We discuss various definitions of decoherence and how it can be measured. We
compare and contrast decoherence in quantum systems with an infinite number of
eigenstates (such as the free particle and the oscillator) and spin systems. In
the former case, we point out the essential difference between assuming
"entanglement at all times" and entanglement with the reservoir occuring at
some initial time. We also discuss optimum calculational techniques in both
arenas.Comment: To be published in Proceedings of the 2004 IEEE NTC Quantum Device
Technology Workshop, IEEE Transactions on Nanotechnology, 4, 77, 200
