With the increased significance of building a global secure information network, free space quantum key distribution provides a brand new approach of transmitting secure information between two distant parties based on quantum mechanics. However, the polarization state of single photon is vulnerable to the complicated atmospheric environment and may lead to failure of key distribution. Therefore an efficient polarization compensation method based on optical rotation effect is proposed. The fundamental-mode Gaussian beam is used as the emission source at the Alice's side which will attenuate the beam into single photon level. In order to demonstrate the effect of the new compensation method, the polarization degree is calculated at different wave length and various distances. The simulation results show that the optical rotation effect can be used to compensating polarization states.
With the increased significance of building a global secure information network, free space quantum key distribution provides a brand new approach of transmitting secure information between two distant parties based on quantum mechanics. However, the polarization state of single photon is vulnerable to the complicated atmospheric environment and may lead to failure of key distribution. Therefore an efficient polarization compensation method based on optical rotation effect is proposed. The fundamental-mode Gaussian beam is used as the emission source at the Alice's side which will attenuate the beam into single photon level. In order to demonstrate the effect of the new compensation method, the polarization degree is calculated at different wave length and various distances. The simulation results show that the optical rotation effect can be used to compensating polarization states.
Quantum cryptography is a new technique for transmitting quantum information. The information is securely transmitted due to the laws of physics. In such systems, the vehicle that transfers quantum information is a single photon. The problem with using photons is that the transmission distance is limited by the absorption of the photons by the optical fiber along which they pass. The maximum demonstrated range so far is approximately 100 km. Using free-space quantum cryptography between a ground station and a satellite is a possible way of sending quantum information farther than is possible with optical fibers. This is because there is no birefringence effect in the atmosphere. However, there is a complication in that the directions of the polarization basis between the transmitter and the receiver must coincide with each other. This polarization changes because the mobile terminals for free-space transmission continuously change their attitudes. If the transmission protocol is based on polarization, it is necessary to compensate for the change in attitude between the mobile terminals. We are developing a scheme to track the polarization basis between the transceivers. The preliminary result is presented.
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.
Abstract Maintaining the polarization state in communication terminals is vital for polarization-encoding free-space quantum key distribution (QKD). Wave plate group phase mismatch caused by manufacturing errors, complex environmental effects, and the working wavelength deviation can reduce the polarization compensation effect. We found in theoretical analysis, that increasing phase mismatch of wave plates leads to the compensation method failure and reduces robustness. We propose a complementary polarization compensation method, which can effectively improve the robustness. Experimental results show that this method can improve the compensation effect by 50% at a slight phase mismatch, and realize a polarization extinction ratio exceeding 250:1 at the ergodic area even if the phase deviates to 0.27π. This method is beneficial to the high-stability design of free-space QKD systems and has the potential to be applied to QKD systems operating at multiple wavelengths.
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.
In quantum key distribution (QKD) systems using polarization entanglement, channel polarization perturbations significantly compromise both efficiency and security. To address the slow response and instability of current entanglement-based polarization compensation techniques for entanglement-based quantum key distribution, we propose a polarization compensation scheme based on an adaptive gradient descent algorithm. The system's performance is tested over a 10 km single-sided fiber channel. Results show that this scheme maintains long-term stability of the polarization state under both random and periodic perturbations. The average compensation time is 248 ms, with an entanglement source average visibility of 83.43% and an average quantum error rate of 8.28%. This method effectively mitigates polarization perturbation in entanglement-based quantum key distribution (QKD). These results are pivotal for the development of long-distance, multi-node quantum communication networks.
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.