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For a 2-level atom in a cavity with a single mode quantized electromagnetic field (in the zero photon state-Vacuum state) there exists QED Rabi oscillations, where the atom constantly emits and absorbs a photon. (from Jaynes-Cummings model)

The average vacuum energy is $\hbar\omega/2$ where $\omega$ is the frequency of the electromagnetic field. In the case of resonance, the frequency of transition is $\omega=(E_{f}-E_{i})/\hbar$, where $E_{i}$ and $E_{f}$ are the energy states of the atom and $E_f > E_i$.

The atom is said to exhibit spontaneous emission and absorption. I do not understand how the absorbed energy from the vacuum state is sufficient to raise it to the higher energy level.

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  • $\begingroup$ Spontaneous absorption is the case when there is a photon present in the field (over and above the vacuum energy), whose energy gets transferred to the atom. You seem to be implying that the vacuum energy itself can be used to excite the atom, which is not the case. What are your reasons for making that claim? I.e. what literature, or otherwise, gave you that impression? $\endgroup$
    – Emilio Pisanty
    Commented Jun 14, 2017 at 11:45
  • $\begingroup$ My reference is 'Introduction to Quantum Optics' by C.Gerry and PL Knight. $\endgroup$
    – S.Ajay
    Commented Jun 14, 2017 at 12:19
  • $\begingroup$ That's a pretty long book - you should be specific to the section and page level. $\endgroup$
    – Emilio Pisanty
    Commented Jun 14, 2017 at 12:25
  • $\begingroup$ While discussing the Jaynes Cummings model for an atom in a cavity with a single mode quantized electromagnetic field (in the zero photon state), there exists QED Rabi oscillations, where the atom constantly emits and absorbs a photon. This is what the author says, marks the difference between a semi-classical field and a quantum field. $\endgroup$
    – S.Ajay
    Commented Jun 14, 2017 at 12:25
  • 2
    $\begingroup$ You should incorporate that information into your question. $\endgroup$
    – Emilio Pisanty
    Commented Jun 14, 2017 at 12:35

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The model you are using evidently has a ground state energy as you say "the average vacuum energy is given by $\frac{1}{2}\hbar\omega$.

Then you go ahead and identify this "assumed" ground state of a quantum oscillator to be of the frequency necessary for the transition.Looking at this link for the Jaynes-Cumming model there is not a vacuum invoked, there exists just an external field with the frequency mode, and that field of course carries energy which can be transferred to the two state atom, which then can relax and give it back to the field.

Since by construction the energy of the field (your vaccum) is equal to the difference in the energy levels there is no problem with the energy balance.

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