Physics Nobel 2025: How winners revealed quantum physics in action

This year’s Nobel Prize in physics has gone to three scientists who showed that it was possible even for large systems, made up of billions of these small particles, to exhibit quantum behaviour under carefully controlled conditions. John Clarke, Michel Devoret and John Martinis have been awarded the 2025 Nobel Prize in Physics “for the discovery of macroscopic quantum mechanical tunnelling and energy quantisation in an electric circuit”.

Their work, done in the mid-1980s, set the stage for the development of quantum computers, which is one of the most active areas of scientific research right now.

Tunnelling

John Clarke, Michel Devoret and John Martinis were working together at the University of California, Berkeley, when they performed a series of experiments in 1984 and 1985 on special electric circuits. Martinis was a PhD candidate, Devoret was a postdoctoral fellow, and Clarke was their professor and supervisor when they did this work.

For their experiments, they used a set-up very similar to one that had already won a Nobel Prize in Physics for Brian Josephson in 1973. It involved an electric circuit containing two superconductors, materials which, under special conditions, allow free flow of electricity without any resistance. The two superconductors were separated by a thin insulator, a material that does not allow the flow of any electricity.

Under normal circumstances, this kind of circuit would not allow the flow of current because the insulator acts as a barrier. But the use of superconductors (instead of normal conducting materials) does allow the flow of current — a discovery that won Josephson the Nobel Prize, and gave the superconductor-insulator-superconductor set-up the name Josephson junction.

“Superconductivity itself, in a sense, is a quantum behaviour. All the electrons in the material move together in a coordinated way as if part of a single quantum wave. In the kind of electric set-up that Josephson had created, the electric current exhibits a tunnelling effect, which is not unexpected for a quantum particle,” explained Rajamani Vijayraghavan, an associate professor in the Department of Condensed Matter Physics and Material Sciences at the Tata Institute of Fundamental Research. Vijayraghavan obtained his PhD under the guidance of Devoret and has collaborated with the other two winners as well.

Josephson’s work was revealing and gave rise to a whole range of new applications, for example in precise measurements of fundamental physical constants and magnetic fields.

Prediction and confirmation

Clarke, a professor at the University of California, Berkeley, had been exploring some of these phenomena with the Josephson junction in the 1980s, when he was joined by Devoret and Martinis. Influenced by the predictions of another Nobel Prize-winning physicist, Tony Leggett, who had suggested that quantum behaviour on a macroscopic level could be observed in a Josephson junction kind of set-up, the trio began working to validate this experimentally.

They used a similar set-up as that of Josephson, but had to take meticulous care to isolate the entire set-up from its environment to avoid interference. The slightest interference could destroy the quantum effects.

What they were eventually able to demonstrate is that the entire circuit, not just the flowing electrons in the Josephson set-up, exhibited quantum behaviour.

The circuit could exist only in certain discrete energy states, each corresponding to a particular value for current, not in any in-between energy states, which is a distinctly quantum behaviour. Also, the circuit could jump between these discrete energy states across the insulating barrier, demonstrating macroscopic quantum tunnelling.

“This was the first time quantum behaviour was demonstrated on a macroscopic scale. It was very big. However, it did not cause much sensation at that time. But with time, the importance of their result was recognised, and its implications began to become clear,” said Vijayaraghavan.

“From a fundamental physics perspective, it provided the first clear answer to the question of how large a system can be while still exhibiting quantum behaviour. That question is still not resolved, but at least this experiment showed that systems as large as this set-up could show quantum behaviour. From an impact perspective, I would say this really set the stage for building quantum bits, or qubits, that are units of information in a quantum computer. At the time this work was done, quantum computers were still some distance away. But today, superconducting circuits are one of the most popular platforms for creating qubits,” he said.