This is a guest post by Daniel Oi
Quantum computing threatens the security of public key cryptosystems that secure the internet. But what quantum takes away, it can also give back. The technique of quantum key distribution (QKD) promises codes that are guaranteed by physics to be, in principle, unbreakable.
In EPJ Quantum Technology, we propose a CubeSat Quantum Communications Mission (CQuCoM) with a vision towards a globe-spanning constellation of QKD satellites. We are an international consortium of six research entities and one company across six countries*.
Companies already sell QKD systems that secure point-to-point links over optical fibers. However, intrinsic losses within fiber limits the practical effective range to a hundred kilometers. QKD relies on photons being sent and received one by one: individual photons cannot be amplified without affecting their delicate quantum state.
An attractive possibility for long distance quantum communication is to send quantum signals through free-space. A pioneering experiment in the Canary Islands transmitted quantum entanglement between two volcanic mountain tops 144km apart. But the Earth’s curvature and atmospheric scattering prevents longer distances being achieved on the ground.
For global QKD, then, the answer is space. A photon travelling up or down through the atmosphere experiences the equivalent of only a few kilometers of ground-level air. A satellite can act as a node exchanging optical quantum signals with ground stations separated by thousands of kilometers.
Experimental quantum physics and satellite engineering are challenging on their own, but researchers are not deterred from trying to combine them. Quantum satellite development is underway in several countries.
With CQuCoM, we propose to take advantage of the NewSpace movement that exploits miniaturization and use of conventional off-the-shelf components, agile development, short iteration cycles, and cheap access to space, to make swift progress.
In August 2016, China launched Micius, a 631kg quantum satellite intended to demonstrate QKD between two ground stations (no results have been released publicly yet). A large satellite like this is expensive to develop and launch.
CQuCoM is based on CubeSats, a type of small satellite with a mass of a few kilograms. Our mission would use a 10kg 6U CubeSat (10cm x 20cm x 30cm) to demonstrate quantum communication between space and Earth.
The main challenge is aiming single photons from a satellite travelling at over 7.6km/s and 400km above the Earth at a telescope on the ground 1000km away. The required pointing and tracking accuracy has not previously been available on small satellites. Another challenge is making the quantum source of photons rugged enough for launch and operation in space.
We have proposed two launches to test and refine the satellite, enabled by the comparatively low cost of CubeSats. The first satellite would carry a weak coherent pulse (WCP) source to confirm the ability to send single photons from space to ground. A second launch would instead use a generator of entangled photon pairs: one photon of each pair would be measured on the satellite, the other being sent to the ground. Such entanglement distribution is useful for QKD and for fundamental tests of quantum physics.
Success for CQuCoM would dramatically lower the development risk for large-scale space quantum communication systems.
*The CQuCoM consortium consists of the University of Strathclyde and the Scottish Centre for Excellence in Satellite Applications, Clyde Space Ltd, IQOQI Austrian Academy of Sciences, TU Delft, Ludwig-Maximilians University, University of Padua, and the Centre for Quantum Technologies at the National University of Singapore.
Daniel Oi is a lecturer at the Department of Physics, University of Strathclyde, Glasgow, where he works on a wide range of topics in quantum information spanning fundamental aspects of quantum mechanics, quantum information theory, and implementations of quantum technology. After a doctorate at the Centre for Quantum Computation, University of Oxford, he was a researcher at DAMTP, University of Cambridge, and a Fellow of Sidney Sussex College. His interests include the structure of quantum theory, implementations of quantum information processing, and the development of quantum technologies for space applications and fundamental tests of physics.