B Is information lost when a photon is absorbed?

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The discussion centers on whether information is lost when a photon is absorbed by a hydrogen atom and then emitted again. It is noted that while photons have spin 1, their polarization can be treated similarly to qubits in quantum computing. The conversation highlights that if the original spins of the photons are known, deducing them post-absorption is unnecessary. However, if the photons are part of an entangled pair and their spins are unknown before absorption, it raises questions about the irreversibility of quantum measurements and the potential loss of information. Ultimately, the nature of quantum mechanics and the collapse of the wave function are debated, with implications for understanding irreversible events in quantum theory.
  • #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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