The authors of the reviewed paper are Eutelsat, one of the world's leading satellite operators, and NTO, a Dutch deep-tech organization, and the paper was published in the International Conference on Space Optics in 2021.
Both companies are working together here, to research quantum cryptographic key distribution form a geostationary Earth orbit. Quantum Key Distribution, or QKD for short, means that the encryption keys for encrypted data access are not distributed via conventional methods, such as hardcopies or vulnerable transmissions over the web, but rather using the quantum characteristics of photons.
There are several different protocols for QKD that have been developed. One such protocol, BB84, follows these very simplified steps. Firstly a bit (0 or 1) is created. This bit is then quantum encoded into a photon using one of two encoding basis. It can then be transmitted. In order to read this encoded bit value from the photon, it has to be decoded also using the correct basis, because according to quantum indeterminancy, if the wrong basis is used, then the decoded bit value is random and the photon is fixed in the wrong state. Thus the receiver randomly chooses a basis to decode all the received photons for bit values. Once the photons have been decoded, half the decoded bit values will be correct on average, and once the sender communicates what basis the bits were encoded in and the receiver what basis they were decoded in, then the correct bits can be used as a shared key.
The increased safety in this method comes from the fact that if there is a third party that tampers with the photon in transit, then if the wrong encoding basis is used for decoding, the photon will be locked in the wrong state, adding extra error on top of the expected ~50% on the receiving side. This additional error can thus be used as a flag to detect tampering, and a new key can be generated by the sender.
When these keys are distributed via satellite, there exist two main configurations: trusted sender (satellite) and untrusted sender (satellite). If the sender is trusted, then the satellite records the key, and can easily transmit it to the receiving end. This however leaves the satellite as a point of vulnerability, so untrusted mode is used more often, where the satellite merely relays the intact photon to the receiver and receives no information about the key itself. This untrusted mode however, is more complicated, because transferring the photon with an intact quantum state is difficult. Thus trusted mode is faster for key creation but untrusted mode is safer.
This paper also specifically explores QKD from GEO. QKD from LEO has already been researched and shown to work. GEO however provides several advantages from LEO, mainly higher link availability and low satellite tracking requirements for senders and receivers. However GEO QKD loses out in transmission rate and has more transmission issues due to the large distances involved.
The specific goal of this paper was to theorize a system that has at least 1 bit/s data rate in untrusted mode on a GEO. Key characteristics impacting bitrate were cloud coverage, source rate - which was taken as 10 GHz - and loss rate - taken as max 41 dB. These parameters were plugged into a numerical model considering link time based on cloud coverage of the Amsterdam - Budapest link, and it was confirmed that the goal bitrate was possible to be achieved in that specific case.
An example photon beam source/receiver that could accomplish this bitrate was also theorized and design plans were drawn up.
Overall this paper proved the numerical and technical feasibility of a GEO QK distributor. They showed clear requirements for ground stations and found the theoretical bitrates for trusted and untrusted modes, 300 bit/s and 1 bit/s, while creating a design proposal for a beam source capable of reaching these bitrates.
QKD: https://qt.eu/discover-quantum/underlying-principles/quantum-key-distribution-qkd/
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