While swelling potassium atoms, the Rochester workers noticed that after a few orbits, the wave packet would disperse, only to come back to life again as two smaller packets on opposite ends of its large orbit. With his colleague Michael W. Noel, Stroud showed last September that the two packets constituted a Schrödinger’s cat state—a single electron in two locations.
An electron, though, is essentially a mere point. Closer to the macroscopic realm is an ion (a charged atom), which consists of many elementary particles. In May 1996 Chris Monroe, David J. Wineland and their colleagues at the National Institute of Standards and Technology (NIST) in Boulder, Colo., created a Schrödinger’s cat out of a beryllium ion. They first trapped the ion with electromagnetic fields, then hit it with a laser beam that stifled the ion’s thermal jitters and thereby cooled it to within a millikelvin of absolute zero. Then the researchers fired two laser beams, each of a slightly different frequency, at the ion to manipulate its spin, an intrinsic, quantum feature that points either up or down. With the lasers, the researchers made the ion take on a superposition of spin-up and spin-down states.
So much for the preparations; next came the more macroscopic part. By manipulating the tuning of the two lasers, the NIST team could swing the spinup state to and fro in space, and the spin-down state fro and to. A snapshot would show the ion in the spin-up state at one physical location and simultaneously in the spin-down state at a second position. The states were 80 nanometers apart—large on the atomic scale. “We made one ion occupy two places that are very far separated compared with the size of the original ion,” Monroe says.
Last December, Michel Brune, Serge Haroche, Jean-Michel Raimond and their colleagues at the Ecole Normale Supérieure (ENS) in Paris took matters a step further. “We were able to monitor the washing-out of quantum features,” Haroche explains. To see how the superposition collapsed to one state or another, they in effect dangled a quantum mouse in front of their Schrödinger’s cat to check whether it was alive or dead.
The cat was a trapped electromagnetic field (a bunch of microwave photons in a cavity). The researchers sent into the cavity a Rydberg atom that had been excited into a superposition of two different energy states. The Rydberg atom transferred its superposed state to the resident electromagnetic field, putting it into a superposition of two different phase, or vibrational, states. With its two phases, the field thus resembled the Schrödinger’s cat in its odd superposition between life and death.
For the mouse, the ENS team fired another Rydberg atom into the cavity. The electromagnetic field then transferred information about its superposed phases to the atom. The physicists compared the second atom with the first to glean superposition information about the electromagnetic field.
More interesting, however, was the team’s ability to control crucial variables and to determine how coherent states become classical ones. By varying the interval between the two atoms sent into the cavity (from 30 to 250 microseconds), they could see how the collapse of the superposition varied as a function of time, and by enlarging the electromagnetic field (by putting more photons in the cavity), they could see how the collapse changed with size. “This is the first time we can observe the progressive evolution of quantum to classical behavior,” Haroche says.
“This is a breathtaking experiment,” Zurek enthuses. “Seeing a Schrödinger’s cat is always surprising, but being able to see the cat forced to make a choice between ‘dead’ and ‘alive,’ to observe for the first time quantum weirdness going away, is the real coup.” Moreover, the ENS results jibed with most theorists’ technical expectations. “What it tells me,” Zurek remarks, “is that the simple equations we’ve been writing down seem to be a good approximation.”