The Study Of Spontaneous Feedback In Cavity And Collective Dissipation Effect

With the development of the quantum optics,many effects which are distinct from the classical physics are found.These effects are the leading edge of the nowadays technology.Researches focus on the areas such as quantum information,quantum computation,and high-precision measurement.Quantum squeezing and entanglement,which is one common theoretical basis of all these new technologies,obviously receives widespread concern.Due to the great performance and the broad application of the quantum squeezing and entanglement,many schemes based on the coherent evolution are proposed in order to generate the squeezed and entangled states.However,the coherent evolution is easily destroyed by the environment dissipation or the decoherence which exists in all kinds of experiments.The generated squeezed and entangled states is vulnerable and fragile.The coherence time is actually shorter than the cavity lifetimes.Recently,the Bogoliubov dissipation have been explored to prepare the squeezed and entangled states in different schemes.Typically,dissipation process is harmful when the system components are well correlated.Thus,one hopes that the dissipation process can be suppressed or even be eliminated.In sharp contrast,as the Bogoliubov mode is dissipated by the engineered reservoir,its two components are entering their own squeezed and entangled states.The physical mechanism is that the Bogoliubov mode dissipation establishes a two-photon process in the system.It is interesting to find out if the long-lived coherence is essential for obtaining squeezing and entanglement.On the other hand,as the system evolves,the spontaneous emission induced by the vacuum reservoir is another major issue both theoretically and experimentally.To our knowledge,linewidth narrowing and intensity elevation of fluorescence spectra turn out to be related important aspects to enhance the accuracy and efficiency in spectra-based high-precision measurements.Usually,one can change the spontaneous emission by putting the atom in different environment or adding extra coherent interaction.Previous works mainly focus on the so-called "doubly coherent scheme".However,this kind of schemes requires strict experimental conditions,and it lowers the intensity of the narrow spectral lines at the same time.Thus,its application is severely limited.The innovative contents are shown as follows:First,we reveal that the spontaneous feedback effect in the below-threshold atom-cavity system is the direct physical mechanism which induces the ultranarrow and high fluorescence spectral line(s).The narrowing and elevation of the fluorescence spectral line(s)can significantly improve both the accuracy and efficiency of the spectral based high-precision measurements.The strict experimental requirement in the " double coher-ence schemes" is reduced.We use this mechanism in various atom-cavity systems and successfully obtain the ultranarrow and high spectral lines.Detailed parameter require-ments of these systems as well as the comparison between these systems are given.We start our discussion with an ensemble of independent two-level bare-state atoms which are coupled to an single mode cavity.The fluorescent photons can be absorbed by the cavity field below threshold,and the cavity field couples back to the atom.If the system approaches the threshold continuously while keeps below the threshold,the effective decay rate of the atom decreases.In this situation,the population is not trapped or the spectral line has no extra splits due to the absence of the double coherence in the system.Conse-quently,the fluorescence spectral lines are narrow and high simultaneously as the decay rate becomes extremely small.The responding mechanism is that we use the fluorescent photons from the atomic spontaneous emission as its own control source,thus we call it the "spontaneous feedback mechanism".Next,we generalize this mechanism to two-level and three-level dressed atoms in which the spontaneous feedback mechanism makes a pair of Rabi sidebands become sharp and high.Additionally,we give a brief discussion on the possibility of the spontaneous feedback in molecule system.Second,we use engineered reservoir dissipation process to generate two-mode squeezing and entanglement of atomic spins or cavity fields as well as the excited-state spin squeezing in quantum beat system.The essence of this work is that the long-lived coherence is not necessary for obtaining squeezing and entanglement.Instead,the dissipation effect hidden behind the detuning induced nonlinearities is responsible for the squeezing and entanglement in the detuned quantum-beat system.We obtain almost ideal squeezed and entangled states in the near-resonance region in which the nonlinearities is created and the coherence is still relatively large.We place an ensemble of N three-level atoms in V configuration at the intersection of two optical cavities which are pumped by their respective external fields.As we choose symmetric detunings in the system,we find that the cavity or the dressed atomic spins can act as engineered reservoir in respective adiabatic conditions.In this situation,the Bogoliubov modes,which are formed by the fluctuation of the collective atomic spins or the collective cavity fields,are dissipated by this engineered reservoir.This leads the Bogoliubov modes reach their vacuum state.Thus we obtain two-mode squeezing and entanglement for the dressed atomic spins or the cavity fields.Additionally,excited-states spin squeezing may occur as the two-mode spins reach its squeezed and entangled states.