Cool light to bring quantum magic into the real world

Light tends to heat things up; less obvious are its dramatic cooling capabilities. Now it seems light's cold side could provide a cheaper, easier way to explore the limits of quantum theory.

Quantum mechanics describes the behaviour of electrons and atoms, but not everyday "macroscopic" objects. One way to probe where and why quantum laws break down is to induce quantum behaviour in ever-larger objects. At room temperature, thermal vibrations destroy delicate quantum states. So the largest object to attain a quantum state, a microscopic lever in a "superposition" of still and vibrating states, was cooled cryogenically to 25 millikelvin.

As cryogenics are expensive and tricky to use, a team led by Oskar Painter of the California Institute of Technology in Pasadena etched a pattern onto a silicon strip about 15 micrometres long. This allowed it to trap light, as well as mechanical vibrations due to heat, but only at certain, resonant frequencies.

Conventional cryogenics chilled the strip to 20 kelvin but cheaper lasers completed the cooling. Applying a laser of a frequency just below the strip's resonant optical frequency prompted the laser's photons to siphon energy from the strip's trapped mechanical vibrations and also reach resonant optical frequency.

Entangled light

The reduction in vibrational energy cooled the strip to 200 millikelvin. In principle, this is cold enough to induce quantum behaviour.

At this temperature, photons from the laser might be "entangled" with the strip, a type of quantum behaviour in which two or more objects become linked no matter how far apart they are in physical space, says team member Simon Gr?blacher of the University of Vienna in Austria.

The device could help determine the size at which objects cease to exhibit quantum behaviour ? if they do at all. Erika Andersson of the Heriot-Watt University in Edinburgh, UK, says, "One of the big unresolved questions in physics concerns the boundary between quantum mechanics and classical physics. This work is a major step in trying to push quantum mechanics to its limits."

Journal reference: Nature, DOI: 10.1038/nature10461.

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