Nobel Prize in Physics 2025: Quantum properties on a human scale

In a ceremony held in Stockholm on 9 December, the Royal Swedish Academy of Sciences awarded the 2025 Nobel Prize in Physics to John Clarke, Michel Devoret and John Martinis for demonstrating that quantum mechanics can be observed in a hand‑sized electrical circuit. Their experiments, carried out in the early 1980s at the University of California, Berkeley, showed that a superconducting loop containing billions of paired electrons could tunnel through an energy barrier and that its energy was quantised in discrete steps, phenomena that had until then been confined to the realm of single atoms.

A Hand‑Sized Quantum Leap

The trio’s key device was a tiny chip, only a centimetre across, that housed a Josephson junction, a pair of superconductors separated by a microscopic insulating layer. When a weak electrical current was pushed through the junction, the electrons inside the superconductors paired up into so‑called Cooper pairs. These pairs behaved as a single, collective wave function that spanned the entire loop. In classical physics, a loop like this would sit in a zero‑voltage state forever unless an external voltage were applied. Quantum mechanics, however, allowed the collective wave function to tunnel through the insulating barrier, emerging on the other side as a measurable voltage. By repeatedly measuring the time the circuit stayed in its zero‑voltage state before tunnelling, Clarke, Devoret and Martinis gathered a statistical distribution that matched the predictions of quantum theory.

Equally striking was the observation that the circuit’s energy levels were not continuous but came in discrete quanta. By irradiating the loop with microwaves of different frequencies, the researchers could selectively excite the system from its ground state to higher energy states. Each excitation produced a sharp jump in the probability of tunnelling, confirming that the energy of the collective wave function was quantised. The experiments thus brought two cornerstone concepts of quantum mechanics, tunnelling and quantisation, into a macroscopic, laboratory‑scale system that could be handled with ordinary tools.

From Superconductors to Super‑Atoms

The experiments were more than a curiosity; they forged a new kind of artificial atom. In a conventional atom, electrons occupy distinct shells, and the atom’s energy levels are set by the balance of electromagnetic forces. In the superconducting circuit, the Cooper pairs formed a macroscopic wave function that behaved as if it were a single particle. The loop could therefore be treated as an “artificial atom” whose ground and first excited states could be addressed with microwaves. This insight laid the groundwork for the first superconducting qubits, a technology that has become one of the leading platforms for building quantum computers.

John Martinis, who later became a professor at the University of California, Santa Barbara, leveraged the same energy‑quantised circuit to encode binary information. The ground state represented logical zero, the first excited state logical one, and the system’s coherence times were long enough to perform quantum gates. Over the past decade, superconducting qubits have scaled from a handful to several dozen units, with companies like Google and IBM pushing towards fault‑tolerant architectures. The 2025 Nobel laureates’ original experiments remain the conceptual foundation of this progress.

Implications for Computing and Fundamental Physics

The implications of macroscopic quantum tunnelling extend beyond the laboratory. In quantum computing, the ability to maintain coherence in a macroscopic system is essential for scaling up the number of qubits. The demonstration that a system of billions of electrons can exhibit coherent quantum behaviour suggests that larger, more complex quantum devices are feasible. It also offers a testbed for exploring quantum error correction schemes that rely on collective states, potentially accelerating the arrival of practical quantum machines.

On the theoretical front, the experiments challenged the long‑standing notion that quantum effects fade away at macroscopic scales. By creating a tangible analogue of Schrödinger’s cat, an entire superconducting loop that could be in a superposition of two voltage states, the work bridged the gap between abstract quantum theory and observable phenomena. It also sharpened the understanding of decoherence, the process by which quantum systems lose their coherence to the environment, a key obstacle in both quantum computing and quantum metrology.

The Nobel recognition of Clarke, Devoret and Martinis underscores the enduring relevance of foundational physics. Their 1980s experiments, performed with modest equipment in a Berkeley lab, opened a new chapter in quantum technology. As quantum processors grow in size and capability, the principles they uncovered will continue to guide researchers toward devices that harness the strange, yet powerful, rules of the quantum world.