Lifetime of a quantum cat

An atomic mouse probes a Schrodinger's cat, which exists in two quantum states at once

Published: Friday 28 February 1997

how do you tell whether a cat is alive or dead without looking at it directly? Serge Haroche, a physicist at the Ecole Normale Superieure (ens) in Paris has a simple answer: "Let a mouse run past its nose and see what happens to the mouse. Haroche is not, however, thinking of an ordinary cat. The cat in this case is a Schrodinger's cat -- a cat shut in a box with a radioactive atom that has a 50-50 chance of decaying in an hour. If the atom decays, it kills the cat. If it doesn't, the cat lives. This set up leaves the cat neither dead nor alive but in a superposition of both states: dead and alive.

To detect this stage, says Haroche, you make a small hole in a box and send in the mouse: "You should have one probability for the mouse to escape if the cat is alive and another one -- presumably larger -- if the cat is dead. With the cat in a quantum superposition, both dead and alive, these probabilities would combine in a strange way, incompatible with classical logic, in an effect called quantum interference." He adds, however, that such an experiment will never work with such macroscopic systems as cats or mice. A ubiquitous process known as decoherence will instantly destroy the quantum superposition, making the cat either dead or alive and washing out the quantum interference between the two outcomes.

But by constructing minute versions of Schrodinger's cat and mouse, Haroche, Jean Michel Raimond, Michel Brune and their ens colleagues have actually measured this decoherence process. They created a Schrodinger's cat consisting of a few microwave photons in an indeterminate quantum state and sent in a mouse -- an atom prepared so that it can react to the dead-and-alive state of the cat. Investigators have caught glimpses of Schrodinger's cat before, but the mouse allows the ens group to see how long the quantum superposition survives before collapsing into one state or the other. "The experiment is one of the first very controlled measurements of decoherence," says physicist Chris Monroe of the National Institute of Standards and Technology (Science, Vol 274, No 5293).

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