How are photons measured

Non-destructive detection of individual optical photons

Scientists from the Max Planck Institute for Quantum Optics in Garching were able to detect individual photons from the visible part of the spectrum for the first time without destroying them. So far, this has only been successful in the microwave range, for which physicist Serge Haroche received the Nobel Prize in 2012. The new detection method, which Andreas Reiserer and his colleagues are now reporting on in the journal “Science”, could be used in optical quantum information processing.

Illustration of the measuring principle

The scientists used an optical resonator to detect the photons. Light can be reflected back and forth between two mirrors and stored in this way. Inside the resonator, Reiserer's team placed a single rubidium atom and prepared it in such a way that it assumed two quantum mechanical states at the same time. With a laser set up a few meters away, they then sent weak light pulses, which on average contained less than one light particle, to the resonator. These were reflected there, which resulted in a change of state in the rubidium atom, which the scientists were able to measure. In this way they could indirectly detect the photons. The reflected photons were then detected a second time for comparison with a conventional detector. 74 percent of all light particles could be detected with the structure used, whereby this efficiency can be increased, according to the researchers.

Detectors for single photons, especially for those in the optical range, have been available for a long time and are used in many areas of science. Usually, however, the energy of the photon is completely absorbed, so that it no longer exists after its detection. Haroche and his team showed that non-destructive detection is possible as early as 1999, using microwaves. “The differences between optical and microwave photons are of course also reflected in the possible applications,” emphasizes co-author Stephan Ritter. The new method can be used in particular in optical quantum information processing and for quantum networks. "On the basis of the mechanism we have demonstrated, it should also be possible in the future to implement a quantum gate, i.e. the elementary building block of a quantum computer, between a photon and an atom."