The technological development of QKD (Quantum Key Distribution) is accelerating from a research topic towards commercial deployment. QKD initiatives are increasingly addressing real-world use cases. Besides lots of literature on theory and lab-demos, first high Technology-Readiness Level systems are available. Due to the security guarantees it has to provide, the performance of any QKD implementation is particularly sensitive to any imperfections that may lead to an increase of the QBER (Quantum Bit Error Rate). Achieving high performance, or a low QBER value, constitutes a challenging engineering task, as the operating systems need to be highly optimized in many aspects. In particular, accurate phase modulation is the foundation of high system performance in case of phase-encoding protocols. In most cases, the overall QBER is measured with a QKD receiver, which accumulates errors from transmitter, quantum channel and receiver. This requires an operational transmitter system and additionally prevents the investigation of the pure transmitter output signal’s characteristics. However, for support of transmitter integration and test activities as well as for validation tasks, a more suitable method remains to be developed. In this paper, we propose such a method by making use of a coherent receiver. This device provides extended signal information including the signal amplitude and its current relative phase. Therefore, it is possible to monitor phase-encoding correctness and precision, among other relevant signal properties. We analyze the reasonability of using a coherent receiver, by outlining its capabilites to measure QBER contributors occurring in a given basic reference QKD transmitter system. Our findings indicate that employing a coherent receiver is a simple method to accelerate development and validation of phase-encoding QKD transmitter systems.
The technological development of QKD (Quantum Key Distribution) is accelerating from a research topic towards commercial deployment. QKD initiatives are increasingly addressing real-world use cases. Besides lots of literature on theory and lab-demos, first high Technology-Readiness Level systems are available. Due to the security guarantees it has to provide, the performance of any QKD implementation is particularly sensitive to any imperfections that may lead to an increase of the QBER (Quantum Bit Error Rate). Achieving high performance, or a low QBER value, constitutes a challenging engineering task, as the operating systems need to be highly optimized in many aspects. In particular, accurate phase modulation is the foundation of high system performance in case of phase-encoding protocols. In most cases, the overall QBER is measured with a QKD receiver, which accumulates errors from transmitter, quantum channel and receiver. This requires an operational transmitter system and additionally prevents the investigation of the pure transmitter output signal’s characteristics. However, for support of transmitter integration and test activities as well as for validation tasks, a more suitable method remains to be developed. In this paper, we propose such a method by making use of a coherent receiver. This device provides extended signal information including the signal amplitude and its current relative phase. Therefore, it is possible to monitor phase-encoding correctness and precision, among other relevant signal properties. We analyze the reasonability of using a coherent receiver, by outlining its capabilites to measure QBER contributors occurring in a given basic reference QKD transmitter system. Our findings indicate that employing a coherent receiver is a simple method to accelerate development and validation of phase-encoding QKD transmitter systems.
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 this work, a reliable and low computational õ complexity frame synchronization method for quantum key distribution (QKD) systems is proposed. The proposed method is based on the computation of the quantum bit error rate (QBER) and requires no more hardware than the one used for qubit coding and decoding. In a initial stage, pre-shared and received sequences are shifted until a minimum QBER value is reached. In a second stage, to confirm that the frames are correctly positioned, and to ensure that during key transmission a correct association of each received bit is made with the corresponding bit sent by the transmitter, a pre-determined number of inverted frames is sent, rising the QBER value to maximum values. As the number of inverted frames is a pre-agreed number, after detecting the first inverted frame it is possible to determine exactly when the secret key transmission will start. We use a polarization-encoding QKD testbed to assess the overall performance of the method. With the proposed frame synchronization method, the synchronization is achieved after roughly 104clock cycles, despite the concerns imposed by the low probability detection inherent to the QKD systems.
Quantum key distribution (QKD) is a technology that allows two users to communicate with theoretically perfect security using standard optical fibres. This is possible by transmitting the key on single photons, meaning a measurement by an eavesdropper disturbs the system in a way observable to the legitimate parties. The technology has advanced since the first protocol proposed in 1984, to the stage where there are now many protocols that can be experimentally implemented. These protocols have allowed secure keys to be generated over distances greater than 400 km and with secure key rates over 10 Mbit/s. In a metropolitan QKD network, it would be desirable for as many users to be connected as possible. Unfortunately, each protocol comes with different requirements on the transmitter and receiver. Even within a single protocol, different clock rates can require individualised transmitter and receiver hardware. This prohibits users from communicating with all receivers, unless they have complex transmitters with hardware for many protocols. This thesis develops a transmitter for practical QKD that is able to adapt to a number of different protocols with no changes to the hardware. The transmitter works using optical injection locking, where a pulse preparation laser adopts the phase of a phase preparation laser. Controlling the phase and intensity of the pulses in this way removes the side channel ordinarily present with direct modulation, in that the phase, intensity and frequency simultaneously change in response to an applied current. The cavity-enhanced electro-optic effect allows for the first demonstration of sub-volt half-wave phase modulation at high clock rates. The transmitter successfully demonstrates phase encoding, intensity encoding and on-demand phase randomisation. This allows for the experimental realisation and direct comparison of different QKD protocols, including one that has never before been implemented due to experimental complexity. A stable intensity modulator is also developed, based on a Sagnac interferometer. This removes a side channel in QKD systems and integrates well with the directly-modulated quantum transmitter. This development also means that the transmitter can implement all current two-party QKD protocols based on weak coherent pulses. The transmitter has the potential to become the standard transmitter for future quantum communication networks due to its stability, versatility and power efficiency. The design could also be demonstrated on a photonic chip, making it compact enough to fit in small transmitter units.
The increasing efforts by different organizations and countries regarding quantum key distribution (QKD) for secure communications are likely to result in diverse implementations for distinct users. To address this, the development of a software framework for a multi-encoding QKD system is proposed. Part of this framework are the time synchronization between transmitter and receiver, input generation for the arbitrary waveform generator (AWG) used in the implementation, as well as computing the quantum bit error rate (QBER) and secret key rate (SKR) for the employed protocol. This framework support various protocols, such as decoy-state BB84, differential phase-shift (DPS) or coherent one-way (COW), enabling a transmitter to securely exchange keys with different types of receivers. Our approach involves designing a modular and scalable architecture accommodating multiple QKD protocols to ensure compatibility and flexibility. The framework is a step towards interoperability and future-proofing in case current protocols become obsolete.
The EAGLE-1 mission aims to deliver Europe’s first autonomous space based quantum key distribution (QKD) system launching the EAGLE-1 satellite in 2026. A key element of this demonstration mission is the development and space qualification of a QKD transmitter, essential for enabling secure quantum communications. This paper presents the subsystem-level design, component qualification, system calibration, and functional testing of the QKD transmitter developed by the Institute of Communications and Navigation (KN) of the German Aerospace Center (DLR). The QKD transmitter is responsible for converting quantum key information into an optical signal for transmission to ground. The quantum key and QKD protocol is based on a quantum random number generator (QRNG) and the key bits are encoded in single photons. The transmitter hardware consists of various electrical and optical components, including modulators, attenuators, amplifiers, and passive elements. We discuss component selection criteria, as well as vibration and shock testing to verify launch resilience and thermal vacuum testing to assess operational stability under space conditions. One of the key challenges is the time and bias drift of critical components, which occurs after system boot-up and varies with temperature and time. These drifts directly affect the timing, amplitude, and phase of the optical signal, impacting overall QKD performance. We present the design of the calibration methodologies which ensure autonomous calibration of the transmitter in orbit, which provides the system stability required to adhere to the QKD protocol requirements. Finally, we describe the laboratory tests assessing quantum bit error rates (QBER).
Quantum key distribution (QKD) enables the establishment of shared keys between two distant users, Alice and Bob, based on the fundamental principles of quantum mechanics, and it has proven to possess information-theoretic security. In most of QKD systems, Alice and Bob require a shared reference frame, and real-time calibration of the reference frame increases system costs and reduces its performance. Fortunately, the reference-frame-independent QKD protocol has been proposed, overcoming reference-frame drift issues and receiving widespread attention. However, in practical QKD systems, the non-ideal characteristics of realistic devices introduce certain inconsistency between the theory and the practice. In real-world quantum key distribution systems, device imperfections can lead to security vulnerabilities, thereby reducing system security. For example, imperfections in the encoding apparatus at the source end may result in errors in the quantum states. The inherent defects in the detection part may cause after-pulse effects and dead-time effects, thus reducing the key rate. Therefore, in this work, we propose a practical state-preparation error tolerant reference-frame-independent quantum key distribution protocol by taking imperfections in both the source and the detectors into account. Moreover, a three-intensity decoy-state scheme for modeling analysis and numerical simulations is employed. In this protocol, we reduce the influence of state-preparation errors on the key rate by utilizing virtual state methods to precisely estimate the phase-error rate. Furthermore, by characterizing the effects of after-pulses and dead-time on the key rate, our protocol exhibits higher robustness and can effectively address issues related to detector imperfections. This approach can also be extended to other quantum key distribution protocols with higher security levels, such as measurement-device-independent quantum key distribution protocol and twin-field quantum key distribution, further mitigating the influence of device imperfections on practical implementation of QKD system. Therefore, our present work provide important reference value for putting the quantum key distributions into practical application.