John Clarke, an emeritus professor of physics at the University of California, Berkeley, was awarded the 2025 Nobel Prize in Physics for his work on quantum tunneling, one of many strange aspects of quantum mechanics.
Clarke shared the prize with two other physicists, Michel H. Devoret and John M. Martinis, who at the time of their prize-winning research were at UC Berkeley. Devoret is now at Yale University and UC Santa Barbara, while Martinis is at UC Santa Barbara.
The Nobel Prize committee honored the three “for the discovery of macroscopic quantum mechanical tunneling and energy quantization in an electric circuit.”
Quantum tunneling is the ability of particles, such as electrons, to move or tunnel through barriers that, according to classical physics, they should not be able to breach. This had been demonstrated many times on the microscopic scale, and is the basis of transistors. In the early 1980s, the three Nobel laureates demonstrated this effect in a larger system — a simple electrical circuit incorporating a superconductor, which allows current to flow without resistance.
“This work laid the foundation for exploring macroscopic quantum physics in superconducting circuits,” the Nobel committee wrote in its summary of the research.
In a phone call with the Nobel committee patched into the press conference in Stockholm, Clarke said that the award took him by surprise.
“To put it mildly, it was the surprise of my life,” he said. That his work was Nobel Prize worthy “had not occurred to us in any way.”
He noted that this discovery would not have happened without the work of the other two Nobelists. “I was in principle the leader of the group, of course, but their contributions are just overwhelming.”
John Clarke standing in front of a magnified image of an amplifier based on a superconducting quantum interference device, or SQUID.
Clarke, who has been on the UC Berkeley faculty since 1969, led one of several groups in the 1980s that were trying to demonstrate macroscopic quantum tunneling (MQT) in superconducting circuits, based on predictions by Anthony Leggett. Working with Martinis, then a graduate student, and postdoctoral fellow Michel Devoret from the Centre d’Etudes Nucleaires de Saclay, France, Clarke devised an experiment that he thought would confirm the existence of MQT beyond a reasonable doubt.
They succeeded in 1984 and 1985, demonstrating that “a superconducting circuit … could be isolated well enough to observe both energy quantization of a macroscopic degree of freedom as well as macroscopic quantum tunneling out of a metastable state,” the Nobel committee wrote.
In analogy with the single-particle quantum tunneling — that of an alpha particle emerging from a heavy nucleus — the researchers described their system as a “macroscopic nucleus” and foresaw the possibility of building exotic “macroscopic nuclei with wires.”
Clarke is also known for his work on ultrasensitive detectors called SQUIDs, or superconducting quantum interference devices. He has used SQUIDs in many applications, including detection of NMR signals at ultralow frequencies, geophysics, the nondestructive evaluation of materials and as biosensors.
He is currently collaborating with the Axion Dark Matter Experiment, for which he developed a low-noise superconducting quantum amplifier based on SQUIDS. ADMX is searching for a possible dark matter candidate, the axion. The high-frequency, low-noise quantum SQUID amplifiers he invented for ADMX have since been employed in another area of physics, to read out the superconducting quantum bits, or qubits, for quantum computers.
Clarke is a Fellow of the Royal Society of London and an Honorary Fellow of Christ’s College, Cambridge. His honors include the University of California’s Distinguished Teaching Award, 1987 California Scientist of the Year, and numerous technology awards including the Fritz London Memorial Award for low temperature physics and the National Academy of Sciences Comstock Prize in physics.