Global quantum networks for secure communication can be realized using large fleets of satellites distributing entangled photon pairs between ground-based nodes. Because the cost of a satellite depends on its size, the smallest satellites will be most cost-effective. This Letter describes a miniaturized, polarization entangled, photon-pair source operating on board a nano-satellite. The source violates Bell’s inequality with a Clauser–Horne–Shimony–Holt parameter of 2.60 ± 0.06 . This source can be combined with optical link technologies to enable future quantum communication nano-satellite missions.

Quantum key distribution (QKD) is a method for establishing secure cryptographic keys between two parties who share an optical, "quantum" channel and an authenticated classical channel. To share such keys across the globe, space-based links are required and in the near term these will take the form of trusted node, key management satellites. We consider such channels between two nanosatellite spacecraft for polarization entanglement-based QKD, and the optical channel is described in detail. Quantum channels between satellites are useful for balancing keys within constellations of trusted node QKD satellites and, in the future, may have applications in long-distance qubit exchange between quantum computers and in fundamental physics experiments. The nanosatellite mission proposed uses an optical link with 80-mm diameter optical terminals. If such a link could be maintained with 10-μrad pointing accuracy, then this would allow QKD to be performed for satellite separations up to around 400 km. A potential pointing and tracking system is also described although currently this design would likely limit the satellite separation to 100 to 150 km.

In order to communicate information in a quantum network effectively, all network nodes should share a common reference frame. Here, we propose to study how well m nodes in a quantum network can establish a common spatial reference frame from scratch, even though t of them may be arbitrarily faulty. We present a protocol that allows all correctly functioning nodes to agree on a common reference frame as long as they are fully connected and not more than < t m 3 nodes are faulty. Our protocol furthermore has the appealing property that it allows any existing two-node protocol for reference frame agreement to be lifted to a protocol for a quantum network.Keywords: quantum networks, reference frame agreement, quantum communication Quantum networks are gaining importance [1] for a variety of tasks such as quantum distributed computing [2], quantum cloud computing [3] and quantum key distribution (see e.g. [4][5][6][7]). From the current architecture of the internet one can assume that any such network will contain a large number of nodes that are distributed over widespread geographical locations on Earth or on satellites [8][9][10][11][12] and connected via quantum and classical communication channels [13]. Some of the many challenges in building a quantum network spanning long distances include Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.how to perform quantum error correction [14] and construct quantum repeaters (see e.g. [15]). Yet, before we can hope to implement even such basic building blocks effectively, we would like all nodes in the quantum network to agree on a common reference frame to enable easy quantum communication.A significant research effort has been devoted to developing protocols for agreeing on a reference frame between just two nodes [16][17][18][19][20][21][22][23]. Such protocols demand quantum communication because in the absence of a pre-shared reference frame, a node cannot meaningfully share directional information with a distant node by exchanging only classical data. Instead, a quantum system must be sent, for example, a qubit with its Bloch vector pointing in the required direction. A simple two-node protocol is thus to send many copies of the same qubit such that the receiver can approximate the direction with a certain level of accuracy.Here, our goal is to allow > m 2 number of nodes in a quantum network to agree on a common spatial reference frame, where in this first work we assume a fully connected network graph. That is, every node is connected to every other node using both classical and quantum communication channels. Why is this problem any more difficult than solving the problem for two nodes? Note that in an ideal case, where all the nodes are perfect and the channels connecting them are error-free, one node can send a reference frame to everyone else, and everyone can subsequently use th...