Can Entropy Decrease After a Projective Measurement?

[StarsRuler]

¿ What´s the matter if after decoherence the measured system coherences again with the bath ¿ Is measure erased

 

[mfb]

Decoherence cannot be reversed, otherwise it is not a proper decoherence. You can prepare a new system in a coherent state, if you like.

 

[VantagePoint72]

Decoherence cannot be reversed, otherwise it is not a proper decoherence.

Well, in practical terms, it cannot. Decoherence is still a unitary process so it's reversible in principle. The point is just that you don't have the necessary control of your system's environment.

 

[Naty1]

http://en.wikipedia.org/wiki/Density_matrix#Entropy

"This entropy can increase but never decrease with a projective measurement, however generalized measurements can decrease entropy. The entropy of a pure state is zero, while that of a proper mixture always greater than zero. Therefore a pure state {entropy zero} may be converted into a mixture by a measurement, but a proper mixture can never be converted into a pure state. Thus the act of measurement induces a fundamental irreversible change on the density matrix; this is analogous to the "collapse" of the state vector, or wavefunction collapse."