# I am doing a measurement in a system S with the apparatus A, in an

1. Aug 5, 2012

### StarsRuler

I am doing a measurement in a system S with the apparatus A, in an entorn E. If the system is at [STRIKE]t0[/STRIKE] in Sithe measurement of the aparatus will be, after the decoherence time

2. Aug 5, 2012

### Staff: Mentor

Re: Decoherence

Is that a question? The state of the apparatus will depend on the interaction with the system.
What is "entorn"? Google suggest "environment", translated from catalan.

3. Aug 5, 2012

### StarsRuler

Re: Decoherence

I don´t understand why the collapse of the wavefunction and the measure increase entropy. I start with a quantum system S and an aparattus A. The initial state of [ S+A ] is [(c_1|s_1&gt;+c_2|s_2&gt;+...)|a_0&gt; ] and after the decoherence implies than apparatus end with a density operator [ p_1|1&gt;&lt;a_1|+p_2|a_2&gt;&lt;a_2|] where [p_n=|c_n|^2] ...His entropy goes up by shannon entropy to [-(p_1 ln (p_1)+p_2 ln (p_2)+....] that is [S&gt;0] , in the premeasurement. But when collapse occurs , the final state of apparatus is [|a_n&gt;&lt;a_n|] , for a random [n] , then it has entropy [S=0] . The entropy goes down and the same occurs to the system. Then why it is said that entropy goes up in a measurement?

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