Nobel Prize Celebrates Superconducting Circuits Bringing Quantum Quirks to Macroscopic World

On Oct. 7, the 2025 Nobel Prize in physics was awarded to John Clarke, Michel Devoret and John Martinis for their foundational work demonstrating that crucial quirks of quantum physics could stretch across the comparatively colossal scale of electrical circuits.

In a series of experiments during the 1980s at the University of California, Berkeley, the trio showed that whole groups of particles spread out in electrical circuits could exhibit quantum behavior normally associated with extremely small particles—like individual atoms or electrons. The group studied the quantum properties of currents in electrical circuits made with superconducting components, which can carry an electrical current without any resistance. Superconductivity was useful for stretching the reach of quantum physics because it is the result of the quantum coordination of electrons: In a superconductor, electrons pair up into “Cooper pairs” that coordinate their behavior in a way that individual electrons can’t. Their experiments laid the groundwork for decades of follow-on research, including modern quantum computing experiments.

“These results were incredibly important,” says JQI Fellow Alicia Kollár, whose research uses superconducting circuits descended from those original devices. “Everything in modern superconducting-circuit devices relies on them.”

In their experiments, Clarke, Devoret and Martinis used the coordination of Cooper pairs in a superconducting current to demonstrate that a collective group of particles can sometimes just ignore a barrier—a phenomenon called quantum tunneling. When a quantum particle tunnels, it surpasses a barrier despite not having enough energy to pass it in a conventional way. Thanks to tunneling, a quantum particle that normally bounces off a wall will occasionally just pass through instead—with no Wile E. Coyote-style hole or other hint of its passing left in its wake. Tunneling allows a quantum particle to circumvent a variety of barriers, from jumping out of a literal box to pushing through an electric field to pulling free from an atomic nucleus. 

When the trio performed their experiments in the 1980s, it was already known that individual particles could tunnel. In fact, a crucial part of the circuits they constructed was an insulating layer between superconductors that Cooper pairs had to tunnel through to form the current. However, tunneling hadn’t been observed for coordinated groups of many particles—like a whole current containing a multitude of Cooper pairs. 

The researcher’s experiments were the first demonstration that a group of many particles that were spread out in space can quantum tunnel as a unit. Specifically, their results demonstrated that an electrical current containing many Cooper pairs could change its collective quantum state despite not having enough energy to do so without tunneling. 

In their experiments, the relevant barrier was more abstract than the insulating wall that the Cooper pairs had to tunnel across in the circuit. The barrier was a deficit in energy required for the current to switch between quantum states. The experiment used the fact that there were states of the current with less energy than its initial state but intermediate states with more energy existed between them—the wall to be tunneled through. 

The distinct states with their different behaviors were the result of the way the superconducting current depended on the quantum states of the two superconductors on either side of the insulator. In the experiment, the current started in a state where both superconductors remain in synch and the current flowed stably. But there were other states with the same or less energy where the two superconductors are out of synch and the current started changing over time and losing energy. 

When the circuit was warm enough, occasional temperature fluctuations could supply the energy to shift the current between the intervening states without tunneling being involved. One of the challenges Clarke, Devoret and Martinis had to overcome to conclusively demonstrate the effect was ensuring the circuit was so cold and so isolated that there was no source of energy to drive the change without tunneling being responsible.

In the experiment, they observed that even when there wasn’t sufficient energy available, the current still left its original state—the current tunneled across the energy barrier of the intervening states. This new form of tunneling involving many coordinated particles is called macroscopic quantum tunneling. Crucially, the current kept a recognizable identity as a single quantum entity throughout the tunneling process.

The researchers also demonstrated that the currents could only absorb or release energy in specific quantized amounts—the eponymous trait of quantum physics. They used microwaves to supply energy to the currents and observed different tunneling behaviors for the discrete states with different energies. In the experiments, the telltale sign of the quantized states was that increasing the available energy increased the likelihood of macroscopic quantum tunneling occurring, as had been predicted by quantum theory. 

The experiments proved that despite an electric current being spread throughout a circuit, it can still have a quantum state with quantum behaviors resembling those of individual quantum particles.

These days, studying the collective quantum states of superconducting currents is an established field of research built on the insights supplied by Clarke, Devoret and Martinis. For instance, JQI Emeritus Fellow Fred Wellstood, who worked as a graduate student in Clarke’s lab, has spent his career studying ways to harness the quantum properties of superconducting circuits for tasks like measuring magnetic fields and quantum computing. Similarly, Kollár, who is also a Chesapeake Assistant Professor of Physics at the University of Maryland, leads a lab that studies superconducting circuits, including their applications in  quantum computing and quantum simulation. In current research, the details of the superconducting circuits and how they are used have changed from those in the original experiments, but the modern devices still rely on the underlying physics celebrated by this year’s Nobel prize in physics.

This year’s award is the latest in a flurry of Nobel prizes given out over the past decade that celebrate research rooted in quantum physics. And it also comes during the United Nations-declared International Year of Quantum Science and Technology, which is celebrating a century of science and technology developments spurred by quantum physics. 

“It is wonderful to be able to celebrate the way that century-old quantum mechanics continually offers new surprises,” said Olle Eriksson, the chair of the Nobel Committee for Physics, in a press release.

For more information about this year’s prize winners and the research that the award recognizes, see the press release from the Royal Swedish Academy of Sciences.

Story by Bailey Bedford