A celebrated experiment in 1,801 showed that light passing through two thin slits interferes with itself, forming a characteristic striped pattern on the wall behind. Now, physicists have shown that a similar effect can arise with two slits in time rather than space: a single mirror that rapidly turns on and off causes interference in a laser pulse, making it change colour.

The result is reported on 3 April in Nature Phys ics¹. It adds a new twist to the classic double-slit experiment performed by physicist Thomas Young, which demonstrated the wavelike aspect of light, but also — in its many later reincarnations — that quantum objects ranging from photons to molecules have a dual nature of both particle and wave.

The rapid switching of the mirror — possibly taking just 1 femtosecond (one-quadrillionth of a second) — shows that certain materials can change their optical properties much faster than previously thought possible, says Andrea Alù, a physicist at the City University of New York. This could open new paths for building devices that handle information using light rather than electronic impulses.

Imperial physicists have recreated the famous double-slit experiment, which showed light behaving as particles and a wave, in time rather than space.

The experiment relies on materials that can change their optical properties in fractions of a second, which could be used in new technologies or to explore fundamental questions in physics.

The original double-slit experiment, performed in 1,801 by Thomas Young at the Royal Institution, showed that light acts as a wave. Further experiments, however, showed that light actually behaves as both a wave and as particles—revealing its quantum nature.