(a) schematic depiction of a Josephson junction: two superconductors (with order parameter amplitude Δ and phase φ) are separated by a thin insulating barrier. When a phase difference is present, a non-dissipative current, carried by Cooper pairs, can flow across the structure (Josephson effect). (b) Potential energy landscape of a current-biased Josephson junction (tilted-washboard potential). The phase difference behaves like a particle in a potential well. The 2025 Nobel Prize in Physics was awarded for demonstrating two macroscopic quantum effects: quantization of the energy levels and quantum tunnelling between wells.