# B Is information lost when a photon is absorbed?

#### wywong

It depends, did you measure the spin of the electron in the initial ground state?
To make matter simple, I just assume it to have zero spin. So let's say I did and the measured spin is zero.

Information is not lost by absorption, it is just carried by another carrier. Information carried by a photon before absorption is carried by electron after the absorption.
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?

#### vanhees71

Science Advisor
Gold Member
Photons have no spin!

You can derive the intensities of emitted photons in various transitions using perturbation theory (usually in atomic physics the dipole approximation is enough). You get certain selection rules, telling you which transitions are possible and what are the probabilities to emit photons with specific polarization (not spin!) states.

#### PeterDonis

Mentor
To make matter simple, I just assume it to have zero spin.
This is not possible. An electron either needs to have spin + 1/2 or spin - 1/2 when measured.

Photons have no spin!
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.

#### PeterDonis

Mentor
the left- and right-handed polarization states of helicities $\pm 1$. These are the left- and right-circular polarization states.
And for our purposes here, since it's a "B" level thread, calling these states of "spin" +1 and -1 should be sufficient.

#### wywong

This is not possible. An electron either needs to have spin + 1/2 or spin - 1/2 when measured.
Oops! Let me correct my scenario by adding that I measure the electron spin before firing the first photon and after the second emission, and the electron spin is found to be unchanged (say both +1/2).

#### vanhees71

Science Advisor
Gold Member
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.

#### Demystifier

Science Advisor
2018 Award
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.

#### wywong

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?

#### Demystifier

Science Advisor
2018 Award
Now that the spins are not conserved, can the emitted photons still be entangled?
The emitted photon is entangled with the electron.

#### DrClaude

Mentor
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.

#### wywong

I now fully understand. Thanks folks. Really appreciate your help.

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