If you love science and are near London, the Royal Society runs a wonderful series of public events that are free of charge. This week, I had the pleasure of attending the Royal Society Milner Prize Lecture, which was given by the quantum cryptography pioneer Artur Ekert. The prize is described as “the premier European award for outstanding achievement in computer science” and his lecture was called “Privacy for the paranoid ones: the ultimate limits of secrecy“. I travelled up from Bristol to see the lecture and I enjoyed it very much.
Ekert has academic appointments at the University of Oxford, the National University of Singapore and the Okinawa Institute of Technology. He bagged this year’s prize, “For his pioneering contributions to quantum communication and computation, which transformed the field of quantum information science from a niche academic activity into a vibrant interdisciplinary field of industrial relevance”.
Ekert is perhaps most famous for his invention in 1991 of entanglement-based quantum cryptography. However, his lecture kicked-off several millennia earlier with an example of a permutation cypher called a scytale. Used by the ancient Greeks, the cypher conceals a message in a series of letters written on a strip of paper. When the paper is wound around a cylinder of the correct radius, the message appears – so not that difficult to decipher if you have a set of cylinders of different radii.
Several hundred years later things had improved somewhat, with the Romans using substitution cyphers whereby letters are substituted for each other according to a secret key that is shared by sender and receiver. The problem with this, explained Ekert, is that if the same key is used to encrypt multiple messages, patterns will emerge in the secret messages. For example, “e” is the most common letter in English, and if it is substituted by “p”, then that letter will be the most common letter in the encrypted messages.
Maths and codebreaking
Ekert said that this statistical codebreaking technique was developed in the 9th century by the Arab polymath Al-Kindi. This appears to be the start of the centuries-long relationship between mathematicians and code makers and breakers that thrives today at places like the UK’s Government Communications Headquarters (GCHQ).
Substitution cyphers can be improved by constantly changing the key, but then the problem becomes how to distribute keys in a secure way – and that’s where quantum physics comes in. While classical key distribution protocols like RSA are very difficult to crack, quantum protocols can be proven to be unbreakable – assuming that they are implemented properly.
Ekert’s entanglement-based protocol is called E91, and he explained how it has its roots in the Einstein–Podolsky–Rosen (EPR) paradox. This is a thought experiment that was devised in 1935 by Albert Einstein and colleagues to show that quantum mechanics was “incomplete” in how it described reality. They argued that classical physics with extra “hidden variables” could explain correlations that arise when measurements are made on two particles that are in what we now call a quantum-entangled state.
Ekert then fast-forwarded nearly three decades to 1964, when the Northern Irish physicist John Bell came up with a mathematical framework to test whether an entangled quantum state can indeed be described using classical physics and hidden variables. Starting in the 1970s, physicists did a series of experiments called Bell tests that have established that correlations observed in quantum systems cannot be explained by classical physics and hidden variables. This work led to John Clauser, Alain Aspect and Anton Zeilinger sharing the 2022 Nobel Prize for Physics.
Test for eavesdropping
In 1991, Ekert realised that a Bell test could be used to reveal whether a secret communication using entangled photons had been intercepted by an eavesdropper. The idea is that the eavesdropper’s act of measurement would destroy entanglement and leave the photon pairs with classical, rather than quantum, correlations.
That year, Ekert along with John Rarity and Paul Tapster demonstrated E91 at the UK’s Defence Research Agency in Malvern. In the intervening decades E91 and other quantum key distribution (QKD) protocols have been implemented in a number of different scenarios – including satellite communications – and some QKD protocols are commercially available.
However, Ekert points out that quantum solutions are not available for all cryptographic applications – they tend to work best for the exchange of messages, rather than the password protection of documents, for example. He also said that developers and users must ensure that QKD protocols are implemented properly using equipment that works as expected. Indeed, Ekert points out that the current interest in identifying and closing “Bell loopholes” is related to QKD. Loopholes are situations where classical phenomena could inadvertently affect a Bell test, making a classical system appear quantum.
So, there is much more work for Ekert and his colleagues to do in quantum cryptography. And if the enthusiasm of his talk is any indication, Ekert is up for the challenge.
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