In 1975 Tai-Tsun Wu and Chen-Ning Yang formulated the non-Abelian Aharonov–Bohm effect, and in 2019 this was experimentally reported in a system with light waves rather than the electron wave function. The effect was produced in two different ways. In one light went through a crystal in strong magnetic field and in another light was modulated using time-varying electrical signals. In both cases the phase shift was observed via an interference pattern which was also different depending if going forwards and backwards in time.