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Improved Two-Way Phase Coding

Salim Ali Al-Kathiri

Published May 17, 2011• 0 citations
OpenAlex

Abstract

Quantum Key Distribution (QKD) system is capable of generating secrete keys between two parties and enables them to sense the presence of any third party providing unconditionally secure communications. During the development process of constructing the enhanced fully polarized QKD configuration, few problems were confronted and solved. The developed optical structure proved to have auto-compensation in terms of photons polarization because of utilization of polarization maintaining fiber and Faraday mirror. It was also observed that phase coding scheme is implementable for QKD over 100 km long of optical fiber with high visibility, trusted system fidelity and low QBER.

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Quantum Key Distribution (QKD) system is capable of generating secrete keys between two parties and enables them to sense the presence of any third party providing unconditionally secure communications. During the development process of constructing the enhanced fully polarized QKD configuration, few problems were confronted and solved. The developed optical structure proved to have auto-compensation in terms of photons polarization because of utilization of polarization maintaining fiber and Faraday mirror. It was also observed that phase coding scheme is implementable for QKD over 100 km long of optical fiber with high visibility, trusted system fidelity and low QBER.
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.
An improved DPS-QKD scheme is proposed and demonstrated in this paper. At Alice's site，we use a M-Z interferometer to produce two coherent pulses. At Bob's site，a Faraday-mirrors-based Michelson interferometer is used instead of a M-Z interferometer to auto-compensate for the phase drifts and polarization mode dispersions in the fiber. It enhances the security of the whole system to use the two quantum cryptography protocols (namely the two pulse DPS-QKD scheme associated with the BB84 scheme). It is shown in our experiment that such a system features perfect stability with a quantum bit error rate less than 5% and high key generation rate. This scheme has been implemented successfully during the 80km fiber transmission.
We demonstrate the decoy-state quantum key distribution (QKD) with one-way quantum communication in polarization space over 102 km. Further, we simplify the experimental setup and use only one detector to implement the one-way decoy-state QKD over 75 km, with the advantage to overcome the security loopholes due to the efficiency mismatch of detectors. Our experimental implementation can really offer the unconditionally secure final keys. We use 3 different intensities of 0, 0.2, and 0.6 for the light sources in our experiment. In order to eliminate the influences of polarization mode dispersion in the long-distance single-mode optical fiber, an automatic polarization compensation system is utilized to implement the active compensation.
A one-way quantum key distribution scheme based on intrinsically stable Faraday-mirror-type Michelson interferometers with four-port polarizing beam splitters has been demonstrated that can compensate for birefringence effects automatically. The encoding is performed with phase modulators, but decoding is accomplished through measurement of the polarization state of Bob's photons. An extinction ratio of about 30 dB was maintained for several hours over 50 km of fiber at 1310 nm without any adjustment to the setup, which shows its good potential for practical systems.