Quantum mechanics says the world is innately random: an isolated system will remain in a fuzzy “superposition” of all possible states until we measure it, at which point it will collapse with a certain probability into just one state. Recently physicists have discovered that this collapse can be witnessed step-by-step if a series of special quantum non-demolition (QND) measurements are performed. Now, Serge Haroche and colleagues from the Ecole Normale Supérieure have developed a new QND technique to see, for the first time, the step-by-step collapse of a coherent light field.

Their system is a box lined with superconducting mirrors, which can keep the photons from a coherent microwave field bouncing around inside for a fraction of a second. Because of the Heisenberg uncertainty principle, which states that two “conjugate” properties cannot be known simultaneously with precision, the microwave field is coherent at the expense of having a well-defined number of photons. This means that, before measurement, the system is in a superposition of several different photon numbers -- in this case between zero and seven.

To watch this superposition collapse the researchers used a method they pioneered in 1999, in which individual atoms are sent in to interact with the field’s photons without destroying any of them. Electrons in these atoms oscillate between two excited states, and the rate of the oscillation is governed by the photon number. For each measurement of an atom’s oscillation rate, therefore, the researchers get an answer for how many photons are in the box.

For the first few measurements the answers are evenly distributed from zero to seven. This means that not enough information has been gathered to ascertain the number of photons, since the system is still in a superposition. But after many measurements the cumulative distribution of answers begins to centre on a particular number, revealing that the system is collapsing into a well-defined state. And as predicted by the probabilistic nature of quantum mechanics, if the whole measurement procedure is performed again, this particular number could well be different.

“All previous methods for counting discrete photons have been destructive,” Haroche told physicsworld.com. “The photon, like the marathon soldier, was dying delivering its message. Our experiment demonstrates that counting is not a curse…this was predicted by quantum mechanics but never demonstrated.”