The establishment of a world-wide quantum communication network relies on the synergistic integration of satellite-based links and fiber-based networks. The first are helpful for long-distance communication, as the photon losses introduced by the optical fibers are too detrimental for lengths greater than about 200 km. This work aims at giving, on the one hand, a comprehensive and fundamental model for the losses suffered by the quantum signals during the propagation along an atmospheric free-space link. On the other hand, a performance analysis of different quantum key distribution (QKD) implementations is performed, including finite-key effects, focusing on different interesting practical scenarios. The specific approach that we chose allows to precisely model the contribution due to different weather conditions, paving the way towards more accurate feasibility studies of satellite-based QKD missions. IntroductionQuantum key distribution (QKD) and quantum communication in general have the potential to revolutionise the way we communicate confidential information over the internet. The natural carriers for quantum information are photons, that are already widely used in classical networks of optical fibers to achieve high communication rates. Unfortunately, even though enormous improvements have been obtained in the last years [1, 2], scaling quantum communication protocols over long distances is very challenging, due to the losses experienced during the propagation inside the optical fibers. Several schemes for the realization of quantum repeaters have been proposed in recent years, that could allow to bridge long distances and naturally be implemented inside a quantum communication network [3][4][5][6][7]. Considering the important technological hurdles that quantum repeaters should overcome before becoming useful, satellite-based free-space links look like the most practical way to achieve long-distance QKD in the short term [8]. They can take advantage of the satellite technology and the optical communication methods developed in the last decades in the classical case. Various feasibility studies had addressed this topic in the last twenty years [8][9][10][11] and several experiments have definitely proved that the technology involved is ready for deployment [12][13][14][15][16].Optical satellite-based links have the important drawback of being strongly dependent on the weather conditions [17][18][19][20]. The presence of turbulent eddies and scattering particles like haze or fog generates random fluctuations of the relative permittivity of the air, on different length-and time-scales. This phenomenon affects the light propagation in a complicated way, inducing random deviations and deformations of any optical beam sent through the atmosphere. It results in reduced transmittance, because of geometrical losses due to the finite collection aperture, and random modifications of the phase front. A comprehensive model of these effects is then necessary, in order to precisely evaluate the performance of the link w...
We present a new optical scheme enabling the implementation of highly stable and configurable non-Markovian dynamics. Here one photon qubit can circulate in a multipass bulk geometry consisting of two concatenated Sagnac interferometers to simulate the so called collisional model, where the system interacts at discrete times with a vacuum environment. We show the optical features of our apparatus and three different implementations of it, replicating a pure Markovian scenario and two non-Markovian ones, where we quantify the information backflow by tracking the evolution of the initial entanglement between the system photon and an ancillary one.
Long-distance entanglement is a very precious resource, but its distribution is very difficult due to the exponential losses of light in optical fibres. A possible solution consists in the use of quantum repeaters, based on entanglement swapping (ES) or quantum error correction. Alternatively, satellite-based free-space optical links can be exploited, achieving better loss-distance scaling. We propose to combine these two ingredients, quantum repeaters and satellite-based links, into a scheme that allows to achieve entanglement distribution over global distances with a small number of intermediate untrusted nodes. The entanglement sources, placed on satellites, send quantum states encoded in photons towards orbiting quantum repeater stations, where ES is performed. The performance of this repeater chain is assessed in terms of the secret key rate achievable by the BBM92 cryptographic protocol. We perform a comparison with other repeater chain architectures and show that our scheme, even though more technically demanding, is superior in many situations of interest. Finally, we analyse strengths and weaknesses of the proposed scheme and discuss exemplary orbital configurations. The integration of satellite-based links with ground repeater networks can be envisaged to represent the backbone of the future quantum internet.
A novel bulk optics scheme for quantum walks is presented. It consists of a one-dimensional lattice built on two concatenated displaced Sagnac interferometers that make it possible to reproduce all the possible trajectories of an optical quantum walk. Because of the closed loop configuration, the interferometric structure is intrinsically stable in phase. Moreover, the lattice structure is highly configurable, as any phase component perceived by the walker is accessible, and finally, all output modes can be measured at any step of the quantum walk evolution. We report here on the experimental implementation of ordered and disordered quantum walks.
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