A SINGLE atom that acts as a light switch could be key to finding the divide between the quantum and classical realms.
Arno Rauschenbeutel of the Vienna University of Technology, Austria, and colleagues created an optical switch by placing a silica cylinder 35 micrometres across in contact with two optical fibres – one for input, one for output. The cylinder's resonant frequency ensures that when light is in the input fibre, it enters the cylinder, spirals around and then exits via the output fibre. This is the "on" state for the switch. For the off state, the team put a single atom of rubidium near the outer wall of the cylinder. This changed the cylinder's resonant frequency, stopping the light from entering (arxiv.org/abs/1306.1357).
The next step is to put the atom, whose small size gives it quantum properties, into a , one which turns the switch on and the other off. This would put the light pulse in a superposition too.
A single photon has quantum properties, but the point at which a more intense pulse would stop behaving quantum mechanically isn't known. Upping the number of photons in the pulse and monitoring when it loses its superposition should reveal this.
This article appeared in print under the headline "Atomic switch seeks quantum frontier"
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