Quantum key distribution (QKD) using weak coherent states and homodyne detection is a promising candidate for practical quantum‐cryptographic implementations due to its compatibility with existing telecom equipment and high detection efficiencies. However, despite the actual simplicity of the protocol, the security analysis of this method is rather involved compared to discrete‐variable QKD. This article reviews the theoretical foundations of continuous‐variable quantum key distribution (CV‐QKD) with Gaussian modulation and rederives the essential relations from scratch in a pedagogical way. The aim of this paper is to be as comprehensive and self‐contained as possible in order to be well intelligible even for readers with little pre‐knowledge on the subject. Although the present article is a theoretical discussion of CV‐QKD, its focus lies on practical implementations, taking into account various kinds of hardware imperfections and suggesting practical methods to perform the security analysis subsequent to the key exchange. Apart from a review of well‐known results, this manuscript presents a set of new original noise models which are helpful to get an estimate of how well a given set of hardware will perform in practice.
Optical quantum communication utilizing satellite platforms has the potential to extend the reach of quantum key distribution (QKD) from terrestrial limits of ∼200 km to global scales. We have developed a thorough numerical simulation using realistic simulated orbits and incorporating the effects of pointing error, diffraction, atmosphere and telescope design, to obtain estimates of the loss and background noise which a satellite-based system would experience. Combining with quantum optics simulations of sources and detection, we determine the length of secure key for QKD, as well as entanglement visibility and achievable distances for fundamental experiments. We analyze the performance of a low Earth orbit (LEO) satellite for downlink and uplink scenarios of the quantum optical signals. We argue that the advantages of locating the quantum source on the ground justify a greater scientific interest in an uplink as compared to a downlink. An uplink with a ground transmitter of at least 25 cm diameter and a 30 cm receiver telescope on the satellite could be used to successfully perform QKD multiple times per week with either an entangled photon source or with a weak coherent pulse source, as well as perform long-distance Bell tests and quantum teleportation. Our model helps to resolve important design considerations such as operating wavelength, type and specifications of sources and detectors, telescope designs, specific orbits and ground station locations, in view of anticipated overall system performance.2. Photons undergo a rotation to simulate imperfectly aligned polarization optics, and appropriate losses are applied to the quantum channel.3. Photons are measured resulting in count rate statistics, with added noise accounting for background light and detector dark counts. A realistic detector efficiency is used.4. These statistics are taken in various loss and background count rate regimes to assess optimal and typical expected performance.
Non-classical states of light, such as entangled photon pairs and number states, are essential for fundamental tests of quantum mechanics and optical quantum technologies. The most widespread technique for creating these quantum resources is spontaneous parametric down-conversion of laser light into photon pairs. Conservation of energy and momentum in this process, known as phase-matching, gives rise to strong correlations that are used to produce two-photon entanglement in various degrees of freedom. It has been a longstanding goal in quantum optics to realize a source that can produce analogous correlations in photon triplets, but of the many approaches considered, none has been technically feasible. Here we report the observation of photon triplets generated by cascaded down-conversion. Each triplet originates from a single pump photon, and therefore quantum correlations will extend over all three photons in a way not achievable with independently created photon pairs. Our photon-triplet source will allow experimental interrogation of novel quantum correlations, the generation of tripartite entanglement without post-selection and the generation of heralded entangled photon pairs suitable for linear optical quantum computing. Two of the triplet photons have a wavelength matched for optimal transmission in optical fibres, suitable for three-party quantum communication. Furthermore, our results open interesting regimes of non-linear optics, as we observe spontaneous down-conversion pumped by single photons, an interaction also highly relevant to optical quantum computing.
Quantum networks scale the advantages of quantum communication protocols to more than just two distant users. Here we present a fully connected quantum network architecture in which a single entangled photon source distributes quantum states to a multitude of users. Our network architecture thus minimizes the resources required of each user without sacrificing security or functionality. As no adaptations of the source are required to add users, the network can readily be scaled to a large number of clients, whereby no trust in the provider of the quantum source is required. Unlike previous attempts at multi-user networks, which have been based on active components, and thus limited to some duty cycle, our implementation is fully passive and thus provides the potential for unprecedented quantum communication speeds. We experimentally demonstrate the feasibility of our approach using a single source of bi-partite polarization entanglement which is multiplexed into 12 wavelength channels to distribute 6 states between 4 users in a fully connected graph using only 1 fiber and polarization analysis module per user. I. QUANTUM KEY DISTRIBUTION NETWORKSQuantum Key Distribution (QKD) [1,2] has reached the level of maturity required for deployment in realworld scenarios [3][4][5][6][7], and has been shown to operate alongside classical communication in the same deployed telecommunication fiber [8-10] and even over long distances in both fiber [11,12] and free-space links [13][14][15][16][17].Despite these great advances, the practical applicability of QKD is severely curtailed by the fact that most implementations and protocols are limited to two communicating parties.The pressing need to adapt quantum communication to more than two users has motivated several attempts at quantum networks. The QKD networks demonstrated thus far can be roughly grouped into four types of configurations [18]:First, Quantum repeater networks [19] which use quantum memories and entanglement swapping to extend and route quantum states and form arbitrary network topologies. However, technological advancement in quantum memories are needed until quantum repeater networks can be considered practical. Note that quantum repeaters can also be used to improve the performance of the following types of quantum networks.Another approach to multi-user networks is to use high-dimensional/multi-partite entanglement to share entanglement resources between several users [20][21][22]. This way different users share different subspaces of the Hilbert space to generate their keys. However, adding * Correspondence and requests for materials should be addressed to Sören Wengerowsky and Rupert Ursin. † Soeren.Wengerowsky@oeaw.ac.at ‡ Rupert.Ursin@oeaw.ac.at or removing users requires changes in the dimensionality of the system which makes complex alterations of the source necessary.The third approach are trusted node networks: They amount to a mesh of point-to-point links, each requiring a complete two-party communication setup. While trusted nodes have been used t...
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