Nobel Prize in Physics 2025

This post is about the award of the Nobel prize in physics 2025 to John Clarke, Michel Devoret and John Martinis.

From the announcement, it seems that the prize was given most directly for their observation of macroscopic quantum tunnelling in 1985, when Clarke was a professor at Berkeley, Devoret was a postdoc in his group, and Martinis was a senior graduate student.

The announcement also referred to the impact this discovery had on the subsequent development of superconducting circuits and quantum computing, a field in which Devoret and Martinis are world leaders. Apart from their academic affiliations (to Yale and Santa Barbara, respectively) they are associated with Google, which is interested in quantum computing.

Background

The story goes back quite a bit, but we may pick it up in the early days of quantum mechanics. This was when it was realized that particles can quantum tunnel through barriers which would be too high for them to climb over if the particles were behaving classically. An early and famous example of the identification of quantum tunneling was in the process of alpha decay, a type of radioactivity.

A more sophisticated type of quantum tunneling was proposed by Brian Josephson after the discovery of superconductivity. Superconductors allow flow of current without resistance, basically because electrons pair up in a coordinated fashion. Josephson predicted that if a thin slice of insulating material was placed between two superconductors, pairs of electrons would pass through the barrier, in a form of quantum tunneling. This effect was eventually observed and the effect was named after Josephson (who is still alive, though no longer a practicing physicist), who received the Nobel for his discovery in 1973.

Following suggestions by Tony Leggett, Clarke's group investigated macroscopic (large) Josephson junctions at various temperatures and demonstrated quantum tunneling as well as quantization of energy in the device. This pioneering experiment led, over decades of development, to the Josephson (phase, flux and charge) qubits realized using electrical circuits (the Josephson junction can be electrically thought of as a nonlinear inductor).

Today, based on these qubits, superconducting circuits pose themselves as one of the two leading platforms for the implementation of a quantum computer. The other candidate consists of trapped ions, which has been acknowledged with a couple of Nobels (Paul, Dehmelt, Wineland). This year's prize may be understood as an acknowledgement of the large community of superconducting circuits researchers who are using these cool devices to investigate fundamental physics as well as life-changing applications.

From a broad perspective, the Josephson qubits are sometimes thought of as artificial atoms whose properties can be engineered via nanofabrication, as opposed to real atoms, whose properties are determined by nature.

Afterword

One of the cool experiments I heard Devoret give a talk on some years ago was how To catch and reverse a quantum jump mid-flight, eventually published in Nature. Transitions in quantum systems were thought to be fundamentally random and unpredictable, but using his superconducting system Devoret showed (it was actually his graduate student's idea) that such jumps could be predicted and indeed reversed deterministically. Of course, this is a nontrivial insight into the fundamental nature of quantum mechanics. But one can also apply it to practical problems such as error correction in a quantum computer.