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The future quantum internet will leverage existing communication infrastructures, including deployed optical fibre networks, to enable novel applications that outperform current information technology. In this scenario, we perform a feasibility study of quantum communications over an industrial 224 km submarine optical fibre link deployed between Southport in the United Kingdom (UK) and Portrane in the Republic of Ireland (IE). With a characterisation of phase drift, polarisation stability and the arrival time of entangled photons, we demonstrate the suitability of the link to enable international UK–IE quantum communications for the first time. [ABSTRACT FROM AUTHOR]

The theoretically proven security of quantum key distribution (QKD) could revolutionize the way in which information exchange is protected in the future (1,2). Several field tests of QKD have proven [...]

Random numbers are an essential resource to many applications, including cryptography and Monte Carlo simulations. Quantum random number generators (QRNGs) represent the ultimate source of randomness, as the numbers are obtained by sampling a physical quantum process that is intrinsically probabilistic. However, they are yet to be widely employed to replace deterministic pseudo random number generators (PRNG) for practical applications. QRNGs are regarded as interesting devices. However, they are slower than PRNGs for simulations and are typically seen as clumsy laboratory prototypes, prone to failures, and unreliable for cryptographic applications. Here, we overcome these limitations and demonstrate a compact and self-contained QRNG capable of generating random numbers at a pace of 8 Gbit/s uninterruptedly for 71 days. During this period, the physical parameters of the quantum process were monitored in real time by self-checking functions implemented in the generator itself. At the same time, the output random numbers were analyzed with the most stringent suites of statistical tests. The analysis shows that the QRNG under test sustained the continuous operation without physical instabilities or hardware failures. At the same time, the output random numbers were analyzed with the most stringent suites of statistical tests, which were passed during the whole operation time. This extensive trial demonstrates the reliability of a robustly designed QRNG and paves the way to its use in practical applications based on randomness. [ABSTRACT FROM AUTHOR]

We report the first quantum-key-distribution (QKD) systems capable of delivering sustainable, real-time secure keys continuously at rates exceeding 10 Mb/s. To achieve such rates, we developed high-speed postprocessing modules, achieving maximum data throughputs of 60 MC/s, 55 Mb/s, and 108 Mb/s for standalone operation of sifting, error correction, and privacy amplification modules, respectively. The photonic layer of the QKD systems features high-speed single-photon detectors based on self-differencing InGaAs avalanche photodiodes, phase encoding using an asymmetric Mach–Zehnder interferometer, and active stabilization of the interferometer phase and photon polarization. An efficient variant of the decoy-state BB84 protocol is implemented for the security analysis, with a large dataset size of $10^8$ bits selected to mitigate finite-size effects. Over a 2-dB channel, a record secure key rate of 13.72 Mb/s has been achieved averaged over 4.4 days of operation. We confirm the robustness and long-term stability on a second QKD system continuously running for 1 month without any user invention. [ABSTRACT FROM AUTHOR]

Secure communication networks enabled by commercial quantum key distribution (QKD) are already available. However, their widespread deployment will require great efforts towards reducing the currently prohibitive cost of QKD systems. Here, we propose a compact and cost-effective alternative to the asymmetric Mach-Zehnder interferometer commonly used to implement phase encoding in the Bennett-Brassard 1984 (BB84) QKD protocol. Our solution consists of an all-fiber, in-line, highly birefringent interferometer (HBI). The HBI shows improved tolerance to length mismatches and a simpler assembly, making it particularly desirable for the fabrication of multi-user systems where several interferometers must have matched delays and where cost and space considerations can be most critical, such as quantum access networks. As a proof-of-principle, we demonstrate point-to-point QKD operation with HBIs over 15.5 km drop fiber and an 8-port passive optical network splitter. We achieve a secure key generation rate of 299.4 ± 16.4 kbit/s with a quantum bit error rate of 2.89 ± 0.31% for a continuous 25 h operation period. [ABSTRACT FROM AUTHOR]

Distributed-phase-reference (DPR) systems were introduced as a method of decreasing the complexity of quantum key distribution systems for practical use. However, their informationtheoretic security has only been proven when the added requirement of block-wise phase randomisation is met. Realisation of this with a conventional approach would result in a cumbersome transmitter, removing any practical advantage held by DPR systems. Here, we solve this problem using a light source that allows the coherence between pulses to be controlled on a pulse-by-pulse basis without the need for additional bulky components. The system is modulator-free, does not require a complex receiver, and features an excellent stability without an active stabilisation mechanism. We achieve megabit per second key rates that are almost three times higher than those obtained with the standard Bennet-Brassard 1984 protocol. [ABSTRACT FROM AUTHOR]

We present a scheme for secure deterministic quantum communication without using entanglement, in a plug-and-play fashion. The protocol is completely deterministic, both in the encoding procedure and in the control one, thus doubling the communication rate with respect to other setups; moreover, the deterministic nature of transmission, apart from rendering unnecessary bases revelation on the public channel, allows the realization of protocols like 'direct communication' and 'quantum dialogue.' The encoding exploits the phase degree of freedom of a photon, thus paving the way for an optical fiber implementation, feasible with present-day technology. [ABSTRACT FROM AUTHOR]

We present results from the first field-trial of a quantum-secured DWDM transmission system, in which quantum key distribution (QKD) is combined with 4 × 10 Gb/s encrypted data and transmitted simultaneously over 26 km of field installed fiber. QKD is used to frequently refresh the key for AES-256 encryption of the 10 Gb/s data traffic. Scalability to over 40 DWDM channels is analyzed.

An asymmetric preparation of the quantum states sent through a noisy channel can enable a new way to monitor and actively compensate the channel noise. The paradigm of such an asymmetric treatment of quantum information is the Bennett 1992 protocol, in which the counts in the two separate bases are in direct connection with the channel noise. Using this protocol as a guiding example, we show how to correct the phase drift of a communication channel without using reference pulses, interruptions of the quantum transmission, or public data exchanges.

We describe the experimental test of a quantum key distribution performed with a two-way protocol without using entanglement. An individual incoherent eavesdropping is simulated and induces a variable amount of noise on the communication channel. This allows a direct verification of the agreement between theory and practice.

We propose a protocol for deterministic communication that does not make use of entanglement. It exploits nonorthogonal states in a two-way quantum channel to attain unconditional security and high efficiency of the transmission. We explicitly show the scheme is secure against a class of individual attacks regardless of the noise on the channel. Its experimental realization is feasible with current technology.