2025 Nobel Prize in Physics: how the discovery of macroscopic quantum mechanical tunnelling inspires the X today

This October 8th, the 2025 Nobel Prize in Physics was awarded to a trio of physicists for “the discovery of macroscopic quantum mechanical tunnelling and energy quantisation in an electric circuit.” Among the laureates is Michel Devoret, a researcher who is very familiar with the Saclay plateau, having earned an engineering degree from the École Nationale Supérieure des Télécommunications (now Télécom Paris) and later a doctorate degree from the CEA Paris-Saclay research center. A long-time research director at Yale University in the United States, his influence has continued to be felt in France and elsewhere, even deep into the heart of the Ecole Polytechnique itself. 

To understand the importance of his work with his co-laureates John Clarke and John Martinis, we interviewed two researchers from the QCMX team at the Condensed Matter Physics Laboratory (PMC) at École Polytechnique. Landry Bretheau and Jean-Damien Pillet are well versed in the subject, having both completed their doctorates in the Quantronics group at CEA Saclay, a group co-founded by Michel Devoret.

What is this macroscopic quantum mechanical tunnelling and the energy quantisation in an electric circuit ?

The importance of this Nobel Prize-winning discovery does not rest on, as is sometimes mistakenly believed, the quantum tunneling effect itself (which has been recognized in previous Nobel Prizes, such as in 1973), but rather in the word “macroscopic.” Their work proved that an electrical circuit, visible to the naked eye and therefore on a macroscopic scale, could exhibit the same quantum behavior as atoms or other objects on a microscopic scale. 

“In an atom, energies are quantized, which is why we call it quantum mechanics,” explains Jean-Damien Pillet, professor and researcher at the École Polytechnique. “This behavior works for very elementary quantum objects such as atoms. What they showed was that it also worked for a very large system, a superconducting electrical circuit.”

When we say that the energy levels of an atom are quantized, it means that the energy take on very precise values and do not fluctuate over a continuous range of possible values. To take an extremely simplified example, this means that on a scale of 1 to 10, it is equivalent to systematically choosing the values 2, 4, and 10 and not varying over the infinite range of possible values between 1 and 10. We can therefore analyze, measure, and even influence the precise value of an atom's energy level.  

Before these experiments, there was no unanimous consensus that this behaviour would extend to macroscopic objects. Indeed, the notion that an electrical circuit, with billions upon billions upon billions of electrons traveling through its wires, could adopt very precise energy values and not vary within a wide range of values may seem counterintuitive.

“A macroscopic superconducting circuit, composed of a considerable number of atoms and electrons, can therefore be described by an extremely elementary energy level diagram,” continues Landry Bretheau, also a professor and researcher at École Polytechnique. "Quantum physics should tell me that each of my electrons will have energy levels. But thanks to superconductivity, we obtain a single macroscopic quantum state for the entire circuit rather than one for each electron. To simplify, it is as if the entire electrical circuit behaved like a single large atom with a single energy level."

Superconductivity is a characteristic of electrical circuits exposed to extremely cold temperatures, close to absolute zero (−273.15 °C), which causes electrical resistance to be canceled out and therefore electricity to be conserved without energy loss. From a quantum perspective, the electrons in such a circuit pair up and condense into a single macroscopic state, which explains their collective behavior.

And what about the macroscopic tunnel effect? Michel Devoret, John Clarke, and John Martinis observed it by creating what we call “Josephson” junctions, key components of superconducting electrical circuits, using a layer of insulating material. In their experiment, the electrical circuit transitions from a state where the voltage across the Josephson junction is zero (V=0) to a state where the electrical voltage is non-zero (V≠0). This transition between these two different macroscopic states for the circuit occurs through tunnelling, i.e., by crossing an energy barrier. In a second experiment, they showed that this behaviour could be magnified by exciting the circuit at certain specific frequencies, thus demonstrating that the electrical circuit is quantized in energy levels, with macroscopic tunneling. 

This pioneering discovery will prove crucial in the development of technologies that already shape our present—such as the design of future quantum computers—but it will probably take many more years for the general public to realize its impact.

A legacy both human and scientific

Shortly after publishing his work in 1984, which would lead to the Nobel Prize forty years later, Michel Devoret returned to France to create, with Daniel Esteve and Cristian Urbina, the Quantronique (QUANTum electRONIQUE) group at the CEA Saclay research center. Instead of following the norm and integrating them into existing teams, the CEA trusted them despite their young age to start a team from scratch—an extremely rare occurrence in the world of research— to explore the previously almost unknown field of superconducting circuits. And the legacy of this group is felt more than ever today. “It was a very niche field, with very few people in the world working on it,” says Jean-Damien Pillet. “And it remained niche for a very long time, during which the Quantronique group was one of the best in the world in this field they had pioneered.”

This pioneering research group was responsible for creating one of the very first superconducting qubits, marking the first step toward future quantum computers. The quality of this research team at the CEA was such that Michel Devoret was invited in 2002 to create and lead a similar laboratory this time at the prestigious Yale University in the United States, where he is still a professor today. In the years that followed, he continued to create teams in France and elsewhere to explore all facets of superconducting circuits. He also encouraged young researchers to get started and even supported getting adequate equipment for these newly created laboratories, such as the QCMX (Quantum Circuits & Matter) team created by Landry and Jean-Damien in 2017 at X and supported by the Ecole Polytechnique Foundation. “When we started out, Michel helped us obtain Helium 3 from the U.S. Department of Energy,”recalls Landry Bretheau. “Today, there is a tremendous legacy with a large number of teams around the world working in this field. It's pretty fantastic.”

A few years later, the field of superconducting circuits exploded with the creation of large American and Chinese research groups, and American technology giants such as IBM, Google, and Microsoft threw massive amounts of investments into it. But academic researchers have not forgotten the origins of this innovative field of research. “Once he left for the US, Michel naturally published less of his research in France, but his spirit and depth of thought continued to influence research groups here through his seminars and discussions,” continues Jean-Damien Pillet.

The legacy at École Polytechnique 

The QCMX team at École Polytechnique is a direct descendant of Michel Devoret's work, as its founders, Landry Bretheau and Jean-Damien Pillet, are both doctoral students from the Quantronics group he co-founded at the CEA. With their team, they are also continuing to explore superconducting electrical circuits at the macroscopic level based on the seminal work published in 1984.

Thanks to superconductivity, these electrical circuits can be manipulated to achieve certain energy levels. A system with two energy levels can be considered a quantum bit, equivalent to 0 and 1 in a classical bit, and can therefore be used as an information carrier. Such a system is called a qubit, which will become the basic unit of the quantum computer. Thanks to their high-performance equipment, such as cryostats that can reach temperatures very close to absolute zero, and advanced lithography techniques, researchers at École Polytechnique can design and test their own qubits. Their advances have recently led to the creation of a new type of superconducting qubit based on a carbon nanotube, which could generate innovative applications. 

Another area of interest for these researchers is understanding and exploring the Josephson effect, which is the basis for Josephson junctions. To this end, they use devices that combine superconducting circuits and nanostructures, such as carbon nanotubes, to probe and control the Josephson effect at the elementary scale of a single electron pair.

“Michel explored macroscopic electrical circuits composed of millions of microscopic particles. Now we are trying to use these macroscopic objects to isolate a microscopic object,” concludes Jean-Damien Pillet. “We're trying to reverse history a little bit.”