Carlo Liorni

The security of prepare-and-measure satellite-based quantum key distribution (QKD), under restricted eavesdropping scenarios, is addressed. We particularly consider cases where the eavesdropper, Eve, has limited access to the transmitted signal by Alice, and/or Bob’s receiver station. For instance, Eve can only receive an attenuated version of the transmitted signals. This results in settings where an uncharacterized bypass channel, inaccessible to Eve, can also carry signals to Bob. We obtain generic bounds on the key rate in the presence of bypass channels and apply them to continuous-variable QKD protocols with Gaussian encoding as well as to the family of BB84 protocols. We find regimes of operation in which the above restrictions on Eve can considerably improve system performance. Our work opens up new security frameworks for spaceborne quantum communications systems.

Quantum key distribution (QKD) is an innovative technology allowing information-theoretically-secure key sharing among distant users relying only on fundamental rules of quantum physics, without assumptions about the computational power of the attacker. This work presents the architecture of the Rome QKD Metropolitan Area Network (QMAN), the different layers required to sustain such network, including the Key Management System developed by Leonardo and the interconnection with the Italian Quantum Backbone to intregrate this network in the EuroQCI program. The Rome QMAN is a quantum-safe hybrid network, made up of several nodes that are connected with both commercial fibre and a free-space link.

Twin-Field Quantum Key Distribution (TF-QKD) is an innovative family of protocols characterized by a weaker dependence of the achievable secret key rate on the channel loss, with respect to conventional QKD solutions. In this work, we discuss several important aspects encountered in TF-QKD when transitioning from point-to-point links to a network configuration. 1) The effects of path length mismatch between the two arms of the link (A-C and B-C) is discussed in several configurations. 2) The noise contributions (stronger in in-field deployment) are meticulously analyzed, their effect on the final key rate is estimated and solutions to mitigate the problem are implemented. 3) The topic of building complex and large networks with TF-QKD is tackled to find advantageous configurations. Interconnected macro-star networks based on TF-QKD are simulated by means of the “qkdnetsim” package of the network simulator “ns3”. The upcoming deployment of national QKD networks requires dedicated studies in this direction to build efficient and long-range solutions, compatible with current telecom standards.

Entanglement distribution between very distant parties allows several interesting quantum-enabled protocols to be performed, in the fields of quantum communication, metrology and distributed computation. However, achieving this task over global distances (thousands of km) is very daunting, due to the exponential losses of light in optical fibres. The concept of a quantum repeater has been introduced to counter this problem. Such a device allows, using quantum memories and protocols based on entanglement swapping or quantum error correction, to connect several elementary links and enlarge the achievable distance. An alternative solution is represented by satellite-relayed free-space channels, that have already been proven to be feasible with current technology. Using a double downlink from a single satellite, however, the maximum distance between the ground stations is limited to {1500-2000} km, due to the additional losses encountered at low elevation angles. Through quantum repeaters, few of these satellite links can be chained together to reach global distances. In this work we propose and study a scheme in which entanglement sources and quantum repeaters are placed on board of satellites, orbiting around the Earth in the string of pearls configuration. This allows to connect two users on the ground via free-space optical links outside the atmosphere, achieving far superior distance-to-loss ratio with respect to the standard fibre-based implementation. In this way, a small number of intermediate nodes is enough to achieve entanglement distribution over global distances at a reasonable rate. The performance of this repeater chain is assessed in terms of the secret key rate achievable by the BB-84 cryptographic protocol, taking into account the most important sources of noise. We perform a comparison with other repeater chain architectures and show that our scheme is superior in almost every situation, achieving higher key rates, reliability and flexibility. These results have been obtained assuming reasonably conservative values of the parameters of the setup, such as the size of the optical elements and the efficiency of the quantum memories. The feasibility of the implementation in the mid-term future is analysed, based on recent developments in space-borne technology. We finally discuss some exemplary orbital configurations to connect several pairs of cities around the world with very small satellite constellations and estimate the cost of such an infrastructure. The integration of satellite-based links with ground repeater networks can be envisaged to represent the backbone of the future Quantum Internet. C. Liorni, H. Kampermann, D. Bruß, arXiv:2005.10146, 2020