Quantum communication on a CubeSat: The Q^3 Sat mission proposal
Distributing quantum states of light over long distances is of fundamental interest for experiments on the intersection of quantum mechanics and relativity, as well as for realizing a quantum internet and commercially usable quantum cryptography. However, optical losses in glass fibres or the atmosphere substantially degrade the quality of quantum states sent via ground-to-ground links. Optical links to satellites can overcome this problem, since they consist mainly of loss-free vacuum. Therefore, already a decade ago China has put high priority on sending a dedicated quantum mission to space. This has resulted in the launch of the Micius satellite in August 2016. It was able to send entangled photon pairs to two different ground stations simultaneously and violating a Bell’s inequality, thereby confirming the quantum nature of their correlation. These promising results show that the future of quantum communications is very likely to take place in space. The group of Rupert Ursin (IQOQI-Vienna) is now looking into possibilities to perform quantum experiments with mini-satellites, so-called “CubeSats”. Their name originates from the 10x10x10cm cubic units (U) they are made up of. The Ursin group believes that the equipment necessary to perform an uplink experiment can be fit into a 12U or even a 3U CubeSat. In such an uplink mission, only a quantum receiver is placed on board the satellite. The technically more complex source of photons stays on ground, which strongly relaxes the requirements for the satellite. While the Chinese quantum satellite houses several experiments and has a mass of more than 600kg, a CubeSat would only weigh 5kg in the 3U configuration or 20kg in 12U. Therefore the launch of such a mini-satellite is substantially less costly than that of a fully-fletched quantum satellite, since several CubeSats can be loaded onto one single space rocket at once and then be released in the correct orbit. Observing entanglement and quantum correlations between space and ground would be possible with these less complex mini-satellite at a fraction of the cost. A quantum CubeSat could even be used to test different quantum cryptography protocols without having to make adaptions to the satellite – all the changes necessary could be made to the easily accessible optical ground station. But what might be even more important is the invaluable experience collected in such an experiment. Such a mission could be the key for European researchers to keep up with the developments in the Chinese quantum community.
 R. Ursin, T. Jennewein, J. Kofler, J. M. Perdigues, L. Cacciapuoti, C. J. de Matos, M. Aspelmeyer, A. Valencia, T. Scheidl, A. Acin, C. Barbieri, G. Bianco, C. Brukner, J. Capmany, S. Cova, D. Giggenbach, W. Leeb, R. H. Hadfield, R. Laflamme, N. Lütkenhaus, G. Milburn, M. Peev, T. Ralph, J. Rarity, R. Renner, E. Samain, N. Solomos, W. Tittel, J. P. Torres, M. Toyoshima, A. Ortigosa-Blanch, V. Pruneri, P. Villoresi, I. Walmsley, G. Weihs, H. Weinfurter, M. Zukowski, and A. Zeilinger, “Space-quest, experiments with quantum entanglement in space | Europhysics News,” Europhysics News, vol. 26, no. 3, pp. 40–40 (3), 2009.
 F. Steinlechner, P. Trojek, M. Jofre, H. Weier, D. Perez, T. Jennewein, R. Ursin, J. Rarity, M. W. Mitchell, J. P. Torres, H. Weinfurter, and V. Pruneri, “A high-brightness source of polarization-entangled photons optimized for applications in free space,” Opt. Express, vol. 20, no. 9, pp. 9640–9649 (2012).
 T. Scheidl, E. Wille, and R. Ursin, “Quantum optics experiments using the International Space Station: a proposal,” New Journal of Physics, vol. 15, no. 4, p. 043008 (2013)
1. S. Neumann et al., Quantum Communication Uplink to a 3U CubeSat: Feasibility & Design, arXiv:1711.03409 [quant-ph] (https://arxiv.org/abs/1711.03409)
2. E. Kerstel et al.,Nanobob: A Cubesat Mission Concept For Quantum Communication Experiments In An Uplink Configuration, arXiv:1711.01886 [quant-ph] (https://arxiv.org/abs/1711.01886)
3. Oi DK, Ling A, Vallone G, Villoresi P, Greenland S, Kerr E, et al. CubeSat quantum communications mission. EPJ Quantum Technology. 4(1):6 (2017)