# Decoherence for dummies and experts

1. Aug 21, 2004

### kurious

2. Aug 22, 2004

### meteor

"The decoherence effect explains how a superposition of states becomes a mixed state"
Is this true? It's not possible taht decoherence acts in a superposition to produce a pure state?

3. Aug 23, 2004

### kurious

From the first link:
a superposition = sum of two wavefunctions which is then squared to give
probability amplitude function
mixed state = square wavefunctions first then add them

decoherence is superposition becoming mixed state

no pure state because we are talking about more than one particle
in this system
also I think generally, mixed state includes observer and observed

Last edited: Aug 23, 2004
4. Aug 24, 2004

### altered-gravity

Interesting!

I´m learning about femtochemistry, excuse my inexperience in quantum physics.

Imagine a real sample of diatomic ideal gas (suppose harmonic oscillator for simplicity). You want to prepare a superposition state between two or more eigenstates of the system and probe it at various time delays.

You use an ultrashort femtosecond laser pulse to prepare the state.

Why is that superposition induced, instead of a simple spectroscopic transition between two stationary states?
What energy does this state have? Are population of the sample "oscillating" between the two eigenstates?

Thanks for your interest

5. Aug 24, 2004

### kurious

The superposition is induced because the femtosecond wavepacket from the laser is classically considered to be a mixture of frequencies and each frequency can cause a different spectroscopic transition,so the wavefunction of the molecules has to reflect this.The population does not oscillate between eigenstates it exists in
the superposition - that is wavefunctions for each spectroscopic state are added together and then squared to give the probability amplitude function.Superpositions are very strange indeed! One day,hopefully, superpositions will be given a more intuitive explanation rather than just a mathematical one.

6. Aug 24, 2004

Staff Emeritus
We employ superpositions of sound waves all the time. They're called chords.

7. Aug 25, 2004

### vanesch

Staff Emeritus
One has to be careful. When looking at the combination (system+macroscopic measurement apparatus), a superposition remains a superposition of course, because the hamiltonian is a linear evolution operator. But what decoherence shows you is that *when you restrict your attention to the system*, then what formerly was a superposition, is now best described (best described because you are neglecting part of it, namely the measurement system) by a statistical mixture of eigenstates of the operator that is measured by the measurement apparatus.

cheers,
Patrick.

8. Aug 25, 2004

### altered-gravity

Ok! The reason is the energy uncertainity of the pulse, right?

I undrestand. Controlling the shape of that pulse you can adjust the eigenstates that you want to take part in the superposition (or, what is the same, the initial coefficients of the terms of the superposition).

Decoherence can be also controlled whith sequences of pulses, but that´s still too difficult for me. I´m working on it. It´s the optical analogous of the NMR multipulse thechniques.

9. Aug 25, 2004

### altered-gravity

As a chemist (well, still student) i have to say that experimental detection, and control of this states is the future of chemistry or at least important part of it (in my opinion). Frontiers between physics and chemistry are now nonsense.

10. Aug 25, 2004

### ZapperZ

Staff Emeritus
I hate to advertize my own site, but Scott (the author of the website in question) and I had a discussion along this line almost a year ago. You can read a part of it here:

http://groups.yahoo.com/group/undernetphysics/message/465

Zz.