Schrödinger's kittens. A single photon, split in two, can make a mirror behave like a quantum object.

Putting Schrödinger's Cat to the Test

Quantum physicists have just proposed a scheme to create a version of Erwin Schrödinger's famous cat that is billions of times bigger than anything attempted in the past. The scheme is within the reach of current technology, and when an experiment is up and working, it might reveal why quantum phenomena don't seem to apply to large things such as cats and bricks.

The laws of quantum mechanics state that a quantum object, such as a photon, can be two opposite things at the same time, something like a cat being dead and alive at the same instant. Physicists don't know why this principle, called superposition, works for photons but not large objects. To find out, physicists have been trying to put increasingly large objects in superposition. Anton Zeilinger of the University of Vienna, Austria, for instance, has made tiny Schrödinger's cats out of individual molecules of C70, enormous by quantum standards (ScienceNOW, 25 May 2001).

Now Roger Penrose of Oxford University, Dik Bouwmeester of the University of California, Santa Barbara, and their colleagues have proposed an experiment to make a Schrödinger's cat of 1014 atoms--billions of times larger than Zeilinger's record-holding molecules. In the experiment they propose, a single photon hits a beam splitter, and, thanks to superposition, the photon travels in two directions at once, down two arms of an interferometer. Each arm has a cavity at the end where the photon bounces around for a while before escaping. One of the cavities, however, is capped with a tiny mirror at the end of a cantilever. Because the photon takes both paths simultaneously, the mirror moves and doesn't move at the same time--it becomes a Schrödinger's cat.

"If this works, you've extended the validity of quantum mechanics by nine orders of magnitude. It's a very exciting prospect," says Max Tegmark, a theorist at the University of Pennsylvania in Philadelphia. "This is the size of a cell already. If quantum mechanics hasn't gone wrong at the size of a cell, it probably won't go wrong with something the size of a mouse or a human." Cat owners don't have to hide their kittens just yet, but pretty soon, Schrödinger's thought experiment might become more than a mere flight of fancy.

Related sites
Abstract of the paper by Penrose and colleagues
Abstract of Zeilinger's C70 paper