Photons caught in the act

Physicists have made the first “movie” of a microwave pulse transitioning from the quantum-physics world to the classical-physics world.

Reporting in the Sept. 25 Nature, the researchers say that their method may help in understanding at what point in nature quantum physics ends and classical physics begins. It could also shed light on how to keep information inside future computers that would take advantage of quantum physics.

Quantum objects — generally, anything that’s small enough to be ruled by quantum physics — can exist in multiple forms at the same time. An atom, for example, can be in two places at the same time, as can the crests and troughs of electromagnetic waves, such as in a microwave pulse.

Any disturbance from the outside world can cause a loss of this quantum innocence — loss of coherence, in physics parlance. The state of the object becomes progressively more definite, until the object picks one state, as would be expected from everyday experience. Normally, physicists cannot capture all the information contained in quantum coherence, since a measurement produces an answer that’s just one in a range of possible outcomes.

Serge Haroche of the Ecole Normale Supérieure and the Collège de France in Paris and his collaborators have now observed this transition in a microwave pulse trapped between two mirrors. The researchers probed the pulse by shooting thousands of rubidium atoms across it, one atom at a time. Each atom extracted a small amount of information from the pulse, without destroying its coherence.

The near-perfect mirrors allowed the photons in the microwave pulse to bounce back and forth, establishing a standing wave that lasted several milliseconds. Through the reflections, the pulse, bit by bit, lost coherence, and the position of the peaks and troughs came closer to being definite.

At the same time, the path the pulse follows to lose coherence is also different each time. To obtain the most complete picture of the process, the researchers repeated the measurement thousands of times on identical pulses.

“This is fascinating work,” comments physicist Mikhail Lukin of Harvard University. It is unique, he adds, “in that it allows one to look directly, in real time, into what happens with a quantum state of light as it loses coherence.”

Haroche says that the team is constantly improving the apparatus so it can preserve the coherence of pulses of higher intensities. Higher-intensity pulses tend to behave more like classical than quantum objects. Thus, the researchers hope to learn more about the boundary between the quantum and the classical world.

Haroche also says that his team might be able to learn how to use the atoms to restore a pulse’s coherence before it is completely lost. This ability could help researchers design quantum data storage for future quantum computers. Such machines would use the multiple states of quantum objects to essentially perform myriad calculations all at once.