2025 Nobel Physics Prize: Quantum Tunnelling in a Circuit Explained
John Clarke, Michel H. Devoret and John M. Martinis received the 2025 physics prize for experiments demonstrating macroscopic quantum tunnelling and quantized energy in an electrical circuit.
Timeline
- 1984–1985: Clarke, Devoret and Martinis performed the prize-recognized experiments at the University of California, Berkeley.
- October 7, 2025: The Royal Swedish Academy of Sciences announced the three physicists as joint laureates.
- December 10, 2025: The laureates received the prize during the Nobel ceremony in Stockholm.
The 2025 Nobel Prize in Physics was awarded jointly to John Clarke, Michel H. Devoret and John M. Martinis for discovering macroscopic quantum mechanical tunnelling and energy quantisation in an electric circuit. Their experiments showed that quantum effects could be measured in a collective electrical system containing many particles, rather than only in an isolated atom or subatomic particle. [1][2]
Quantum tunnelling describes a system crossing a barrier that classical physics says it lacks enough energy to overcome. A familiar analogy is a ball appearing on the other side of a wall without going over it. The analogy is imperfect, but it captures why tunnelling seems counterintuitive: quantum mechanics assigns a finite probability to outcomes that ordinary objects do not display in daily life. [2][3]
The laureates built a circuit with two superconducting components separated by a thin insulating layer, a device known as a Josephson junction. In a superconductor, large numbers of electrons form paired states and can move without electrical resistance. Under carefully controlled conditions, the circuit's many charged particles behaved together like one quantum-mechanical system spanning the device. [1][2]
Initially, the system occupied a state in which current flowed without a voltage. Classically, it would remain trapped behind an energy barrier. The experiments detected the system escaping through quantum tunnelling, producing a measurable voltage. Repeating and controlling the measurements allowed the researchers to distinguish the quantum process from ordinary thermal activation over the barrier. [1][2][3]
The team also showed that the circuit absorbed and emitted energy in specific amounts. That energy quantisation is another hallmark of quantum mechanics. Instead of accepting any continuous energy value, the circuit occupied separated levels. The combination of tunnelling and quantized levels demonstrated that the same system behaved in multiple recognizably quantum ways. [1][2]
Calling the effect macroscopic does not mean that household objects can routinely tunnel through walls. The circuit was engineered, cooled and isolated so that fragile collective quantum behavior could survive interactions with its environment. Its physical scale and large number of participating electrons made it macroscopic relative to individual particles, while the extreme experimental conditions remained essential. [2][3]
The work became foundational for using superconducting circuits as controllable quantum systems, including later research on artificial atoms and quantum technologies. The Nobel citation recognized the experimental discovery itself rather than claiming that the 1980s circuit was a modern quantum computer. Its lasting achievement was to make quantum tunnelling and energy levels visible and testable in a deliberately constructed electrical device. [1][2][3]
Sources
- Nobel Prize — 2025 physics press release
- Nobel Prize — Popular background on quantum properties at a macroscopic scale
- Royal Swedish Academy of Sciences — Scientific background for the 2025 physics prize