# Vacuum Rabi oscillation

A vacuum Rabi oscillation is a damped oscillation of an initially excited atom coupled to an electromagnetic resonator or cavity in which the atom alternately emits photon(s) into a single-mode electromagnetic cavity and reabsorbs them. The atom interacts with a single-mode field confined to a limited volume V in an optical cavity. Spontaneous emission is a consequence of coupling between the atom and the vacuum fluctuations of the cavity field.

## Mathematical treatment

A mathematical description of vacuum Rabi oscillation begins with the Jaynes–Cummings model, which describes the interaction between a single mode of a quantized field and a two level system inside an optical cavity. The Hamiltonian for this model in the rotating wave approximation is
    ${\hat {H}}_{\text{JC}}=\hbar \omega {\hat {a}}^{\dagger }{\hat {a}}+\hbar \omega _{0}{\frac {{\hat {\sigma }}_{z}}{2}}+\hbar g\left({\hat {a}}{\hat {\sigma }}_{+}+{\hat {a}}^{\dagger }{\hat {\sigma }}_{-}\right)$
where ${\hat {\sigma _{z}}}$ is the Pauli z spin operator for the two eigenstates $|e\rangle$ and $|g\rangle$ of the isolated two level system separated in energy by $\hbar \omega _{0}$; ${\hat {\sigma }}_{+}=|e\rangle \langle g|$ and ${\hat {\sigma }}_{-}=|g\rangle \langle e|$ are the raising and lowering operators of the two level system; ${\hat {a}}^{\dagger }$ and ${\hat {a}}$ are the creation and annihilation operators for photons of energy $\hbar \omega$ in the cavity mode; and
    $g={\frac {\mathbf {d} \cdot {\hat {\mathcal {E}}}}{\hbar }}{\sqrt {\frac {\hbar \omega }{2\epsilon _{0}V}}}$
is the strength of the coupling between the dipole moment $\mathbf {d}$ of the two level system and the cavity mode with volume $V$ and electric field polarized along ${\hat {\mathcal {E}}}$.
The energy eigenvalues and eigenstates for this model are
    $E_{\pm }(n)=\hbar \omega \left(n+{\frac {1}{2}}\right)\pm {\frac {\hbar }{2}}{\sqrt {4g^{2}(n+1)+\delta ^{2}}}=\hbar \omega _{n}^{\pm }$
    $|n,+\rangle =\cos \left(\theta _{n}\right)|g,n+1\rangle +\sin \left(\theta _{n}\right)|e,n\rangle$
    $|n,-\rangle =\sin \left(\theta _{n}\right)|g,n+1\rangle -\cos \left(\theta _{n}\right)|e,n\rangle$
where $\delta =\omega _{0}-\omega$ is the detuning, and the angle $\theta _{n}$ is defined as
    $\theta _{n}=\tan ^{-1}\left({\frac {g{\sqrt {n+1}}}{\delta }}\right).$
Given the eigenstates of the system, the time evolution operator can be written down in the form
    ${\begin{aligned}e^{-i{\hat {H}}_{\text{JC}}t/\hbar }&=\sum _{|n,\pm \rangle }\sum _{|n',\pm \rangle }|n,\pm \rangle \langle n,\pm |e^{-i{\hat {H}}_{\text{JC}}t/\hbar }|n',\pm \rangle \langle n',\pm |\\&=~e^{i(\omega -{\frac {\omega _{0}}{2}})t}|g,0\rangle \langle g,0|\\&~~~+\sum _{n=0}^{\infty }{e^{-i\omega _{n}^{+}t}(\cos {\theta _{n}}|g,n+1\rangle +\sin {\theta _{n}}|e,n\rangle )(\cos {\theta _{n}}\langle g,n+1|+\sin {\theta _{n}}\langle e,n|)}\\&~~~+\sum _{n=0}^{\infty }{e^{-i\omega _{n}^{-}t}(-\sin {\theta _{n}}|g,n+1\rangle +\cos {\theta _{n}}|e,n\rangle )(-\sin {\theta _{n}}\langle g,n+1|+\cos {\theta _{n}}\langle e,n|)}\\\end{aligned}}.$
If the system starts in the state $|g,n+1\rangle$, where the atom is in the ground state of the two level system and there are $n+1$ photons in the cavity mode, the application of the time evolution operator yields
    ${\begin{aligned}e^{-i{\hat {H}}_{\text{JC}}t/\hbar }|g,n+1\rangle &=(e^{-i\omega _{n}^{+}t}(\cos ^{2}{(\theta _{n})}|g,n+1\rangle +\sin {\theta _{n}}\cos {\theta _{n}}|e,n\rangle )+e^{-i\omega _{n}^{-}t}(-\sin ^{2}{(\theta _{n})}|g,n+1\rangle -\sin {\theta _{n}}\cos {\theta _{n}}|e,n\rangle )\\&=(e^{-i\omega _{n}^{+}t}+e^{-i\omega _{n}^{-}t})\cos {(2\theta _{n})}|g,n+1\rangle +(e^{-i\omega _{n}^{+}t}-e^{-i\omega _{n}^{-}t})\sin {(2\theta _{n})}|e,n\rangle \\&=e^{-i\omega _{c}(n+{\frac {1}{2}})}{\Biggr\cos {{\biggr{\frac {t}{2}}{\sqrt {4g^{2}(n+1)+\delta ^{2}}}{\biggr )}}{\biggr{\frac {\delta ^{2}-4g^{2}(n+1)}{\delta ^{2}+4g^{2}(n+1)}}{\biggr ]}|g,n+1\rangle +\sin {{\biggr{\frac {t}{2}}{\sqrt {4g^{2}(n+1)+\delta ^{2}}}{\biggr )}}{\biggr{\frac {8\delta ^{2}g^{2}(n+1)}{\delta ^{2}+4g^{2}(n+1)}}{\biggr ]}|e,n\rangle {\Biggr ]}\end{aligned}}.$
The probability that the two level system is in the excited state $|e,n\rangle$ as a function of time $t$ is then
    ${\begin{aligned}P_{e}(t)&=|\langle e,n|e^{-i{\hat {H}}_{\text{JC}}t/\hbar }|g,n+1\rangle |^{2}\\&=\sin ^{2}{{\biggr{\frac {t}{2}}{\sqrt {4g^{2}(n+1)+\delta ^{2}}}{\biggr )}}{\biggr{\frac {8\delta ^{2}g^{2}(n+1)}{\delta ^{2}+4g^{2}(n+1)}}{\biggr ]}\\&={\frac {4g^{2}(n+1)}{\Omega _{n}^{2}}}\sin ^{2}{{\bigr{\frac {\Omega _{n}t}{2}}{\bigr )}}\end{aligned}}$
where $\Omega _{n}={\sqrt {4g^{2}(n+1)+\delta ^{2}}}$ is identified as the Rabi frequency. For the case that there is no electric field in the cavity, that is, the photon number $n$ is zero, the Rabi frequency becomes $\Omega _{0}={\sqrt {4g^{2}+\delta ^{2}}}$. Then, the probability that the two level system goes from its ground state to its excited state as a function of time $t$ is
    $P_{e}(t)={\frac {4g^{2}}{\Omega _{0}^{2}}}\sin ^{2}{{\bigr{\frac {\Omega _{0}t}{2}}{\bigr )}.}$
For a cavity that admits a single mode perfectly resonant with the energy difference between the two energy levels, the detuning $\delta$ vanishes, and $P_{e}(t)$ becomes a squared sinusoid with unit amplitude and period ${\frac {2\pi }{g}}.$

## Generalization to N atoms

The situation in which $N$ two level systems are present in a single-mode cavity is described by the Tavis–Cummings model
, which has Hamiltonian
    ${\hat {H}}_{\text{JC}}=\hbar \omega {\hat {a}}^{\dagger }{\hat {a}}+\sum _{j=1}^{N}{\hbar \omega _{0}{\frac {{\hat {\sigma }}_{j}^{z}}{2}}+\hbar g_{j}\left({\hat {a}}{\hat {\sigma }}_{j}^{+}+{\hat {a}}^{\dagger }{\hat {\sigma }}_{j}^{-}\right)}.$
Under the assumption that all two level systems have equal individual coupling strength $g$ to the field, the ensemble as a whole will have enhanced coupling strength $g_{N}=g{\sqrt {N}}$. As a result, the vacuum Rabi splitting is correspondingly enhanced by a factor of ${\sqrt {N}}$.