Satellite Applications

My post on quantum random number generation in satellites a few weeks ago garnered a few comments (though some were asking for a way around a paywall) so this post will be on satellite applications.

That post was about satellites with QRNG capability, generating QRNG and broadcasting them for use in terrestrial applications, a space-bound resource available for public use, a bit like GPS I suppose. The real beauty and security in the approach is that the QRNG generation cannot be interfered with remotely and it is extremely difficult to physically interfere with a satellite! 

Most satellite applications in quantum-related cybersecurity are to do with Quantum Key Distribution (QKD). QKD uses quantum phenomena to establish a cryptographic key exchange, the neat part being that eavesdropping is inherently detectable by the properties of quantum mechanics.

There is a variety of protocols but they all involve generating a pair of entangled states, usually photons, and sharing them between both parties who measure them to infer a common cryptographic key. There are variations in how this can be done, such as whether the pair are generated by one party who transmits half the pair to the other, or generated at a third location who then sends one part of the entangled state to each of them. Whichever approach is used an eavesdropper cannot avoid irreparably disrupting the state, nor can they replicate it. This means that when Alice and Bob check the security of their key exchange, typically by sending a pre-arranged message, they will find that their keys do not match. 

This is easiest to do though optical fibres but without the use of trusted relays they are currently limited to a range of about 120 km. An alternative way to signal over greater distances is to send signals via satellite. The satellites then exchange photon states with specific ground stations to run a QKD protocol. I shall be focussing here on work by QEYSSat and by QTLabs.

If the entangled states, usually pairs of photons generated by shining a laser through non-linear materials, are generated at the ground station then one of the photons from each entangled pair is transmitted to the satellite to establish a key between the satellite and that station. Another key is then established in the same way at a different station. All that remains now is for each station to be given the other’s key. 

The satellite achieves this by combining the two keys, written in binary, with an XOR (exclusive-or) operation. Suppose we have two keys, 1101001 and 1001100. Combining these with an XOR operation, equivalent to addition modulo 2, gives us 1101001 ⊕ 1001100 = 0100101. Anyone receiving this number and knowing one of the keys can perform another XOR to find the other key (go on - try it!), but it is useless otherwise. Sharing the combination with the two ground stations, which may be done via insecure means, allows them to communicate securely since they now know each other’s key.

Technical obstacles aside, this is très élégant except that the satellite is a trusted exchange. 

QTLabs have developed a trustless alternative where the entangled pair are generated on board the satellite and shared between the two ground stations. QTLabs claim the only successful such downlink in the EU and have also achieved horizontal (no satellite) free space links up to 20km.

Quantum applications in communication and security are no longer laboratory curiosities, they are real world commercial developments.