Quantum Communications Feasibility Tests over a UK-Ireland 224 km Undersea Link.

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.

Coherent phase transfer for real-world twin-field quantum key distribution.

Quantum mechanics allows distribution of intrinsically secure encryption keys by optical means. Twin-field quantum key distribution is one of the most promising techniques for its implementation on long-distance fiber networks, but requires stabilizing the optical length of the communication channels between parties. In proof-of-principle experiments based on spooled fibers, this was achieved by interleaving the quantum communication with periodical stabilization frames.

Patterning-effect mitigating intensity modulator for secure decoy-state quantum key distribution.

Quantum key distribution (QKD) is a technology that allows two users to exchange cryptographic keys securely. The decoy state technique enhances the technology, ensuring keys can be shared at high bit rates over long distances with information theoretic security. However, imperfections in the implementation, known as side-channels, threaten the perfect security of practical QKD protocols.

Testing the photon-number statistics of a quantum key distribution light source.

A commonly held tenet is that lasers well above threshold emit photons in a coherent state, which follow Poissonian statistics when measured in photon number. This feature is often exploited to build quantum-based random number generators or to derive the secure key rate of quantum key distribution systems. Hence the photon number distribution of the light source can directly impact the randomness and the security distilled from such devices.

Overcoming the rate-distance limit of quantum key distribution without quantum repeaters.

Quantum key distribution (QKD) allows two distant parties to share encryption keys with security based on physical laws. Experimentally, QKD has been implemented via optical means, achieving key rates of 1.26 megabits per second over 50 kilometres of standard optical fibre and of 1.

Experimental measurement-device-independent quantum digital signatures.

The development of quantum networks will be paramount towards practical and secure telecommunications. These networks will need to sign and distribute information between many parties with information-theoretic security, requiring both quantum digital signatures (QDS) and quantum key distribution (QKD). Here, we introduce and experimentally realise a quantum network architecture, where the nodes are fully connected using a minimum amount of physical links.

Quantum key distribution with hacking countermeasures and long term field trial.

Quantum key distribution's (QKD's) central and unique claim is information theoretic security. However there is an increasing understanding that the security of a QKD system relies not only on theoretical security proofs, but also on how closely the physical system matches the theoretical models and prevents attacks due to discrepancies. These side channel or hacking attacks exploit physical devices which do not necessarily behave precisely as the theory expects.

Ultra-high bandwidth quantum secured data transmission.

Quantum key distribution (QKD) provides an attractive means for securing communications in optical fibre networks. However, deployment of the technology has been hampered by the frequent need for dedicated dark fibres to segregate the very weak quantum signals from conventional traffic. Up until now the coexistence of QKD with data has been limited to bandwidths that are orders of magnitude below those commonly employed in fibre optic communication networks.

Near perfect mode overlap between independently seeded, gain-switched lasers.

We drastically improve the mode overlap between independently seeded, gain-switched laser diodes operating at gigahertz repetition rates by implementing a pulsed light seeding technique. Injecting pulsed light reduces the emission time jitter and enables frequency chirp synchronization while maintaining random optical phases of the emitted laser pulses. We measure interference of these pulsed sources both in the macroscopic regime, where we demonstrate near perfect mode overlap, and in the single photon regime, where we achieve a Hong-Ou-Mandel dip visibility of 0.

Quantum key distribution over multicore fiber.

We present the first quantum key distribution (QKD) experiment over multicore fiber. With space division multiplexing, we demonstrate that weak QKD signals can coexist with classical data signals launched at full power in a 53 km 7-core fiber, while showing negligible degradation in performance. Based on a characterization of intercore crosstalk, we perform additional simulations highlighting that classical data bandwidths beyond 1Tb/s can be supported with high speed QKD on the same fiber.

Quantum secured gigabit optical access networks.

Optical access networks connect multiple endpoints to a common network node via shared fibre infrastructure. They will play a vital role to scale up the number of users in quantum key distribution (QKD) networks. However, the presence of power splitters in the commonly used passive network architecture makes successful transmission of weak quantum signals challenging.

High speed prototype quantum key distribution system and long term field trial.

Securing information in communication networks is an important challenge in today's world. Quantum Key Distribution (QKD) can provide unique capabilities towards achieving this security, allowing intrusions to be detected and information leakage avoided. We report here a record high bit rate prototype QKD system providing a total of 878 Gbit of secure key data over a 34 day period corresponding to a sustained key rate of around 300 kbit/s.

Field trial of a quantum secured 10 Gb/s DWDM transmission system over a single installed fiber.

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.

Efficient decoy-state quantum key distribution with quantified security.

We analyse the finite-size security of the efficient Bennett-Brassard 1984 protocol implemented with decoy states and apply the results to a gigahertz-clocked quantum key distribution system. Despite the enhanced security level, the obtained secure key rates are the highest reported so far at all fibre distances.

A quantum access network.

The theoretically proven security of quantum key distribution (QKD) could revolutionize the way in which information exchange is protected in the future. Several field tests of QKD have proven it to be a reliable technology for cryptographic key exchange and have demonstrated nodal networks of point-to-point links. However, until now no convincing answer has been given to the question of how to extend the scope of QKD beyond niche applications in dedicated high security networks.

Experimental inhibition of decoherence on flying qubits via "bang-bang" control.

Decoherence may significantly affect the polarization state of optical pulses propagating in dispersive media because of the unavoidable presence of more than a single frequency in the envelope of the pulse. Here we report on the suppression of polarization decoherence in a ring cavity obtained by properly retooling for photonic qubits the "bang-bang" protection technique already employed for nuclear spins and nuclear-quadrupole qubits. Our results show that bang-bang control can be profitably extended to quantum information processes involving flying polarization qubits.

Experimental test of two-way quantum key distribution in the presence of controlled noise.

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.

Secure deterministic communication without entanglement.

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.