Coherent Control Of Atomic Properties In Cavity

Quantum optics,as a discipline,uses quantum theory to describe the light,which are distinct from the classical physics in many aspects.Their appearance has greatly promoted the scientific progress in every area.Quantum computer,quantum communication,precision measurement and other fields has attracted considerable attention,both theoretically and experimentally.In precision measurement,it is found that if the input state is a classical state,the accuracy of the final measurement is limited to the standard quantum limit.However,the introduction of quantum states in precise measurement allows a precision gain over the standard quantum limit.Since the observed spin-squeezed states can be used to improve measurement accuracy,a theory based on Quantum NonDemolition measurement is proposed to prepare spin-squeezed states in the laboratory.However,since the spin-squeezed state,which is prepared by QND measurement is conditional,the obtained squeezed state is different in each time.Recently,the one-and two-axis twisting interactions have been proposed to generate the spin-squeezed states.on the other hand,narrowing the fluorescence spectrum by manipulating the fluorescence radiation is also of great significance for improving the accuracy and efficiency of highprecision measurement.In general,the nature of spontaneous emission can be changed either by changing the external environment of the atom or by using additional coherent interactions.For a two-level atom as a basic model,fluorescence emits from a doublet to the adjacent lower-lying doublet.A particularly interesting mechanism for the spectral narrowing is to tune a cavity resonant with an inverted dressed-state sub-transition and to create a dressed-state laser.The essential mechanism is the cavity feedback into the fluorescence emission dynamics.The innovative contents are shown as follows:First,in the common cases,on resonance or close to resonance,atoms are excited and their spontaneous emission will destroy any possible squeezing of the ground-state spin.In order to overcome the spontaneous emission,one has to work in the far-off-resonance regimes,where Ω/Δ(Ω is small,real Rabi frequencies associated with the driving fields and Δ is the large detunings of the atomic frequencies from the driving field frequencies).In the far-off-resonance regimes,the dispersion is dominant over the absorption although both of them are sufficiently weak,when the atoms are hardly excited and spontaneous emission is almost avoided.Consequently,the strength of one-and two-axis nonlinear interactions are very small.Once,the damping rates are much bigger than them,spin squeezing arising from the nonlinear twisting Hamiltonian can be washed out by the damping.This means that we are faced with a challenge to increase the strengths.we show that spin two-axis-twisting can be established for the long lived ground states in a nearresonant system that displays coherent population trapping(CPT).In a A configuration from the two ground states to a common excited state,the near-resonant dipole interactions with two classical dressing fields trap the atoms almost in a dark state,which is the coherent superposition of the involved ground states and persists for a long time.With two cavity vacuum fields used for virtual transitions,the CPT atoms are induced into the ground state spin two-axis-twisting interaction,for which the effective Hamiltonian is extracted from the coupled system and the spin interaction strength is one-order higher than in the usual far-off resonant driving cases.Since the atoms stay dominantly in the ground states,the present scheme is robust to spontaneous emission.Second,however,the method of narrowing spectral lines by laser above the threshold is only applied to two-level atoms,and there is no scheme or experimental application that can make fluorescence spectral lines narrow and high simultaneously in three-level atom system.We consider three-level atoms that are dressed in Λ configuration by two strong coherent fields.We show that the coherence between the triplets of dressed states of atoms which can be established by the degenerate cascade dressed laser.The established coherence is the essential physical mechanism which leads to narrowing of all spectral lines in the above-threshold atom-cavity system.A three-level atom usually has two nondegenerate(even microwave and optical)electric dipole allowed transitions,the fluorescence spectrum of either of which is well described in terms of spontaneous transitions from a triplet of dressed sublevels to an adjacent lower-lying triplet.As a remarkable feature,degenerate cascade fluorescence happens and displays a five-peaked structure when the three dressed sublevels are equally spaced from each other.We show that a single cavity can make all spectral lines extremely narrow whether they come from the cavity-coupled or cavityfree transitions.This effect is based on the intrinsic cascade lasing feedback and makes it possible to use a single microwave cavity(even bad cavity)to narrow the spectral lines in the optical frequency regime.