It sounds like a simple task: Count the number of photons or particles of light in a light beam without destroying them in the process. But in fact, it took 17 years to accomplish the feat, researchers report this week in Nature.

A team at the École Normale Superiéure in Paris fired specially primed atoms through a pair of the most reflective mirrors ever built [see image], gradually revealing the number of photons bouncing between their reflective surfaces. Their method provides a high-resolution glimpse of the eerie "collapse" of a quantum system and may be useful in developing future quantum-based technologies.

"It's a textbook illustration of quantum field theory," says physicist Serge Haroche, who led the Paris team.

Counting photons in itself is nothing new. A standard optics tool absorbs incoming photons and converts them to electricity, resulting in an audible click—a photon's death knell. An "ideal" measurement would allow repeated sampling of the same photon, Haroche says.

In an attempt to achieve that, his group employed a trick known as quantum nondemolition in which one property of a system—in this case the number of photons in an electromagnetic field—is measured precisely by obscuring the value of a second property, the phase of the electric field, which is sort of like the position of the hands on a clock. Nondemolition is the flip side of the famous uncertainty principle, which states that certain properties such as position and momentum cannot be precisely measured at the same time.

The Paris physicists, including Michel Brune and Jean-Michel Raimond, prepared cold rubidium atoms, each containing a high-energy electron that oscillated much like a ticking clock between two energies at a certain frequency—a so-called Rydberg state. They adjusted the "hands" of each clock so that they were at a specific phase and shot up to 100 atoms individually through a golf ball–wide mirrored cavity filled with laserlike microwaves.

Initially, the light was in a typically nutty quantum state that simultaneously numbered zero photons, one photon and so on up to seven photons. But each passing atom slightly scrambled the microwaves' combined phase, nudging the light toward a definite number of photons. The scrambling also shifted the phase of each atom, which the team measured. After several dozen atoms, the number of photons settled into one value.

True to quantum weirdness, when the researchers repeated their count, the field randomly collapsed to a number between zero and seven photons each time (with an 80 percent chance of numbering from two to four). The scientists, who demonstrated nondemolition for a single photon eight years ago, had long dreamed of counting multiple photons, Haroche says.

Key to the team's success, he explains, was the high quality of the mirrored cavity, because it allowed photons to bounce in lockstep for a tenth of a second, or long enough for the light to circle Earth's equator—plenty of time for enough atoms to zoom through before the photons disorganized.

"There were quite a few technological feats," says experimental physicist Luis Orozco of the University of Maryland. "That convergence into a definite answer was to me a very impressive thing."

The technique may help create exotic quantum states, he says, that could play a role in quantum-based computers or communications systems. If only it could explain why a quantum measurement yields a random outcome.