Macroscopic Quantum Tunnelling: The Science Behind the 2025 Nobel Prize in Physics

 

Even though physicists do not believe in ghosts, they have no way of denying the strange, almost ghost-like behaviour of quantum mechanics. To mark the centenary of quantum mechanics—especially of its working equations—this year (2025) is being celebrated worldwide as the Quantum Year: Year of Quantum Science and Technology.

Quantum science occupies the deepest layer of all theories of science and technology in the modern world. From the most fundamental particles, the quarks, to the vast universe itself, quantum science extends across every scale.

Yet the theories of quantum science are sometimes so unbelievable that even a Nobel Prize–winning scientist for research in quantum mechanics once exclaimed in frustration: “There is actually no such thing as the quantum world. The quantum state exists only inside my head, where I use it to do calculations. At best, a quantum state describes some information—it has very little resemblance to the real world.”

Very recently, these remarks about quantum mechanics were made by Anton Zeilinger, a quantum physicist at the University of Vienna, who received the 2022 Nobel Prize in Physics for the experimental verification of principles of quantum mechanics [1].

From modern mobile phones to space-faring satellites, quantum mechanics is used everywhere today. Yet there was a time when even quantum scientists themselves did not believe that quantum science could have any practical applications. Two of the principal architects of quantum mechanics, Erwin Schrödinger and Paul Dirac, received the Nobel Prize in 1933. Dirac was only 31 years old at the time, while Schrödinger was 46. When journalists asked what use quantum mechanics would be to humanity, Paul Dirac replied rather curtly, “It will be of no use.” When asked whether it might be useful in the future, he answered, “It will be of no use in the future either” [2]. And yet today we have reached a point where the question must instead be: “Is there any field in which quantum mechanics is not used?”

For the past hundred years, scientists have been working to turn quantum mechanics into something practically useful. What began in 1900 with Max Planck as merely a set of strange, almost ghost-like theories has, a century later, become a vehicle for immensely powerful technologies. In the twenty-first century, we are on the verge of seeing the dream of quantum computers become reality. This reality is built upon the foundations of quantum electronics, or quantronics, and three of its leading architects—John Clarke, Michel Devoret, and John Martinis—have received the Nobel Prize in Physics this year.

Nobel Prizes for quantum science are by no means a surprising or new phenomenon. Over the past hundred years, more than forty scientists have received the Nobel Prize in Physics for research directly connected with quantum mechanics. Beginning in 1918 with Max Planck, who received the first Nobel Prize for the quantisation of energy, the list continues with Albert Einstein (1921—photoelectric effect), Niels Bohr (1922—atomic model), Arthur Compton (1927—Compton effect), Louis de Broglie (1929—matter waves), Werner Heisenberg (1932—uncertainty principle), Paul Dirac and Erwin Schrödinger (1933—quantum theory of the atom), Wolfgang Pauli (1945—exclusion principle), Max Born (1954—quantum wave function), Shin’ichirō Tomonaga, Julian Schwinger, and Richard Feynman (1965—quantum electrodynamics), John Bardeen, Leon Cooper, and Robert Schrieffer (1972—BCS theory of superconductivity), Klaus von Klitzing (1985—quantised Hall effect), Gerd Binnig and Heinrich Rohrer (1986—scanning tunnelling microscope), Robert Laughlin, Horst Störmer, and Daniel Tsui (1998—quantum fluids), Gerardus ’t Hooft and Martinus Veltman (1999—quantum structure of the electroweak interaction), Alexei Abrikosov, Vitaly Ginzburg, and Anthony Leggett (2003—theory of superconductors and superfluids), Roy Glauber (2005—quantum theory of optical coherence), Serge Haroche and David Wineland (2012—experimental methods for individual quantum systems), Alain Aspect, John Clauser, and Anton Zeilinger (2022—experimental tests of quantum mechanics), and John Clarke, Michel Devoret, and John Martinis (2025—macroscopic quantum tunnelling).

As you can see, up to 1972 all Nobel Prizes awarded for quantum mechanics were based on theoretical work. There is a reason for this. In the first fifty years of the twentieth century, quantum theory was enriched by the interpretations and analyses of many scientists. By analysing the wave function in Schrödinger’s equation, numerous physical phenomena could be explained that classical physics had previously failed to account for—for example, the emission of alpha particles in nuclear reactions.

Soon after Schrödinger’s equation was published in 1926, the German physicist Friedrich Hund discovered in 1927 the possibility of quantum tunnelling within molecular energy levels. Analysis of the wave function showed that certain seemingly impossible processes could, in theory, become possible. Quantum tunnelling is one such phenomenon. Suppose a quantum particle encounters a high barrier that it lacks the energy to cross. Classical physics would state with certainty that the probability of the particle crossing the barrier is zero—that is, the particle must remain on the near side of the wall. But according to the quantum wave function, a particle can also exist in the form of a wave. In that case, the probability that the particle, in its wave form, reaches the other side of the barrier is not zero. A non-zero probability means that the particle can indeed pass through the barrier to the other side—despite not having the required energy. An unquestionably unbelievable possibility.

But it was precisely this astonishing phenomenon of quantum tunnelling that physicists George Gamow, Edward Condon, and Ronald Gurney used in 1928 to explain nuclear alpha decay. In their joint paper, they showed that when alpha particles are spontaneously emitted from radioactive isotopes, they escape from the nucleus through quantum tunnelling. How does this happen? Let us understand it with an example.

We know that Marie and Pierre Curie discovered radium. From the isotope radium-226, alpha particles are emitted spontaneously. The half-life of radium-226 is about 1,600 years. That is, if someone possesses two grams of radium-226, no matter where or how it is kept, it will become one gram after 1,600 years. The emission of alpha particles from the radium nucleus occurs through quantum tunnelling; otherwise, the alpha particle would not be able to overcome the intense nuclear force and binding energy of the radium nucleus. As with all quantum-mechanical phenomena, tunnelling occurs based on probability. We know that alpha particles will be emitted from radium atoms, but it is not possible to say exactly which atom will emit an alpha particle at what precise moment. However, when many such events are considered together, the exponential decay curve obtained from the pattern of alpha emission shows that the probability of alpha emission never becomes zero. This means that quantum tunnelling continues to occur.

The nuclear fusion taking place at the centre of the Sun—where two hydrogen nuclei fuse together to form a helium nucleus—also happens through quantum tunnelling. Otherwise, it would not be possible for the nuclei to overcome the repulsive Coulomb force between two protons. Biologists are now also offering new explanations, suggesting that quantum tunnelling underlies most biochemical interactions inside our bodies.

Although Hund, Gamow, Condon, and Gurney discovered the theory of quantum tunnelling, none of them received a Nobel Prize for this discovery. However, many scientists later received Nobel Prizes for the practical applications of quantum tunnelling. The 2025 Nobel Prize, too, has quantum tunnelling at its core.

Almost immediately after the Bardeen–Cooper–Schrieffer (BCS) theory of superconductivity was established in 1957; the practical potential of quantum theory began to expand rapidly. The BCS theory provides a microscopic explanation of superconductivity.

From the basic properties of electrical conduction, we know that the rate of flow of electrons through a conductor is called electric current. Accordingly, a current of one ampere means that one coulomb of charge flows in one second. The charge of a single electron is 1.9 × 10⁻¹⁹ coulombs. To have one coulomb of charge flow, 6.25 × 10¹⁸ electrons must pass through in one second—six billion billion electrons per second. By their very nature, electrons prefer to stay apart from one another. Yet, due to the electric potential, they all move in an orderly manner from one terminal to another. However, as the temperature of the conductor increases, the electrons gain additional energy, become agitated, and disorder sets in. Because they experience mutual repulsion, this disorder obstructs the flow of electrons. This obstruction is called resistance. Hence, at ordinary temperatures, electrical current through a conductor encounters resistance. If the resistance becomes infinite, the flow of electricity stops completely, and the conductor turns into an insulator.

What happens in the opposite case? If resistance increases with rising temperature, how do electrons behave when the temperature is reduced to extremely low values? It was observed that at very low temperatures (close to absolute zero), the behaviour of electrons changes. In certain materials, electrons can move without any obstruction at all. Such materials become superconductors, exhibiting zero resistance. In this state, electrons no longer experience mutual repulsion. How is this possible?

In 1956, the scientist Leon Cooper discovered the existence of “Cooper pairs” of electrons in superconductors. Named after him, a Cooper pair is a bound pair of electrons. In a superconductor, electron flow occurs through the crystal lattice. In the presence of even a slight positive ionic charge, electrons are attracted toward the positive ions, causing a mild distortion of the crystal lattice. As a result, a region of positive charge is created, to which another electron becomes attached. This produces an indirect attractive force between two electrons, forming a paired state known as a Cooper pair. The spins of the electrons in a Cooper pair are opposite, so their total spin becomes zero. Their momenta are also opposite, giving a total momentum of zero. While individual electrons have spin 1/2 and are fermions, in a Cooper pair the two electrons combine to give zero spin and behave like bosons. Bosons obey Bose–Einstein statistics, and any number of bosons can occupy the same state without interacting with one another. As a result, millions of Cooper pairs can move in the same quantum state without collisions, giving rise to superconductivity.

According to the BCS theory, when a large number of Cooper pairs come together, they form a macroscopic quantum state. Although composed of fermions, because they behave like bosons they cannot be distinguished from one another. Therefore, all of them can be described collectively by a single large wave function. In this way, the theory of a macroscopic—or visible—quantum state emerging from microscopic states was established. For this work, John Bardeen, Leon Cooper, and Robert Schrieffer received the Nobel Prize in Physics in 1972. This was John Bardeen’s second Nobel Prize; earlier, in 1956, he had received the Nobel Prize in Physics for the invention of the transistor. To this day, John Bardeen remains the only scientist to have received the Nobel Prize in Physics twice.

As soon as the BCS theory was published, laboratory research into the creation of superconductors began. In 1962, British theoretical physicist Brian Josephson had just begun his PhD research at the University of Cambridge. At the age of only 22, he discovered a new possibility of quantum tunnelling by electrons. He showed that if an insulating layer is placed between two superconductors, electrons can flow from one superconductor to the other through the insulating material by quantum tunnelling. This process was named the Josephson effect in his honour, and the device itself came to be known as a Josephson junction. Experimental confirmation of this effect was obtained in 1963 at Bell Laboratories in the United States.

Earlier, in 1957, the Japanese scientist Leo Esaki invented the Esaki diode, or tunnel diode, by exploiting the quantum tunnelling effect in semiconductors. The Norwegian scientist Ivar Giaever demonstrated the tunnelling effect in superconductors. In 1973, Josephson, Esaki, and Giaever were awarded the Nobel Prize in Physics for the tunnelling effect.

The scanning tunnelling microscope was invented by making use of quantum tunnelling. For this achievement, Gerd Binnig and Heinrich Rohrer received the Nobel Prize in Physics in 1986.

This year’s Nobel Prize in Physics has also been awarded for research related to the quantum tunnelling effect. Between 1982 and 1984, under the supervision of John Clarke at the University of California, Berkeley, his student John Martinis and postdoctoral fellow Michel Devoret discovered macroscopic quantum mechanical tunnelling. This phenomenon has become a central foundation of the electrical circuits of quantum computers—quantum electronics, or quantronics. This year’s Nobel Prize recognises the far-reaching impact of that discovery made forty years ago.

John Clarke and his research group have made major contributions to the development of macroscopic quantum phenomena—that is, ways of displaying quantum properties at a visible, macroscopic scale. They demonstrated that the ghostly events of quantum mechanics, which usually occur beyond our direct perception, can in fact be made observable before our eyes. In the laboratory, they created superconducting quantum circuits that not only display the strange properties of quantum mechanics but also lay much of the practical groundwork for quantum computing in the near future.

The leader of this year’s Nobel trio, John Clarke, was born on 10 February 1942 in Cambridge. After completing his schooling at the renowned Perse School in Cambridge, he earned his bachelor’s degree in physics from Christ’s College, University of Cambridge, in 1964. In 1965, he began his PhD research at Cambridge under the supervision of Sir Alfred Brian Pippard, a pioneer of condensed matter physics.

During his PhD research, John Clarke invented an extremely sensitive voltmeter capable of measuring exceedingly small voltages. He later named it the SLUG (Superconducting Low-inductance Undulatory Galvanometer). After completing his PhD in 1968, he joined the University of California, Berkeley, as a postdoctoral fellow and has not changed institutions since. There, he was promoted to assistant professor in 1969, associate professor in 1971, and full professor in 1973. For the past sixty years, he has been conducting research on superconducting quantum interference devices, or SQUIDs (Superconducting Quantum Interference Devices).

He has received extensive recognition for his research. Early in his career, he was awarded the Alfred Sloan Fellowship, followed by the Guggenheim Fellowship. He became a Fellow of the Royal Society in 1986. In 1987, he was named Scientist of the Year in California. In 1998, he received the Joseph Keithley Award from the American Physical Society. In 1999, he was awarded the Comstock Prize of the National Academy of Sciences. In 2004, he received the Hughes Medal of the Royal Society. This year, he was awarded the Nobel Prize in Physics.

The second member of this year’s Nobel trio is Michel Devoret, who was born in Paris, France, in 1953. After earning his undergraduate degree in 1975 from the prestigious French engineering and research institution Télécom Paris, he completed his PhD in condensed matter physics in 1982 at the University of Orsay (now Paris-Sud). His PhD supervisor was Anatole Abragam, one of the pioneers of nuclear magnetic resonance research. In 1982, he joined the University of California, Berkeley, as a postdoctoral fellow in Professor John Clarke’s group. It was there that macroscopic quantum tunnelling was discovered for the first time.

From 1982 to 1984, he worked in Professor John Clarke’s group, after which he returned to France. From 1984 to 1995, he led research as head of the Quantronics Group at the French Atomic Energy Commission. From 1995 to 2002, he served there as head of the Quantronics Group and director of research. Since 2002, he has been affiliated with Yale University in the United States as a research professor. At the same time, he has also been working as a research professor at the University of California, Santa Barbara, on superconducting circuits. For the past 43 years, he has been conducting research in experimental solid-state physics, and has pioneered developments in circuit quantum electrodynamics, quantum amplification, and the single-electron transistor.

Michel Devoret has received worldwide recognition for his research. He is a Fellow of both the American Academy of Arts and Sciences and the French Academy of Sciences. For his work on the entanglement of superconducting qubits and microwave photons, he received the John Bell Prize in 2013; the Fritz London Memorial Prize for low-temperature physics in 2014; the Comstock Prize in 2024; and this year, the Nobel Prize.

The third hero of this year’s Nobel Prize in Physics is John Martinis. The discovery for which he received the Nobel Prize was made when he was only twenty-five years old. John Martinis was born in California in 1958. After earning his bachelor’s degree in physics from the University of California, Berkeley, in 1980, he began his PhD research under the supervision of Professor John Clarke. He received his PhD in 1987. The title of his doctoral thesis was “Macroscopic quantum tunnelling and energy-level quantization in the zero-voltage state of the current-biased Josephson junction.” There is a striking resemblance between this title and the wording used by the Nobel Committee in this year’s Nobel Prize announcement. Essentially, what John Martinis discovered during his PhD research, in collaboration with John Clarke and Michel Devoret, is what has earned them the Nobel Prize.

After completing his PhD, at Michel Devoret’s invitation, he worked as a postdoctoral fellow at the French Atomic Energy Commission, then returned to the United States to join the National Institute of Standards and Technology. In 2004, he joined the University of California, Santa Barbara, where his research on quantum computing and superconducting qubits continued. From 2014 to 2020, John Martinis led quantum computer research at Google’s Quantum AI Lab. In 2020, Martinis joined an Australian silicon quantum computing company founded in 2017 by Professor Michelle Simmons of the University of New South Wales. After working at the company for two years, John Martinis founded his own company, Qolab, in 2022. Established to provide engineering support for quantum computers, the company has John Martinis serving as its Chief Technology Officer.

John Martinis carried out the core research that underlies the 2025 Nobel Prize nearly forty years ago, between 1982 and 1984. After the Josephson effect was experimentally verified at Bell Laboratories in the United States in 1963, a wide range of applications of Josephson junctions rapidly emerged, particularly in the precise measurement of magnetic fields.

At that time, John Clarke had just completed his undergraduate studies and had begun his PhD in Cambridge. Scientists James Zimmerman and John Mercereau at Ford Scientific Laboratory in the United States built the first practical SQUID (Superconducting Quantum Interface Device) using a pair of Josephson junctions. A SQUID can measure extremely small magnetic fields. From the very beginning of his PhD research, John Clarke not only developed expertise in the use of SQUIDs but also introduced many technological improvements. Today, SQUIDs are used in a wide range of applications, from studying neural activity in the brain to any field that requires the measurement of extremely subtle magnetic fields.

For many years, efforts had been underway to elevate quantum tunnelling from the microscopic to the macroscopic scale. In 1978, British physicist Anthony Leggett proposed that it might be possible to realise a physical version of Schrödinger’s cat experiment. In superconductors or superfluids, quantum tunnelling between two macroscopic objects should be possible—especially at temperatures close to absolute zero. Quantum properties at the macroscopic scale have indeed been demonstrated in superfluid helium-3. For his theory of superfluids, he received the Nobel Prize in 2003. He had proposed that quantum tunnelling could be demonstrated on a macroscopic scale.

In 1982, Professor John Clarke took up a project to prove this idea experimentally. In his group were the young postdoctoral fellow Michel Devoret and the PhD student John Martinis. They began the work as John Martinis’s PhD project.

Using superconducting electrical circuits, the experiment was designed with extreme precision so that no external disturbance could influence the results. To observe quantum behaviour, they passed very small electrical currents through a Josephson junction and measured the resulting voltage, from which the resistance could be easily calculated. As expected, the initial voltage across the Josephson junction was zero. They then measured how long it took for the system to escape from the zero-voltage state with the help of quantum tunnelling. When quantum tunnelling occurs, the voltage changes from zero. Since quantum states depend on probability, data from many measurements were collected. During the experiment, microwaves of different wavelengths were applied to the system in the zero-voltage state. When the system absorbed some of the microwave energy, it transitioned to a higher energy level. As the energy of the system increased, the lifetime of the zero-voltage state decreased—that is, quantum tunnelling occurred. The experiments demonstrated that quantum tunnelling does indeed occur on a macroscopic scale in superconductors. Their results, published in Physical Review Letters in 1984–85 [3–5], are what have earned them this year’s Nobel Prize.

Although the experiments recognised by this year’s Nobel Prize were carried out nearly forty years ago, over these four decades the macroscopic applicability of quantum tunnelling has increased enormously. Quantum electronics is now a reality. In the near future, qubit-based quantum computers are expected to come within our reach. When that happens, perhaps the charge that quantum mechanics is incomprehensible will diminish at least to some extent.

References

[1] Biggan Chinta, October 2022; September 2025.
[2] Pradip Deb, Quantum Bhalobasa, Meera Prokashani, Dhaka, 2014.
[3] M. H. Devoret, J. M. Martinis, D. Esteve, J. M. Clarke, Physical Review Letters, Vol. 53, p. 1260 (1984).
[4] J. M. Martinis, M. H. Devoret, J. Clarke, Physical Review Letters, Vol. 55, p. 1543 (1985).
[5] M. H. Devoret, J. M. Martinis, J. Clarke, Physical Review Letters, Vol. 55, p. 1908 (1985).
[6] www.nobelprize.org.