According to quantum theory, microscopic objects exist in a superposition o
f distinct states until they are "observed". Nobody knows whether such quan
tum coherence can actually exist in a macroscopic system. In the experiment
described here, a superconducting quantum interference device is extremely
well isolated from any interaction with internal or external modes, and a
superposition state can be demonstrated even if it lasts for less than 1 ns
. This is accomplished by using superconducting digital electronic circuitr
y as the experimental apparatus. If successful, it is the first step toward
a future full-scale quantum computer fabricated using integrated circuit m
anufacturing techniques. (C) 2001 Elsevier Science B.V. All rights reserved
.