How do you tell whether a cat is alive or dead without looking directly at it? Simple. Let a mouse run past its nose and see what happens. In the 9 December Physical Review Letters, a group of physicists reports taking this strategy a step further, to test whether a cat is both dead and alive.
This peculiar cat is a metaphorical creature imagined by Austrian physicist Erwin Schrödinger, who pictured 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 setup is supposed to transfer the atom's indeterminate state to the cat, leaving it neither dead nor alive but straddling both states: dead and alive. In the macroscopic world of real cats and mice, this cat could never be detected. A ubiquitous process known as decoherence would instantly destroy the quantum superposition, making the cat dead or alive, but certainly not both.
But in the microscopic world, the dead-and-alive cat persists for long enough for the right sort of mouse to detect it, says Serge Haroche, a physicist at the École Normale Superieure (ENS) in Paris. "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," he says. With the cat in a quantum superposition--both dead and alive--these probabilities combine in an effect called quantum interference.
Haroche and his colleagues created a Schrödinger's cat consisting of a few microwave photons in an indeterminate quantum state and sent in a mouse--an atom prepared so that it could react to the dead-and-alive state of the cat. Investigators have caught glimpses of Schrödinger's cat before, but the atomic mouse allowed the ENS group to monitor its condition: to see how long the quantum superposition survived 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, who has also been involved in creating laboratory versions of Schrödinger's cat. The ENS experiment marks the beginning of exploration of the weird and mysterious "transition from quantum to classical," adds Wojciech Zurek of the Los Alamos National Laboratory. This is territory that is attracting technological interest as researchers discuss tapping into the quantum world for computation and cryptography. "What [Haroche and colleagues] have established now is only a beachhead," he says.