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Quantum Optics and Photon Counting

Preparing the deployment of quantum key distribution over a classical network infrastructure in Prague

Tomáš Novák, Elisabeth Andriantsarazo, Carlos Guerra-Yánez, Petr Pospisil, Josef Vojtěch

Published Jun 6, 2023• 0 citations
Publisher

Abstract

Quantum Key Distribution (QKD) is a promising tool for secure communication in the near future. In combination with one-time-pad technique, it provides an unconditionally secure communication channel (meaning that it is secure against an adversary, even with unlimited computational power), at least in principle. However, a major implementation challenge for some schemes is the reliable creation, transportation, and measurement of entangled photon pairs over long-distance fiber networks. Our project aims to explore the possibilities for distributing quantum information on an existing network infrastructure while measuring the effects of real-world conditions. We characterized a commercial source of entangled photons. We measured its spectrum, brightness (1.6±0.3)×104 pairs/s/μW, and performed quantum state tomography (QST) to reconstruct the density matrix of the quantum state. Our implementation focuses on an all-fiber solution, which would enable a simplified QKD implementation. In laboratory conditions, we achieved the visibility equal to (0.957 ± 0.004) as a mean in both bases with a coincidence rate of (275 ± 4) counts/s and successfully ran QKD protocol with secret key rate of (86 ± 1) bits/s and average quantum bit error rate (QBER) of (4.8 ± 0.7) %.

DOI
10.1117/12.2665653

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Quantum Key Distribution (QKD) is a promising tool for secure communication in the near future. In combination with one-time-pad technique, it provides an unconditionally secure communication channel (meaning that it is secure against an adversary, even with unlimited computational power), at least in principle. However, a major implementation challenge for some schemes is the reliable creation, transportation, and measurement of entangled photon pairs over long-distance fiber networks. Our project aims to explore the possibilities for distributing quantum information on an existing network infrastructure while measuring the effects of real-world conditions. We characterized a commercial source of entangled photons. We measured its spectrum, brightness (1.6±0.3)×104 pairs/s/μW, and performed quantum state tomography (QST) to reconstruct the density matrix of the quantum state. Our implementation focuses on an all-fiber solution, which would enable a simplified QKD implementation. In laboratory conditions, we achieved the visibility equal to (0.957 ± 0.004) as a mean in both bases with a coincidence rate of (275 ± 4) counts/s and successfully ran QKD protocol with secret key rate of (86 ± 1) bits/s and average quantum bit error rate (QBER) of (4.8 ± 0.7) %.
Current technological progress is driving Quantum Key Distribution towards a commercial and worldwide scale expansion. Its capability to deliver secure communication regardless of the computational power of the attackers will be a fundamental feature in the next generations of telecommunication networks. Nevertheless, demonstrations of QKD implementation in a real operating scenario and their coexistence with the classical telecom infrastructure are of fundamental importance for reliable exploitation. Here we present a Quantum Key Distribution application implemented over a classical fiber-based infrastructure. We exploit a 50 MHz source at 1550 nm, a single 13 km-long fiber cable for both the quantum and the classical channel, and a simplified receiver scheme with just one single-photon detector. In this way, we achieve an error rate of approximately 2% and a secret key rate of about 1.7 kbps, thus demonstrating the feasibility of low-cost and ready-to-use Quantum Key Distribution systems compatible with standard classical infrastructure.
Current technological progress is driving Quantum Key Distribution towards a commercial and worldwide scale expansion. Its capability to deliver secure communication regardless of the computational power of the attackers will be a fundamental feature in the next generations of telecommunication networks. Nevertheless, demonstrations of QKD implementation in a real operating scenario and their coexistence with the classical telecom infrastructure are of fundamental importance for reliable exploitation. Here we present a Quantum Key Distribution application implemented over a classical fiber-based infrastructure. We exploit a 50 MHz source at 1550 nm, a single 13 km-long fiber cable for both the quantum and the classical channel, and a simplified receiver scheme with just one single-photon detector. In this way, we achieve an error rate of approximately 2% and a secret key rate of about 1.7 kbps, thus demonstrating the feasibility of low-cost and ready-to-use Quantum Key Distribution systems compatible with standard classical infrastructure.
Quantum Key Distribution (QKD) is an emerging technology that enables secure data encryption by basing key generation on the transmission of quantum states rather than computational complexity, thereby anchoring security in the laws of physics and offering resilience against quantum computing attacks. Current research efforts focus on transitioning QKD from laboratory-based demonstrations to real-world deployment, thereby establishing it as a practical communication technology. Quantum entanglement plays a central role in QKD by enabling robust security assurance and providing a path towards scalable quantum networks. This thesis presents a polarisation-entanglement system designed for operation in practical research scenarios, comprising state preparation from a commercial source, transmission, reception and data analysis. The system's capabilities are demonstrated through measurement of polarisation correlation visibility, violation of CHSH-type inequalities, and quantum state tomography, culminating in the generation of the first discrete-variable secret key generated within the Experimental Quantum Communication Group at the University of York. Using the developed system, the integration of quantum communication into existing information infrastructure is experimentally investigated in two distinct scenarios. First, the coexistence of quantum and classical signals in the shared optical fibre is explored in the context of a White Rabbit time-distribution system, recovering entanglement visibilities above 96\% over co-propagation distances of up to 1~km. Second, the transmission of photon pairs from the entanglement source is demonstrated over a 224~km deployed submarine fibre link, connecting the UK and Ireland used for classical data traffic. Together, these efforts provide a versatile, robust research tool for future experimental studies of polarisation-based QKD and demonstrate the viability of integrating quantum communication into existing optical infrastructure, paving the way for practical QKD systems.
In this paper we present our work demonstrating the transfer of quantum cryptographic information over a 78 km optical fiber, as a base for future long distance public quantum links, using a commercial quantum key distribution (QKD) system based on the BBM92 protocol. The system communicates with pairs of polarization-entangled photons generated by the nonlinear optical process of spontaneous parametric down-conversion. The system was put to the test in the laboratory using optical fiber of different lengths up to 112 km and with different link losses up to 29 dB. The QKD system was implemented on single mode standard optical fiber link of 78 km length deployed between the two cities of Braunschweig and Hannover. Characterizations and optimizations of the QKD system and the fiber link have been carried out in order to improve the synchronization, to reduce the quantum bit error rate, and to successfully establish the intercity quantum communication.
Quantum key distribution (QKD) offers unconditional information security by allowing two distant users to establish a common encryption key resilient to hacking. Resultingly, QKD networks interconnecting critical infrastructure and enabling the secure exchange of classified information, can provide a solution to the increasing number of successful cyberattacks. To efficiently deploy quantum networks, the technology must be integrated over existing communication infrastructure, such as optical fibre links. Yet, QKD poses stringent requirements on the conditions of the network over which it is deployed. This work demonstrates the first quantum communication network in Cyprus via the deployment of a multi-node quantum network, exploiting existing commercial underground optical fibre. The network employs bidirectional occupation of fibres and wavelength multiplexing in a ring architecture to achieve, with minimal use of dark fibres, high-rate QKD. Results obtained reveal consistent key generation rates across all nodes, confirming reliable operation in a real-world environment. This deployment highlights the feasibility of leveraging existing telecom infrastructure for quantum-secured communication, marking a significant step toward scalable and cost-effective quantum networks suited for critical applications.
Quantum cryptography uses the quantum properties of individual photons to provide two or more users with means to communicate securely and efficiently. Specifically, quantum key distribution (QKD) focuses on the secure communication between two distant parties and guarantees the security of a transmitted message between them. Although QKD is the most mature application of quantum cryptography, applications of quantum mechanics in communication systems are not limited to QKD and other cryptographic protocols can be implemented as well. This thesis focuses on the implementation of quantum cryptographic systems over deployed fiber. A first QKD system was built and used to demonstrate the BB84 protocol over deployed fiber between the University of Calgary and SAIT. The system implemented a novel tool called quantum frames that allow for the quantum signals to be routed, for clock synchronization, and for channel stabilization in a network scenario. The same system was used to examine the scalability of the secret key generation rate in different steps of the process in order to detect potential bottlenecks for key generation rate of high speed systems. A second QKD system was developed to demonstrate the measurement-device independent QKD protocol. The system worked over deployed fiber between the University of Calgary, SAIT, and Foothills Hospital in the City of Calgary. The advantage of the measurement-device independent QKD protocol over previous QKD protocols is that it eliminates the possibility of side-channel attacks that target single photon detectors used in QKD systems. This demonstration also involved the first demonstration of a Bell state measurement over deployed fiber. Bell state measurements were further studied by using novel single photon detectors, which allowed demonstrating a highly efficient Bell measurement. Finally, a cryptographic protocol known as private queries was demonstrated between the University of Calgary and SAIT. Quantum private queries were demonstrated for the first time over deployed fiber by developing and implementing an error correction scheme to make the protocol noise and loss tolerant.