The 2025 Nobel Prize in Physics has been awarded to John Clarke, Michel H. Devoret, and John M. Martinis for their groundbreaking discovery of ‘macroscopic quantum mechanical tunnelling and energy quantisation in an electric circuit’. Their experiments, conducted in the mid-1980s at the University of California, Berkeley, demonstrated that the strange and counter-intuitive principles of quantum physics could manifest not just in single particles but in systems large enough to be held in one’s hand. The laureates’ work bridged the microscopic world of atoms and electrons with the macroscopic world of everyday objects, revealing that quantum behaviour could emerge on scales far larger than previously imagined.
Quantum mechanics describes the behaviour of matter and energy at the smallest scales. Particles such as electrons can exist in multiple states simultaneously (superposition) or even pass through barriers that would seem impenetrable (tunnelling). However, such phenomena are usually confined to the microscopic domain, disappearing when large numbers of particles interact. Clarke, Devoret, and Martinis overturned this conventional boundary by demonstrating that under carefully controlled conditions, an electrical circuit comprising billions of particles could display distinctly quantum behaviour. Their discovery has since laid the experimental foundation for the development of quantum computers, quantum cryptography, and quantum sensors that promise to redefine computation, communication, and measurement.
The Landmark Experiment
The experiments that earned the 2025 Nobel Prize were performed between 1984 and 1985 in John Clarke’s laboratory at Berkeley. At that time, he was a professor exploring superconductivity and the Josephson junction, an electrical circuit consisting of two superconductors separated by a thin insulating barrier. Michel Devoret had joined Clarke’s group as a postdoctoral researcher after completing his doctorate in Paris, while John Martinis was a doctoral student under Clarke. Together, they sought to test a bold theoretical prediction made by physicist Anthony Leggett, who proposed that quantum behaviour might be observable on a macroscopic scale in Josephson junction systems.
The trio refined an existing experimental setup first developed by Brian Josephson, whose discovery of quantum tunnelling in superconductors had won him the Nobel prize in Physics in 1973. A Josephson junction allows electrons, paired as so-called ‘Cooper pairs’ within superconductors, to flow without electrical resistance. In this state, all the electrons move in synchrony, behaving as a single quantum wave. The Josephson Junction forms the core of ultra-sensitive magnetic detectors called Superconducting Quantum Interference Device (SQUID) and is now fundamental to quantum computer chips. Clarke, Devoret, and Martinis sought to probe whether entire macroscopic circuits could display similar effects.
Demonstration of Quantum Tunnelling on a Macroscopic Scale
To achieve this, the researchers built a superconducting electrical circuit separated by a thin insulating layer and isolated it meticulously from environmental interference. Even the slightest vibration or electromagnetic disturbance could destroy the fragile quantum effects. The resulting system, though composed of countless particles, behaved as a single quantum entity, an enormous particle-like wave function encompassing the entire circuit.
When the scientists passed a weak current through this circuit, the voltage across the junction initially remained zero. In classical physics, a system trapped in such a state would stay there indefinitely unless sufficient energy were supplied to overcome the potential barrier. However, the circuit escaped its trapped state spontaneously, generating a measurable voltage. This escape was not due to any classical mechanism but through quantum tunnelling, the same principle that allows subatomic particles to pass through barriers in a microscopic world. This was analogous to a single microscopic particle passing through a barrier that a macroscopic object, such as a ball, could never cross, a phenomenon first theorised in the 1920s by physicist George Gamow, who showed that tunnelling explains certain types of nuclear decay. This experiment, thus, provided the first unambiguous demonstration of quantum tunnelling in a macroscopic system.
Their experiment further revealed that the circuit’s energy was ‘quantised’. It could exist only in discrete energy states, each corresponding to a specific current value, with no in-between states allowed. By introducing microwaves of varying frequencies, the circuit absorbed or emitted energy only in fixed quanta, precisely as quantum mechanics predicts. This behaviour was the first clear demonstration of energy quantisation at a scale far beyond individual atoms or electrons.
Energy Quantisation
In the quantum world, energy changes occur in fixed amounts called quanta. Clarke’s circuit absorbed microwave energy only in these discrete jumps, revealing ‘artificial atom’ behaviour in a man-made device.
Significance and Theoretical Implications
The results of these experiments addressed one of physics’ most enduring questions: how large could a system be and still exhibit quantum mechanical effects? The researchers showed that quantum behaviour could persist in a circuit comprising billions of particles, blurring the classical-quantum boundary. Their findings also provided empirical support for Anthony Leggett’s theories on macroscopic quantum tunnelling, earning him the Nobel Prize in 2003.
The results have profound implications of their discovery extend deep into the philosophical and practical realms of physics. Erwin Schrödinger’s famous ‘cat in a box’ thought experiment, which questioned whether a large object could exist in a quantum superposition, had long illustrated the apparent absurdity of applying quantum laws to macroscopic systems. While no real-life cat has the ability to display quantum properties, the laureates’ circuits demonstrated that a carefully engineered macroscopic system could behave consistently with quantum theory. In essence, their superconducting circuit functioned as a tangible ‘artificial atom’ capable of exhibiting quantised states and tunnelling on a visible scale.
From Fundamental Discovery to Quantum Technology
Beyond resolving theoretical puzzles, the laureates’ research provided a practical foundation for the rise of quantum technology. Their superconducting circuits became the precursors to the ‘qubit’, the fundamental unit of information in quantum computing. Unlike classical bits, which represent either zero or one, qubits could exist in superpositions of both states 0 and 1 simultaneously, allowing quantum computers to perform certain calculations exponentially faster than traditional computers.
In subsequent years, John Martinis further developed this concept, creating superconducting circuits that operated as qubits. These advances have made superconducting technology a leading approach to building functional quantum computers. Today, the same principles first demonstrated in Clarke’s Berkeley laboratory underpin quantum processors worldwide, including India’s National Quantum Computing Mission, aiming to realise practical quantum computers by 2031.
Quantum tunnelling, as shown in these circuits, also forms the basis for next-generation quantum sensors and cryptographic systems. These devices exploit the sensitivity of quantum states to detect minute changes in physical parameters, such as magnetic or gravitational fields, with unprecedented precision. In this way, the experiments of the mid-1980s have evolved from a fundamental curiosity into the cornerstone of modern quantum engineering.
Recognition and Legacy
The Royal Swedish Academy of Sciences recognised the laureates of experiments ‘on a chip that revealed quantum physics in action’. The official citation praised them for making quantum mechanical properties ‘concrete on a macroscopic scale’, marking a pivotal step in the evolution of physics from theoretical abstraction to tangible technology. The 2025 Nobel Prize carries an award of 11 million Swedish kronor, to be shared equally among the three winners—
- John Clarke, born in Cambridge, in 1942, completed his Ph.D. at the University of Cambridge in 1968 and became a professor at the University of California, Berkeley, specialising in superconductivity and magnetic field measurements.
- Michel H. Devoret, born in 1953 in Paris, France earned his Ph.D. from Paris-Sud University in 1982 and is presently a professor at Yale University and the University of California, Santa Barbara. He is further recognised for mentoring young scientists under the European Union’s Marie Sklodowska-Curie Actions (MSCA) fellowship programme, which supports international research collaboration.
- John M. Martinis, born in 1958, received his Ph.D. from UC Berkeley in 1987 and later became a professor at the University of California, Santa Barbara, where he continues to pioneer work on superconducting qubits.
In acknowledging their contributions, Olle Eriksson, Chair of the Nobel Committee for Physics, noted that quantum mechanics, though more than a century old, ‘continually offers new surprises’ and remains the foundation of all digital technology. This recognition situates the laurates’ work within a broader lineage of quantum discoveries that have progressively shaped the modern technological world, from transistors to semiconductors and now to quantum computers.
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