Spontaneous Emission + Reversibility

[StevieTNZ]

Hi there,

In regards to the Spontaneous Emission of a photon from an atom, after that process occurs, is it possible according to the reversibility of the laws of Quantum Mechanics that the photon is re-absorbed by the atom? According to the reversible equation governing the process, would that be with the same probability of it being emitted from the atom, or different?

 

[StevieTNZ]

Or would the probability, because of so many routes the photon can take after leaving the atom, be the same probability that it returns via that route back to the atom?

 

[Cthugha]

In regards to the Spontaneous Emission of a photon from an atom, after that process occurs, is it possible according to the reversibility of the laws of Quantum Mechanics that the photon is re-absorbed by the atom?

That depends. In free space not really. The gazillion of possible vacuum modes the atom can emit to make this process pretty irreversible. Things are different when you put the atom into a good cavity. This may modify the spectrum of vacuum modes such that the number of significant vacuum modes is drastically reduced. It also increases the effective light-matter interaction strength as the photon goes back and forth and passes the atom pretty often. If the cavity photon lifetime is longer than the mean time needed for reabsorption, spontaneous emission becomes reversible and you end up in the strong coupling regime. This new system will now have two new resonance modes. Instead of the old emission line, you will find two new states of the system: One at higher and one at lower energy. The atom and the cavity photon behave more or less like two coupled oscillators and the two modes describe in-phase and out-of-phase energy exchange between the atom and the cavity mode.

Such systems have been treated intensively. The prototype model for them is the Jaynes–Cummings model. You should find lots of information on it by just doing a google search.

 

[craigi]

That depends. In free space not really. The gazillion of possible vacuum modes the atom can emit to make this process pretty irreversible. Things are different when you put the atom into a good cavity. This may modify the spectrum of vacuum modes such that the number of significant vacuum modes is drastically reduced. It also increases the effective light-matter interaction strength as the photon goes back and forth and passes the atom pretty often. If the cavity photon lifetime is longer than the mean time needed for reabsorption, spontaneous emission becomes reversible and you end up in the strong coupling regime. This new system will now have two new resonance modes. Instead of the old emission line, you will find two new states of the system: One at higher and one at lower energy. The atom and the cavity photon behave more or less like two coupled oscillators and the two modes describe in-phase and out-of-phase energy exchange between the atom and the cavity mode.

Such systems have been treated intensively. The prototype model for them is the Jaynes–Cummings model. You should find lots of information on it by just doing a google search.

This intrigues me. When you say "not really", do you mean with very low probability, but still possible? Or do you mean that it's not conclusive as to whether it takes place at all?

Also, when a spontaenously emitted photon is reabsorbed by the atom that emitted it, in either the free space in a cavity, does this involve information loss?