Philip Walther

The theory of gravity by Einstein — general relativity — rarely intersects with quantum physics in direct experiments. Here, scientists built a 50-kilometer glass labyrinth on a table and sent single photons through it.

Light is a stream of photons. In a special waveguide, resembling a narrow corridor, a 'gatekeeper' atom usually lets them through one at a time. This phenomenon is the domain of single-particle spectroscopy.