Is information lost when a photon is absorbed?

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SUMMARY

The discussion centers on the absorption of photons by hydrogen atoms and whether information is lost in the process. It is established that photons possess spin 1 and their polarization is a representation of this spin. The conversation highlights that while photons can be entangled, the original spins of the photons can be known prior to absorption, making the question of deducing their spins post-absorption somewhat moot. The consensus leans towards the idea that information is not lost, as quantum mechanics asserts that information cannot be destroyed, although irreversible events complicate this understanding.

PREREQUISITES
  • Understanding of quantum mechanics principles, particularly regarding photon behavior.
  • Familiarity with concepts of quantum entanglement and polarization.
  • Knowledge of the Hilbert space and its application to quantum states.
  • Awareness of the implications of irreversible events in quantum measurements.
NEXT STEPS
  • Explore the concept of quantum entanglement in greater detail, focusing on photon pairs.
  • Study the implications of irreversible events in quantum mechanics, particularly in measurement theory.
  • Investigate the role of polarization in quantum computing and its applications.
  • Learn about the quantum measurement problem and various interpretations of quantum mechanics.
USEFUL FOR

Quantum physicists, researchers in quantum computing, and students of advanced quantum mechanics will benefit from this discussion, particularly those interested in the nature of information in quantum systems.

  • #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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