Quantum airborne comms makes breakthrough

The Do228 aircraft equipped with the ight terminal. One can see the optical dome underneath the fuselage (marker). a shows a closeup of this dome housing the coarse pointing assembly. b shows a schematic section view of the ight terminal. Credit: Sebastian Nauerth et al.

The first air to ground quantum key distribution exchange (QKD) was achieved on September 18 by German physicists using an aircraft in flight and a ground station.

The team which presented its results at the recent QCrypt convention said it could pave the way to developing a future global quantum-based secure communications network.

Led by Sebastian Nauerth at the Ludwig Maximilian University of Munich, the researchers achieved a stable connection over 20km for ten minutes, and in that time achieved a key rate of 145 bits/s.

While far too slow for a data channel, this only refers to the rate at which the keys are transmitted. However, if the system were to be made secure against eavesdropping, the authors note, the key exchange rate would fall to 5 bits/second.

The experiment was conducted just after sunset at Munich’s Oberpfaenhofen airport to avoid errors that could be introduced by sunlight. The researchers also had to create a mechanism of moving mirrors to compensate for the movement of the aircraft. The qubits were encoded on the polarisation of the beam transmitted from the aircraft (Alice) to the ground (Bob).

The kit the researchers used included a free-space laser terminal developed by the German Aerospace Centre (DLR), modified to implement the QKD transmitter communicating with a ground-based receiver on a DLR building.

Phys.org reports that scientists have over the years developed some very powerful encryption schemes that are almost impossible to crack, yet virtually all of them rely on one weak point, the key that is used to unlock them.

“For this reason, cryptologists have been pinning their hopes on QKD. This is because of the nature of quantum bits; looking at or measuring them causes them to be changed, which would alert the true owners of the key that a breach has occurred. QKD is where encryption keys are generated based on the polarization of photons to represent 1s and 0s,” the website reports.

The photons in such a system are converted to cubits on the receiving end where they can be read and interpreted.

“While the idea has been around for quite some time, it’s only recently that workable devices have actually been built. Prior to this new development, another team had achieved a QKD exchange between two stationary buildings,” states the report.

To make such a system work, the photons must be sent from one station and received by another. In this case, they were sent from a moving aircraft using the laser and mirrors to a ground station that also used mirrors.

The researchers were able to hold on to the connection for ten minutes; long enough to transmit 10 kilobytes of data (at 145 cubits per second) with an error rate of just 4.8 per cent.

“That’s enough to send a key, and that’s all that’s really needed to build quantum based network,” states phys.org. “Based on these results alone, it’s clear that a truly secure network could be put in place, in a battle zone for instance, with keys being transmitted from drones hovering overhead. But of course, that won’t be enough, the real goal will be to see if the same feat can be achieved between satellites and ground stations giving us a world where all communications can be made almost perfectly secure.”

Read More: ESA breaks world quantum teleportation record


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