Controllable Josephson junction for photon Bose-Einstein condensates

Josephson junctions are the basis for the most sensitive magnetic flux detectors, the definition of the unit volt by the Josephson voltage standard, and superconducting digital and quantum computing. They result from the coupling of two coherent quantum states, as they occur in superconductors, superfluids, atomic Bose-Einstein condensates and exciton-polariton condensates. In their ground state, Josephson junctions are characterised by an intrinsic phase jump. Controlling this phase jump is fundamental for applications in computing. Here, we experimentally demonstrate controllable phase relations between two photon Bose-Einstein condensates resulting from particle exchange in a tuneable potential landscape with all-optical control. Our experiment realises an optical analogue of a controllable 0,\pi-Josephson junction, which can form a building block for ultrafast spin glass simulators and oscillatory neural networks. Further applications include the use as a phase battery for engineering topological states of light.