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 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.
Abstract Quantum key distribution (QKD) is a method for secure communication that utilizes quantum mechanics principles to distribute cryptographic keys between parties. Integrated photonics offer benefits such as compactness, scalability, energy efficiency and the potential for extensive integration. We have achieved BB84 phase encoding and decoding, time-bin phase QKD, and the coherent one-way (COW) protocol on a planar lightwave circuit (PLC) platform. At the optimal temperature, our chip successfully prepared quantum states, performed decoding and calculated the secure key rate of the time-bin phase-decoding QKD to be 80.46 kbps over a 20 km transmission with a quantum bit error rate (QBER) of 4.23%. The secure key rate of the COW protocol was 18.18 kbps, with a phase error rate of 3.627% and a time error rate of 0.377%. The uniqueness of this technology lies in its combination of high integration and protocol flexibility, providing an innovative solution for the development of future quantum communication networks.
Quantum key distribution (QKD) is an effective solution to ensure the secure transmission of information. However, for the large-scale application of QKD, the interoperability and flexibility of the transmitter and receiver are urgent issues to be solved. Here, for the first time, to the best of our knowledge, we experimentally verify the feasibility of one continuous-variable (CV) QKD system to achieve multiple protocols and rates. The flexibility of the system comes from the modulator realizing multiple protocols and a broadband coherent detector realizing multiple symbol rates. The results show that this system can switch between different rates and protocols to generate the secure key, and reveal its similarity to classical optical communication. Therefore, It can be adjusted according to user needs and provides a system-level solution for building a flexible quantum network.
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 existence of quantum computing presents an existential threat to the traditional cryptographic algorithms, and there is a need to create communication mechanisms that are fundamentally secure. In this regard, the concept of Quantum Key Distribution (QKD) provides information-theoretic security, which is founded on quantum mechanics. Nevertheless, conventional QKD protocols have serious shortcomings such as lack of distance in communication, vulnerability to side-channel attacks, and inability to support the large-scale heterogeneous network. We suggest that HQNet-QKD a Hybrid Quantum Key Distribution framework can help solve these issues by offering a more secure, scalable and future-proof key exchange framework in hybrid classical-quantum networks. To support multi-layered security, the proposed architecture is a network ensuring the smoothness of decoy-state BB84, Measurement-Device-Independent QKD (MDI-QKD), entanglement/twin-field-based QKD with verifiable Secret Sharing (VSS) and Post-Quantum Cryptography (PQC). It provides a complete methodology such as system architecture, mathematical modeling, and end-to-end key synthesis. When used over a distance of 300 km, simulation and testing have shown that HQNet-QKD can achieve up to2.7×higher secret key rate (SKR) than conventional methods, secret communication at 700 km and resilience to both quantum and classical attacks more than 96%. In addition, the framework guarantees forward secrecy, detector-side immunity, contextbound key derivation, and composability and thus resistant to current and future adversaries. It has a low-computational overhead modular design, making it trivial to deploy in a real world, in applications like quantum-secured 5G backbones, cloud data centers, critical infrastructure, and satellite-based communication systems. The findings make HQNet-QKD a new generation solution to quantum-safe secure communication and the first step towards the implementation of the quantum internet.
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.
The high pace of quantum computing development presents a serious risk to classical cryptographic systems like RSA and ECC in that quantum-resilient communication systems must be developed. The present paper will suggest Quantum Blind Signature-based Hybrid Quantum Key Distribution (QKD -QBS) to provide secure and authenticated quantum communication. The proposed architecture uses both BB84 and Decoy-State QKD protocols with Quantum Blind Signature (QBS) authentication layer in order to support quantum key generation and verification of entities in the same session. The framework is simulated and tested with IBM Qiskit Aer simulator in realistic channel noise situations. Quantum Bit Error Rate (QBER), Secret Key Retention (SKR), Quantum Entropy (H), Authentication Latency and Security Index (SI) are used as performance evaluation metrics. The experimental data show that the QBER is low (0.34%), SKR is high (92.4%), entropy is nearly ideal (H=0.99) with the lowest authentication latency (0.17 ms), which proves the fact that the integration of QBS does not reduce the efficiency of the key generation but the reliability of authentication. The hybrid framework proposed has information-theoretic security using the principles of quantum mechanics and strong resistance to impersonation, re-play and man in the middle attacks. The findings show that QKDQBS model provides a scalable and solid platform of next generation secure communication systems, such as IoT,5G/6Gnetworks, and cloud-based quantum systems.