An introduction to quantum key distribution and its challenges (invited)

Hugo Zbinden, University of Geneva

Abstract: This talk tries to set the scene for this workshop. I will give a short introduction to QKD for non-specialist and present the state of the art. Then I will outline a few technical and less technical challenges the QKD has to address in order to have a chance to be widely implemented in networks. Finally, I will slip into the role of devil's advocate and ask some provocative questions!

Quantum key distribution networks (invited)

Andrew Shields, Toshiba Research Europe Ltd

Abstract: We present a scheme for large-scale quantum networks based on a nodal mesh of high bandwidth point-to-point links, which are connected to the customer premise by point-to-multipoint access networks. Low cost Quantum Access Networks (QANs) may be realised by connecting multiple transmitters to a single quantum receiver using passive optical (or DWDM) combiners. We demonstrate continuous operation of a QAN by pre-compensation of phase and polarisation fluctuations at each transmitter and show that it is possible to connect up to 64 users to a single QAN. We report also on recent progress to use wavelength multiplexing to distribute quantum keys on fibre transmitting conventional data with high bandwidths.

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QKD in classic optical networks: Two different worlds forever? (invited)

Andreas Poppe, Austrian Institute of Technology

Abstract: In my talk I will try to give a brief overview of the concepts to bring QKD to optical networks and will address some major problems. I will outline the need for higher tolerance to loss of QKD-links to span typical access networks. At the same time a high demand for better resistance against noise photons must be fulfilled in order to develop the next generation of QKD-devices.

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Quantum communications from ultralow cost to ultralong range (invited)

John Rarity, University of Bristol


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Quantum friendly architectures for passive optical networks

Mohsen Razavi, University of Leeds

Abstract: Three network architectures, compatible with passive optical networks, for future hybrid quantum-classical networks are proposed and compared. These setups rely on three different schemes for quantum key distribution (QKD): BB84, entanglement-based QKD, and measurement-device-independent QKD (MDI-QKD). It turns out that, while for small-to-moderate size networks BB84 supports the highest secret key generation rate, it may fail to support large numbers of users. Its cost implications are also expected to be higher than other setups. For large networks, MDI-QKD offers the highest key rate if fast single-photon detectors are employed. Entanglement-based networks offer the longest security distance among the three setups. MDI-QKD is, however, the only architecture resilient to detection loopholes and possibly the most favorable with its less demanding end-user technology. Entanglement-based and MDI-QKD setups can both be combined with quantum repeater systems to allow for long-distance QKD with no trust constraints on the service provider. Finally, with a small modification to PONs, we show how to make them more quantum friendly.

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Quantum key distribution with continuous variables: long distance, side channels and network integration (invited)

Eleni Diamanti, CNRS, Telecom Paristech

Abstract: The distribution of secret keys with information-theoretic security is arguably one of the most important achievements of the field of quantum information processing and communications. Encoding the key information on continuous variables, such as the values of quadrature components of coherent states, presents the major advantage that implementations require only standard telecommunication technology, albeit at the expense of complex post-processing procedures. In this talk, we describe recent long-distance continuous-variable QKD experiments, which take into account all aspects of a practical scenario, including finite-size effects. Furthermore, we discuss progress towards the identification of side channels present in our system and development of suitable countermeasures, and demonstrate experimentally the coexistence of continuous-variable QKD with intense wavelength division multiplexed classical channels. These results illustrate the suitability of this technology for securing communications in emerging quantum information networks.

Satellite quantum communications (invited)

Paolo Villoresi, University of Padova

Abstract: The modelling of the quantum communications for the ground-to space and the intersatellite links has been analysed on the base of extensive calculations and the previous experience on the single-photon exchange with orbiting retroreflectors. For this purpose, the exploitation of novel resources as non maximally entangled states in the device independent QKD as well, on classical side, of the exploitation of the statistic transformation in the photon arrival time is expected to pave the way for a significant advancement of free-space QKD.

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Measurement-device-independent quantum key distribution and its application to quantum networks (invited)

Xiongfeng Ma, Tsinghua University

Abstract: The measurement-device-independent (MDI) QKD protocol closes all loopholes on detection at once. In fact, the detectors in a MDI-QKD setup can even be assumed to be in Eve's possession. Here, we present recent experimental realizations of MDI-QKD. By developing up-conversion single-photon detectors with high efficiency and low noise, the MDI-QKD protocol is faithfully demonstrated. Meanwhile, the decoy-state method is employed to defend attacks on non-ideal source, such as photon-number-splitting attacks. In the end, the system generates more than 25 kbits secure key over a 50 km fiber link. The MDIQKD can adapt to a quantum network setting.

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Long-distance measurement-device-independent quantum key distribution without quantum memories

Feihu Xu, University of Toronto

Abstract: We present a novel and practical method that can make quantum key distribution (QKD) both ultra-long-distance and immune to all attacks in the detection system. This method is called measurement-device-independent QKD (MDI-QKD) with entangled photon sources in the middle. By proposing a model and simulating a QKD experiment, we find that MDI-QKD with one entangled photon source can tolerate 77dB loss (367km standard fiber) in the asymptotic limit and 60dB loss (286km standard fiber) in the finite-key case with state-of-the-art detectors. Our method does not require quantum memories and thus can be easily implemented in practice.

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Quantum repeaters and QKD: analysis of secret key rates (invited)

Dagmar Bruß, Universität Düsseldorf

Abstract: This talk gives an overview of our recent work on secret key rates in the context of quantum repeaters. We analyse the influence of various parameters on the key rate: on one hand, experimental imperfections such as gate errors, imperfect detectors and memories are studied [PRA 87, 052315 (2013)], and on the other hand variations of the repeater protocol (different realisations, different distillation strategies [PRA 87, 062335 (2013)]) are investigated. Furthermore, the performance of a two-segment quantum repeater in the measurement-device- independent scenario [arXiv:1306.3095] is analysed, and the influence of multiplexing with short range [arXiv:1309.1106] is discussed.

How to get performance out of your quantum network? (invited)

William Munro, NTT Basic Research Lab, Japan

Abstract: Quantum communication—the ability to transmit quantum information—is a primitive necessary for any quantum internet. At its core, quantum communication generally requires the formation of entangled links between remote locations. The performance of these links is limited by the classical signalling time between such locations, necessitating the need for long-lived quantum memories. Here, we present the design of a communications network that neither requires the establishment of entanglement between remote locations nor the use of long-lived quantum memories. The rate at which quantum data can be transmitted along the network is only limited by the time required to perform efficient local gate operations.

The role of quantum memories in quantum networks (invited)

Kae Nemoto, National Institute of Informatics, Japan

Abstract: Until recently, it had been believed that long-lived quantum memory was necessary for long-distance quantum communications.The requirements for quantum memory is dependent on the quantum communication scheme, and the feasibility of schemes is dependent on the quantum repeater-node technology. In our presentation, with a concrete model of global quantum network using a cavity-based device, we discuss the trade off in resources with and without quantum memories.

Quantum networks with spins in diamond

Hannes Bernien, Delft University of Technology

Abstract: We present our recent results towards the realization of scalable quantum networks with solid-state qubits. We have entangled two spin qubits, each associated with a nitrogen vacancy center in diamond. The two diamonds reside in separate setups three meters apart from each other. With no direct interaction between the two spins to mediate the entanglement, we make use of a scheme based on quantum measurements: we perform a joint measurement on photons emitted by the NV centers that are entangled with the electron spins. The detection of the photons projects the spins into an entangled state. We verify the generated entanglement by single-shot readout of the spin qubits in different bases and correlating the results.

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Ultrafast and fault-tolerant quantum communication across long distances

Sreraman Muralidharan, Yale University

Abstract: We investigate a new approach to ultrafast quantum repeaters in which information can be transmitted through a noisy channel without the use of long distance entanglement [Nat. Photon. 6, 777-781 (2012)]. With small encoding blocks, our approach can fault-tolerantly correct both operational and photon loss errors using a teleportation based error correction procedure at each repeater station [arXiv:1310.5291]. Furthermore, we optimize the resource requirements for this quantum repeater scheme for the generation of a secure key. Finally, we discuss using multi-level quantum systems and quantum polynomial codes for more efficient ultrafast quantum repeaters.

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Quantum digital signatures without quantum memory

Erika Andersson, Heriot-Watt University

Abstract: Digital signatures ensure that messages cannot be forged or tampered with. They are widely used to provide security for electronic communications, for example in financial transactions and electronic mail. Signed messages are also transferrable, meaning that if one recipient accepts a message as genuine, then she is guaranteed that others will also accept the same message if it is forwarded. Digital signatures are different from encryption, which guarantees the privacy of a message. Both are important cryptographic tasks. Currently used classical digital signature schemes, however, only offer security relying on unproven computational assumptions. In contrast, quantum digital signatures (QDS), similar to quantum key distribution (QKD), offer information-theoretic security based on the impossibility of perfectly distinguishing between non-orthogonal quantum states. A serious drawback of previous QDS schemes is however that they require long-term quantum memory, making them unfeasible. We present a scheme that does not need quantum memory and which uses only standard linear optical components and photodetectors, together with an experimental proof-of-principle demonstration. In the experimental realisation, the recipients measure the distributed quantum signature states using a new type of quantum measurement, quantum state elimination. This shows that QDS and QKD are similar in terms of experimental feasibility.

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Measurement-device independent quantum cryptography with continuous variables

Stefano Pirandola, University of York

Abstract: We extend the field of continuous variable quantum cryptography to a more robust formulation which can be applied to untrusted networks. We consider two remote parties connected to an untrusted relay by insecure quantum links. To generate correlations, they transmit coherent states to the relay where a continuous-variable Bell detection is performed. Despite the working mechanism of the relay could be fully tampered and the links subject to optimal coherent attacks, the parties are still able to extract a secret key. Furthermore, our analysis shows that very long distances can be reached when the relay is proximal to one of the parties, configuration typical of a mobile device connecting to a public access point. Thus, using the cheapest possible quantum resources, our findings demonstrate the possibility of long-distance high-rate quantum key distribution in network topologies where direct links are missing between two end-users and intermediate relays cannot be trusted.

Continuous-variable quantum key distribution based on EPR entanglement

Vitus Handchen, Max-Planck Institute

Abstract: Continuous-variable (CV) quantum key distribution (QKD) gained a lot of interest in recent years due to its high detection efficiencies and low dark noise levels. Current implementations use for example standard telecommunication lasers and modulators and PIN photo detectors, thereby achieving efficient QKD links over several tens of kilometers. The scheme I will present in this talk uses entangled states of light at 1550 nm which enables low-loss transmission of the quantum states through telecommunication networks. The setup involves two squeezed-light sources providing more than 10 dB non-classical noise reduction, each. The two squeezed states are superimposed at a beam splitter, thereby achieving the strongest CV entanglement source currently available at the desired detection band. A secure key is generated exploiting the correlations/anticorrelations between the field quadratures measured by homodyne detection. A complex locking scheme provides long term stability of the entanglement level. Furthermore, a switching process was implemented to enable the random basis choise for QKD measurements. By applying a recent security proof by Furrer et al., a secret key with security against collective attacks was generated using a post-selection technique. Futhermore, the setup even enables QKD with security against most general attacks wherefor the postprocessing is currently under developement. By coupling one of the entangled beams into a km-scale fibre we plan to demonstrate the feasibility of QKD based on EPR entanglement in local area networks.

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Quantum key distribution using microwaves

Freya Wilson, University of Leeds

Abstract: We are developing a method of key distribution using microwave regimes allowing for long distance communication and implementation on current hardware technologies; most continuous variable quantum key distribution focuses on optical exchanges however microwave communication links have a ubiquitous presence. The method exploits quantum shot noise properties of measuring a microwave exchange and reconcilliation techniques to secure the exchange from an eavesdropper. If biased noise is distributed over the system a correlation of signals between communicants Alice and Bob can be identified and it is this we intend to exploit for secrecy. This should allow us to create a secure system for communication using microwaves with applications across wi-fi networks, mobile phones and satellites.

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Spacetime effects on satellite-based quantum communications

David Bruschi, Hebrew University of Jerusalem


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