A realistic experiment to demonstrate macroscopic quantum coherence

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 .