Probing the quantum realm

You might not know it, but we’re in the midst of a revolution; a quantum revolution. Powered by the physics of the subatomic world, it could drastically change how we store and send information. Its impact is already being felt in the metrology community – the new definition of the ampere¹, which came into effect in 2019, was enabled by our ability to count individual electrons.

The mutually-beneficial relationship between quantum technologies and measurement science has been around for decades, according to MSL Principal Research Scientist Dr Vladimir Bubanja. “Metrology played a role in the earliest days of the first quantum revolution, led by Max Planck and his contemporaries,”  he says. “As soon as people started to probe the atom, they relied on accurate measurement.”  Control of electrons and photons led to countless technological breakthroughs – LEDs, semiconductors, lasers and nuclear magnetic resonance imaging are all based on quantum physics. Later discoveries in metrology, such atomic clocks, led to the development of entirely new quantum technologies, including the now-ubiquitous satellite navigation systems that encircle our planet.

Single electrons

A hunger for more powerful computers has driven further developments in our understanding of the quantum realm, as Dr Bubanja explains, “The basic element in any computer chip is a transistor, which operates like a tap – current either flows or it doesn’t. As transistors continue to shrink in size, they need fewer electrons for switching operations. The ultimate goal, and the end of the road of classical digital electronics, is for a single electron to represent one bit of information.”

Bubanja has been working on single electron transport for more than two decades, in labs across the Netherlands, Japan and New Zealand. As a theoretician, one of the key tools at his disposal is the supercomputer owned by NeSI, New Zealand eScience Infrastructure. In 2019, collaborating with colleagues in India, he used it to run simulations on graphene² – the one-atom-thick ‘wonder material’ –to probe its fundamental behaviour.

Such two-dimensional materials can also offer metrologists a unique opportunity to observe quantum phenomena that support the new International System of Units (SI). “Now that the SI is based solely on fundamental constants of nature, quantum physics is central,”  says Bubanja. “Take the Kibble Balance – it allows mass to be defined in terms of the Planck constant instead of a physical artefact. It requires precise measurement of the voltage and resistance, which we can do by quantum means, via the Josephson effect and the quantum Hall effect, respectively.”

The effort to realise the new definition of the ampere is a global one, and for Dr Bubanja, single electron devices are leading the way. He recently published a new theory, based on a ‘single electron turnstile’³, that expands our understanding of the Josephson effect. Working with experimentalists in Japan and Finland, he has shown that there is a maximum Josephson current that can flow through this device. His results may not only have implications for metrology, but also for the future of quantum computers.

Qubit by qubit

This second quantum revolution is all about the bit. In today’s computer architecture, information is stored in bits, with each one able to take a value of either 0 or 1. But a qubit – the unit of quantum information – can be both a 1 and 0 simultaneously. This state, known as the superposition, has a dramatic impact on computation. Just 300 qubits could hold 2³⁰⁰ values at once, which is greater than the number of atoms in the universe⁴. And as Dr Bubanja explains, superposition, and related processes like entanglement and teleportation, could also enable “absolutely secure channels of communication.”

MSL’s expertise in this area recently attracted the attention of the Defence Technology Agency (DTA) – an arm of the New Zealand Defence Force. “Our study assessed the opportunities and risks posed by quantum communications and computation technologies,” says Bubanja. Dr Branislav Jovic from the DTA says, 'MSL offered valuable insight into a rapidly developing area of research and development. Vladimir highlighted significance of quantum technologies to the military by identifying threats and opportunities posed. His enthusiasm for the subject matter helped identify a number of future potential collaborative opportunities. We appreciate Vladimir’s expertise, and his in-depth knowledge of quantum systems.”

[1] https://measurement.govt.nz/metrology/si-units/ampere/

[2] https://www.nesi.org.nz/case-studies/exploring-remarkable-properties-graphene

[3] V. Bubanja, “Effect of quantum fluctuations on the critical supercurrent through a mesoscopic normal-metal island,” PHYSICAL REVIEW B 97, 224516 (2018)

[4] https://www.nobelprize.org/uploads/2018/06/popular-physicsprize2012.pdf