Press release: The Nobel Prize in Physics 2025

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   Tunnelling is not only for tiny particles. In 1984–1985 John Clarke, Michel Devoret and John Martinis showed that an electric circuit on a one-centimetre chip, with billions of electron pairs, tunnels like a single particle and takes in energy only in steps. For that they received the 2025 Nobel Prize in Physics.

   Press release: The Nobel Prize in Physics 2025 · 2025

   A circuit you can hold in your hand can tunnel like a single particle, if it is cold and quiet enough.

   Until now, tunnelling has been the business of tiny particles: an electron, a proton, an alpha particle. When you put many particles together, quantum effects usually wash out, which is why a ball does not pass through a wall. In the early 1980s the physicist Anthony Leggett asked a precise question: can a large, human-made object, made of very many particles, behave quantum-mechanically as a whole? If so, we might see it tunnel. At the University of California, Berkeley, professor John Clarke, postdoctoral researcher Michel Devoret and doctoral student John Martinis built a circuit out of superconductors, materials through which current flows with no loss at all when they are very cold. Between two pieces of superconductor was a thin insulating layer, a so-called Josephson junction. Cooled close to absolute zero, the circuit behaves strangely: billions of electron pairs move together, like a single giant ‘particle’ that fills the whole circuit. At first the current flows with no voltage at all, like a marble caught in a dip. Classically, the marble should stay there. Yet it escaped by tunnelling, and the escape showed up simply, on an instrument: a voltage suddenly appeared. They also showed the second quantum feature: the circuit took in energy only in portions of precise sizes, not in any amount, like an atom. That is why people speak of ‘artificial atoms’. Later Martinis used exactly these energy steps to build a qubit, the unit of information in a quantum computer: the lowest step means zero, the first step up means one. Superconducting circuits are today one of the main routes to quantum computers. The 2025 Nobel Prize in Physics went to the three ‘for the discovery of macroscopic quantum mechanical tunnelling and energy quantisation in an electric circuit’.

   “It is wonderful to be able to celebrate the way that century-old quantum mechanics continually offers new surprises. It is also enormously useful, as quantum mechanics is the foundation of all digital technology.”

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