Counting photons without destroying them

Is the light in the cavity or not? With quantum mechanics, the answer is often "yes."

Schibli lab, University of Colorado

Detecting photons is destructive. From telescope cameras through phosphorescent screens to detectors inside particle colliders, measuring and counting photons requires absorbing them, at which point they are gone. This process obviously precludes repeat measurements, including tracking photons through space or determining if their properties change, things we can often do with other particle types. Non-destructive measurements would be a boon to quantum computing and communication, which frequently rely on photons but often lose many of them due to inefficiencies.

However, quantum physics provides a loophole involving the wave-like character of photons. Andreas Reiserer, Stephan Ritter, and Gerhard Rempe reflected light off the outside of a cavity containing a single atom. Reflection doesn't affect any of the absolute properties of the photon, but the nature of the cavity causes a resonant effect on the atom it contains, much as a wine glass can be set to ringing by sound produced some distance away. The experiment showed that multiple detections for a single photon was possible, a profound accomplishment.

Performing measurements on photons—counting them, determining their polarization or wavelength, etc.—requires interacting with them in some way. A typical example is a photodetector called a charge-coupled device (CCD), which absorbs a photon and uses its energy to generate a measurable electrical signal. That signal can be used to construct digital photographs, but the photons that produced it are destroyed in the process. Even polarizing filters, which allow light to pass through, absorb half of the incident photons and alter the polarization of the remainder.