Advancing the robustness of polarization and time bin quantum key distribution for free-space channels

Abstract

Quantum networks are an emerging technology that aims to harness the power of quantum mechanics to revolutionize communication and computation. Many countries are establishing national quantum networks to modernizing their communication and computational infrastructure. Satellites are necessary to extend the distances between communication nodes to a global scale. Creating a global quantum network requires many such nodes to be built, increasing the overhead of a network. Thus, to increase adoption, reducing the overhead and increasing the robustness of the systems employed by these nodes are necessary. In this thesis, we begin by developing an upgraded polarization modulation system for the weak coherent pulse source that will be used to connect with the Quantum Science and Encryption Satellite (QEYSSat). This new system is an inline optical fiber solution that completely avoids the stability and alignment issues that were present in previous versions. The inline scheme reduces the need for realignment and maintenance. The performance of the prototype system is analyzed and investigated. Another aspect of the QEYSSat mission is investigated. Particularly the feasibility of the 6-state 4-state reference frame independent (RFI) protocol for a moving free-space channel. By using RFI protocols, the random polarization rotations that occur in optical fibers can be compensated for, particularly in the optical fiber that connects the source to the QEYSSat ground station telescope. Thus eliminating the need for active polarization compensation systems. The robustness of the protocol to overcome polarization misalignment is investigated in the context of a QEYSSat pass. Second, a fully passive time bin quantum key distribution scheme is developed and investigated. This scheme removes the need for active phase alignment of the interferometers between the two communication parties. Proof-of-concept experiments are conducted over several challenging channels, particularly highly multi mode optical fibers. This scheme is then used to investigate the feasibility of using near-infrared time bin encoded photons in a standard telecommunication optical fiber. Near-infrared is particularly interesting as many single quantum sources produce photons within this regime. The passive scheme is also tested in a moving free-space time bin demonstration. The results of these demonstrations are discussed, including the challenges that were encountered. Third, a novel optical design for a field widened interferometer is investigated. The new optical design employs a fully reflective imaging system that is similar to an Offner relay. The new optical design allows for long relative path delays while maintaining a relatively compact physical footprint. The performance of the interferometer is tested for both single mode and multi mode signals. In addition, the achromatic performance of the design is tested. The device is also tested in a quantum sensing scenario, demonstrating its practicality beyond quantum communications. Finally, a prototype of a monolithic chassis for the Offner relay interferometer is built using additive manufacturing with the objective of increasing the robustness of the interferometer. As part of the monolithic chassis, flexure devices are studied to be used instead of standard optomechanical components to provide the necessary degrees of freedom for optical alignment purposes. In addition, the thermal stability of the chassis is studied using finite element analysis with standard materials and an analytical analysis with functionally graded materials. Through various studies, experiments, and component design, this thesis has advanced the practicality of both polarization and time bin encoding for free-space channels. Particularly increasing the potential for satellite deployable time bin interferometers. This work contributes to the long line of progress leading towards realizing a global quantum network.