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We propose a method for extracting an errorless secret key in a continuous-variable quantum key distribution protocol, which is based on Gaussian modulation of coherent states and homodyne detection. The crucial feature is an eight-dimensional reconciliation method based on the algebraic properties of octonions. Since the protocol does not use any post-selection, it can be proven secure against arbitrary collective attacks by using well-established theorems on the optimality of Gaussian attacks. By using this coding scheme with an appropriate signal-to-noise ratio, the distance for a secure continuous-variable quantum key distribution can be significantly extended.

Reconciliation is an essential part of any secret-key agreement protocol and hence of a Quantum Key Distribution (QKD) protocol, where two legitimate parties are given correlated data and want to agree on a common string in the presence of an adversary, while revealing a minimum amount of information.In this paper, we show that for discrete-variable QKD protocols, this problem can be advantageously solved with Low Density Parity Check (LDPC) codes optimized for the BSC. In particular, we demonstrate that our method leads to a significant improvement of the achievable secret key rate, with respect to earlier interactive reconciliation methods used in QKD.

We demonstrate experimentally the feasibility of continuous variable quantum key distribution (CV-QKD) in dense-wavelength-division multiplexing networks (DWDM), where QKD will typically have to coexist with several co-propagating (forward or backward) C-band classical channels whose launch power is around 0 dBm. We have conducted experimental tests of the coexistence of CV-QKD multiplexed with an intense classical channel, for different input powers and different DWDM wavelengths. Over a 25 km fiber, a CV-QKD operated over the 1530.12 nm channel can tolerate the noise arising from up to 11.5 dBm classical channel at 1550.12 nm in the forward direction (9.7 dBm in backward). A positive key rate (0.49 kbits s −1 ) can be obtained at 75 km with classical channel power of respectively −3 and −9 dBm in forward and backward. Based on these measurements, we have also simulated the excess noise and optimized channel allocation for the integration of CV-QKD in some access networks. We have, for example, shown that CV-QKD could coexist with five pairs of channels (with nominal input powers: 2 dBm forward and 1 dBm backward) over a 25 km WDM-PON network. The obtained results demonstrate the outstanding capacity of CV-QKD to coexist with classical signals of realistic intensity in optical networks.Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence.Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.1 The notion of 'distance limitation of QKD ' is, as such, ill-defined. If one wants to establish a comparison between the performance of different QKD systems, a large number of parameters have to be jointly considered, starting with the security model and the secure key rate, together with important more 'practical' parameters that include channel loss, noise environment and the photodetector technology. The numbers we indicate are approximative estimates of the maximum distance at which QKD could be operated with state-of-the art QKD systems, ten years ago [6] and now [7], with a key rate of at least 100 bits s −1 sufficient for practical key renewal, on a standard dark fiber (loss 0.25 dB km −1 ) and for QKD systems operating without He-cooled detectors, under conservative security model (coherent attacks including finite-size effect, even if such analysis was not yet performed 10 years ago). © 2015 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft allow to deploy QKD on lit fiber. This would boost the compatibility of quantum communications with existing optical infrastructures and lead to a significant improvement in terms of cost-effectiveness and addressable market for QKD.However, coexistence with intense classical channels raises new challenges for QKD. The optical power used on optical classical channels is orders of magnitude higher than for quantum communication. Multiplexing classical and quantum signals on a single fiber can result in very important additional noise for the quant...

We propose a method for extracting an errorless secret key in a continuous-variable quantum key distribution protocol, which is based on Gaussian modulation of coherent states and homodyne detection. The crucial feature is an eight-dimensional reconciliation method based on the algebraic properties of octonions. Since the protocol does not use any post-selection, it can be proven secure against arbitrary collective attacks by using well-established theorems on the optimality of Gaussian attacks. By using this coding scheme with an appropriate signal-to-noise ratio, the distance for a secure continuous-variable quantum key distribution can be significantly extended.

We present a full implementation of a quantum key distribution (QKD) system with a single photon source, operating at night in open air. The single photon source at the heart of the functional and reliable setup relies on the pulsed excitation of a single nitrogenvacancy color center in diamond nanocrystal. We tested the effect of attenuation on the polarized encoded photons for inferring longer distance performance of our system. For strong attenuation, the use of pure single photon states gives measurable advantage over systems relying on weak attenuated laser pulses. The results are in good agreement with theoretical models developed to assess QKD security.

The appealing feature of quantum key distribution (QKD), from a cryptographic viewpoint, is the ability to prove the information-theoretic security (ITS) of the established keys. As a key establishment primitive, QKD however does not provide a standalone security service in its own: the secret keys established by QKD are in general then used by a subsequent cryptographic applications for which the requirements, the context of use and the security properties can vary. It is therefore important, in the perspective of integrating QKD in security infrastructures, to analyze how QKD can be combined with other cryptographic primitives.The purpose of this survey article, which is mostly centered on European research results, is to contribute to such an analysis. We first review and compare the properties of the existing key establishment techniques, QKD being one of them. We then study more specifically two generic scenarios related to the practical use of QKD in cryptographic infrastructures: 1) using QKD as a key renewal technique for a symmetric cipher over a point-to-point link ; 2) using QKD in a network containing many users with the objective of offering any-to-any key establishment service. We discuss the constraints as well as the potential interest of using QKD in these contexts. We finally give an overview of challenges relative to the development of QKD technology that also constitute potential avenues for cryptographic research.

We develop a comprehensive framework to model and optimize the performance of CV-QKD with a local local oscillator (LLO), when phase reference sharing and QKD are conjointly implemented with the same hardware. We first analyze the limitations of the only existing approach, called LLO-sequential, and show that it requires high modulation dynamics and can only tolerate small phase noise, leading to expensive hardware requirements. Our main contribution is to introduce two original designs to perform LLO CV-QKD with shared hardware" respectively called LLO-delayline and LLO-displacement, and to study their performance. Both designs rely on a self-coherent approach, in which phase reference information and quantum information are coherently obtained from a single optical wavefront.We show that these designs can lift some important limitations of the existing LLO-sequential approach. The LLO-delayline design can in particular tolerate much stronger phase noise and thus appears as an appealing alternative to LLO-sequential that can moreover be deployed with affordable hardware. We also investigate, with the LLO-displacement design, how phase reference information and quantum information can be multiplexed in a single optical pulse. By studying the trade-off between phase reference recovery and phase noise induced by displacement we however demonstrate that this design can only tolerate low phase noise. On the other hand, the LLO-displacement design has the advantage of minimal hardware requirements and can be applied to multiplex classical and quantum communications, opening practical path towards the development of coherent quantum communications systems compatible with next-generation networks requirements. * Electronic address: adrien.marie@telecom-paristech.fr arXiv:1605.03642v2 [quant-ph] 30 Aug 2016 on the reference frame allows the receiver to more faithfully translate the received physical signals into logical information. It can for example consist in the knowledge of the relative angle between spatial two-dimensional cartesian reference frames [16], in the synchronization of spatially separated clocks [17], or information about the relative phase between two lasers, respectively at emitter and receiver side, when coherent optical communication is performed [15]. This latter problem, phase reference frame sharing considered in the context of CV-QKD, will be the main focus of this article .The problem of sharing a reference frame is specific in the sense that reference frame information constitutes unspeakable information, that can only be shared through physical carriers exchanged between emitter and receiver [18]. On the other hand, it is important to emphasize that although quantum mechanics gives a precise framework to formulate the question of reference frame sharing, in relation with quantum metrology [18], this question can be solved "classically", using macroscopic signals to exchange reference frame information. The type of questions related to phase reference sharing is not whether it is possible, bu...