High-Speed Quantum Communication Over Free-Space Optical Links and Mitigating Atmospheric Turbulence Effects

Abstract

Quantum communication in free-space optical (FSO) link is the primary focus of my research as explained in this dissertations. In particular, the research is concerned with the reliable and practical high-speed FSO communication systems in both quantum and classical communication domains. Exploring the possibilities of communicating under strong atmospheric turbulence regime in urban terrestrial environment was the main objective of various experiments that in future can potentially be extended to terrestrial-to-space, satellite, and inter-planetary optical communications. The superiority of the FSO communication system against the RF communication lies in its capacity for higher data rates, lower power overheads, reliability, range of wavelengths, and simplicity of installation. Although RF communication is effective when it comes to broadcast type links, the data rate capacity of laser communication are still far superior as we know from optical fiber communication. This research work is another step in advancing the possibilities of FSO communication systems. To accomplish these aforementioned goals, I have developed a 1.5 km long FSO link testbed between Department of Electrical and Computer Engineering (ECE) and Optical Sciences (OpSci) Meinel buildings at the University of Arizona campus for the practical demonstrations of FSO communication. The single most significant nemesis of an FSO communication system is the deterioration of link quality that arises due to atmospheric turbulence effects that severely limits its range, quality of transmission, and overall functioning. Typically, in most of the FSO experiments we start with phase modulation of a laser beam, ASE (amplified spontaneous emission) noise source, or signal photons from the entanglement pair (in case of quantum communication) imposing the phase-shift keying (PSK) signal. Now this information carrying beam traverses through the FSO link that is getting heated variably from the heat radiating from the surface below. These processes vary spatially, temporally, time of the day, and seasonally. Atmospheric turbulence effects exhibit as phase and wavefront distortion and scintillation at the receive side which leads to poor power reception, beam wandering, and ultimately loss of information. We characterize our FSO link’s operational quality in terms of bit-error rate and we treat it as a benchmark. To mitigate the losses incurred due to phase aberrations, we make use of adaptive optics (AO) techniques for azimuthal phase compensation. Our AO system is based on deformable mirror and a Shack-Hartmann wavefront sensor. I always strive to improve the data-rate and link efficiency with every new experiment, which I have demonstrated in many papers. The link currently operates at over 35% transmissivity, and the link is capable of performing experiments and research work with data rates up to 10 Gb/s and even 20 Gb/s. We demonstrated the first ever outdoor FSO based entanglement assisted communication and a radar system with quantum sensing features using entanglement. Our entanglement assisted systems outperform both the standard Shannon limit for communication and standard quantum limit for sensing applications. In classical communication we developed methods from basic FSO link that demonstrate BER improvements by application of AO. We developed a unique method to generate bright entangled photons by using only telecom devices that can survive strong atmospheric turbulence effects. To improve the FSO quantum system reliability we perform phase-conjugation on idler photons rather than signal photons that are affected by atmospheric turbulence effects. We developed a low probability of intercept (LPI) system that keeps communication covert and secure from an adversary. We also developed a unique method of information encoding-decoding called “wavelength-diversity” that shows significant improvement over conventional laser communication in FSO channel that matches improvements from AO at a fraction of its cost.