Polychromatic Electromagnetically Induced Transparency And The Preparation Of Entangled States

Atomic coherence and quantum interference is one of the most important research subjects in the field of laser physics and quantum optics.Electromagnetically induced transparency makes an otherwise opaque medium almost transparent.Theoretical and experimental work has demonstrated that this phenomenon can be used to slow down light pulses dramatically,or even bring them to a complete halt.Interactions between photons in such an atomic medium can be many orders of magnitude stronger than in conventional optical materials.Thus,electromagnetically induced transparency plays an important role in slow light,nonlinearity at low light level,light storing and retrieving in quantum information processing.Quantum entanglement reflects the nonclassical quantum correlation between two or more quantum systems.The problems related to quantum entanglement are basic theories in quantum mechanics such as reality,locality,hidden variables and measurement. Quantum entanglement also lies in the key of quantum computation and quantum information. The research about the properties of entangled states can be used to test the basic rules in quantum mechanics,while entangled states themselves are basic resources in quantum information processing.Thus entanglement preparation has received extensively research.Otherwise,due to decoherence came from the interaction between the system with the environment,the entangled states prepared are fragile and can not be kept for a long time.Thus,the preparation of robust,steady and bright entangled states has been paid more attention.In this thesis,we have studied the amplitude and phase control of electromagnetically induced transparency with polychromatic driven fields and presented several schemes to prepare entangled states in different systems.The main initiative results are presented as follows.1.The work about electromagnetically induced transparency with polychromatic driven fields:The first work about electromagnetically induced transparency with polychromatic driven fields is to study the absorption and dispersion properties of the three-level A atomic system when a trichromatic driven field and a monochromatic probe field couple two different transitions,respectively.The dependence of the absorption and the dispersion on the amplitude and phase of the driven field is checked.Numerical method is adopted to calculate the spectra.Multiple electromagnetically induced transparency is obtained.Two characteristic features are found.First,the central transparency can be made to appear or disappear by utilizing the amplitudes and phases of the components of the driven field. Secondly,so long as we fix the sum of two relative phases of two sideband excitation components to the central component,the absorption and dispersion spectra keep their own lineshapes unchanged no matter how we vary the respective relative phases.The second work about electromagnetically induced transparency with polychromatic driven fields is to study the two-photon absorption in a three-level cascade atomic system driven by a pair of bichromatic fields with equal frequency difference.The high-frequency component of one bichromatic field and the low-frequency component of the other are on two-photon resonance.We adopt the method of harmonic expansion and matrix inversion. The main result is that when the sums of the phases of the different pairs of field components on the two-photon resonance are equal to each other,two-photon absorption is dramatically suppressed and the atomic system becomes transparent against two-photon absorption. Due to dynamic Stark splitting,the two-photon transitions induced by the different pairs of field components experience different dressed states with phase difference ofπ.As a result,destructive interference occurs between the two pathways and leads to the inhibition of two-photon absorption.2.The work about preparation of entangled states:In the first scheme of entanglement preparation,we show that a quantum-beat system with incoherent pump or coherent driving produces an entangled sub-shot-noise laser that operates well above threshold.We adopt the combination modes of the lasing fields to present the numerical results and physical analysis.The relative mode is decoupled from the active medium and stays in its vacuum state,while the sum mode operates well above threshold and has sub-shot-noise.The quantum beat and the sum mode intensity noise reduction combine to yield entanglement between two bright beams and sub-Poissonian photon statistics of the respective beams.In the second work about preparation of entangled states,we propose a scheme for generating a two-atom entangled state and an N-atom W state using adiabatic evolution of dark eigenstates in cavity QED.This scheme has two advantages.Firstly,the time required to complete the process does not need precise control.Secondly,since the cavity modes are never excited during the operations by engineering adiabatic evolution and controlling the atom-cavity couplings,the decoherence due to the cavity decay can be suppressed.The last work about preparation entangled states is a concentration scheme for unknown atomic states through the Laman interaction of two atoms with a cavity mode and an external laser field.We obtain a two-atom maximally entangled sate from a pair of unknown partially entangled atomic states.In the concentration process,as the cavity mode is virtually excited and the atomic excited states are eliminated,our scheme is robust against the cavity decay,the thermal field and the atomic spontaneous emission.