Weirdness Expands Its Turf

Big surprise. Like small atoms, bulky C

Erwin Schrödinger's pipe dreams aside, no sane physicist would try to do a quantum-mechanical experiment with a cat. Being a large object, a cat appears to follow the classical laws of prequantum physics; quantum mechanics tends to hold sway over small objects, such as atoms. But now, physicists are breaking down that tidy distinction by showing that a cluster of 70 carbon atoms--a veritable monster by quantum-theory standards--is governed by quantum-mechanical rules.

molecules obeyed quantum-mechanical rules.
CREDIT: Sussex Fullerene Research Centre

Small things like neutrons and protons clearly obey Werner Heisenberg's dictum: The better you understand their position, the less you're able to predict their momentum. As objects get larger, though, that principle and other quantum effects become harder to measure. Some physicists, including Roger Penrose of the University of Cambridge, U.K., argue that quantum effects simply break down as objects get larger, causing big things to behave classically. But Anton Zeilinger of the University of Vienna in Austria disagrees. He thinks it's possible to show quantum effects in bulky objects: "It's just a question of the skill of the experimenter and how much money he has to perform the experiment."

Grant in hand, Zeilinger and his colleagues decided to probe the hazy border using the C70 molecule, the ungainly bigger brother of C60 (buckminsterfullerene). The team shot a beam of C70 through an adjustable slit, an experiment that has been used to reveal quantum effects with small objects such as atoms. Using a sensitive laser detector, they measured the range of momenta of the C70 molecules that had passed through the slit. Sure enough, as the slit size decreased, the range of the molecules' momenta got broader and broader. In other words, the more that was known about the molecules' positions, the less was known about their momenta. The C70 was behaving "very, very precisely" as a quantum-mechanical object should, says Zeilinger, who has posted the results on the Los Alamos preprint archives and submitted them to Physical Review Letters.

"I think these are wonderful experiments," says Christopher Monroe, a physicist at the University of Michigan, Ann Arbor. And although Zeilinger notes that C70 is still too small to disprove Penrose's breakdown theory, it has extended the quantum domain farther than before. "We can work upward slowly," Monroe says. Cat lovers beware.

Related sites

Abstract of the paper
Anton Zeilinger's home page