Nobel physics prize awarded for pioneering experiments that paved the way for quantum computers

The Conversation UK reports that the 2025 Nobel Prize in Physics has been awarded to John Clarke, Michel Devoret, and John Martinis for experiments that demonstrated macroscopic quantum tunnelling in superconducting circuits. Their work on Josephson junctions shows a quantum effect in the world we can see, with wide-ranging implications for memory technologies, qubits, and superconducting devices used in medicine and industry. The prize coincides with the 100th anniversary of quantum mechanics and underscores how quantum principles are now shaping practical technologies from sensors to future quantum computers. — The Conversation UK

Original article published in The Conversation:

The 2025 Nobel prize in Physics has been awarded to three scientists for the discovery of an effect that has applications in medical devices and quantum computing.

John Clarke, Michel Devoret and John Martinis conducted a series of experiments around 40 years ago which would go on to shape our understanding of the strange properties of the quantum world. It’s a timely award, since 2025 is the 100th anniversary of the formulation of quantum mechanics.

In the microscopic world, a particle can sometimes pass through a barrier and appear on the other side. This phenomenon is called quantum tunnelling. The laureates’ experiments demonstrated tunnelling in the macroscopic world – in other words, the world that’s visible to the naked eye. They showed that it could be observed on an experimental electrical circuit.

Quantum tunnelling has potential future applications in improving memory for mobile phones and has been important for the development of “qubits”, which store and process information in quantum computers. It also has applications in superconducting devices, those that conduct electricity with very little resistance.

British-born John Clarke is Professor of Physics at the University of California, Berkeley. Michel Devoret was born in Paris and is the F. W. Beinecke Professor of Applied Physics at Yale University. John Martinis is Professor of Physics at the University of California, Santa Barbara.

What is quantum tunnelling?

Quantum tunnelling is a counter-intuitive phenomenon where the tiny particles which make up everything we can see and touch can appear on the other side of a solid barrier, which you would otherwise expect to stop them.

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Since it was first proposed in 1927, it has been observed for very small particles and it is responsible for our explanation of the radioactive decay of large atoms into smaller atoms and something else called an alpha particle. However, it was also predicted that we might be able to see this same behaviour for larger things. We call this macroscopic quantum tunnelling.

How can we see quantum tunnelling?

The key to observing this macroscopic tunnelling is something called a Josephson junction, which is essentially a fancy broken wire. The wire is not a typical wire which you might use to charge your phone, instead it is a special type of material known as a superconductor. A superconductor has no resistance, which means that a current can flow through it forever without losing any energy. They are used, for example, to create the very strong magnetic fields in magnetic resonance imaging (MRI) scanners.

Extremely weak magnetic fields emitted from the brain can be used to locate the sources of epileptic seizures.

So how does this help us to explain this strange quantum tunnelling behaviour? If we put two superconducting wires end to end, separated by an insulator, we create our Josephson junction. This is normally manufactured in a single device which, with a basic understanding of electricity, shouldn’t conduct electricity. However, thanks to quantum tunnelling we can see that current can flow across the junction.

The three prize winners demonstrated quantum tunnelling in a paper published in 1985 (it’s common to have such large gaps in time before Nobel prizes are awarded). Quantum tunnelling had previously been suggested to be caused by a breakdown in the insulator. The researchers started by cooling their experimental apparatus to within a fraction of a degree of absolute zero, the coldest temperature which can be achieved.

Heat can give the electrons in conductors just enough energy to get through the barrier. So it would make sense that the more the device is cooled, the fewer electrons would escape. If however quantum tunnelling is taking place, there should be a temperature below which the number of electrons which escape should no longer decrease. The three prize winners found exactly this.

Why is this important?

At the time, the three scientists were trying to prove this developing theory about macroscopic quantum tunnelling through experiments. Even during the announcement of the 2025 prize, Clarke downplayed the importance of this discovery, even though it has been pivotal in so many developments which are at the forefront of quantum physics today.

Quantum computing remains one of the most exciting opportunities which is promised for the near future, and is the source of significant investment worldwide. It comes with much speculation about the risks to our encryption technologies.

It will also ultimately solve problems which are outside the reach of even the largest of today’s supercomputers. The handful of quantum computers which are in existence today, rely on the work of the three 2025 physics Nobel laureates and no doubt will be the subject of another physics Nobel prize in the coming decades.

We are already exploiting these effects in other devices such as superconducting quantum interference devices (SQuIDs) which are used to measure small variations in magnetic fields from the Earth, allowing us to find minerals below the surface. SQuIDs also have uses in medicine since they can detect the extremely weak magnetic fields emitted from the brain. This technique, known as magnetoencephalography, or MEG, can be used for example to find the specific area of the brain from which epileptic seizures emanate.

We can’t predict if and when we will have quantum computers in our homes, or indeed in our hands. One thing that is for certain, though, is that the speed of development of this new technology is thanks in no small part to the winners of the 2025 Nobel prize in physics, demonstrating macroscopic quantum mechanical tunnelling in electric circuits.

Author

Rob Morris
Professor of Physics, School of Science and Technology, Nottingham Trent University