2025 Nobel Prize in Physics goes to the quantum world
In the mid-1980s, US researchers John Clarke, Michel H. Devoret, and John M. Martinis were able to prove in experiments that a counterintuitive phenomenon from the quantum world also occurs in macroscopic systems: the tunnel effect, in which individual quanta can “pass through walls”. Prior to their research, it was assumed that this was only possible in systems with single particles. Their observation is of great importance for quantum computing, among other things. Now the trio has been awarded the 2025 Nobel Prize in Physics.
To make the tunnel effect understandable, physics uses comparisons. Imagine a ball that bounces off a wall many times and suddenly finds itself on the other side without breaking through or flying over the wall. Or imagine a marble reaching the other side of a hill without the kinetic energy needed to overcome or roll around it.
In the quantum world, this effect, which seems to be guided by ghostly forces, was already known when the three newly crowned Nobel Prize winners conducted their experiments. Among other things, this tunnel effect plays a central role in explaining radioactive decay, in which alpha (α) particles escape from the atomic nucleus, overcoming the barrier surrounding it. Through systematic observation of many nuclei of the same type, the time until the tunneling effect occurs can be measured and expressed in terms of the so-called half-life: this indicates how long it takes for half of all nuclei in a sample to decay.
What is possible for individual α particles, electrons, and atoms—physics refers to these as microscopic quantum systems—could also be effective in systems with many particles, i.e., in macroscopic systems. To investigate this hypothesis, Clarke, Devoret, and Martinis, then a doctoral student, conducted their groundbreaking experiments at the University of California, Berkeley, beginning in 1984. They integrated an electrical circuit with two superconductors—materials in which electricity flows without resistance—onto a chip measuring approximately one centimeter. They separated the superconductors with a thin insulating layer that did not conduct any electricity.
The team was able to build on Nobel Prize-winning research
At the time of the experiments, it was known from Nobel Prize-winning research that electrons in superconducting materials form pairs (Cooper pairs). Leon Cooper received the Nobel Prize in Physics for this discovery in 1972, together with his partners John Bardeen and Robert Schrieffer. Their observation: although electrons actually repel each other, they form a solid unit as Cooper pairs in a superconductor. In this form, they can be described as a wave function. Brian Josephson received the Nobel Prize the following year for discovering the “Josephson junction” named after him: Cooper pairs are able to overcome gaps and insulators and form an electrical circuit with electron pairs on the other side of the "Josephson junction,” behaving like a single particle. They enter a collective state with a common wave function.
For their experiments, the three US researchers not only constructed the silicon chip with the two superconductors separated by a “Josephson junction." They also surrounded it with a structure that shielded their measurements from any interference. In the experiments, they then observed that initially—as expected in a macroscopic system—no electrical voltage was measurable, but that voltage then built up without any further intervention. In many repetitions, they were able to determine the duration until the zero-voltage state ended, analogous to half-life. This could only be explained by the tunnel effect, which in this case occurred in a quantum mechanical system with many billions of Cooper pairs in the macroscopic superconductor on the chip. They had thus proven that the effect is also effective on a macroscopic level. In addition, their system exhibited quantized energy levels. To measure them, the researchers fed microwaves of different wavelengths into the zero-voltage state. Some of these absorbed, causing the energy level to rise in each case. The zero-voltage state in the system ended more quickly when the system contained more energy. This phenomenon was previously only known from microscopic quantum systems. The findings gained in the Nobel Prize winners' experiments on the tunnel effect and energy quantization now play a central role in the implementation of quantum computers.
The Royal Swedish Academy of Sciences presents here the research of Clarke, Devoret, and Martinis in a clear and detailed manner. This includes easy-to-understand explanatory videos and slideshows, as well as scientific background information.