Photons detected without damage
Technique to sense particles without changing their quantum states holds promise for quantum networks.
One of the cornerstones of quantum theory is the principle that you cannot measure any property of an object without affecting the object itself. Physicists, however, have now devised a way to detect single photons of visible light without changing any of the information that they carry. Others had done the same with microwave photons, but this is the first time that it has been done in the part of the spectrum that could matter for a future ‘quantum Internet’.
The conventional way to detect a single particle of light is to catch it with a sensor, absorbing its energy but destroying the particle in the process. In recent years, physicists have developed ways to extract only part of the information carried by a quantum particle such as a photon, thus affecting its quantum state without completely destroying it — a set of methods known as weak measurement.