Nobel Prize in Physics 2025

The Nobel Prize in Physics has been awarded to John Clarke, Michel H. Devoret and John M. Martinis. "for the discovery of macroscopic quantum mechanical tunnelling and energy quantisation in an electric circuit". Synopsis by M. Zgirski and M. Foltyn.

The Schrödinger’s Cat gets bigger
Synopsis by Maciej Zgirski and Marek Foltyn, CoolPhon Group, MagTop, ON6.4

Usually effects of quantum tunneling and energy quantization are associated with behavior of single atoms or molecules. Electrons occupy orbitals of definite energy and can change their state in the process of absorbing or emitting well-defined energy quanta – photons. In the Sun, two protons fuse by tunneling over the Coulomb barrier to form deuterium. But can we extend the direct applicability of quantum mechanics to macroscopic objects, i.e. objects visible with a bare eye?

Yes, we can. This year Nobel Prize has been awarded “for the discovery of macroscopic quantum tunneling and energy quantization in an electric circuit”. In their two seminal experiments (Phys. Rev. Lett. 55, 1908 (1985), Phys. Rev. Lett. 55, 1543 (1985)) laureates John Clarke, Michel Devoret and John Martinis showed that these two hallmarks of quantum mechanics can be observed also for macroscopic artificially-defined objects. They studied the escape rate from the superconducting state for current biased Nb-NbO_x-PbIn tunnel Josephson junctions embedded in a properly defined electric circuit (Fig.1). The escape rate measured at the lowest temperatures (< 30 mK) appeared significantly larger than that expected from thermal activation, signaling appearance of a new escape mechanism. The laureates identified this mechanism as Macroscopic Quantum Tunneling (MQT) i.e. the process in which a collective state of many Cooper pairs switches between two macroscopic wavefunctions, although the two configurations are separated by a barrier which forbids the classical transition (Fig.2). Unlike familiar tunneling observed in real space, such MQT happens in a space of the superconducting phase and involves its abrupt change leading to the appearance of a non-zero voltage measured across the junction (Fig.2).

Before the escape happens the electric current and superconducting phase across the junction oscillate around the local energy minimum. It is basically the behavior of a harmonic oscillator. At sufficiently low temperatures the oscillator becomes dominated by quantum effects and the energy of the macroscopic oscillatory current becomes quantized (Fig.2). The higher the energy of the quantum state the easier the escape. The laureates were able to capture this effect by resonantly activating the energy levels with microwaves and measured the enhancement of the escape rate at the expected photon frequencies. The ability to address the junction with microwave photons and force the transitions makes the designed circuit similar to an atom. Since the trapping potential is not ideally parabolic, the oscillator is anharmonic and the spacing between energy levels is not equal.

This pioneering study initiated the field of quantum electronics, in which electric circuits are described fully quantum-mechanically and their components can be treated like artificial atoms. The best known example of such a circuit is a superconducting qubit, one of the current leaders in the race for a quantum computer.

Fig.1. Left: A Josephson tunnel junction with a difference in the superconducting phase across it. Right: An electric circuit in which the junction is embedded. From the circuit point of view the junction acts as a nonlinear inductor (denoted with the symbol X in the scheme). Together with a capacitor it creates an anharmonic oscillator, for which the energy of the oscillating current is quantized.

Fig.2. Left: A collective wavefunction of many Cooper pairs trapped in a local minimum of a confining potential corresponds to a superconducting current flowing through the junction. The wavefunction leaks out through the potential barrier allowing system to macroscopically tunnel in the phase space. this leads to the emergence of a voltage across the junction, which is detected experimentally. The superconducting ground-state wavefunction and 4 energy levels are depicted schematically. A quantum blurring of the superconducting phase is analogous to the delocalization of a quantum particle confined in a potential defined in a real space and means that the current flowing through the junction is not a classical variable, but instead it is given by the superposition of many macroscopic currents, each with probability described by the wavefunction. Right: The junction is probed with a current bias. When the testing supercurrent is sufficiently high, the junction is in a metastable state, characterized by its escape rate. An escape event leads to the development of a voltage across the junction.