The experimental setup © Johan Jarnestad/The Royal Swedish Academy of Sciences.

For their experiments, the Nobel Prize winners built an electrical circuit made of superconductors in which the electrons join together to form so-called Cooper pairs and act together as a single, large particle.

What are Cooper pairs?

In a normal metal, electrons move independently of each other and constantly collide with the lattices of the atoms as they flow, which creates electrical resistance. At very low temperatures, however, the behavior of electrons can change fundamentally: Two electrons with opposite spin and momentum can combine to form a Cooper pair.

The special feature: Although electrons normally repel each other, a weak attraction acts in a superconductor via the vibrations of the atomic lattice. This pair formation ensures that the two electrons glide through the material in a synchronized and coordinated manner - without resistance. In other words, instead of chaotically racing around individually, the Cooper pairs move like a "team" that transports the current together.

Graphic shows the principle of quantum tunneling © Johan Jarnestad/The Royal Swedish Academy of Sciences

Physicists wondered early on whether tunneling could also show collective effects of many particles. One approach was developed using materials that become superconducting at extreme temperatures. In a superconductor, the individual electrons form pairs, known as Cooper pairs, which move synchronously and conduct electricity without resistance. Unlike individual electrons, Cooper pairs can be described as a single quantum mechanical system, represented by a common wave function.

When two superconductors are connected by a thin insulator layer, a Josephson junction structure is created that exhibits interesting quantum mechanical effects. The Josephson junction structure has been used, among other things, for precise measurements of physical constants and magnetic fields. Theoretical work on macroscopic quantum tunnels, such as that of Anthony Leggett (Nobel Prize 2003), inspired new experiments - including the work of Clarke, Devoret and Martinis.

The experiment of the Nobel Prize winners

John Clarke, professor at the University of California, Berkeley, led a research group specializing in superconductors and Josephson junctions. Michel Devoret joined as a postdoc in the mid-1980s, together with doctoral student John Martinis. Together they wanted to demonstrate macroscopic quantum tunneling.

They fed a weak current into the Josephson junction and measured the voltage, which was initially zero. The system was in a state that did not allow any voltage. They then investigated how long it took for the system to escape from this state by tunneling and generate voltage. Since quantum mechanics contains an element of chance, they took numerous measurements and presented them in diagrams - similar to the determination of half-lives of radioactive nuclei.

The tunneling showed that the Cooper pairs in the system act synchronously as a single large particle. The researchers received further confirmation when they observed the quantized energy levels of the system. They excited the system with microwaves of different wavelengths: some waves were absorbed, causing the system to jump to a higher energy level. This demonstrated that a system with more energy stays in the zero-volt state for a shorter time - exactly as predicted by quantum mechanics.

Consequences for theory and practice

The experiment has both theoretical and practical significance: it shows that macroscopic effects - such as a measurable voltage - can arise directly from a quantum mechanical system comprising many particles. Leggett compared the prizewinners' system to Schrödinger's cat to illustrate that quantum effects can also be measured in large systems, albeit much smaller than a real cat.

Such macroscopic quantum states open up new possibilities for experiments and applications in quantum research. They can serve as artificial atoms on a large scale, for example in the simulation of other quantum systems or in quantum computers.

Martinis used precisely this energy quantization in circuits that functioned as quantum bits, with the lowest energy state representing "0" and the first increased state representing "1".

Superconducting circuits are among the most promising approaches for future quantum computers. The 2025 Nobel Laureates have thus both enabled practical applications in physics laboratories and expanded our theoretical understanding of the quantum world.

Source: Royal Swedish Academy of Sciences