HD-QKD(high-dimensional quantum key distribution) is an alternative to two-dimensional QKD with advantages such as higher data transmission per pulse [54] and noise tolerance [13, 66]. It has been shown that MCFs (multicore fibers) are a viable platform for performing HD-QKD through phase encoding across modes of the same fiber [10]. MCFs have also gathered attention due to their increased data transmission rates and are likely to become part of classical telecom infrastructure in the future [35, 50, 14, 51]. In this thesis we report a proof-of-concept decoy-state BB84 HD-QKDexperiment with an all-fiber setup using installed 4C-MCFs (four core multicore fibers) in a realistic environment. We introduce the theory of QKDand the motivations for decoy states as well as the security bounds for systems with different dimensions. We describe the experiment in detail, including active phase stabilization, state preparation, MCF-based technologies, channels and the detection system. We report a maximum short-range key rate of 7.8 kbit/s and a maximum secure key transmission distance of 105 km, as well as a short-range stability of over 16 hours. A comparison of our experiment with several others is provided, showing that our results are comparable to other works with similar detection technology. Wealso include two papers where we used our experimental platform for experiments simulating non-Markovian noise and implementing single setting quantum state tomography, showing the flexibility of our setup and potential for further work.
HD-QKD(high-dimensional quantum key distribution) is an alternative to two-dimensional QKD with advantages such as higher data transmission per pulse [54] and noise tolerance [13, 66]. It has been shown that MCFs (multicore fibers) are a viable platform for performing HD-QKD through phase encoding across modes of the same fiber [10]. MCFs have also gathered attention due to their increased data transmission rates and are likely to become part of classical telecom infrastructure in the future [35, 50, 14, 51]. In this thesis we report a proof-of-concept decoy-state BB84 HD-QKDexperiment with an all-fiber setup using installed 4C-MCFs (four core multicore fibers) in a realistic environment. We introduce the theory of QKDand the motivations for decoy states as well as the security bounds for systems with different dimensions. We describe the experiment in detail, including active phase stabilization, state preparation, MCF-based technologies, channels and the detection system. We report a maximum short-range key rate of 7.8 kbit/s and a maximum secure key transmission distance of 105 km, as well as a short-range stability of over 16 hours. A comparison of our experiment with several others is provided, showing that our results are comparable to other works with similar detection technology. Wealso include two papers where we used our experimental platform for experiments simulating non-Markovian noise and implementing single setting quantum state tomography, showing the flexibility of our setup and potential for further work.
Quantum key distribution (QKD) provides information-theoretic security rooted in quantum physics, while high-dimensional (HD) encoding increases both noise tolerance and secret-key yield. Multicore fibers (MCFs), a leading platform for next-generation telecom networks, are a natural substrate for HD-QKD. Field demonstrations over deployed MCFs have so far relied on a hybrid qudit encoding strategy that combines two path (core modes) with the time-bin photonic degree of freedom, rather than exploiting the full set of available core modes. Although practical, this current approach incurs intrinsic efficiency penalties that grow with dimension. Encoding a qudit in the full set of core modes removes these penalties, but it requires the coherent control of all cores at once and has so far been restricted to laboratory settings. Here we implement a four-dimensional ( d = 4 ) QKD protocol that directly exploits the full set of core modes of a four-core MCF, operating over an installed MCF network across the Universidad de Concepción campus under continuous environmental perturbations. We further benchmark the scheme using superconducting nanowire detectors at 10 dB channel loss, achieving a composable finite-key rate of R = 6.19 × 10 − 3 bits / pulse , the highest per-pulse rate reported to date for deployed HD-QKD at comparable loss. This result establishes core-mode encoding as a viable architecture for realistic, high-rate quantum-secure communications.
Quantum key distribution (QKD) provides information-theoretic security rooted in quantum physics, while high-dimensional (HD) encoding increases both noise tolerance and secret-key yield. Multicore fibers (MCFs), a leading platform for next-generation telecom networks, are a natural substrate for HD-QKD. Field demonstrations over deployed MCFs have so far relied on a hybrid qudit encoding strategy that combines two path (core modes) with the time-bin photonic degree of freedom, rather than exploiting the full set of available core modes. Although practical, this approach incurs intrinsic efficiency penalties that grow with dimension. Here we implement a four-dimensional (d=4) QKD protocol that directly exploits the full set of core modes of a four-core MCF, operating over an installed MCF network across the Universidad de Concepción campus under continuous environmental perturbations. We further benchmark the scheme using superconducting nanowire detectors at 10dB channel loss, achieving a composable finite-key rate of R=6.19×10−3bits/pulse, the highest per-pulse rate reported to date for HD-QKD at comparable loss. This result establishes core-mode encoding as a viable architecture for realistic, high-rate quantum-secure communications.
Higher Dimensional Quantum Key Distribution (HD-QKD) improves information capacity and security compared to binary QKD systems. Previous HD-QKD implementations have required d+1 (d refers to encoding dimension) single-photon detectors, limiting large-scale implementation. In contrast, another HD-QKD implementation using coherent one-way (COW) protocol hardware with two detectors achieved a secret key rate (SKR) of 155 kbps for an 8-D system and 2.7% QBER tolerance for a 16-D system with qubit scrambling across a time window. In this paper, we theoretically simulate and experimentally demonstrate time-bin encoded HD-QKD using COW protocol hardware with two single-photon detectors operating at standard telecom wavelengths, achieving a remarkable maximum encoding of 6 bits per photon (i.e., 56-d system). Our HD-QKD system incorporates a practical decoy-state technique to counter the photon number splitting attack, allowing greater flexibility in choosing higher mean photon number thereby achieving higher SKR and distances. This implementation demonstrates a remarkable ninefold increase in SKR for a 16-d system (210 kbps over 25 km of standard single-mode fiber) compared to qubit-based systems, with a QBER tolerance of 13.5% for d = 16. Notably, we achieved positive key rates for emulated loss up to 131 km of fiber length for d = 8. By optimizing detector deadtime and mean photon number using our theoretical model, we achieve enhanced key rates and security. Our findings highlight the superiority of our approach and lay the foundation for high throughput HD-QKD implementation, offering a secure and efficient method for transmitting sensitive information.
In contemporary society, communication security has become increasingly important. The security of the current cryptosystems, based on mathematical assumptions, will not be guaranteed when quantum computers become available [1]. This has spurred investigations into new security technologies based on quantum physics. In order to exchange secure information between users, quantum key distribution (QKD), a branch of Quantum Communications (QCs), provides good prospects for ultimate security based on the laws of quantum mechanics [2–7]. Most of QKD systems are implemented in a point-to-point link using bulky, discrete and expensive devices. Consequently, a large scale deployment of this technology has not been achieved. In a future scenario, where QCs will become standard technology, and where infrastructures like banks and government buildings, will be connected through a quantum network, different requirements in terms of key generation are needed. A solution may be represented by new technologies applied to quantum world. In particular multicore fiber (MCF) open a new scenario for quantum communications, from high-dimensional (HD) spatial entanglement generation, to HD QKD and multi-user key generations, to HD-entanglement distribution. Furthermore, MCFs are expected as a good candidate for overcoming the capacity limit of a current optical communication system, as example the record capacity of 661 Tbits/s was obtained last year with a 30-cores fiber [8]. Proof of concept experiment has already proved the coexistence of classical and quantum communications transmitted into different cores of MCF [9]. On the other hand, photonic integration has played a critical role in recent quantum information revolution by integrating functionalities of traditional discrete bulky components into ultra-compact chips [10, 11]. In fact, integrated photonic circuits provides excellent performances (compacts, good optical phase stability, access to new degrees of freedom), and are particularly suitable for the manipulation of quantum states. Some recent experiments have already demonstrated conventional binary QKD systems, using polarization and phase reference degrees of freedom [12, 13]. Moreover, by using integrated solution new high-dimensional quantum states can be generated and propagated. Based on compact silicon photonic integrated circuits, we here show how a MCF can be used for quantum communications protocols by proving decoy-state HD-QKD and multi-users quantum key generations.
High-dimensional quantum key distribution (HD-QKD) allows two parties to generate multiple secure bits of information per detected photon. In this work, we show that decoy-state protocols can be practically implemented for HD-QKD using only one or two decoy states. HD-QKD with two decoy states, under realistic experimental constraints, can generate multiple secure bits per coincidence at distances over 200 km and at rates similar to those achieved by a protocol with infinite decoy states. Furthermore, HD-QKD with only one decoy state is practical at short distances, where it is almost as secure as a protocol with two decoy states. HD-QKD with only one or two decoy states can therefore be implemented to optimize the rate of secure quantum communications.
High-dimensional quantum key distribution (QKD) allows achievement of information-theoretic secure communications, providing high key-generation rates, which cannot, in principle, be obtained by QKD protocols with binary encoding. Nonetheless, the amount of experimental resources needed increases as the quantum states to be detected belong to a larger Hilbert space, thus raising the costs of practical high-dimensional systems. Here, we present an alternative scheme for fiber-based four-dimensional QKD, with time and phase encoding and one-decoy state technique. Quantum state transmission is tested over different channel lengths up to 145 km of standard single-mode fiber, evaluating the enhancement of the secret key rate in comparison to the three-state two-dimensional BB84 protocol, which is tested with the same experimental setup. Our scheme allows measurement of the four-dimensional states with a simplified and compact receiver, where only two single-photon detectors are necessary, thus making it a cost-effective solution for practical and fiber-based QKD.