Crueltyfree: counting photons without killing them.

Disclaimer. No particles were harmed in the making of this experiment. Physicists have found a way to count photons as they zip along, without destroying them. The researchers say that the technique will enable scientists to probe quantum effects that so far have been the subject only of speculation.

In physics labs, detecting light has long been synonymous with absorbing photons. Typically, the photons cease to exist and the light's energy transforms into an electrical signal. Physicists can count single photons--but they haven't been able to count them and keep them.

"Up to now, when you measure light, it's a destructive process," says Serge Haroche of the Ecole Normale Superieure of Paris. Now, Haroche and his colleagues have shown how to count photons nondestructively while they bounce back and forth between two mirrors.

Haroche's team began by introducing small numbers of photons into the space between two niobium-coated screens. Kept at less than 1 kelvin, the niobium became superconducting, which made the screens into virtually perfect mirrors. The photons could bounce back and forth up to a billion times, lingering inside their hall of mirrors for more than a tenth of a second.

The team then shot rubidium atoms one by one across the photons' path. The atoms were in a highly excited state in which their electrons were especially sensitive to the photons' electric fields. The electrons responded with a shift in the timing of their orbits, essentially acting as the hands of microscopic clocks. The amount of shift was proportional to the number of photons between the two mirrors.

Quantum uncertainty dictates that the number of photons could not be well defined at the start of the experiment. Measuring the influence of the photons on a single rubidium atom yielded only incomplete information about the number of photons. But after the researchers had shot about 100 atoms through the chamber--gaining information and reducing uncertainty at each step--the number of photons converged to a definite value. Subsequent measurements confirmed that count. So far, the team has managed to count up to seven photons, Haroche says.

While the photons didn't die, their lives would never be the same. In any experiment, measuring one physical quantity with increasing precision leads to increased fuzziness in a related quantity. In this case, obtaining a precise count of the photons came at the expense of losing knowledge about the relative timing, or phase, of the photons' wavelike fluctuations. The findings appear in the Aug. 23 Nature.

David Hume of the National Institute of Standards and Technology in Boulder, Colo., says that the results are "an elegant demonstration of the measurement process in quantum mechanics."

The experiment highlights a little-known aspect of quantum physics: When quantifies go from a fuzzy state to one with a precise value, the transition can take place in small increments. In that way, measurements can extract partial information (SN: 5/12/07, p. 292). Haroche says that his team's setup could be a means for testing new quantum phenomena in which photons occupy multiple states simultaneously. "Quantum physics textbooks are illustrated by thought experiments," Haroche says. "Now we are doing those experiments."