Control of the switch between optical multistability and bistability in three-level V-type atoms

We theoretically investigated a hybrid absorptive-dispersive optical bistability and multistability behaviour in a three-level V-type system using a microwave field driving a hyperfine transition between two upper excited states inside a unidirectional ring cavity. We find that the intensity and the frequency detuning of the coupling field as well as the intensity of the microwave field can affect the OM behaviour dramatically, which can be used to control the transition from OM to OB or vice versa without need to resort the effect of the quantum interference. The effects of the phase, the quantum interference and the atomic cooperation parameter on the OM are also studied. Our scheme may be used for building more efficient all-optical switches and logic-gate devices for optical computing and quantum information processing.     

Control of group velocity of light via magnetic field

We have theoretically studied the effects of quantum coherence in a driven quasi-degenerate two-level atomic system. We have shown that the quantum interference, which can be destructive or constructive, can be controlled by an externally applied magnetic field allowing one to implement both electromagnetically induced transparency and electromagnetically induced absorption in the same atomic system. Determined by frequency dispersion of the index of refraction of the system, the group velocity of light pulses ranges from ultra-slow to superluminal with changing of the magnitude of the magnetic field.     

Entanglement dynamics and quasiprobability distribution for the degenerate Raman process

In this paper, we consider the model which consists of a degenerate Raman process involving two degenerate Rydberg energy levels of an atom interacting with a single-mode cavity field. The influence of the atomic coherence on the von Neumann entropy of the atom and the atomic inversion is investigated. It is shown that the atomic coherence decreases the amount of atom-field entanglement. It is also found that the collapse and revival times are independent of the atomic coherence, while the amplitude of the revivals is sensitive to this coherence. Moreover, the Q function and the entropy squeezing of the field are examined. Some new conclusions can be obtained.     

Tripartite Entanglement via Microwave Driven Atomic Coherence

We propose a new scheme to achieve the tripartite entanglement based on the standard criteria [Phys. Rev. A 67（2003） 052315] in a inverse-tripod atomic system. In our scheme, the atomic coherence is introduced by two microwave fields which drive the upper three levels of atom. By numerically simulating the dynamics of system, we investigate the generation and evolution of entanglement in the presence of atom and cavity decay. As a result, the present research provides an efficient approach to achieve fully tripartite entanglement with different frequencies and initial states for each entangled mode, which may have impact on the progress of multicolored multi-notes quantum information networks.     

Spectral line suppression and sideband narrowing via quantum interference

We reveal that for a realistic system, interference effects are obtained such as the suppression of central line and inner sidebands and the narrowing of the outer fluorescence sidebands. For this purpose, we consider a spontaneous decay from an excited state to a metastable state when the excited and metastable states are resonantly coupled to an auxiliary metastable state by a laser field and a microwave field, respectively. The fluorescence spectrum evolves from a five-peaked structure into a doublet of ultrasharp lines as the ratio of the laser field Rabi frequency to the microwave Rabi frequency is decreased. The physical origin is presented in terms of dressed states.     

Controllable Optical Bistability and Multistability in a Four-Level Atomic System with Closed-Loop Configuration

We theoretically investigate optical bistability （OB） and multistability （OM） behaviour of a closed-loop configuration atomic system driven by a degenerate coupling- field and a degenerate probe field inside a unidirectional ring cavity. It is found that the OB and OM behaviour can be controlled by adjusting- the intensity and the frequency detuning of the coupling- field, respectively. Interestingly, our numerical results show that it is easy to realize the transition from OB to OM or vice versa by adjusting- the intensity of the coupling- field under a appropriate frequency detuning. The effect of the atomic cooperation parameter on the OB behaviour is also discussed.     

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.     

Phase and amplitude control of the inversionless gain in a microwave-driven <Emphasis Type="Bold">Λ</Emphasis>-type atomic system

In order to achieve the phase-sensitive probe gain without population inversion, we investigate a three-level Λ-type atomic system driven by a coherent field and a microwave field. It is shown that, by modulating the relative phase of applied fields, we can obtain quite high inversionless gain at different probe detunings and change the gain behavior of the probe correspondingly. We find that amplitudes of the coherent field and the microwave field are also important factors that can result in different gain behavior of the probe. Here, we use the microwave field to induce the quantum coherence between the two ground levels, which is necessary for phase-sensitive effects, since it can result in the interference between two different transition channels. Received 20 June 2002 / Received in final form 5 December 2002 Published online 4 March 2003     

Six-Fold Degenerate Dark States in a Four-Level Atomic System

With all driving fields on Raman resonance, a tripod-type atomic system quickly evolves into a dark state decoupled from the lossy excited level. The dark state depends strongly on field Rabi frequencies, spontaneous decay rates, and the initial atomic population in a complicated way. Analytical results reveal that it is a sixfold degenerate dark state with its three components superposed both coherently and incoherently due to population redistribution from spontaneous emission.     

Entanglement and Its Dynamics with Atomic Spontaneous Decay

The atomic decay for a two-level atom interacting with a single mode of electromagnetic tield is considered. For a chosen initial state, the exact solution of the master equation is found. Therefore, effect of the atomic damping on entanglement （purity loss）, degree of entanglement by the negativity, mutual information and atomic coherence through the master equation are studied.     

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.     

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.     

Observation of collimation and decollimation of an atomic beam in a misaligned standing wave

This paper reports an experimental study on the collimation and decollimation of an atomic beam in a misaligned standing wave, in which the effective detuning caused by the Doppler effect is affected by the longitudinal velocity of the atomic beam. The experiment shows that in a strong field with red detuning between laser field and atomic transition frequency, laser heating in a normal standing wave becomes laser cooling in a misaligned standing wave for an approriate misalignment angle. For blue detuning, laser cooling in a standing wave can also become laser heating in a misaligned standing wave for an appropriate condition. These results ca be used in controling atomic motion.     

Quantum nondemolition measurement of transverse atomic position in Kapitza-Dirac atomic beam scattering

It is demonstrated that one can measure the distribution of the transverse position of an atom crossing one or more optical cavities by monitoring the phase of the standing wave fields in the cavities. For the atom-field interaction the Kapitza-Dirac regime is assumed; it is shown that in this regime the method represents a quantum nondemolition measurement of the atomic position. On the other hand it can be applied to prepare narrow distributions of the transverse atomic position. In order to show this, a numerical simulation is performed, which illustrates the collapse of a broad initial Gaussian wavepacket, which can be coherent or incoherent, to a distribution with narrow peaks. Preparing the cavity fields in a squeezed state, one can greatly enhance the impact of the cavity field measurements on the atomic density matrix.     

Three-Photon Resonant Nondegenerate Six-Wave Mixing in a Dressed Atomic System

We report a three-photon resonant nondegenerate six-wave mixing （NSWM） in a dressed cascade five-level system. It has advantages that phase match condition is not stringent and NSWM signal is enhanced tremendously due to the multiple resonance with the atomic transition frequencies. In the presence of a strong coupling field, the threephoton resonant NSWM spectrum exhibits＂ Autler-Townes splitting. This technique provides a spectroscopic tool for measuring not only the resonant frequency and dephasing rate but also the transition dipole moment between two highly excited atomic states.     

Scheme for Direct Measurement of the Wigner Function via Resonant Interaction

We propose a scheme for direct measurement of the Wigner function for a cavity mode. In the scheme the cavity field resonantly interacts with an atomic ensemble. Under certain conditions, the state of the cavity mode is transferred to the atomic system. After a displacement the measurement of the parity of the atomic excitation number directly yields the Wigner function of the initial state of the cavity mode.     

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.     

Obtaining high refractive index with vanishing absorption via spontaneously generated coherence and incoherent pumping

A three-level ∧-model atomic system with incoherent pumping is proposed to achieve high refractive index without absorption. In this kind of model, two lower levels are near-degenerate levels. It is found that high refractive index accompanied by vanishing absorption can be always accomplished by adjusting some related parameters. Although probe field is very weak, the SGC effect is prominent in the presence of incoherent pumping.     

Generation of strongly squeezed states for a cavity field with a single atom

We propose a scheme to realize strong squeezing for a cavity field with a single three-level atom. In the scheme the atom is sent through the cavity initially filled with a coherent field. The atom dispersively interacts with the cavity field, which is displaced by a microwave resource during the interaction. Then, a selective measurement on the atom collapses the field to a superposition of an even coherent state with a vacuum state, which exhibits strong squeezing. The scheme can also be generalized to the two-mode case.     

Manipulation of optical fields by atomic interferometry: Quantum variations on a theme by young

We analyze the principle of a very general and conceptually simple method for manipulating optical fields by coupling them into a matter waves Young double slit apparatus. The field, non resonant with the atoms, acts as a phase-retarding medium in one of the arms of the interferometer and shifts the atomic fringe pattern. The method constitutes a simple quantum nondemolition measuring scheme of the photon number. Non classical states such as Schrödinger cats and Fock states of the field are generated in the measurement process. The analysis of the atomic interferometer with optical retarding fields provides a very simple and striking illustration of basic concepts of the quantum measurement theory and of the principle of complementarity. This scheme, which would be very difficult to implement in the optical domain, is equivalent to a more feasible and recently proposed Ramsey interference method to measure small microwave fields with beams of Rydberg atoms.Associé au Centre National de la Recherche Scientifique et à l'Université Pierre et Marie Curie