Quantum communication on a CubeSat: The Q³Sat mission proposal

Design study of the Q³Sat. Left: Exploded view. The first part (“unit”, U) consists of a Cassegrain mirror telescope to collect the photonic qubits. The red box shows the second U consisting of the optical analysis module: The photons pass a beam splitter which separates them according to their polarization, then they are collected by single-photon detectors. The third U is used for housekeeping: Classical communication, attitude control, GPS positioning, batteries, temperature control and detection electronics. Right: The assembled Q³Sat. When no experiment is conducted, the deployable solar panels are oriented towards the sun to charge the batteries. Four UHF-band antennas (back) and one S-band antenna (front) are used for classical communication with the ground.

Quantum Communication (QC) allows for unconditionally secure exchange of messages. Establishing an optical QC link to a satellite is the most promising way to bridge large distances. This has recently been shown by the Chinese quantum satellite Micius in the course of QUESS, a 100-million-dollar project with involvement of the Austrian Academy of Sciences. Missions of this order of magnitude are however prohibitively expensive for most institutions or companies. Physicists of Rupert Ursin’s group at IQOQI therefore suggest to use a so-called CubeSat with dimensions of only 10×10×34 cm³ for similar quantum experiments.

 

“CubeSat” is a size and mass standard for picosatellites introduced by California Polytechnic State University and Stanford University in 1999. In this standard, one unit (1U) is a 10 cm cube (excluding protrusions) with a maximum mass of 1.33 kg. Many CubeSats can be stacked in a carrier rocket at once and launched in bulk, which greatly decreases the mission costs (launches for a 3U CubeSat are available from 300.000 Euro). The picture shows a design developed by IQOQI’s Ursin group in collaboration with Carsten Scharlemann’s group at FH Wiener Neustadt and Erik Kerstel’s group at the University of Grenoble. This Q³Sat (pronounced Q-Cube-Sat) is capable of receiving photon qubits from an optical ground station (OGS), analysing them with single-photon detectors, and creating a key with the OGS.

Until recently, the main problem hindering QC with a CubeSat was the pointing precision of the small satellites: Their orientation could not be made stable enough for the CubeSat to “see” the ground station interruption-free. The latest advancements in attitude control via spinning wheels however make it possible to align a CubeSat to a precision of 40 microradian (equivalent to pointing at a sugar cube from a distance of four kilometers). This allows to make the field of view of the satellite narrow enough to only see the OGS and collect little to none noise counts.

Another fundamental difficulty when designing a CubeSat is the limited size available for the payload. In the Q³Sat design, this issue was mitigated by going for an uplink scenario, where the photonic qubits are sent from ground to satellite and not the other way round. This has the disadvantage of higher loss, because atmospheric disturbances act close to the sender, deflecting the photons already at the start of their journey rather than at their end; however, the huge advantage is that the satellite design simplifies drastically by equipping it with a simple receiver instead of a highly complex sender.

 

The IQOQI physicists have conducted an extensive study of tolerable losses (a so-called “link budget”), including different weather conditions and simulating the effect of atmospheric turbulences during each satellite pass. They concluded that even with losses increased by the uplink, one kilobit of secret key would amount 60 Euro if the satellite worked for one year only. The Q³Sat design poses an important step towards commercial applications and could, with building and launch costs of only about 500.000 Euro, even be realized by private entities.

References

1. S. Neumann et al.,  Q³Sat: Quantum Communications Uplink to a 3U CubeSat – Feasibility & Design. EPJ Quantum Technology accepted 2018, 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)