A SUPERCONDUCTING "SCHRODINGER'S CAT" has been demonstrated in the lab by a group at Stony Brook. Quantum phenomena can be big things; examples include supercurrents, consisting of billions of electron pairs, moving around a macroscopically sized superconductor, or ensembles of billions of photons making up a pulse of laser light, all residing in a single quantum state. By contrast, quantum superposition, in which the system exists in two states (such as having two different values of angular momentum or being in two different places) at the same time, has mostly been a small thing, or a thing of few parts. Examples: a single ion simultaneously in two places (several nm apart) within an atom trap (David Wineland, NIST); or wavelike manifestations of C-60 molecules split and sent along separate paths of an atom interferometer (Anton Zeilinger, Univ Vienna).
In the Stony Brook experiment (Jonathan Friedman, 631-632-8079, jonathan.friedman@sunysb.edu) the superposition of quantum states is both big in size and in the number of parts. The quantum system in question is a supercurrent (containing billions of electron pairs) flowing around a 140-micron-sized superconducting quantum interference device (SQUID) circuit. As for the superposition of states in this case, it consists of the fact (improbably enough) that the supercurrent can flow in both directions at the same time (note: the current is in a superposition of clockwise or anticlockwise; it is never zero).
Normally the two supercurrent quantum states (clockwise and counterclockwise flow) sit in two separate potential wells (in the abstract space of quantum states). But the Stony Brook researchers (James Lukens heads the team) apply a gentle blast of microwaves that nudges the quantum current states part of the way out of their valleys, high enough to make quantum tunneling between the states possible (facilitating currents flowing in both directions at the same time) but not so high as to break up the electron pairs which are the heart of the superconducting condition. One hope is that this type of large coherent quantum state, well isolated from the outside (nonquantum) environment, could be put to service in quantum computing. (Friedman et al., Nature, 6 July /pnu/2000/.)
