Grenoble university space centre

NanoBob: Quantum secure communication using photon entanglement between a ground station and a CubeSat

The Grenoble University Space Center (CSUG) is leading the effort to develop, in collaboration with the Institute for Quantum Optics and Quantum Information in Vienna (AT) and a dozen other partners, including notably the Institute of Physics of the University of Nice (INPHYNI) and Thales Alenia Space France (TAS-F), a nanosatellite that aims at demonstrating quantum secure communication using a CubeSat terminal.

The security of current cryptographic techniques relies on numerical complexity. In this respect the eminent arrival of the quantum computer presents a serious threat as will be able to crack such problems many orders of magnitude faster than current binary computers (the first quantum code was written in 1994). Using a single key of the same length as the message is the only provably secure way to circumvent this issue, but the distribution of the key between two parties (commonly referred to as Alice and Bob) remains vulnerable to attacks (by a party known as Eve). The latter concern is addressed by quantum key distribution (QKD). The most advanced protocols use single photons that are generated in highly correlated, i.e., entangled, pairs with the information coded into the polarization state of the photons. The laws of quantum mechanics dictate that the properties of an individual photon cannot be measured without impacting its state, making the exchange in theory immune to eavesdropping by Eve: a quantum mechanical statistical test (the Bell test) can be used to certify the quantum nature of the link and therewith the security of the communication channel. Such systems have been implemented in fiber and are nowadays commercially available. However, on Earth, fiber and free-space transmission are both limited to ~100 km, limited by optical losses and Earth’s curvature, unless trusted relay stations are used. Going to space enables increasing this distance on a global scale with just a single trusted satellite relay.

The initial proposal put forward by the NanoBob consortium placed the entangled photon source in an optical ground station. One of the photons of a pair is send to a local detection unit (Alice), while the other photon is send to the satellite. An important advantage of this approach is that the space segment is relatively simple as its payload consists of Bob’s detection unit only. Consequently, it can be implemented in a nanosatellite, NanoBob, that adheres to the CubeSat standard. In addition, the configuration is compatible with a variety of quantum communication protocols, but also with experiments in fundamental physics, such as the search for decoherence of entanglement by gravitation (i.e., the interrelation between quantum mechanics and general relativity). The results of a mission definition and feasibility study have appeared in the literature [Ker2018].

CSUG is currently engaged in a more ambitious project, financed by the French Space Agency CNES and with TAS-F as the leading partner, that investigates the requirements and specification of a future Quantum Information Network that includes one or more Space links [Par2022]. NanoBob or its successor could re-appear in this context as a convenient in-orbit demonstrator.