John Clarke, Michel Devoret and John Martinis’s experiments in the 1980s proved that the strange laws of quantum mechanics could govern not just subatomic particles but entire circuits visible to the eye. Their discovery of macroscopic quantum tunnelling and energy quantisation in an electric circuit won them the 2025 Physics Nobel Prize. This marks a significant interval since quantum mechanics last directly featured in a Physics Nobel, noticeable given the field’s enduring vitality, and at a time when the world anticipates profound revolutions in computing and communications. Their experiments at the University of California, demonstrated quantum behaviour in a circuit comprising two superconductors separated by an ultrathin insulating barrier, also known as a Josephson junction. In classical physics, a current flowing through this system would be trapped in a zero-voltage state unless it had enough energy to cross the barrier. But at temperatures near absolute zero, they found that the current could escape by ‘tunnelling’ through the barrier, a uniquely quantum phenomenon. The system also behaved as if it were a single large particle, with discrete energy levels instead of a continuous range. To ensure these effects were not artefacts of noise in the circuits, the team took elaborate pains to isolate them from stray microwave radiation. Their results confirmed that a superconducting phase difference, a collective property of the trillions of pairs of electrons that sustained superconductivity, behaved as a single quantum variable.