For the first time, physicists have observed in photons one of the weirdest and most basic phenomena in quantum mechanics: that field of physics where subatomic objects follow strange, unfamiliar laws. In the quantum world, particles like photons spend most of their time in a bizarre condition called a superposition, meaning the particles exist in several possible states at once. The superposition collapses into one of its component states only when the particle is measured. Amazingly, physicist Serge Haroche and his team at École Normale Supérieure in Paris reported in August that they were able to watch the process of this collapse as it happened in a photon, one of the most difficult—and most useful—particles to work with in experimental physics.
The physicists used a variant of a complicated method for measuring photons they had first devised in 1999—the only existing way to count photons without destroying them in the process. They shoot atoms, each with a widely orbiting electron, through a photon stream, and then measure how much the photons knock the electrons out of phase. The degree to which the electrons move out of kilter indicates the number of photons present.
This ability to count photons repeatedly without destroying them now allows physicists to observe the superposition collapse. A microwave field is put into a superposition of eight states, in which the states represent the presence of zero to seven photons. While studying photons trapped in a cavity for a tenth of a second, the scientists noticed that at first the electrons were knocked out of phase in a random way, indicating that not enough information had been gathered from the photons to create an actual measurement. Gradually, the possible existing states of the photons became more limited as the measuring continued, until finally all the electrons were being affected in the same way. This suggests that the superposition had finally collapsed into a well-defined, and now measured, state, with a particular number of photons.
“This is the kind of research that people will immediately start teaching in physics classes when talking about quantum mechanics,” says Mikhail Lukin, a physicist at Harvard University who specializes in quantum optics. “This is the first time this basic building block of physics has been directly observed in a very beautiful, clean, textbook kind of way.”
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