Is information lost when a photon is absorbed?

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Discussion Overview

The discussion revolves around the question of whether information is lost when a photon is absorbed by an atom, specifically in the context of quantum mechanics. Participants explore the implications of photon absorption on quantum information, considering scenarios involving photon polarization and entanglement, as well as the nature of irreversible events in quantum mechanics.

Discussion Character

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

Main Points Raised

  • Some participants propose that it may be possible to deduce the polarization of the original photons by measuring the emitted photons, suggesting that information is not necessarily lost.
  • Others argue that since the spins of the photons are specified in the scenario, the question of deducing them is meaningless, as the information is known prior to absorption.
  • A later reply questions whether information can be destroyed in quantum mechanics, particularly in the context of irreversible events, indicating a potential conflict with the principle that information cannot be lost.
  • Some participants discuss the implications of irreversible events on quantum mechanics, suggesting that if such events do not adhere to quantum rules, it could resolve various quantum paradoxes.
  • One participant mentions Lene Hau's experiment as evidence that information can be stored even when light is absorbed, implying that absorption does not equate to loss of information.

Areas of Agreement / Disagreement

Participants express differing views on whether information is lost upon photon absorption, with some asserting that it is retained while others contend that it may be lost under certain conditions. The discussion remains unresolved, with multiple competing perspectives presented.

Contextual Notes

Participants highlight the complexities of quantum mechanics, particularly regarding the definitions of information and the nature of irreversible events. There is an acknowledgment of the limitations in current understanding and the need for further exploration of these concepts.

  • #31
PeterDonis said:
This is not possible. An electron either needs to have spin + 1/2 or spin - 1/2 when measured.
This is a "B" level thread so the technicalities involved with photon polarization are beyond the scope of this discussion. For our purposes here saying that a photon can have "spin" +1 or -1 should be sufficient.
Also in a B-level thread one should not provide wrong information. Usually it's confusing for students thinking a photon would have a spin rather than polarization states.
 
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  • #32
wywong said:
What puzzles me is how the spin information carried by the electron can be transferred to the emitted photon given that the photon is emitted in a random direction. For example, if the emitted photon has spin +1, can we conclude that the absorbed photon must have spin +1, regardless of the angle between the two paths?
Spin is not conserved, so the spin of photon (or polarization, as vanhees prefers to call it for some technical reasons that are not essential here) does not need to be equal to the spin of electron. What is conserved is the total angular momentum, which a sum of all spins and all orbital angular momenta. So when electron absorbs or emits a photon, the electron changes its orbital angular momentum such that the total angular momentum is conserved.
 
  • #33
Demystifier said:
Spin is not conserved,
In my scenario, the absorbed photons were entangled and had opposite spins (or polarization). Now that the spins are not conserved, can the emitted photons still be entangled?
 
  • #34
wywong said:
Now that the spins are not conserved, can the emitted photons still be entangled?
The emitted photon is entangled with the electron.
 
  • #35
Depending on the polarization of the photon, different atomic states can be reached. If the photon is in a superposition of states, then after absorption, the atom will be in a superposition of the different atomic states. If the photon was initially entangled with another photon, after absorption, the atom is now entangled with that other photon.
 
  • #36
I now fully understand. Thanks folks. Really appreciate your help.
 
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