The unconditional security in the creation of cryptographic keys obtained by quantum key distribution (QKD) protocolswill induce a quantum leap in free-space communication privacy in the sameway that we are beginning to realize secure optical fiber connections. However, free-space channels, in particular those with long links and the presence of atmospheric turbulence, are affected by losses, fluctuating transmissivity, and background light that impair the conditions for secure QKD. Here we introduce a method to contrast the atmospheric turbulence in QKD experiments. Our adaptive real time selection (ARTS) technique at the receiver is based on the selection of the intervals with higher channel transmissivity.We demonstrate, using data from the Canary Island 143-km free-space link, that conditions with unacceptable average quantum bit error rate which would prevent the generation of a secure key can be used once parsed according to the instantaneous scintillation using the ARTS techniqu

Quantum interference arising from the superposition of states is striking evidence of the validity of\ud quantum mechanics, confirmed in many experiments and also exploited in applications. However, as for\ud any scientific theory, quantum mechanics is valid within the limits in which it has been experimentally\ud verified. In order to extend such limits, it is necessary to observe quantum interference in unexplored\ud conditions such as moving terminals at large distances in space. Here, we experimentally demonstrate\ud single photon interference at a ground station due to the coherent superposition of two temporal modes\ud reflected by a rapidly moving satellite a thousand kilometers away. The relative speed of the satellite\ud induces a varying modulation in the interference pattern. The measurement of the satellite distance in real\ud time by laser ranging allows us to precisely predict the instantaneous value of the interference phase. We\ud then observed the interference patterns with a visibility up to 67% with three different satellites and with a\ud path length up to 5000 km. Our results attest to the viability of photon temporal modes for fundamental\ud tests of physics and quantum communication in space

The quantum wave-particle duality of light is confirmed at the unprecedented distance of low-Earth-orbit satellites.

Entanglement is an invaluable resource for fundamental tests of physics and the implementation of quantum information protocols such as device-independent secure communications. In particular, time-bin entanglement is widely exploited to reach these purposes both in free-space and optical fiber propagation, due to the robustness and simplicity of its implementation. However, all existing realizations of time-bin entanglement suffer from an intrinsic postselection loophole, which undermines their usefulness. Here, we report the first experimental violation of Bell's inequality with "genuine" time-bin entanglement, free of the postselection loophole. We introduced a novel function of the interferometers at the two measurement stations, that operate as fast synchronized optical switches. This scheme allowed to obtain a postselection-loophole-free Bell violation of more than nine standard deviations. Since our scheme is fully implementable using standard fiber-based components and is compatible with modern integrated photonics, our results pave the way for the distribution of genuine time-bin entanglement over long distances.

Recent interest in quantum communications has stimulated great technological progress in satellite quantum technologies. These advances have rendered the aforesaid technologies mature enough to support the realization of experiments that test the foundations of quantum theory at unprecedented scales and in the unexplored space environment. Such experiments, in fact, could explore the boundaries of quantum theory and may provide new insights to investigate phenomena where gravity affects quantum objects. Here, we review recent results in satellite quantum communications and discuss possible phenomena that could be observable with current technologies. Furthermore, stressing the fact that space represents an incredible resource to realize new experiments aimed at highlighting some physical effects, we challenge the community to propose new experiments that unveil the interplay between quantum mechanics and gravity that could be realizable in the near future.This article is part of a discussion meeting issue 'Foundations of quantum mechanics and their impact on contemporary society'.

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The violation of Bell's inequality requires a well-designed experiment to validate the result. In experiments using energy-time and time-bin entanglement, initially proposed by Franson in 1989, there is an intrinsic loophole due to the high postselection. To obtain a violation in this type of experiment, a chained Bell inequality must be used. However, the local realism bound requires a high visibility in excess of 94.63% in the time-bin entangled state. In this work, we show how such a high visibility can be reached in order to violate a chained Bell inequality with six, eight, and ten terms.

We here present the rate analysis and a proof of principle demonstration of a device-independent quantum key distribution protocol requiring the lowest detection efficiency necessary to achieve a secure key compared to deviceindependent protocols known so far. The protocol is based on a non-maximally entangled state and its experimental demonstration has been performed by twophoton bipartite entangled states. The improvement with respect to protocols involving maximally entangled states has been estimated.