Controllable Kerr Nonlinearity with Perfect Transparency in the Existence of Spontaneously Generated Coherence

We investigate the Kerr nonlinearity of a V-type three-level atomic system where the upper two states decay outside to another state and hence spontaneous generated coherence may exist. It is shown that dark state and hence perfect transparency present under certain conditions. Meanwhile, the Kerr nonlinearity can be controlled by manipulation of the decay rates and the splitting of the two excited states. Therefore, enhanced Kerr nonlinearity without absorption can be obtained under proper parameters.     

Entangled Radiation through an Atomic Reservoir Controlled by Coherent Population Trapping

We show that it is possible to generate Einstein-Podolsky-Rosen （EPR） entangled radiation using an atomic reservoir controlled by coherent population trapping. A beam of three-level atoms is initially prepared in nearcoherent population trapping （CPT） state and acts as a long-lived coherence-controlled reservoir. Four-wave mixing leads to amplification of cavity modes resonant with RabJ sidebands of the atomic dipole transitions. The cavity modes evolve Jnto an EPR state, whose degree of entanglement is controlled by the intensities and the frequencies of the driving fields. This scheme uses the long-lived CPT coherence and is robust against spontaneous emission of the atomic beam. At the same time, this scheme is implemented in a one-step procedure, not in a two-step procedure as was required in Phys. Rev. Lett. 98 （2007） 240401.     

Coherence induced by incoherent pumping field and decay process in three-level Λ type atomic system

Following the method proposed by Kozlov et al. [Victor V. Kozlov, Yuri Rostovtsev, Marlan O. Scully, Phys. Rev. A 74 (2006) 063829], we have investigated the atomic coherence induced by incoherent pump and vacuum spontaneous decay process in a Λ type three-level atomic system. The system can be in a coherent population trapping state and multi-steady states in different conditions. Interestingly, two kinds of new states are derived from the system with different pumping rate and decaying rate. They are the “robust” steady state and the “weak” steady state. Under the action of pump field and vacuum reservoir, these two kinds of states exhibit stable or unstable characteristics, respectively. Moreover, by investigating the difference between these states, we reveal the mechanism of coherence excitation and level-population transition. The special feature of the Λ atomic system will promise fruitful applications in quantum optics.     

Arbitrary quantum superposition state for three-level system using oscillating dark states

An method for adiabatic population transfer and the preparation of an arbitrary quantum superposition state using the oscillating dark states (ODS) in atomic system is presented. Quantum state of a three-level Λ configuration atomic system finally evolves into the same time-dependent state, and oscillates periodically between two ground levels under evolving adiabatic conditions when two pairs of classical detuning laser fields drive the system into the ODS forcedly, whatever the initial states of the system are. The decoherence of the ODS evolution is greatly suppressed and the oscillation is very stable, therefore adiabatic population transfer and the preparation of an arbitrary quantum superposition state of atomic system can be completed accurately and conveniently.     

Influence of the dipole-dipole interaction on velocity-selective coherent population trapping

We analyze the influence of the dipole-dipole interaction between ground and excited state atoms on atomic cooling by velocity-selective coherent population trapping. We consider two three-level atoms in the -configuration, interacting with two counterpropagating laser fields as well as with the electromagnetic vacuum modes. The elimination of these modes in the Born-Markov approximation results in spontaneous decay, which is essential in providing the momentum diffusion necessary for cooling, as well as a two-body dipole-dipole interaction between ground-and excited-state atoms. The corresponding two-body master equation is solved numerically by Monte-Carlo wave-function simulations. Our main result is that although a dark state survives the inclusion of dipole-dipole interactions, the presence of this interaction can significantly slow down the cooling process for sufficiently high atomic densities.Dedicated to H. Walther on the occasion of his 60th birthdayStrictly speaking, VSCPT is not a true cooling mechanism. The final atomic distribution cannot be characterized by a temperature, so that there is some ambiguity in characterizing the cooling efficiency. We return to this point in Sect. 3     

Spontaneous emission of a three-level lambda-type atom coupled to separate reservoirs

Considering the anisotropic dispersion model, the upper state population and spontaneous emission spectrum of three-level lambda-type atom with two transitions coupled to separate reservoirs are investigated using the resolvent operator. The upper state population reaches to a steady state value after a weak oscillation when the decay rate is zero and one transition frequency is inside the bandgap. The spectrum associated with each transition was given. Compared with results that were obtained by using isotropic dispersion model, the shape of spectrum changes significantly, and no dark line appears in the spectra.     

Autler-Townes triplet spectroscopy

We study the effects of quantum interference in the spontaneous emission spectrum of a four-level driven atomic system. We use three strong laser fields to drive the atom and a weak laser field to prepare the initial state of the atom. The atomic system exhibits Autler-Townes triplet in the spectrum. The single Lorentzian peak splits into triplet and their widths are controlled by the relative strengths of the laser fields.     

Quantum interferences in coherent population trapping of a cold double Λ-type atom

We investigate quantum interferences in coherent population trapping of a cold double Λ-type four-level atomic system driven by two counterpropagating laser fields. We study both decoherence and enhanced-coherence actions resulting from the multi-transition pathways in building up the trapping state, and analyze the system operating with and without external coherences in various configurations of the atomic dipole moments.     

Spontaneous emission from a Λ three-level atom in an anisotropic photonic crystal

The spontaneous emission properties of a Λ type atom embedded in a three-dimensional anisotropic photonic crystal are investigated. Only one of the two atomic transition frequencies is considered near the photonic band edge. The atomic decay properties such as the time-evolution of the excited-state population and the instant and effective decay rates are studied in detail. It is found that there exists a wide region for the difference of the transition frequency from the band edge, in which only diffusion fields with frequencies being near or far away from the band edge appear in the emitted field. The spontaneous emitted field and its spectrum depend not only on the detuning of the transition frequency from the band edge, but also on the distance from the atom. Therefore, during the propagating process, the propagating field is partially transferred into the diffusion field.     

Schemes for Generating Cluster States via Cavity Systems

We propose a scheme for generating an N-atom cluster state via cavity quantum electrodynamics （ CQED）. In our scheme, there is no transfer of quantum information between the atoms and the cavity, i.e., the cavity is always in the vacuum state, so the cavity decay can be suppressed. Also, the generated cluster state is the entanglement of the ground states, so the atomic spontaneous emission can be avoided. Therefore, the cluster state generated in our scheme has a longer lifetime. Furthermore, the requirement on the quality factor of the cavity greatly loosened for the cavity is only virtually excited.     

Scheme for Concentration of Unknown Greeberger-Horne-Zeilinger Entangled States via Cavity Decay

We present a scheme for entanglement concentration of an unknown atomic non-maximally entangled GHZ state via cavity decay. In the scheme, the atom trapped in a cavity is manipulated by laser field, so the maximally entangled GHZ state can be obtained by performing certain operation, which can be realized by illuminating the atom in a cavity. Our method is robust against spontaneous atomic decay.     

Coherent control of a light field with electromagnetically induced transparency in a dark state Raman coherent tripod system

A dark state superposition is employed and formed a tripod-like system. A weak probe pulse propagates in it can experience a crossover from absorption to transparent and then to amplification, and its group velocity can be controlled in any desired speed by determining the initial states of the dark state superposition.     

Concentration of Unknown Atomic Entangled States via Entanglement Swapping through Raman Interaction

We show that entanglement concentration of unknown atomic entangled states is achieved via the implementation of entanglement swapping based on Raman interaction in cavity QED. A maximally entangled state is obtained from a pair of partially entangled states probabilistically. Due to Raman interaction of two atoms with a cavity mode and an external driving field, the influence of atomic spontaneous emission has been eliminated. Because of the virtual excitation of the cavity mode, the decoherence of cavity decay and thermal field is neglected.     

Evolution of population in an equispaced three-level ladder-type atomic system

Transient response of nearly equispaced three-level ladder-type atomic system with a broad-band squeezed vacuum (SV) is investigated. We focus our attention in the interplay between the quantum interference and the squeezed field on the population distribution. It is shown that an atomic population inversion can be attained on one of the optical transitions due to the SV. Additionally, we show, with the proper value of the relative phase, the SV can also lead to unexpected population inversion on the transition between two different levels.     

Ground State of Jaynes-Cummings Model: Comparison of Solutions with and without the Rotating-Wave Approximation

The eigenenergy spectrum of the Jaynes-Cummings （JC） model with and without the rotating-wave approx- imation （RWA） is investigated. The numerical analysis indicates that the non-RWA spectrum can only be approximated by the RWA in the range of sufficiently small coupling constant and detuning. In other region, the counter-rotating terms remarkably change the nature of the RWA energy spectrum. A simple expression with high accuracy for ground eigenenergy and eigenvector non-RWA shows that the ground state is not a dark state state. for non-RWA is available. The ground eigenvector for and very different from that of RWA which is a dark state.     

Control scheme for two photon ionization of condensate atoms

The efficient photoionization of a Bose-Einstein condensate requires the creation of ions with the smallest possible transfer of atoms from the condensate into the thermal phase. The spontaneous decay from excited states into atomic states with momentum different from the initial one should be reduced. We investigate theoretically two-photon ionization of a rubidium condensate using near resonant excitation to the 6P state and second photon at 421 or 1002 nm into the continuum. Different ionization schemes with coherent control of the first excitation and reduction of the spontaneous decay are presented.     

Analytical analysis of the “collapse-revival” effect in the Jaynes-Cummings model

The evolution of the atomic state population in a two-level system coupled to a single-mode quantum field is calculated in the analytical form. Essential characteristics of the “collapse-revival” effect are expressed in terms of the physical parameters of the system by means of simple formulas in both the resonant and the non-resonant cases. The obtained results are of great importance for the qualitative analysis of the phenomenon.     

Efimov Superchemistry： Quantum Dynamical Theory for Coherent Atom-Trimer Conversion in a Repulsive Atomic Bose-Einstein Condensate

We show that by making a generalized atom-molecule dark state, coherent creation of triatomic molecules can be enhanced in a repulsive atomic Bose-Einstein condensate. The dynamics of heteronuclear trimer creation is significantly different from the homonuclear case and further enhancement can be realized by controlling its chemical reaction channels. The possibility of manipulating atom-trimer conversion provides an appealing research area for current coherent matter-wave optics.     

Effective Scheme for Generating Cluster States in Cavity QED

We propose a scheme to prepare many two-mode cavities into one-dimensional cluster states in the context of cavity QED. The left-circularly polarized state and right-circularly polarized state of the cavity are encoded as the logic zero and one of the qubits. In the scheme, the atomic spontaneous emission is suppressed, and the fidelity is unaffected by the cavity decay on the assumption that the detection efficiencies of all the photondetectors are 1.