BY ANDREW GRANT

Physicists have seen a single particle of light and then let it go on its way. The feat was possible thanks to a new technique that, for the first time, detects optical photons without destroying them. The technology could eventually offer perfect detection of photons, providing a boost to quantum communication and even biological imaging.

Plenty of commercially available instruments can identify individual light particles, but these instruments absorb the photons and use the energy to produce an audible click or some other signal of detection.

Quantum physicist Stephan Ritter and his colleagues at the Max Planck Institute of Quantum Optics in Garching, Germany, wanted to follow up on a 2004 proposal of a nondestructive method for detecting photons. Instead of capturing photons, this instrument would sense their presence, taking advantage of the eccentric realm of quantum mechanics in which particles can exist in multiple states and roam in multiple places simultaneously.

Ritter and his team started with a pair of highly reflective mirrors separated by a half-millimeter-wide cavity. Then they placed a single atom of rubidium in the cavity to function as a security guard. They chose rubidium because it can take on two distinct identities, which are determined by the arrangement of its electrons. In one state, it’s a 100 percent effective sentry, preventing photons from entering the cavity. In the other, it’s a totally useless lookout, allowing photons to enter the cavity. When photons get in, they bounce back and forth about 20,000 times before exiting.

https://www.sciencenews.org/article/single-photon-detected-not-destroyed