Polarization Entangled Photon Sources for Free-Space Quantum Key Distribution

Free-space quantum key distribution has recently achieved several milestones, such as the launch and results of the first quantum satellite, Micius. The emergence of quantum satellites has certainly made progress towards the realization of a global quantum cryptographic network. In this thesis, two challenges in the development of an optical quantum ground station for a free-space quantum satellite link are studied. The first is the development of a high brightness, fiber pigtailed waveguide that is to be used as a polarization entangled photon source. The high pair production rate is required in order to meet the requirements for a satellite up-link configuration. The portability, robustness and ease of alignment were motivations for choosing a fiber pigtailed source. Certain challenges that are fundamental to the source design were characterized and several solutions to these challenges were investigated. The other main investigation in this thesis, is the development of a passive polarization compensation using polarization maintaining fibers. The birefringence in standard single mode optical fibers causes random polarization rotations to the light passing through the fiber. Polarization maintaining fibers, though very high in birefringence, are used with entangled photons and techniques from reference frame independent quantum key distribution protocols are shown to compensate for random polarization rotations while preserving the entanglement. In addition, the feasibility of the protocol using the polarization maintaining fibers is investigated. Through various studies, experiments, and component design, the feasibility of a pigtailed waveguide entangled photon source has been shown to need further investigation, while the feasibility of implementing polarization maintaining fibers to the ground station has been shown to be effective. It is particularly effective as a passive polarization compensation system that uses entanglement, however a similar concept is effective for non-entangled single photons. This work contributes to a long line of achievements leading towards satellite implementations of quantum key distribution for an eventual global quantum cryptographic network

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

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

Towards Fully Passive Time-Bin Quantum Key Distribution over Multi-Mode Channels

Phase stabilization of distant quantum time-bin interferometers is a major challenge for quantum communication networks, and is typically achieved by exchanging optical reference signals, which can be particularly challenging over free-space channels. We demonstrate a novel approach using reference frame independent time-bin quantum key distribution that completely avoids the need for active relative phase stabilization while simultaneously overcoming a highly multi-mode channel without any active mode filtering. We realized a proof-of-concept demonstration using hybrid polarization and time-bin entangled photons, that achieved a sustained asymptotic secure key rate of greater than 0.06 bits/coincidence over a 15m multi-mode fiber optical channel. This is achieved without any mode filtering, mode sorting, adaptive optics, active basis selection, or active phase alignment. This scheme enables passive self-compensating time-bin quantum communication which can be readily applied to long-distance links and various wavelengths, and could be useful for a variety of spatially multi-mode and fluctuating channels involving rapidly moving platforms, including airborne and satellite systems.Comment: 12 pages, 4 Figures, 1 Tabl

Towards a Multi-Pixel Photon-to-Digital Converter for Time-Bin Quantum Key Distribution

We present an integrated single-photon detection device custom designed for quantum key distribution (QKD) with time-bin encoded single photons. We implemented and demonstrated a prototype photon-to-digital converter (PDC) that integrates an 8 × 8 single-photon avalanche diode (SPAD) array with on-chip digital signal processing built in TSMC 65 nm CMOS. The prototype SPADs are used to validate the QKD functionalities with an array of time-to-digital converters (TDCs) to timestamp and process the photon detection events. The PDC uses window gating to reject noise counts and on-chip processing to sort the photon detections into respective time-bins. The PDC prototype achieved a 22.7 ps RMS timing resolution and demonstrated operation in a time-bin setup with 158 ps time-bins at an optical wavelength of 410 nm. This PDC can therefore be an important building block for a QKD receiver and enables compact and robust time-bin QKD systems with imaging detectors