Collective effects in the dynamics of driven atoms in a high-Q resonator

We study the quantum dynamics of N coherently driven two-level atoms coupled to an optical resonator. In the strong coupling regime the cavity field generated by atomic scattering interferes destructively with the pump on the atoms. This suppresses atomic excitation and even for strong driving fields prevents atomic saturation, while the stationary intracavity field amplitude is almost independent of the atom number. The magnitude of the interference effect depends on the detuning between laser and cavity field and on the relative atomic positions and is strongest for a wavelength spaced lattice of atoms placed at the antinodes of the cavity mode. In this case three dimensional intensity minima are created in the vicinity of each atom. In this regime spontaneous emission is suppressed and the dominant loss channel is cavity decay. Even for a cavity linewidth larger than the atomic natural width, one regains strong interference through the cooperative action of a sufficiently large number of atoms. These results give a new key to understand recent experiments on collective cavity cooling and may allow to implement fast tailored atom-atom interactions as well as nonperturbative particle detection with very small energy transfer.Received: 18 May 2004, Published online: 19 October 2004PACS: 32.80.Pj Optical cooling of atoms; trapping - 42.50.Pq Cavity quantum electrodynamics; micromasers - 42.50.Fx Cooperative phenomena in quantum optical systems     

Quantum memory based on $\mathsf{\Lambda}$-atoms ensemble with two-photon resonance EIT

We study the -atoms ensemble based quantum memory for the quantum information carried by a probe light field. Two atomic Rabi transitions of the ensemble are coupled to the quantum probe field and classical control field respectively with a same detuning. Our analysis shows that the dark states and dark-state polaritons can still exist for the present case of two-photon resonance EIT. Starting from these dark states we can construct a complete class of eigen-states of the total system. A explicit form of the adiabatic condition is also given in order to achieve the memory and retrieve of quantum information.Received: 8 June 2004, Published online: 10 August 2004PACS: 03.67.-a Quantum information - 42.50.Gy Effects of atomic coherence on propagation, absorption, and amplification of light; electromagnetically induced transparency and absorption - 03.65.Fd Algebraic methods     

Nonlinear optical responses due to exciton-phonon interactions in strongly coupled exciton-phonon systems

The excitonic nonlinear optical responses due to exciton-phonon interactions in strongly coupled exciton-phonon systems are investigated theoretically. It is shown that the influence of exciton-phonon interactions on the nonlinear optical absorptions and Kerr nonlinear coefficients is significant as the signal field frequency detuning from the exciton frequency approaches to the optical phonon frequency. How to manipulate the nonlinear optical responses by using the control fields is also presented.Received: 16 March 2004, Published online: 4 May 2004PACS: 42.50.Gy Effects of atomic coherence on propagation, absorption, and amplification of light; electromagnetically induced transparency and absorption - 42.50.Hz Strong-field excitation of optical transitions in quantum systems; multiphoton processes; dynamic Stark shift - 42.25.Bs Wave propagation, transmission and absorption - 72.80.Le Polymers; organic compounds (including organic semiconductors)     

Vacuum Rabi splitting for multilevel electromagnetically induced transparency system

We discuss the vacuum Rabi splitting (VRS) from multilevel atoms under electromagnetically induced transparency condition within the framework of linear absorption-dispersion theory. Sharp resonance features superimposed on usually occurring VRS doublet to three-peaked structure spectra are obtained here which can be engineered to have absorptive, dispersive, or dip like profiles according to the choice of system parameters such as radiative damping constants, atomic detunings and driving field strengths etc.Received: 23 June 2004, Published online: 24 August 2004PACS: 42.50.Pq Cavity quantum electrodynamics; micromasers - 42.50.Ar Photon statistics and coherence theory     

Quantum storage of single photons with unknown arrival time and pulse shapes

We present a scheme for the quantum storage of single photons using electromagnetically induced transparency (EIT) in a low-finesse optical cavity, assisted by state-selected spontaneous atomic emission. Mediated by the dark mode of cavity EIT, the destructive quantum interference between the cavity input-output channel and state-selected atomic spontaneous emission leads to strong absorption of single photons with unknown arrival time and pulse shapes. We discuss the application of this phenomenon to photon counting using stored light.     

Spatial distribution of quantum fluctuations in spontaneous down-conversion in realistic situations

We show that in the limit of negligible pump depletion, the spatial distribution of the quantum fluctuations in spontaneous parametric down-conversion can be computed for any shape of the pump beam by using the Greens function method to linearize the quantum fluctuations, even for very low levels of the intensities measured on the pixels. The results are in complete agreement with stochastic simulations of the Wigner distribution. Both methods show specific quantum effects in realistic situations close to the experiments now in progress, like sub-shot noise correlation between opposite pixels in the far field.Received: 15 January 2004, Published online: 23 March 2004PACS: 42.65.Yj Optical parametric oscillators and amplifiers - 42.50.-p Quantum optics - 42.65.-k Nonlinear optics     

Quantum interference effect on spontaneous emission spectrum in a doubly driven M-type atom

An M-type atomic system with two closely spaced upper levels interacting with the same modes of the vacuum radiation field exhibits the interference effect between the spontaneous decay channels. The phenomenon of this decay-interference along with the dynamically induced quantum interference created in the system by two coherent fields can change significantly the spontaneous emission spectrum. Our results highlight large enhancement of ultranarrow spectral components within the spontaneous emission line shape in various conditions. We have shown the occurrence of two contradistinctive phenomena: compression of two spectral lines towards their mid-position and the spreading of two lines away from each other under different dynamic conditions. The phenomenon of suppression of spectral line and the quenching of spontaneous emission have also been focused.     

Disentanglement of atom-photon via quantum interference in driven three-level atoms

In this paper, the effect of quantum interference on the entanglement of a driven V-type three-level atom and its spontaneous emission field was investigated by using the quantum entropy. The results indicate that, in the absence of quantum interference the atom and its spontaneous emission field are always entangled at the steady-state. But, in the presence of full quantum interference their steady-state entanglement depends on the atomic parameters. Specifically, with appropriate atomic parameters they can be entangled or disentangled at the steady-state. We realized that the steady-state entanglement is due to completely destructive nature of quantum interference. On the contrary, the steady-state disentanglement is due to instructive nature of quantum interference.     

The coherent time evolution of two coupled quantum dots in a two-mode cavity

For two coupled identical quantum dots in a two-mode cavity, we determine the conditions of two-photon and single-photon resonance. It is shown that the exciton-phonon interaction reduces the Rabi frequencies of each model and the Förster interaction between double quantum dots even at absolute zero temperature. The exciton-phonon interaction also makes a contribution to the static exciton-exciton dipole interaction energy. Furthermore, the additional interactions can modify the conditions of photon resonance significantly. A more realistic case of two nonidentical quantum dots is also considered. The influence of parameter misfits on the quantum system is discussed.Received: 8 July 2004, Published online: 3 November 2004PACS: 73.21.La Quantum dots - 71.35.-y Excitons and related phenomena - 42.50.Pq Cavity quantum electrodynamics; micromasers     

Doppler-free spectroscopy in driven three-level systems

We demonstrate two techniques for studying the features of three-level systems driven by two lasers (called control and probe), when the transitions are Doppler broadened as in room-temperature vapor. For -type systems, the probe laser is split to produce a counter-propagating pump beam that saturates the transition for the zero-velocity atoms. Probe transmission then shows Doppler-free peaks which can even have sub-natural linewidth. For V-type systems, the transmission of the control beam is detected as the probe laser is scanned. The signal shows Doppler-free peaks when the probe laser is resonant with transitions for the zero-velocity group. Both techniques greatly simplify the study of three-level systems since theoretical predictions can be directly compared without complications from Doppler broadening and the presence of multiple hyperfine levels in the spectrum.Received: 10 September 2003, Published online: 6 January 2004PACS: 42.50.Gy Effects of atomic coherence on propagation, absorption, and amplification of light; electromagnetically induced transparency and absorption - 42.50.-p Quantum optics     

Controlling the probe-absorption and -dispersion via quantum interference from incoherent pumping field in a four-level lambda-type system

Absorptive and dispersive properties of a four-level Λ-type atomic system with two fold lower levels are investigated. The effect of quantum interference induced by incoherent pump and spontaneous emission upon the absorption, dispersion, and group index is them discussed. In addition, the rate of an incoherent pump field and the intensity of coupling field on light propagation is also presented.     

Coherent control of spontaneous emission: Effect of counter-rotating terms

We study the coherent control of spontaneous emission in a driven three-level atomic system. In particular we consider the quantum interference effects including energy shifts due to the counter-rotating terms. We show that the quantum interference resulting from the energy shifts has great influence on the evolution of the atomic population and the spontaneous emission spectrum. This makes it feasible to observe the effect of the counter-rotating terms in the long time limit.     

Level mixing induced transparency for gamma radiation

The propagation of gamma radiation through a resonant medium in the case of interfering quantum transition paths is considered. The interference is made possible by a field that mixes the crossing spin levels in the excited nuclear state and splits two degenerate transitions into two V-type transitions. If forward resonant scattering allows for a change of the gamma radiation polarization, then the two V-transitions are coupled, which results in destructive interference. In this case the absorption is reduced in a particular frequency domain. PACS 42.50.Gy; 33.45.+x     

Forces and spatial ordering of driven atoms in a resonator in the regime of fluorescence suppression

An atom in a high-Q cavity, which is coherently driven at the frequency of a cavity mode, exhibits strong suppression of fluorescence when the atomic decay rate exceeds the cavity linewidth. This effect is due to destructive interference of cavity and pump field, such that at the atomic position the total field intensity has a local minimum. For atomic ensembles the magnitude of the interference effect grows with atom number and depends on the relative atomic positions. It is strongest for a wavelength spaced array of atoms placed at the antinodes of the cavity mode. This suppresses fluorescence and enhanced collective scattering into the cavity mode. We analyze the mechanical forces in the regime where the interference condition is fulfilled. We show that the atomic pattern is mechanically stable whenever the driving frequency is red detuned with respect to the cavity frequency, irrespective of the atomic transition frequency. Hence atomic selforganization, as predicted in [6] can also occur in the parameter regime where superradiant scattering is suppressed by collective interference. PACS 32.80.Pj; 42.50.Pq; 42.50.Fx     

Squeezing and entangling atomic motion in cavity QED via quantum nondemolition measurement

We propose a scheme for preparing the squeezing of an atomic motion and an Einstein-Podolsky-Rosen state in position and momentum of a pair of distantly separated trapped atoms. The scheme utilizes the quantum nondemolition measurements with interaction between the cavity field and the motional state of the trapped atom in cavity QED. By illuminating the atoms with bichromatic light, the interaction Hamiltonian of the cross-Kerr effect between the cavity and atomic motion is generated to implement quantum nondemolition measurements.Received: 5 February 2003, Published online: 17 July 2003PACS: 03.67.Hk Quantum communication - 32.80.Lg Mechanical effects of light on atoms, molecules, and ions - 42.50.-p Quantum optics     

Effects of the control field on the optical multistability in V-type three-level atomic system

The optical multistability behaviour in a ring cavity for the V-type atomic system, driven by a coherent field and control field (coherent + dc fields), has been analysed. The presence of dc field is having a dominant effect on generating the optical multistability of the system. We show that, the effects of the quantum interference from spontaneous emission and of the relative phase between the two fields of the control field might be of use to control the threshold value and width of the hysteresis cycle, which can adjust the optical switching process when they are taken at optimal values. Also, the optical multistability is predicted for the output field as a function of the cooperative parameter in the presence of the quantum interference of the spontaneous emission.     

Spontaneous interference bremsstrahlung effect in the scattering of a relativistic electron by a nucleus in the field of two light waves

This paper presents, for the general relativistic case, a theoretical study of nonresonance spontaneous bremsstrahlung by an electron scattered by a nucleus in the field of two elliptically polarized light waves propagating in the same direction. We show that there are two significantly different kinematic regions: the noninterference region where the main multiphoton parameters are the Bunkin-Fedorov quantum parameters γ _1,2, and the interference region where interference effects play an important role and where the quantum interference parameters α(±) act as multiphoton parameters. We encounter the spontaneous interference bremsstrahlung effect in two cases: in the special case of the same linear polarization of both waves, and in the general case of elliptical polarization of the waves. The effect manifests itself in the interference region and is due to stimulated, correlated emission and absorption of photons of both waves. For moderately strong fields, we find the cross sections of spontaneous bremsstrahlung by an electron scattered by a nucleus in the given kinematic regions. Finally, we show that the differential cross section in the interference region with correlated emission (absorption) of equal numbers of photons of both waves can be much greater than the corresponding cross section in any other geometry. Zh. éksp. Teor. Fiz. 116, 1210–1240 (October 1999)