Physicists Unveil Schroedinger's SQUID

MINNEAPOLIS--It doesn't purr, but a tiny superconducting ring is the closest thing yet to Erwin Schroedinger's famous dead-and-alive cat. At last week's meeting of the American Physical Society in Minneapolis, physicists announced that, under the right conditions, such rings can carry current in opposite directions at the same time--a property never before observed in an object so big.

For decades, Schroedinger's cat has been the stock example of a paradoxical but fundamental property of quantum mechanics: that an object can be in two or more states at the same time. But physicists' favorite feline remains purely hypothetical, because objects much bigger than individual atoms, photons, and molecules generally interact strongly with their surroundings, which force them to choose one state or another. Now two teams of researchers have induced millions of electrons to flow simultaneously both ways around a small superconducting ring, a gizmo known as a Superconducting Quantum Interference Device, or SQUID.

A SQUID prefers the total amount of magnetic field passing through it to equal an exact multiple of a fundamental constant known as the flux quantum. Add or subtract a fraction of a flux quantum by changing the field the SQUID sits in, and the ring tries to round off to the nearest whole value by creating an electric current that adds or subtracts from the imposed magnetic field. Since SQUID can round up or down, the current can flow in either direction. It's when the SQUID tries to polish off exactly half a flux quantum that the quantum fun begins. In that case, the energy in the ring is equal for current flowing either way, clockwise or counter-clockwise, and the system might be coaxed into flowing in both directions at once.

To test whether they had achieved a two-way flow, two teams of researchers fed their SQUIDs energy by shining microwaves on them. One team, led by physicist Hans Mooij of Delft University of Technology in the Netherlands, and another, led by physicist James Lukens of the State University of New York, Stony Brook, found that energy from the microwaves could bounce SQUIDS between two energy states characteristic of two-way flow.

Their both-ways-at-once currents may earn SQUIDs a starring role in the processors of quantum computers. Whereas ordinary computers use bits that can be either 0 or 1, quantum computers require "q-bits," which can be 0, 1, or 0-and-1. Researchers have fashioned handfuls of q-bits out of individual atoms, molecules, and photons. But SQUIDs should be easier to manipulate because they measure a few millionths of a meter across.