Nobel Prize in Physics 2025: Macroscopic Quantum Tunnelling in Superconducting Circuits

This note covers the Nobel Prize in Physics 2025: why John Clarke, Michel H. Devoret and John M.

Martinis won it, what macroscopic quantum tunnelling and energy quantisation in an electric circuit mean, how their experiments on superconducting chips worked, how the discovery unfolded through the 1980s, why it matters for quantum technology today, and quick facts for exams.

What was the Nobel Prize in Physics 2025 awarded for?

The Royal Swedish Academy of Sciences gave the award jointly to the three laureates "for the discovery of macroscopic quantum mechanical tunnelling and energy quantisation in an electric circuit". This is the exact wording of the official citation.

In plain words, the laureates showed that two strange quantum mechanical effects, normally seen only in single tiny particles such as electrons or atoms, can also appear in a circuit big enough to hold in your hand.

The circuit behaved as if it were one giant particle. The two effects were tunnelling (passing through a barrier without having enough energy to climb over it) and energy quantisation (absorbing or releasing energy only in fixed, specific amounts rather than smoothly).

The prize's official name is the Nobel Prize in Physics, awarded by the Royal Swedish Academy of Sciences.

Who are the laureates?

John Clarke

John Clarke was born in 1942 in Cambridge, United Kingdom. At the time of the award he was a professor at the University of California, Berkeley, USA, where he had worked since joining in 1969, the year after completing his doctorate at the University of Cambridge.

He received one third of the prize. Clarke led the Berkeley research group that built and ran the superconducting circuit experiments, bringing years of expertise in superconductors and Josephson junctions.

Michel H. Devoret

Michel H. Devoret was born in 1953 in Paris, France. At the time of the award he was affiliated with Yale University, New Haven, USA, the University of California, Santa Barbara, and Google Quantum AI in Santa Barbara, where he was Chief Scientist for Quantum Hardware.

He received one third of the prize. Devoret joined Clarke's group as a postdoctoral researcher in the mid-1980s, after his doctorate in Paris, and helped carry out the key tunnelling and spectroscopy measurements.

John M. Martinis

John M. Martinis was born in 1958. At the time of the award he was affiliated with the University of California, Santa Barbara, and with Qolab in Los Angeles, where he was Chief Technology Officer.

He received one third of the prize. Martinis was a doctoral student in Clarke's group during the experiments, and later used the same quantised-energy-level physics to build superconducting quantum bits for quantum computing.

What problem were the laureates trying to solve?

Quantum mechanics describes the behaviour of very small things such as single electrons or atoms.

A well-known quantum effect is tunnelling: a particle can sometimes appear on the far side of a barrier even though, by ordinary physics, it does not have enough energy to cross it.

This had already explained alpha decay, where a piece of a heavy atomic nucleus escapes through a barrier created by the nuclear forces, as the physicist George Gamow realised in 1928.

The open question was how large a system could be and still show such quantum behaviour. As soon as many particles are involved, the committee noted, "quantum mechanical effects usually become insignificant".

An ordinary ball thrown at a wall always bounces back; it never tunnels through, because it is made of an enormous number of particles acting independently rather than as one coordinated system.

The theorist Anthony Leggett (later a Nobel laureate himself, in 2003) proposed in the late 1970s that a special kind of circuit, a Josephson junction, might let a macroscopic number of particles act together as one, making a visible, human-scale system that still tunnels like a single quantum particle.

This idea needed an experiment that could isolate a circuit well enough from outside disturbance to see the effect clearly, which is the challenge Clarke, Devoret and Martinis took on. Vast care and precision were needed to screen the delicate setup from any outside interference that could mask the quantum behaviour they were hunting for.

Earlier attempts by other research groups in the early 1980s had looked for similar signs of macroscopic quantum tunnelling but could not rule out the possibility that what they saw was caused by ordinary noise rather than genuine quantum behaviour, leaving the question open until the Berkeley group's more careful measurements.

How did superconductors and Josephson junctions make this possible?

A superconductor is a material that, when cooled enough, carries electric current with absolutely no resistance.

Inside it, electrons do not move independently; they pair up into so-called Cooper pairs, named after Leon Cooper, who with John Bardeen and Robert Schrieffer explained superconductivity (work honoured by the Nobel Prize in Physics 1972).

Unlike ordinary electrons, which always stay distinguishable from one another, Cooper pairs can all be in exactly the same state, so the whole collection can be described by a single shared wave function, the mathematical description of a quantum state.

A Josephson junction is formed when two superconductors are separated by an extremely thin layer of non-conducting material. Named after Brian Josephson (Nobel Prize in Physics 1973), this junction allows Cooper pairs to tunnel across the thin insulating gap.

Because billions of Cooper pairs move together, the whole junction behaves as though it were a single giant particle filling the entire circuit.

Draw two blocks labelled "superconductor" with a thin vertical strip between them labelled "thin insulating layer".

Show a current arrow entering one block and a voltmeter symbol across the junction, representing the setup the laureates used to pass current through the circuit and measure the voltage.

What did the experiments actually show?

In 1984 and 1985, Clarke, Devoret and Martinis built an electronic circuit of superconductors separated by a Josephson junction at the University of California, Berkeley. By carefully refining and measuring every property of the circuit, they could control exactly what happened when a current was passed through it.

The system started in a state where current flowed with no voltage at all, trapped as if behind a barrier it could not normally cross.

The laureates then watched for the moment the system escaped this zero-voltage state through tunnelling, which was revealed by a sudden appearance of voltage.

Because quantum tunnelling involves chance, they repeated the measurement many times and built up statistics, similar to how scientists measure the half-life of radioactive decay from many nuclei rather than from a single atom.

1. Build a superconducting circuit containing a Josephson junction, cooled to extremely low temperature.
2. Pass a weak current into the junction, keeping it below the critical current so the system stays trapped in a zero-voltage state.
3. Measure how long the system stays in this state before a voltage suddenly appears, repeating this many times to build up a statistical picture.
4. Shine microwaves of different wavelengths onto the junction and observe which are absorbed, moving the system to a higher energy level.
5. Check whether the time spent in the zero-voltage state shortens as predicted when the system holds more energy, confirming quantised energy levels.

The result confirmed both effects the citation names: the escape from the zero-voltage state was macroscopic quantum tunnelling, and the fact that only specific amounts of energy were absorbed or released, with the escape rate changing exactly as theory predicted, demonstrated energy quantisation in the circuit.

Draw a tilted, bumpy surface (a "washboard") with a ball sitting in one dip, representing the zero-voltage state. Draw a dotted arrow showing the ball appearing in the next dip along without climbing over the hump between them, to show tunnelling rather than climbing.

How did the discovery unfold?

Why does Schrödinger's cat come up in this story?

The Austrian physicist Erwin Schrödinger (Nobel Prize in Physics 1933) imagined a cat that would be simultaneously alive and dead inside a sealed box, to show how absurd quantum superposition looks when scaled up to everyday objects.

Normally, he intended this to illustrate that quantum weirdness is often erased at macroscopic scale: a real cat's quantum properties can never be shown in a lab.

Anthony Leggett compared the laureates' circuit to this thought experiment. The system of billions of Cooper pairs is still far smaller than even a kitten, but because the experiment measured quantum properties belonging to the circuit as a whole, rather than to many separate tiny parts, physicists treat it as a genuine, if modest, version of a Schrödinger's-cat-like state.

This matters because many familiar large-scale quantum effects, such as lasers or superfluids, arise from combining many separate microscopic quantum events; the laureates' circuit instead created one macroscopic quantum effect directly from a single shared wave function covering the whole device.

Why does this matter?

The Nobel Committee for Physics chair, Olle Eriksson, said it was "wonderful to be able to celebrate the way that century-old quantum mechanics continually offers new surprises", adding that it is also useful because quantum mechanics underlies all digital technology.

The committee noted that this work created opportunities for the next generation of quantum technology, including quantum cryptography, quantum computers and quantum sensors.

The Josephson junction used in these experiments can be treated as an artificial atom, a human-made device with wires and connectors that behaves like a natural atom and can be wired into new experiments.

John Martinis later used exactly the quantised energy levels demonstrated in 1985 to build a quantum bit (qubit), using the lowest energy state and the first level above it as the "0" and "1" of quantum information.

Superconducting circuits of this kind remain one of the main technologies being explored for building future quantum computers.

The committee also pointed to the transistor inside every computer microchip as an everyday example of established quantum technology already surrounding us, to show that ideas from fundamental quantum physics regularly end up inside useful devices years later.

Beyond computing, artificial atoms built from Josephson junctions are now used to help scientists simulate and understand other quantum systems that are otherwise hard to study directly, widening the discovery's influence well past the original 1980s experiments.

How does this connect to what you study?

Students who study electricity and circuits learn about current, resistance and voltage; this prize shows what happens when resistance drops to exactly zero in a superconductor, and how the familiar idea of a circuit connects to the strange rules of quantum physics.

Students who meet radioactive decay and half-life in chemistry or physics will recognise the same idea of chance-based tunnelling used here, just applied to electrons in a circuit chip instead of particles inside a nucleus. In both cases, a single event cannot be predicted in advance; only the statistics of many repeated trials reveal the underlying pattern.

The laureates' later use of quantised energy levels as a qubit also links directly to any discussion of how modern computers might one day use quantum bits instead of ordinary binary bits made of simple on-off switches.

Anyone who has studied atomic structure and the idea that electrons in an atom occupy fixed, separate energy shells rather than any value in between will find the same principle at work here: the circuit's energy could only take specific, quantised values, never a smooth range, exactly as in a textbook atom.

The idea of a wave function, usually introduced only briefly at school level, becomes concrete in this story: a single mathematical description can stand for the shared state of billions of paired electrons moving together inside one chip, rather than describing just one particle at a time.

Finally, the discovery is a useful example of how pure, curiosity-driven research carried out in a university laboratory in the 1980s, with no obvious practical use at the time, went on decades later to underpin a fast-growing field of quantum technology, a pattern worth remembering when weighing the long-term value of basic science funding.

Quick facts for exams

The Nobel Prize in Physics 2025 was awarded to John Clarke, Michel H. Devoret and John M.

Martinis for discovering macroscopic quantum mechanical tunnelling and energy quantisation in an electric circuit, work carried out in 1984 and 1985 on a superconducting circuit built around a Josephson junction at the University of California, Berkeley.

The prize, worth 11 million Swedish kronor shared equally, was announced by the Royal Swedish Academy of Sciences on 7 October 2025.

The discovery showed that a circuit large enough to hold in one's hand can still obey the quantum rules normally seen only in single particles, opening paths toward quantum computers and quantum sensors.

Note: Source. The prize facts in this note are from the Nobel Prize's official site, nobelprize.org.

Glossary

• Tunnelling — a quantum process in which a particle or system passes through a barrier it does not have enough energy to climb over classically.
• Macroscopic — large scale, involving a visible object made of a huge number of particles, as opposed to a single tiny particle.
• Microscopic — the scale of single particles such as electrons, atoms or nuclei, where quantum effects are usually strongest.
• Superconductor — a material that, below a certain temperature, conducts electric current with no electrical resistance at all.
• Cooper pair — two electrons in a superconductor joined together so they behave as a single quantum unit rather than two separate particles.
• Josephson junction — two superconductors separated by a very thin insulating layer, across which Cooper pairs can tunnel.
• Wave function — the mathematical description of a quantum system's state, giving the probability of finding it with certain properties.
• Energy quantisation — the rule that a quantum system can only absorb or emit energy in fixed, specific amounts, not any amount at all.
• Zero-voltage state — the state of the laureates' circuit in which current flows but no voltage is measured, before tunnelling occurs.
• Qubit — a quantum bit, the basic information-carrying unit of a quantum computer, built here from quantised energy levels.
• Artificial atom — an engineered circuit, such as a Josephson junction, that behaves like a natural atom with discrete energy levels.
• Half-life — the time it takes for half of a large sample of radioactive nuclei to decay, used as an analogy for statistical tunnelling measurements.

Common errors and misconceptions

• Misconception: The laureates discovered tunnelling for the first time. Correct: Tunnelling by single particles, for example in alpha decay, was already known since Gamow's 1928 work; the laureates showed it on a macroscopic, circuit-wide scale.
• Misconception: The circuit used in the experiment was microscopically small. Correct: The circuit was macroscopic, big enough to be held in the hand, which is exactly what made the discovery remarkable.
• Misconception: Superconductors conduct current with very low resistance. Correct: Superconductors conduct current with no electrical resistance at all, below a critical temperature.
• Misconception: Energy quantisation and tunnelling are the same phenomenon. Correct: They are two separate quantum effects that the same experiment demonstrated together in one circuit.
• Misconception: The prize honours a single experiment done by one person. Correct: It honours a joint series of experiments performed in 1984 and 1985 by all three laureates working together as a research group.
• Misconception: The Josephson junction is named after one of the 2025 laureates. Correct: It is named after Brian Josephson, who received the Nobel Prize in Physics 1973 for his theoretical prediction of the effect.

Exam-style questions with model answers

Q1. State the official citation for the Nobel Prize in Physics 2025. [2 marks]

1. The citation reads: "for the discovery of macroscopic quantum mechanical tunnelling and energy quantisation in an electric circuit" (the exact official wording, naming both tunnelling and energy quantisation, is needed for full marks).

Q2. Name the three laureates and their affiliations at the time of the award. [2 marks]

1. John Clarke, University of California, Berkeley; Michel H. Devoret, Yale University and University of California, Santa Barbara and Google Quantum AI; John M. Martinis, University of California, Santa Barbara and Qolab.

Q3. What is a Josephson junction, and why was it important to this discovery? [4 marks]

1. A Josephson junction consists of two superconductors separated by a very thin non-conducting layer.
2. It allows paired electrons, called Cooper pairs, to tunnel across the thin barrier.
3. Because billions of Cooper pairs in the junction move together under one shared wave function, the whole junction behaves as a single giant particle.
4. This made it possible to observe quantum tunnelling and quantised energy levels on a macroscopic, visible scale rather than only in single particles.

Q4. Explain the difference between tunnelling and energy quantisation as shown in the 1984 to 1985 experiments. [4 marks]

1. Tunnelling was shown when the circuit, trapped in a zero-voltage state, suddenly developed a voltage without climbing over the energy barrier classically.
2. Energy quantisation was shown separately by shining microwaves of specific wavelengths on the circuit and seeing it absorb only particular amounts of energy.
3. When the system held more absorbed energy, it tunnelled out faster, exactly as quantum theory predicted for discrete energy levels.
4. Together, both results confirmed the circuit behaved as a single quantum system with quantised states, not as a classical object.

Q5. Discuss how the work of Clarke, Devoret and Martinis built on earlier Nobel Prize winning physics, and describe its later applications. [6 marks]

1. The 1984 to 1985 experiments depended on earlier discoveries: Bardeen, Cooper and Schrieffer's 1957 BCS theory explaining Cooper pairs in superconductors, honoured by the 1972 physics prize, and Brian Josephson's 1962 prediction of Cooper pair tunnelling across an insulating barrier, experimentally confirmed in 1963 and honoured by the 1973 physics prize.
2. Theorist Anthony Leggett, later a 2003 physics laureate, had proposed in the late 1970s that such circuits could show macroscopic quantum tunnelling, giving the Berkeley group a concrete target to test.
3. Clarke, Devoret and Martinis carefully measured all the properties of their circuit so they could control and interpret the tunnelling and energy quantisation they observed, isolating it from outside disturbance.
4. Their work established the Josephson junction as an engineerable artificial atom, usable in new quantum experiments.
5. Martinis later used the same quantised energy levels to build a qubit for quantum computing, and superconducting circuits remain one of the leading technologies explored for quantum computers, quantum sensors and quantum cryptography.

Q6. Who was the Chair of the Nobel Committee for Physics at the time of the announcement, and what did he say about the discovery? [3 marks]

1. Olle Eriksson was the Chair of the Nobel Committee for Physics.
2. He said it was "wonderful to be able to celebrate the way that century-old quantum mechanics continually offers new surprises".
3. He added that quantum mechanics is also enormously useful, since it is the foundation of all digital technology.

Q7. What is a superconductor, and why was it essential to this discovery? [3 marks]

1. A superconductor is a material that conducts electric current with absolutely no electrical resistance once cooled below a certain temperature.
2. Inside a superconductor, electrons pair up into Cooper pairs that can all occupy exactly the same quantum state.
3. This let billions of particles behave as one single quantum system, which was essential for making a macroscopic quantum effect observable.

Q8. Give one real example, mentioned by the Nobel Committee, of how this discovery links to everyday technology. [2 marks]

1. The committee compared this discovery to transistors in computer microchips; transistors are an established, everyday example of quantum technology that already surrounds us in digital devices.

Key takeaways

• John Clarke, Michel H. Devoret and John M. Martinis won the Nobel Prize in Physics 2025 jointly, sharing it equally.
• The citation honours the discovery of macroscopic quantum mechanical tunnelling and energy quantisation in an electric circuit.
• Their 1984 to 1985 experiments used a superconducting circuit built around a Josephson junction, at the University of California, Berkeley.
• A superconductor carries current with zero resistance because electrons pair up into Cooper pairs sharing one quantum state.
• The circuit tunnelled out of a zero-voltage state, a quantum effect normally seen only in single microscopic particles.
• Microwave absorption experiments confirmed the circuit's energy levels were quantised, matching quantum mechanical predictions.
• The work built directly on the 1972 and 1973 physics prizes on superconductivity and the Josephson effect.
• Martinis later applied the same quantised levels to build superconducting qubits for quantum computers.
• The prize was announced on 7 October 2025, worth 11 million Swedish kronor shared between the three laureates.

Test yourself

What two quantum effects does the official citation name?

Macroscopic quantum mechanical tunnelling and energy quantisation in an electric circuit.

In which years were the key experiments performed?

The experiments were performed in 1984 and 1985 at the University of California, Berkeley.

What is a Cooper pair?

Two electrons in a superconductor joined together so they behave as a single quantum unit rather than remaining distinct particles.

Who predicted the tunnelling of Cooper pairs across an insulating barrier, and when was this honoured?

Brian Josephson predicted it; his work was honoured by the Nobel Prize in Physics 1973.

What state was the circuit trapped in before tunnelling occurred?

A zero-voltage state, where current flowed but no voltage could be measured across the junction.

How did Martinis later apply this physics to computing?

He used the quantised energy levels of a similar circuit as the "0" and "1" states of a quantum bit, or qubit, for quantum computers.

Which earlier thought experiment is compared to this discovery?

Erwin Schrödinger's cat thought experiment, illustrating quantum superposition at a scale larger than single particles.