Networks, information and money in the quantum age

Reported by Samuel Ward, CSaP Policy Intern

For the Horn Fellows’ first discussion of 2021, Professor Adrian Kent of the University of Cambridge's Department of Applied Mathematics and Theoretical Physics explored quantum computing, communication and cryptography - emerging technologies based on quantum information . Through the way they manipulate and propagate information, these systems behave fundamentally differently to their classical counterparts, giving theoretical advantages that are now being exploited in practical technologies.

“If you try to cheat the bank by cloning then you will almost certainly fail.”

This is just one way, explained Professor Kent, that the addition of quantum principles may revolutionise some systems – for example currency [1,2]. Unlike today's money, which contains only classical information public to the bearer, such as holograms or unique serial numbers, quantum bank notes would also hold quantum information that is private to the bearer. This new information, possibly eventually stored in the form of a small memory chip on each note, may allow banks to exploit quantum theorems to create unforgeable currency. In this case, by literally translating the “no-cloning” theorem of quantum physics to a classical bank note. Professor Kent has developed more immediately practical versions of money, with security guaranteed by quantum theory but without the need to store quantum information in memory chips, which is beyond present technology. Together with colleagues in the UK Quantum Communications Hub, he is currently trialling implementations.

Applications of quantum principles may extend further than money, to arbitrary networks of information – a quantum internet. In such a network, signals would propagate in exotic ways through multiple paths simultaneously, allowing users to perform clever calculations with respect to time or to communicate with sophisticated encryption schemes. However, designing these systems may be no mean feat. A global quantum internet has to be designed to both allow and exploit new constraints imposed by the very physical principles that enable it, as Professor Kent demonstrated by showing how the “no-superluminal-signalling" and “no-cloning” theorems could introduce a “no-summoning” theorem in quantum networks.

Though money and the internet remain relatively untouched by quantum technologies today, discussions between the fellows highlighted where the field is measuring progress. Secure communication is now possible thousands of miles apart, and a global scale may soon follow. As for the possible teleportation of information across a quantum network, recent experiments at Calgary [3] were described by Professor Kent as having so far achieved high fidelity over more than 20 kilometres of optical fibre The gaze of commercial ventures may also be turning toward the quantum realm, for example as a way for generating truly random numbers - a technology which Professor Kent suggests could make its way into the mobile phone chips of the future.

What will it take to reach a quantum internet?

In terms of measuring overall progress against the capabilities of today's classical technology, Professor Kent cautions that it may be difficult to tell. Two independent groups of researchers have demonstrated so-called quantum supremacy, using quantum systems to solve tasks that are impossible with any existing classical computers. However, these specific tasks were invented in order to achieve the benchmark, rather than for their intrinsic importance. The research community, and the governments and investors who fund much of their work, are ultimately more focussed on practical landmarks and achievements that mark true steps forward, such as the factorisation of large numbers or the implementation of the quantum repeater. It is by adopting this viewpoint, of pre-measured, staged developments, that some parts of the community have outlined a forward-looking roadmap [4], illustrating stages in the future development of a a real, functional global quantum network.

[1] - Wiesner, Stephen. "Conjugate Coding". SIGACT News. 15 (1): 78–88 (1983)

[2] - Kent, Adrian. "Summonable Supermoney: virtual tokens for a relativistic economy." Phys. Rev. A 98, 062332 (2018).

[3] - Valivarthi, R., Davis, S., Pena, C. et al. ,“Teleportation systems towards a quantum internet” PRX Quantum 1, 020317 (2020)

[4] - Wehner, Stephanie, David Elkouss, and Ronald Hanson. "Quantum internet: A vision for the road ahead." Science 362.6412 (2018).