Quantum repeaters in space

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.

Improved analytical bounds on delivery times of long-distance entanglement

“…The 3-over-2 approximation was first used by Jiang et al [39], who mentioned that the factor 3/2 agreed well with simulations in the small-probability regime. Since then, the approximation has been frequently used [8,10,28,32,[40][41][42][43][44][45][46][47][48][49][50][51][52].…”

“…For this scheme, it was empirically known [39] that for small success probabilities of connecting the pairs, the average time to in-parallel create both required initial pairs at each nesting level is roughly 3/2 times the average time for a single pair. This results in an approximation to the average completion time of the repeater scheme which is known as the 3-over-2 formula and has been frequently used since [8,10,28,32,[39][40][41][42][43][44][45][46][47][48][49][50][51][52]. Analytically finding the exact factor, for an arbitrary number of nesting levels and for any value of the success probabilities, has been an open problem for more than ten years [28].…”

Policies for elementary links in a quantum network

“…Furthermore, because our results apply to elementary links consisting of an arbitrary number of nodes and to any noise model for the quantum memories, they can be applied to protocols that go beyond bipartite entanglement distribution, namely to protocols for distributing multipartite entanglement. We also expect our results to be useful in the analysis of entanglement distribution using all-photonic quantum repeaters [69], and in the analysis of entanglement distribution using satellite-based quantum networks [70][71][72][73][74], in which an elementary link can easily be on the order of 1000 km [38] while still having a high fidelity. Initial applications of the results of this work to satellite-based elementary links can be found in [75,Chapter 7].…”

Survey of Promising Technologies for Quantum Drones and Networks

“…Satellitebased free-space optical communications, on the other hand, can be used to achieve improved loss-distance scaling. In [183], it is suggested to combine these two elements, quantum repeaters, and satellite-based communications, into a method that achieves entanglement distribution across long distances with only a few untrusted intermediary nodes. The presence of entanglement between two access points is required for quantum communication.…”