(a) Schematic of two weakly coupled superconductors, where the collective wavefunction of the system has a phase difference between the left and right one. This relative phase, associated with a macroscopic collective wavefunction, becomes an effective macroscopic quantum mechanical quasiparticle. The energy landscape (b) of this phase quasiparticle features a tilted set of valleys, where the tilting is controlled by the current. The valleys lead to quantum mechanical level quantization, that can feature tunneling between levels in different valleys. At the lowest temperatures, the appearance of tunneling between states, observable electrically as a voltage spike, is only possible through macroscopic quantum tunneling of the collective quantum state.