Abstract
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.
Highlights
Entanglement distribution between very distant parties allows several interesting quantum-enabled protocols to be performed, in the fields of quantum communication [1, 2], metrology [3, 4, 5] and distributed computation [6, 7]
Quantum repeaters are placed at the connections between adjacent elementary links, while entanglement sources are in their central points (Fig. 2)
First of all Quantum Non-Demolition (QND) measurement devices herald the arrival of a photon from the elementary link
Summary
Entanglement distribution between very distant parties allows several interesting quantum-enabled protocols to be performed, in the fields of quantum communication [1, 2], metrology [3, 4, 5] and distributed computation [6, 7]. Satellite-to-ground optical links for quantum communication have already been proven to be feasible with current technology [16, 17, 18, 19, 20, 21, 22] They allow, in the double down-link configuration, to share entanglement between two ground stations, at distances that far exceed what can be achieved with direct fibre transmission. In this work we propose and study the scheme pictured, 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 atmo-
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