The Study Of Atomic Coherence And Collective Dissipation Effect

Squeezing is one of the most striking features of quantum optics and widely applied in high-precision measurements and quantum communications.It,can be simply defined as the reduction of quantum fluctuations in a certain quadrature below the vacuum lev-el,at the expense of increasing them in its canonically conjugate variable.Of extreme importance is the close correlation of the two-mode squeezing to the continuous variable entanglement.Moreover,when the squeezing parameter is large one can have Einstein-Podolsky-Rosen(EPR)entanglement.Very often,one obtains parametric interaction using the atom-field system,and furtherly generates two-mode squeezing.In the usual situations,on resonance or close to resonance,atoms are excited and their spontaneous emission destroys any possible squeezing of the coupled optical fields.In order to over-come the spontaneous emission and to seek for squeezing,one ha,s to work in the far-off-resonance regimes.In the far-off-resonance regimes,the atoms are hardly excited and then there is hardly spontaneous emission.At the same time,the dispersive interactions are dominant over the absorptive interactions.As a consequence,the parametric interaction strengths are very small.Once they are too much smaller than the cavity damping rates,squeezing can no longer appear in the output fields.In addition,squeezed and entangled states are generally generated by coherent evolution.In most cases,dissipation,caused by coupling to the environment,leads to a rapid decay of quantum coherence.Squeez-ing and entanglement are destroyed and will very soon disappear.Thus,dissipation is considered to be a serious enemy to quantum-mechanical system.Recently,it is shown that engineered dissipative dynamics can stably generate nonclassical states.This type of quantum-state engineering,driven by dissipation,has attracted considerable interest both theoretically and experimentally.In the reservoir engineering for two-mode squeezing and entanglement,the squeezing parameter and the dissipative rates combine to determine the two-mode quantum correlations and depend strongly on nonlinearities or saturations.However,we notice that the two factors depend on the nonlinearities of the atom-field interactions in drastically different ways.Until now,the conditions of strong quantum correlation in the reservoir engineering have not been given.The innovative contents are shown as follows:Firstly,we obtain the tunable and effective two-mode and four-mode field squeeze operators via coherent population trapping(CPT).Three-level atoms in ? configurat.ion are coupled to two coherent fields.Within CPT windows opened by the dressing fields,atoms stay predominantly in the superposed state of two ground states(dark state)and yield strong nonlinearities for both the dressing fields themselves and the other applied fields.The two-mode squeeze operator is separated from degrees of freedom of atoms in a common CPT windows,and the multimode squeeze operator is segregated in different CPT windows.The resulting two-mode and multimode interaction strengths are at least one order higher than in the previous dispersive schemes,where all fields are far off resonant with the atoms.The present near-resonant scheme is robust to spontaneous emission since atoms are nearly trapped in a long-lived superposition state.Secondly,we optimize the conditions of strong quantum correlation in reservoir en-gineering.In the reservoir engineering for two-mode squee.zing and entangle.ment,the squeezing parameter determines the Bogoliubov-like modes collective interactions and the dissipative rates determine the engineered dissipation of the Bogoliubov modes.They combine to determine the two-mode quantum correlations.Since they have opposite dependences on the atom-field nonlinearities,they have competing effects on quantum correlations.Exemplifying two-level atomic ensembles,we examine the effects of the atom-field nonlinearities on the squeezing parameter and the dissipation rates.The re-sults verify that the balance between them by the moderate nonlinearities remarkably enhances the two-mode squeezing and entanglement of separated spin atomic ensembles or different optical fields.Without the adiabatic elimination,our calculations and results hold for arbitrary rates of the atomic and cavity relaxations.