Macroscopic Quantum Tunneling: Unlocking the Quantum Secret Inside an Electrical Circuit

This prize recognizes the three researchers "for the discovery of macroscopic quantum tunneling and the quantization of energy in an electrical circuit." What are the applications for the development of quantum computing?

Bridging the Divide: The Microscopic and Macroscopic Worlds

Indeed, this prize is all the more important as quantum mechanics now constitutes the heart of the most advanced digital technologies underpinning research in the field of quantum computing.

Clarke, Devoret, and Martinis have completely overturned the perception we held until now regarding the counterintuitive properties of the quantum world. In fact, while we traditionally observed these properties in the realm of atoms and particles, they can manifest in a system large enough to be held in the hand.

The incredible demonstration they achieved marks a massive leap in the development of the next generation of quantum technologies, particularly quantum computing.

The Strange Yet Powerful Foundations of Quantum Mechanics

For uninitiated readers, it is worth recalling that the "counter-intuitive" properties of quantum mechanics we mentioned rest notably on fundamental concepts such as superposition (a particle exists in multiple states at once) or entanglement (two particles can be linked instantaneously, regardless of distance).

Quantum computing draws its power by exploiting these phenomena. For the quantum computer, superposition means that the unit of information (the qubit) can exist simultaneously in states 0 and 1. Unlike the classical bit, which stores only a single value (0 or 1), the qubit can store a combination of these two states, allowing it to represent and process an exponential number of possibilities in parallel.

As for entanglement, it means that two or more qubits are linked in such a way that the state of one instantly depends on the state of the other, regardless of the distance. This correlation enables the creation of complex interactions that form the basis of quantum algorithms, offering formidably efficient computing power for certain complex tasks. The significance of the laureates' work lies precisely in demonstrating the possibility of manipulating these quantum properties not at the level of a single atom but within an integrated circuit. Clearly, this perspective constitutes the hardware foundation necessary to build these qubits.

Quantum Tunneling: A Counterintuitive Phenomenon

Quantum physics, formalized notably by physicists such as Paul Dirac, Werner Heisenberg, Wolfgang Pauli, Louis de Broglie, and Erwin Schrödinger, not to mention Albert Einstein and Alain Aspect, has revolutionized our way of conceiving physical objects. It posits that elementary particles are not classical corpuscles with well-defined positions and trajectories. Rather, they are described as waves, or physical entities completely delocalized in space, known as wave functions.

Definition and Microscopic Origins

The wave function of a quantum particle gives the probability of measuring it at a given location. When this wave encounters an energy barrier that a classical particle could not overcome (due to insufficient energy), a small portion of the wave can penetrate the barrier and emerge on the other side. This phenomenon, where the particle traverses the barrier as if it had dug a tunnel, is called the quantum tunneling effect.

This effect plays a fundamental role in the microscopic world. For example, it enables hydrogen nuclei to fuse in the heart of the Sun, even though they theoretically lack sufficient energy to overcome their electrostatic repulsion. The same principle is exploited in certain tunnel diodes, as well as in scanning tunneling microscopes, among the most precise instruments ever designed to probe matter at the atomic scale.

From Atoms to Circuits: Conquering the Macroscopic World

The work of Clarke, Devoret, and Martinis, inspired by the theoretical research of Anthony Leggett, provided the first proof that quantum mechanics applies not only to the world of the very small, but also to the "mesoscopic" world (that of billions of electrons) and potentially to the wider world.

Anthony Leggett, the 2003 Nobel Laureate in Physics, had deeply explored the question of large-scale quantum coherence. His work aimed to determine to what extent quantum laws (superposition, entanglement, tunneling) could apply to systems composed of numerous constituents. By studying superconducting and superfluid liquids in particular, Leggett introduced models allowing us to understand how quantum mechanics could emerge within collective ensembles.

The three 2025 laureates thus achieved the demonstration of Macroscopic Quantum Tunneling.

The Superconducting Circuit: Creating an Artificial Atom

The Heart of the Experiment: The Josephson Junction

To observe these effects on a large scale, the experiments conducted by Clarke, Devoret, and Martinis at UC Berkeley in the mid-1980s used a superconducting electrical circuit of approximately 1 cm. The central element of this circuit is the Josephson junction. This junction is composed of two superconductors separated by a thin insulating layer (a tunnel barrier).

The Josephson junction takes its name from physicist Brian Josephson, a 1973 Nobel co-laureate who had predicted the tunneling of Cooper pairs through this barrier without resistance (the Josephson effect).

Concretely, Josephson had shown that an electric current could flow through a thin insulating layer separating two superconducting materials, even with no applied voltage. This phenomenon relies on the tunneling of Cooper pairs, those electron duos that move collectively and coherently within a superconductor. Even though the barrier should block them according to classical physics, quantum mechanics allows them to cross it as if they were "passing through a wall."

Electrons Acting as One

This phenomenon is enabled by superconductivity. When the material is cooled to temperatures near absolute zero, electrons, which are typically fermions, pair up to form Cooper pairs. These Cooper pairs behave like bosons, particles capable of occupying the same quantum state without repelling one another. 

While individual electrons obey the Pauli exclusion principle (which prevents them from being in the same state at the same place), Cooper pairs can cluster together within a single, unified collective wave. This unified behavior enables electric current to flow through the material without resistance, the signature of superconductivity.

Inside the superconductor, billions of these Cooper pairs move collectively, described by a single collective wave function. This behavior transforms the entire circuit into a single, large-scale quantum object.

Proving Macroscopic Quantum Tunneling

In their setup, the collective system (the macroscopic quantum object) was initialized in a metastable state at zero voltage, trapped by an energy barrier. The circuit could escape from this state (observable as a sudden voltage).

A metastable state is a situation of temporary equilibrium. The system appears stable, but in reality, it sits perched on a sort of "energy hill." As long as no disturbance occurs, it remains there, motionless. In this specific case, the superconducting circuit remained in a voltage-free state (with no measurable current at its terminals) until it crossed the energy barrier via the tunneling effect. This sudden transition from a "blocked" to a "liberated" state manifests as the appearance of a voltage. It is the sign that quantum escape has occurred.

However, for this observation to be indisputable, it was necessary to distinguish the genuine tunneling effect from simple thermal fluctuations, which are also capable of releasing the system at random. That is why the researchers set out to suppress all parasitic thermal agitation, aiming to isolate the purely quantum signature of the phenomenon.

• Eliminating thermal noise: To isolate the tunneling effect from classical fluctuations, the researchers had to cool the device to extremely low temperatures (in the range of a few tens of millikelvins (mK), or less than 50 mK) using a dilution refrigerator.
• The quantum proof: At higher temperatures, the escape was explained by thermal agitation (classical thermal activation). However, below 50 mK, the escape rate stopped dropping with temperature and remained constant. This constant escape rate in the absence of any measurable thermal activation proves that the object crossed the barrier via macroscopic quantum tunneling (MQT).
• Energy quantization: By irradiating the junction with microwaves, the researchers also demonstrated that the system exhibited quantized energy levels. This proved that the system behaved like an "artificial atom" whose behavior could be tailored.

Schrödinger's Cat: From Paradox to Reality

In many respects, the work honored by the 2025 Nobel Prize lends a tangible reality to one of the most famous paradoxes in physics: Schrödinger's cat. This thought experiment imagined a cat that is both alive and dead as long as one has not observed the inside of its box, illustrating the superposition of quantum states. In the superconducting circuits designed by Clarke, Devoret, and Martinis, this paradox moves beyond the abstract. The system can actually exist in multiple states simultaneously, until a measurement comes to "open the box" and freeze its state.

These experimental results appeared in several seminal articles in Physical Review Letters in 1984 and 1985.

Quantum Legacy and Applications

The Birth of Quantronics

By demonstrating that strange quantum effects could infiltrate our macroscopic world, the work of Clarke, Devoret, and Martinis opened up an entirely new scientific field known as quantronics, or circuit quantum physics. They established an experimental bridge between the electronics of integrated circuits and quantum mechanics.

This work enabled the standardized fabrication and manipulation of artificial atoms (superconducting circuits). One can easily understand that this is far “easier” than manipulating natural atoms individually.

The Foundation of Quantum Computing

The laureates' work is widely regarded as the basis for superconducting qubits. 

As we noted earlier, unlike classical bits that encode information as 0 or 1, qubits can exist in a superposition of these two states simultaneously. To operate, superconducting qubits, which rely on Josephson junctions, must be cooled to temperatures near absolute zero and shielded from environmental noise.

This extreme cooling is not merely a technical constraint. In fact, it addresses one of the greatest challenges in applied quantum physics, which is decoherence. This phenomenon refers to the loss of quantum properties (superposition and entanglement) when a system interacts with its environment. The slightest vibration, the tiniest thermal or electromagnetic variation, is enough to disrupt the qubit's state and cause it to "fall back" into the classical world.

To maintain quantum coherence, researchers must therefore isolate the circuits from all external noise and cool them to temperatures near absolute zero. This constant struggle against decoherence is what makes quantum computers both fascinating and extraordinarily complex to build.

Their approach laid the foundations for all modern superconducting qubit architectures, including the Transmon, a direct evolution of the Quantronium. This type of qubit, simpler and more stable, was designed to reduce sensitivity to electrical fluctuations, which represent one of the main causes of decoherence. Today, the Transmon constitutes the heart of the quantum processors developed by the field's leading players, such as IBM, Google, and European research laboratories.

Architects of the Quantum Future

The laureates themselves have been driving forces in the field. John Martinis led Google's quantum computing effort. His team claimed (controversially) to have achieved quantum advantage in 2019.

When we speak of "quantum advantage," we refer to the moment a quantum computer performs a calculation in a matter of seconds that would take a classical supercomputer years, or even centuries, to complete. That said, this does not yet imply generalized superiority across all computing tasks. Instead, it serves as an experimental demonstration that the principles of quantum mechanics can offer computational power inaccessible to conventional machines for very specific problems.

This major breakthrough was driven by three researchers with complementary backgrounds: John Clarke, a pioneer in quantum sensing; John Martinis, the architect of the first superconducting quantum processors; and Michel Devoret, the artisan who bridged theory, experimentation, and engineering.

All three contributed to transforming an abstract idea into concrete technology. I will take the liberty of dwelling a bit longer on Michel Devoret's path. By following in the tradition of the great French physicists who shaped our understanding of the quantum world, from Louis de Broglie, who laid its theoretical foundations, to Claude Cohen-Tannoudji, Serge Haroche, and Alain Aspect, who revealed its experimental manifestations, Michel Devoret extends this lineage of scientific excellence à la française.

Portrait of the French Laureate: Michel Devoret

Michel Devoret, a French engineer and physicist, is a co-laureate of the 2025 Nobel Prize in Physics. He is the first graduate of Télécom Paris (Class of 1975) to receive this prestigious distinction.

His academic path includes a PhD in Physics from Université Paris-Saclay. After his thesis, he spent part of his career at CEA-Saclay (the French Alternative Energies and Atomic Energy Commission), where he was recruited, before pursuing a post-doctorate with John Clarke, where the award-winning experiments were conducted.

The "Quantronique" Group

Upon returning to the CEA, where he became a Research Director, Michel Devoret founded the "Quantronique" group at CEA-Saclay (IRAMIS/SPEC) together with Daniel Estève and Cristian Urbina. This group is widely regarded as a school of thought on superconducting quantum circuits.

The major advances of the Quantronique group are numerous and pivotal. We notably owe them the invention of the electron pump, a device capable of controlling the passage of electrons one by one, enabling electrical metrology of extreme precision. They also directly observed the charge of Cooper pairs. This experimentally confirmed a phenomenon that had long remained theoretical. Finally, the group developed the first superconducting quantum bit, the Quantronium, designed to withstand decoherence, the loss of quantum information caused by the environment.

These achievements have profoundly transformed the way researchers conceive and manipulate quantum circuits.

"Michel Devoret’s path exemplifies what our school strives to instill: scientific rigor, intellectual audacity, and a passion for exploring the frontiers of knowledge. His work, which led to the discovery of the quantum tunneling effect, has opened fascinating perspectives for research and future quantum technologies. It serves as a source of inspiration for our students and faculty working in fields directly related to this discovery, such as quantum cryptography, quantum sensors, and quantum computing." — Patrick Olivier, Director of Télécom Paris

International Career and Recognition

Michel Devoret is currently a Professor of Applied Physics at Yale University (since 2002) and also holds the position of Scientific Director at Google Quantum AI. He also held the Chair of Mesoscopic Physics at the Collège de France from 2007 to 2012.

His journey exemplifies the mission of the IMT (Institut Mines-Télécom) schools, which is to train engineers capable of scientific rigor and intellectual audacity. His work has thus helped position France in the race for quantum computing.

Conclusion: Science Serving Collective Progress

The recognition of John Clarke, Michel Devoret, and John Martinis in 2025 honors fundamental work carried out over 40 years ago (in 1984 and 1985). This work irrefutably demonstrated that quantum mechanics is not limited to the abstract and microscopic world.

They transformed quantum theory into a tangible physical reality by successfully isolating a macroscopic system, described by a single collective wave, from environmental noise. This opens a wider window into the field of quantum engineering. Their discovery also constitutes the hardware foundations for the superconducting qubits used today by major corporations and research institutions.

Their work serves as a source of inspiration for researchers and students working in direct application fields such as quantum cryptography, quantum sensors, and quantum computing. The prize thus underscores the universality of science and the excellence of fundamental research at the service of collective progress.

Sources

• info.gouv.fr: “Le Français Michel Devoret prix Nobel 2025 de physique,” [FR] [link]
• iramis.cea.fr: “Prix Nobel de physique 2025: Physique quantique,” [FR] [link]
• physics.aps.org: “Nobel Prize: Quantum Tunneling on a Large Scale,” [link]
• scientificamerican.com: “The 2025 Nobel Prize in Physics Goes to Researchers Who Showed Quantum Tunneling on a Chip,” [link]
• NobelPrize.org: “Scientific Background to the Nobel Prize in Physics 2025,” [link]