Decoherence is the process that occurs as a system transitions from
quantum to
classical mechanical behavior.
It is not possible to observe decoherence at normal human scales because the transition occurs at timescales that are shorter than we are currently able to measure. It is possible for a particle at the quantum level to be in two places at the same time (and sometimes I wonder if they can be in two times at the same place).
Decoherence has now been experimentally observed by David Wineland and colleagues at the National Institute of Standards and Technology in Boulder, Colorado, where they used a single beryllium ion confined in a magnetic trap and cooled so that quantum effects dominate its behavior. When the magnetic trap was disturbed the ion decohered.
Applications of decoherence include quantum computers that use quantum superposition.
Source:
Schroedinger's Cat in an Atomic Cage http://physics.nist.gov/News/Update/960528.html 5/25/2001