type-0 PPLN waveguide to generate signal and idler photons at 1570 nm.
DOI
10.1364/fio.2025.fth1b.2

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Quantum 2.0 Conference and Exhibition•Jan 1, 2024•0 citations
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.
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.
We propose a low-complexity degenerate entanglement-assisted free-space optical (FSO) communication system employing a two-pumps approach, a type-0 periodically poled lithium niobate (PPLN) waveguide, a multistage WDM demultiplexer, and a properly biased Mach-Zehnder modulator on transmitter side and, for binary phase-shift keying (BPSK) signaling, a single PIN photodetector or avalanche photodetector (APD) on a receiver side. Two-pumps-based approach utilizes two independent pump lasers operating at telecom wavelengths. The portion of degenerate entangled photons is used as the unmodulated carrier and the rest to transmit the modulated PSK/quadrature amplitude modulation (QAM) signal. The unmodulated portion of entangled photons is selected by properly choosing the bias voltage and phase representing the transmitted bits. In BPSK signaling, the unmodulated carrier photons are used on receiver side to reconstruct the transmitted symbol by beating them with modulated signal photons on either PIN photodetector or APD. We also describe how to implement the M-ary PSK and QAM transmitters for the degenerate entanglement assisted communications. Further, we propose a low-complexity PSK/QAM demodulator compatible with the proposed degenerate entanglement assisted communication concept. To demonstrate the quantum advantage, we conducted 1 Gb/s degenerate entanglement assisted BPSK experiments over a 1.5 km terrestrial FSO link we have developed previously at the University of Arizona, between ECE Dept. and Optical Sciences Meinel buildings. We demonstrate that in the strong atmospheric turbulence regime the proposed low-complexity degenerate entanglement assisted communication system performs significantly better compared to corresponding classical laser and thermal source-based communications counterparts.
In secure communications, the primary focus is on safeguarding the content of the message. In contrast, low probability of intercept (LPI) communications prioritize the privacy of the communication process itself. The most restrictive form of LPI communication, where an adversary attempts to disrupt the communication once its presence is detected, is known as covert communication. To hide the transmitted signal in solar radiation, we propose using the Gaussian quantum states generated by the degenerate parametric down-conversion process with signal photons being modulated with the phase-shift keying (PSK). To detect the transmitted signal, we propose transmitting the unmodulated idler photons (serving as the reference) together with modulated signal photons. To establish the link, we use the spatial light modulator (SLM)-based beam steering. We successfully demonstrate dual-pump-lasers-based degenerate entanglement-assisted (EA) LPI communication at 1 Gb/s over a 1.5 km terrestrial free-space optical (FSO) link. For the same link, the number of achievable EA covert bits for EA covert FSO communication is significantly higher compared to the corresponding classical covert communication scheme.