Decoherence And What happens in reality

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    Decoherence Reality
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Discussion Overview

The discussion revolves around the concept of decoherence in quantum mechanics, specifically questioning whether a hydrogen atom's electron, in a superposition of energy states, actually "chooses" a stationary state through environmental interaction or remains in superposition. The conversation explores theoretical implications and interpretations of decoherence, including its effects on quantum systems.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant questions if decoherence leads to a real-life selection of stationary states for an electron in a hydrogen atom or if it remains in superposition.
  • Another participant suggests that while theories exist, there is a lack of experimental proof regarding the outcomes of decoherence.
  • Some argue that artificial qubits provide a clearer example of decoherence, asserting that such systems end up in one definite state due to decoherence.
  • There is a discussion about the nature of the superposition, with one participant clarifying that it refers to energy states.
  • One participant expresses uncertainty about decoherence occurring in systems with strictly stationary states, noting that interaction with the environment typically causes both decoherence and relaxation.
  • Another participant emphasizes that the final state of a system undergoing decoherence is a mixed state, maintaining that the probabilities of the initial superposition states remain unchanged.
  • There is a contention regarding the interpretation of mixed states, with one participant arguing that decoherence results in an improper mixed state, which complicates the measurement problem in quantum mechanics.
  • Some participants mention alternative interpretations of decoherence, such as Decoherent Histories and Many Worlds, discussing their implications for understanding quantum reality.

Areas of Agreement / Disagreement

Participants express differing views on the implications of decoherence, particularly regarding whether it leads to a definite state or a mixed state. There is no consensus on the interpretation of these outcomes or the applicability of different models to the hydrogen atom versus artificial qubits.

Contextual Notes

Participants highlight limitations in understanding decoherence, including the dependence on definitions of states and the unresolved nature of experimental evidence. The discussion also reflects varying interpretations of mixed states and their implications for quantum measurement.

the_pulp
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I know that the theoretical decoherence mechanism makes the density mattrix diagonal, but my question is if that happens in real life. I mean, let's take the hidrogen atom (lets work in the QM framework And not in the QFT framework in order to have multiple stationary states) And suppose that the electron is in a superposition of energy/stationary states. In this case, does in real life the electron, through the interaction with the environment, spontaneously "choose" one of those stationary states or it remains in a superposition of energy states?

Thanks
 
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I think we do not know, there are theories but we lack experimental proof.
 
I don't think a hydrogen atom is a good example. An artifical qubit would be a better system to study since it then clear what the "classical" state would be (this is never obvious for an atom).

In the latter case then yes, that is pretty much what happens. Due to decoherence, a system that is initially in a superposition of states will end up permanently in ONE of those states.

There are experiments where people have traced out the "trajectory" of a qubit on a Bloch sphere, and this can be used as a nice illustration of this.
 
The superposition you mention is a superposition of Energy states, right?

Thanks
 
the_pulp said:
(lets work in the QM framework And not in the QFT framework in order to have multiple stationary states)
I'm not sure if decoherence is possible if we have strictly stationary states in the system. Usually, the interaction with the environment causes both decoherence and relaxation. The time scale for decoherence is just much much shorter for most environments.

the_pulp said:
[...] suppose that the electron is in a superposition of energy/stationary states. In this case, does in real life the electron, through the interaction with the environment, spontaneously "choose" one of those stationary states or it remains in a superposition of energy states?
The final state of the system is a mixed state. This means that the probability to find the system in any of the states of the initial superposition remains unchanged. I don't think that we should say that it is in only one of them, see also below.

f95toli said:
In the latter case then yes, that is pretty much what happens. Due to decoherence, a system that is initially in a superposition of states will end up permanently in ONE of those states.
I tend to disagree. Consider a Bell state of two entangled qubits. We wouldn't think of a single qubit as having one definite state here. But the reduced density matrix is the same as for a qubit which has undergone decoherence. I don't think that it should matter wether the qubit is entangled with another qubit or with the environment.
 
kith said:
The final state of the system is a mixed state. This means that the probability to find the system in any of the states of the initial superposition remains unchanged. I don't think that we should say that it is in only one of them, see also below.

Yea - but just to elaborate - decoherence transforms a superposition into an improper mixed state. The difference here is that mixed states are usually interpreted as a system where a randomly selected state is given for observation. If that was the case for the mixed states of decoherence then low and behold the measurement problem would be solved, you are observing a reality that exists prior to observation and everything is sweet. But it was not physically prepared that way so you can't do that - bummer - that's why its called improper - Schlosshauer carefully explains it in his book on Decoherence. But wait - it is both observationally and mathematically exactly the same so what you can do is assume it is like that - no observation or mathematical analysis can prove you wrong. This is the simple assumption I make. Other ways of using it exist eg Decoherent Histories and Many Worlds. Many worlds is particularly neat - nothing happens - the mixed state simply keeps evolving but each state of the ensemble is considered a separate world. You can read about Decoherent Histories here:
http://quantum.phys.cmu.edu/CHS/histories.html

Thanks
Bill
 
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