Within the Rabi quantum model, it has been shown theoretically that a two-level atom ultrastrongly coupled to an electromagnetic field generates or absorbs photons. Photons can be generated if the field is initially in the vacuum state. Under certain initial states of the atom + field system, the absorption of photons is possible in the field mode in the ultrastrong atom–field coupling regime. If the atom is initially in the ground (unexcited) state and the field is in the vacuum state, photons can be generated under resonance condition ωa ≈ ωf or ξ ≡ ωa/ωf ≈ 1, where ωa and ωf are the atomic transition and field frequencies, respectively, in the ultrastrong coupling regime. At the negative detuning when ξ
≪ 1 or ωa
≪ ωf, Rabi oscillations of the average number of photons
⟨n^⟩t with
0⩽⟨n^⟩t⩽nmax≫1 are observed in the case of the ultrastrong coupling with the atom–field coupling constant
g~≡|g|/ωf∼1 ; in this case, the population of the excited atomic state is Pe(t) ≈ 0.5. At a large positive detuning when ξ
≫ 1, photons are not generated, i.e.,
⟨n^⟩≈0 , and the atom remains in the initial state, i.e., Pe(t) ≈ 0. The statistics of photons in the generation regime is nearly chaotic: the variance of photons is much larger than the level of the coherent field state (i.e., is super-Poisson). Field photons are absorbed without the excitation of the atom in the ultrastrong coupling regime in the case of the coherent initial state of the field
(⟨n^(t=0⟩>0) for certain positive detuning values. In this case, the field becomes sub-Poisson.
Quantum interference and coherence effects in the interaction of atoms with the quantized electromagnetic field are investigated theoretically. A general master equation for the description of atom-field interactions is introduced. The interplay of the concepts of complementarity and interference in the time-energy domain are studied on the basis of the fluorescence light emitted by a single laser-driven atom, where the coherence of spontaneous processes gives rise to quantum interference in the spectrum of resonance fluorescence. The vacuum-induced dipole-dipole interaction in pairs of multi-level atoms is analyzed. It is shown that the interaction between orthogonal transition dipole moments of different atoms does not only influence the system dynamics crucially, but implies that the few-level approximation in general cannot be applied to near-degenerate Zeeman sublevels of the atomic level scheme. Potential applications of dipole-dipole interacting multi-level atoms for the implementation of decoherence-free subspaces and the generation of entanglement between atomic states are examined. The generation of an entangled state of the radiation field with a macroscopic number of photons is discussed on the basis of a single-atom laser.
In this short communication, a new type of two-channel cavity QED model is derived. Two-channel models are important for they often lead to quantum interference phenomena. The previous models relied on the use of two or more modes of the quantized electromagnetic field, partially because of energy and parity restrictions. As it is shown in this work, such restrictions may be overcome with the use of properly chosen configurations of atomic levels and the aplication of classical external fields. Competing one- and two-photon processes involving one single mode may be obtained.
When a four-level
Y-type atom with two highest nearly degenerate lying levels interacts with the same vacuum radiation field, its excited levels have vacuum-induced coherence due to the quantum interference between the spontaneous decay channels. We consider the effects of the vacuum-induced coherence on the electromagnetically induced transparency against single- and two-photon absorption in the atomic system. We find that the vacuum-induced coherence can lead to probe gain without incoherent pumping. On the other hand, the coherence can suppress the two-photon transparency, even in some cases where it can enhance the two-photon absorption. Another important result is the finding of the crucial role played by the relative phase between the probe and coupling fields: the two-photon transparency and the probe gain spectra can be modulated by this phase.
The quantum dynamics of a hyper-Raman coupled model interacting with two modes of the quantized cavity field is described. The model consists of a four-level atom in a v configuration where transitions between the ground and excited states occur through the absorption (emission) of two photons from one mode and the emission (absorption) of one photon from the other and where two intermediate states are taken to be far off-resonance and are adiabatically removed. This is a multiphoton extension of the Jaynes-Cummings model and is exactly solvable. We study the atomic inversion and investigate the production of non-classical light exhibiting antibunching, violations of the Cauchy-Schwartz inequality, and squeezing.