Quantum Optics - transition from pure to mixed state

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Discussion Overview

The discussion revolves around an experiment involving a two-level atom transitioning from a pure state to a mixed state in the context of quantum optics. Participants explore the implications of time evolution on the state of the atom after applying a Pi/2 pulse and the effects of interaction with vacuum fields.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant describes an experiment where a two-level atom is prepared in state |0>, subjected to a Pi/2 pulse, and then analyzed after a wait time T with a second Pi/2 pulse.
  • It is proposed that if the state remains pure, the atom will transition to state |1> with probability p1=1, while if the state becomes mixed, the probabilities p1 and p2 will both equal 1/2.
  • Another participant notes that the interaction with vacuum fields leads to a loss of information, which may affect the purity of the state.
  • There is a suggestion that using a long-lifetime excited state could mitigate the effects of spontaneous emission, introducing uncertainty instead.
  • A participant questions how the vacuum field 'destroys' the superposition and seeks an intuitive understanding of this process.
  • Another participant clarifies that the vacuum is not a field but a specific state of quantum fields.
  • A later post reiterates the experiment and seeks references for further understanding of the described process.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the effects of vacuum fields on superposition and whether the interpretation of the experiment is correct. There is no consensus on the intuitive understanding of how the vacuum influences the atomic state.

Contextual Notes

Participants discuss the implications of the atom's lifetime and the nature of vacuum fields, but the discussion does not resolve the underlying assumptions or the specifics of the experimental setup.

SchroedingersLion
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Hello guys,

I am trying to understand the following experiment:

1. Prepare a 2 level atom in state |0>
2. Shine in a Pi/2 pulse --> atom goes to 1/√2 (|0>+|1>)
3. Wait time T
4. Shine in second Pi/2 pulse
a) if the state is pure: atom will go to state |1>, p1=1
b) if the state is mixed: p1=1/2, p2=1/2

And below a plot of p1 with respect to T. It starts at p1=1 and decreases with T to 1/2.

I don't understand point 4.
What I understand:
If T=0, the state is a pure superposition and gets to |1> via the second Pi/2 pulse
But as soon as time T passes after the first pulse, I am losing information, since the atom interacts with the vacuum fields (who are also responsible for spontaneous emission).

But what does that mean? Does the interaction with the vacuum 'destroy' my superposition and force the atom into either state |0> or state |1>?
Because if I then apply my second light pulse, it would go either from |0> to 1/√2 (|0>+|1>) or from |1> to 1/√2 (|0>+|1>), and then I would have my probabilities at 1/2.

Regards
 
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SchroedingersLion said:
But as soon as time T passes after the first pulse, I am losing information, since the atom interacts with the vacuum fields (who are also responsible for spontaneous emission).
Yes, but you also have to consider the lifetime of the state with respect to the duration of the experiment. Using a long-lifetime excited state, one can neglect any effect due to spontaneous emission. It can also become simply a source of uncertainty (for instance, in an atomic clock).
 
Ok, let's assume it has a short enough average lifetime to play a role.
How exactly is the vacuum field 'destroying' the superposition? Is it possible to understand it intuitively?
 
The vacuum is not a field but a specific state of quantum fields.
 
SchroedingersLion said:
I am trying to understand the following experiment:

1. Prepare a 2 level atom in state |0>
2. Shine in a Pi/2 pulse --> atom goes to 1/√2 (|0>+|1>)
3. Wait time T
4. Shine in second Pi/2 pulse
a) if the state is pure: atom will go to state |1>, p1=1
b) if the state is mixed: p1=1/2, p2=1/2

I am not sure whether the interpretation is correct. Do you have a reference where the experiment is described?
 
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