Feeling the heat. Laser-warmed C70 molecules lose their quantum characteristics as their energy level rises.

Quantum Transition Heats Up

The quantum world and the classical world are getting harder and harder to distinguish. In a paper published this week, physicists show that there's a smooth transition between the outlandish behavior typical of the very small and the ordinary behavior of larger objects such as cells and baseballs and people.

The quantum world and the everyday world seem very different. A small quantum-mechanical object such as an atom or a photon can do several seemingly contradictory things at the same time (take both a left path and a right path at a beam splitter, be polarized both horizontally and vertically, and be "spin up" and "spin down"), whereas familiar macroscopic objects are always one thing or the other (alive or dead, on the left side of a barrier or the right, prone or supine). It's been a physical and philosophical puzzle to figure out what accounts for this difference in behavior.

In this week's issue of Nature, Anton Zeilinger and his colleagues at the University of Innsbruck, Austria, describe a clever experiment that used medium-sized objects--cagelike carbon molecules known as buckminsterfullerenes--to probe the boundary where quantum behavior ends and classical behavior takes over. The researchers took C70 molecules (70-carbon buckminsterfullerenes, enormous by quantum standards), heated them with a laser beam, and shot them through a series of gratings that essentially forced each molecule to choose among several paths. According to one leading theory, the behavior of the objects should change from quantum to classical as the molecules heat up--the energetic molecules would radiate energy that destroys their delicate multiple-path-at-once superposition.

By tuning the power of the laser, the team could make the C70 molecules as hot as 3000 kelvin or as "cool" as a mere 1000 kelvin. Sure enough, the hot molecules behaved like classical objects, taking one path at a time, and the cool molecules behaved like quantum objects, taking several paths at once, just as theorists expected.

"I love it. It's superb and great," says Wojciech Zurek, a physicist at Los Alamos National Laboratory in New Mexico. The Innsbruck group hopes to push the boundaries between quantum and classical still further, but the dream of getting very large objects to act like quantum creatures is still far away, says Markus Arndt, an Innsbruck physicist and co-author of the paper.

Related sites
More about "decoherence," an effect that many theorists think is responsible for the quantum-classical transition
Details about how to use large molecules to measure the effects of decoherence