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QUESS and Quantum Communications

In mid August China launched “QUESS” (Quantum Experiments at Space Scale), a new type of satellite that it hopes will be capable of “quantum communications” which is supposed to be hack-proof, through the use of “quantum entanglement”. This allows the operator to ensure that no one else is listening to your communications by reliably distributing keys that are then used for encryption in order to be absolutely sure that there is no one in the middle intercepting that information.

According the Chinese scientists involved in the project, quantum encryption is secure against any kind of computing power because information encoded in a quantum particle is destroyed as soon as it is measured. (According to Tibor Molnar a scientist at the University of Sydney), the only way to ‘observe’ a photon is to have it interact with (a) an electron, or (b) an electromagnetic field. Either of these interactions will cause the photon to “decohere” – i.e., interfere with it in a way that will be apparent to the intended recipient.

Gregoir Ribordy, co-founder of Geneva-based quantum cryptography firm ID Quantique, likened it to sending a message written on a soap bubble. “If someone tries to intercept it when it’s being transmitted, by touching it, they make it burst.”

Quantum physicists have recently advanced the use of photons to communicate securely over short distances – 50-150 km – on earth. The satellite, if successful, would vastly expand the range of unhackable communication.

To test whether quantum communications can take place at a global scale, the Chinese team will attempt to beam a quantum cryptographic key through space from Beijing to Vienna.

This topic was also discussed by a group of my international colleagues (USA, UK, Netherlands) and this is a summary of that discussion.

Two of them assisted in explaining what this is all about, one worked on the first quantum key distribution network and one of the world’s best quantum computing teams is situated close to where he works.

The two explained the differences between the various quantum technologies.

  • Quantum communications – sending information encoded in single photons (or equivalent) such that one can determine eavesdropping. Most useful for key exchange though has other uses.  Sometimes called quantum key distribution networks.
  • Quantum cryptography – work to devise cryptographic algorithms that are not affected by the creation of quantum computers. (Generally “quantum cryptography” has tended to mean what is now called – in the context of the Chinese satellite – “quantum communications”). Post-quantum cryptography is the search for algorithms not rendered useless by quantum computation.
  • Quantum computing – a computer that harnesses quantum physics such that certain types of computation can be done more efficiently. There are still some doubts as to whether this is feasible.  (Less so then before, but some say that it might be like nuclear fusion, not forbidden by physical laws, but hard to implement.) And, like fusion, if it could be made practical, certain types of cryptosystems (in particular, the RSA cryptosystem, but also the elliptic curve systems that have become widespread) would have to be abandoned. RSA encryption relies on the practical difficulty of factorising very large numbers, a task which is imagined to be very much easier (or at least faster) with quantum computers. But we do have substitute classical crypto systems that could be used that, as far as we know, are hard to break.

A few other colleagues discussed the concept of “quantum entanglement”. As he explained intuitively you’d think this would work and provide a means of faster-than-light communication. However, it turns out that though the two particles are quantum entangled, you can’t actually convey any information between the two measurement points. Tibor added to this that even Quantum Key Distribution requires two-channel communication: one of “entangled photons” (which may be described as super-luminal), and another classical channel (which is definitely sub-luminal) advising which measurements of those photons are significant.

To take a example, if you measure two quantum entangled photons and find the first photon is “spin up”, the second photon will always be “spin down” and vice-versa. Some clever statistics – the so-called “Bell Inequality” and its further elaboration, the “CHSH Inequality” –  tells you they weren’t in this state to start with, it’s only the act of measuring that forces the first photon into this state, then instantly the second photon will be in the opposite state. Or so it seems: there are other interpretations, e.g., Quantum Bayesianism, but the effect is the same. I won’t go into details here, it’s a fairly long and difficult to get your head around the explanation as to how we know they weren’t in a particular state to start with. The mathematics (and in this example, intuition) also tell you that no information is conveyed from one location to the other by the measurement alone.

The discussion also addressed the implications of this development. One of the experts commented: “This has zero practical significance”.  Classical crypto is occasionally attacked, but the progress against the basic mathematical algorithms is seldom dramatic.  Tibor added that it will become much more significant/dramatic when/if quantum computing becomes a reality, for then the most commonly used ‘classical’ cryptography techniques will no longer be secure. Practically all of the zillions of attacks that we hear about are at higher levels, implementation, protocols, … and, of course, human users (phishing, whaling). So the question could indeed be: why struggle to intercept/decrypt a message when you can just read the Post-It Note stuck on the sender’s screen?

Paul Budde (standing on the shoulders of giants)