The field of quantum key distribution (QKD) has seen many advancements since its conception in 1984. Recently there have been developments towards establishing free space optical links between satellites and earth to enable global QKD networks. Polarization encoded photos are commonly utilized as qubits for these free space links, placing polarization distortion constrains on the optics used. \n \nTwo polarization distortion studies were conducted in this thesis. The first is a simulation of stress induced birefringence in the Window Observational Research Facility window on board the International Space Station (ISS). Atmospherics pressure inside the ISS elicits stress on window leading to optical birefringence, which in turn can distort the polarization of photons as they pass through the window. The model studies the birefringent effects for incoming optical beams over different polarizations, diameters and incoming angles. \n \nAs part of free space optical quantum links, it can be beneficial to utilize beam steering optics to manage the spatial positioning of various components in the system. Implementing beam steering optics can aid in achieving spatial and maneuverability requirements for ground stations and satellites as they track each other. However, these optics can have an adverse effect on incoming photon polarization states. Because of this, the effects of various different beam steering configurations were characterized experimentally, and compared to a corresponding simulated model. \n \nThe studies presented in this thesis have revealed that common optics such as windows and mirrors can distort polarization based quantum signals. The polarization distortion of the studied optics have shown high dependencies on the angular orientation of the incoming optical beam. However, the distortions measured can be noticeably reduced by implementing compensation optics. As a result, their distortions are not detrimental to a quantum link that utilizes them. The studies presented in this thesis are critical to the design of polarization based quantum receivers as they shed light on potential sources of link distortions. This work is part of a large collection of developments bringing the implementation of global quantum key distribution networks closer to reality.
The field of quantum key distribution (QKD) has seen many advancements since its conception in 1984. Recently there have been developments towards establishing free space optical links between satellites and earth to enable global QKD networks. Polarization encoded photos are commonly utilized as qubits for these free space links, placing polarization distortion constrains on the optics used. \n \nTwo polarization distortion studies were conducted in this thesis. The first is a simulation of stress induced birefringence in the Window Observational Research Facility window on board the International Space Station (ISS). Atmospherics pressure inside the ISS elicits stress on window leading to optical birefringence, which in turn can distort the polarization of photons as they pass through the window. The model studies the birefringent effects for incoming optical beams over different polarizations, diameters and incoming angles. \n \nAs part of free space optical quantum links, it can be beneficial to utilize beam steering optics to manage the spatial positioning of various components in the system. Implementing beam steering optics can aid in achieving spatial and maneuverability requirements for ground stations and satellites as they track each other. However, these optics can have an adverse effect on incoming photon polarization states. Because of this, the effects of various different beam steering configurations were characterized experimentally, and compared to a corresponding simulated model. \n \nThe studies presented in this thesis have revealed that common optics such as windows and mirrors can distort polarization based quantum signals. The polarization distortion of the studied optics have shown high dependencies on the angular orientation of the incoming optical beam. However, the distortions measured can be noticeably reduced by implementing compensation optics. As a result, their distortions are not detrimental to a quantum link that utilizes them. The studies presented in this thesis are critical to the design of polarization based quantum receivers as they shed light on potential sources of link distortions. This work is part of a large collection of developments bringing the implementation of global quantum key distribution networks closer to reality.
Free-space quantum key distribution has recently achieved several milestones, such as \nthe 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 \nfor 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 \ncompensation 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. \nThrough 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.
Due to the fast-paced advancements of the quantum computing field, current public key cryptographic protocols need to be rethought. One promising solution is Quantum Key Distribution (QKD), whose security is robust to any computational advancement. The security of QKD relies on the laws of quantum mechanics, ensuring that any eavesdropper attempting to access the data will be detected. In particular, polarization-encoding QKD systems allow to generate secret keys in two remote locations to encode/decode information, using the State of Polarization (SOP) of single photons. Although its security has been theoretically proven, its practical implementation poses some challenges, part of which are the main focus of this PhD thesis. One relevant process in QKD systems is the synchronization between the transmitter and receiver, namely, the frame synchronization. This way, in this thesis, a frame synchronization method is developed and implemented, showing its potential for the use in prepare-and-measure QKD systems. The developed method has been shown to be effective, even with a low number of photons reaching the receiver. Furthermore, its architecture allows it to be adjusted for different attenuation regimes of the QKD system. The ability to encode data on the physical properties of single photons, in this case, polarization, highly depends on the capacity to accurately generate the required polarization states on demand. This way, an automatic SOP generation method, comprising an optical polarization monitoring system and a dedicated algorithm was developed and implemented, capable of generating four SOPs. This method reached an average Quantum Bit Error Rate (QBER) of 0.75%, which is well below the security threshold. After the SOP generation, and their transmission through the quantum channel, polarization alignment algorithms are required to compensate for the random rotations induced by birefringence in the optical fiber. In this thesis, a non-iterative polarization compensation method was implemented. This method requires only three QBER estimations to re-align the polarization, reaching average QBERs of 1.8%. Additionally, multiple iterative methods were implemented for comparison purposes, with the results clearly showing a better efficiency of the deterministic method. Beyond fiber-based transmissions, free-space transmissions were analysed due to their ability to reach sites without installed fiber and due to its potential for satellite-based quantum communications. This thesis demonstrates the feasibility of polarization-encoding QKD over free-space channels, by subjecting quantum signals to atmospheric turbulence to study its impact. In that sense, polarization variations over a 1.8 km outdoor free-space channel were monitored, to understand the effect of atmospheric turbulence on polarization. These results have shown that SOP variations in free-space links are slow, allowing their compensation. Furthermore, it was identified that, under conditions tested, the Secret Key Rate (SKR) could reach values up to 8.7×10ˉ⁴ bits per second, allowing QKD over free space for distances up to 2.31 km.
In polarization-encoding free-space Quantum Key Distribution (QKD), unwanted variations in the states of polarization may cause errors, leading to an increase in the quantum bit error rate (QBER), and a reduction of the Secure Key Rate (SKR). Optical elements that reflect or transmit the quantum signal at the transmitter and/or receiver are the main cause of polarization variations in the quantum states. Although most of these optical elements affect polarization in a constant manner, requiring relatively simple compensation techniques, mobile elements such as steering mirrors, cause time-varying polarization variations that are more complex to characterize and correct. In this work, we simulate the effect of steering mirrors’ typical values of angular rotation on the QBER. In particular, we show that, for most cases, an active correction of the polarization reference frame and a passive correction of the phase shift between the electric field components is sufficient to obtain QBER values due to the steering mirrors below 0.2%. This avoids the need of a fully active correction strategy, which increases the complexity, weight, and cost of the implementation. Furthermore, we present different design considerations to further reduce the steering mirrors’ contribution to the QBER (less than 0.05%).
Satellite quantum key distribution is a promising technique that overcomes the limited transmission distance in optical-fiber-based systems. The polarization tracking technique is one of the key techniques in the satellite quantum key distribution. With free-space quantum key distribution between an optical ground station and a satellite, the photon polarization state will be changed according to the satellite movement. To enable polarization based quantum key distribution between mobile terminals, we developed a polarization-basis tracking scheme allowing a common frame of reference to be shared. It is possible to orient two platforms along a common axis by detecting the reference optical signal only on the receiver side with no prior information about the transmitter's orientation. We developed a prototype system for free-space quantum key distribution with the polarization-basis tracking scheme. Polarization tracking performance was 0.092° by conducting quantum key distribution experiments over a 1 km free space between two buildings in a Tokyo suburb.
Abstract The establishment of a world-wide quantum communication network relies on the synergistic integration of satellite-based links and fiber-based networks. The first are helpful for long-distance communication, as the photon losses introduced by the optical fibers are too detrimental for lengths greater than about 200 km. This work aims at giving, on the one hand, a comprehensive and fundamental model for the losses suffered by the quantum signals during the propagation along an atmospheric free-space link. On the other hand, a performance analysis of different quantum key distribution (QKD) implementations is performed, including finite-key effects, focusing on different interesting practical scenarios. The specific approach that we chose allows to precisely model the contribution due to different weather conditions, paving the way towards more accurate feasibility studies of satellite-based QKD missions.
Satellite networks and quantum communications offer complementary opportunities for enhanced operations. Quantum communications provide security for the transmissions between satellites and ground stations; while the free-space link of satellite networks provide the potential of long distance transmission of quantum bits (qubit) for space communications. However, with the promising advantages of the two approaches, challenges remain to fully develop quantum-based satellite communications such as robust and reliable information detection which is difficult to achieve due to the movement of satellites. In this paper, a tracking algorithm is proposed for polarization-encoded quantum satellite communications where polarization states are used to determine the bit transfer between the transmitter and receiver. The polarization tracking is essential for the decoding of a qubit and the quantum key distribution (QKD). A practical channel model for free-space quantum communications is adopted in this paper. With the estimated polarization, a novel dynamic polarization compensation scheme is also proposed. The results show that our methods can accurately estimate the polarization, providing much lower quantum bit error rate (QBER) by compensation, as compared with the direct qubit detection without polarization tracking and compensation scheme.