Diode lasers exposed to moderate optical feedback, when biased near threshold, exhibit low-frequency intensity fluctuations in the RF spectrum that materialize as dropout events in the intensity time series. We recorded high-quality intensity time traces and frequency spectra of these events that show a new phenomenon in which the dropout events synchronize and appear at regular time intervals. The fine structure of the individual dropouts is recorded with an ultrafast digitizer that reveals the presence of fast oscillations occurring in the picosecond range. Numerical work based on the single-mode single-delay Lang and Kobayashi rate equations shows a locked state consisting of sharp intensity pulses that are slowly modulated and locked to an extraordinarily regular pattern.

By applying a quantum-trajectory approach to a master equation for the cavity mode density operator, we investigate the dynamics and the trapping states of a micromaser or microlaser including the effects of cavity temperature, two-atom events, atomic velocity spread, and partial excitation of the atomic beam. In the description of two-atom collective effects we allow for the nonsimultaneous arrival of atoms in the cavity, generalizing to a mean number of atoms $N_{\mathrm{at}}\approx 1,$ a recent treatment [M. I. Kolobov and F. Haake, Phys. Rev. A 55, 3033 (1997)]. We show that, in trapping regions, our treatment of two-atom events can give strikingly different effects with respect to the approximation of simultaneous arrival of atom pairs; e.g., detrapping instead of trapping, different times of approach to the steady state, and different photonstatistics. Also, we simulate micromaser dynamics with parameter values corresponding to the recent experimental observation of the trapping effect by Walther and collaborators [H. Weidinger et al., Technical Digest of the European Quantum Electronics Conference ’98 (IEEE, Piscataway, NJ, 1998)].

A recently developed scheme [S. Scheel, L. Knöll, and D.-G. Welsch, Phys. Rev. A 58, 700 (1998)] for quantizing the macroscopic electromagnetic field in linear dispersive and absorbing dielectrics satisfying the Kramers-Kronig relations is used to derive the quantum local-field correction for the standard virtual-sphere-cavity model. The electric and magnetic local-field operators are shown to become approximately consistent with QED only if the polarization noise is fully taken into account. It is shown that the polarization fluctuations in the local field can dramatically change the spontaneous decay rate, compared with the familiar result obtained from the classical local-field correction. In particular, the spontaneous emission rate strongly depends on the radius of the local-field virtual cavity.

A superfluorescence effect in a V-type medium with initial Raman inversion, driven by a strong coherent field on one of the transitions, is studied analytically and numerically. It is shown that the collective spontaneous emission evolves in the presence of a minor population of an upper state of a superfluorescent transition. In the limit of the Rabi frequency of a driving field large compared to that of a superfluorescent pulse, the system of two equations for the fields and density-matrix equations for three states of the medium is reduced to Maxwell-Bloch equations for an effective two-level system. The shape of a superfluorescent pulse is perfectly described in terms of a slowly varying envelope function (which is a solution of the Maxwell-Bloch equations), filled with fast oscillations associated with Rabi floppings on the driving transition. In the average, i.e., after integrating over a period of Rabi oscillations, the upper superfluorescent state is populated less than other states, such that the phenomenon may be called “superfluorescence without inversion” by analogy with lasing without inversion.

We have studied the intensity correlation between two lasers induced by the pumping mechanism. This correlation is studied through the noise spectrum of the intensity difference between the two lasers. We have shown, in the context of atomic lasers, that this spectrum may be smaller than the corresponding shot noise if the pumping noises of the two lasers are correlated. 85% noise reduction with respect to shot noise is predicted under reasonable operating conditions.

We quantize the electromagnetic field in an absorbing resonant medium with translation symmetry broken along one direction. In this case the general polarization of the material system is driven by the field via a nonlocal complex susceptibility. We extend recently developed schemes for field quantization in absorbing dielectric media with local susceptibilities to dielectric systems described by a nonlocal susceptibility. Dissipation inside the medium is described by considering the coupling of polarization quanta of the system with the reservoir oscillators in the usual Langevin approach. The method is applied to the case of a semiconductor quantum well embedded in infinite barriers. Expressions for the electromagnetic-field operators are obtained in the different spatial regions and the spatial and frequency dependencies of the vacuum-field fluctuations are calculated.

We analyze the spatial aspects of the down-converted field emitted by a cavityless optical parametric amplifier, both from theoretical/numerical and experimental viewpoints. Our model neglects pump depletion and includes a classical description of the noise which initiates the process. The ringshaped angular distribution in the far field is determined basically by the phase-matching conditions. In the near field, the configuration arises from the superposition of contributions from a broad frequency band; it is shown that the finite duration of the pump pulse introduces a correlation among neighboring frequencies that allows for the appearance of a spotlike spatial modulation. When the spatial configuration of the pump field is changed from a plane wave to a narrow Gaussian, the near-field pattern assumes the form of a roll structure. The experimental observations obtained with a lithium triborate crystal agree qualitatively well with the theoretical picture.

We suggest a variant of the recently proposed experiment for the generation of a kind of mesoscopic superposition quantum state (a Schrödinger-cat-type state), using two coupled parametric down-converter nonlinear crystals [F. De Martini, Phys. Rev. Lett. 81, 2842 (1998)]. We study the parametric oscillator case and find that an entangled Schrödinger-cat-type state of two cavities, whose mirrors are placed along the output beams of the nonlinear crystals, can be realized under suitable conditions.

With the aim of obtaining a better insight in the optical tunneling process the near-field electrodynamics of a current-density (equivalently polarization) sheet is investigated, taking as a starting point the near-field optics of a single atom, and afterwards the tunneling field of a macroscopic medium is determined integrating over a distribution of sheets. The total electric field hitherto used to study tunneling times and effective tunneling velocities is divided into a nonretarded (matter attached) longitudinal part of standing-wave character, and a retarded (detached) transverse part propagating away from the matter-vacuum interface with the vacuum speed of light. For a current-density distribution phase shifted with a wave number $q_{\Vert }$ along the interface, the transverse part is nonzero in the vacuum and decays exponentially with a decay constant $q_{\Vert }^{-1}$ as a function of the distance from the interface. Since the source domain of photons is precisely the domain of the transverse current density, the optical tunneling process attains an important contribution associated with the lack of spatial localizability of a photon in the evanescent regime. It is shown that in an observationally equivalent electromagnetic propagator description of the space-time dynamics, where the source domain of the photons is identified with the domain of the total electron current density, the retarded transverse dynamics necessarily must include spacelike couplings in the evanescent regime. Since these are destroyed with the vacuum speed of light as the light-cone coupling moves away from the matter-vacuum interface, the Einstein causality is always obeyed. The link to previous studies of the optical tunneling process is established by investigating the transverse and longitudinal dynamics in the frequency domain. Finally, it is shown that surface currents may play an important role in the optical tunneling process, in particular in cases where the incident electromagnetic field generates divergence-free currents in the bulk of the source medium.

We present a theoretical investigation of optical phase conjugation by use of nearly degenerate four-wave mixing in a two-level atomic system. The description is based on the semiclassical dressed-state approach, which offers a very transparent picture of the physics involved in the nonlinear process. The theory provides an explanation in terms of a quantum interference between two channels for the strong reduction of the optical phase-conjugate yield when the pump fields are tuned to the atomic resonance. In addition, the model predicts an efficient generation of the optical phase conjugate if the system is prepared in a pure dressed state.

The emission frequency of an optical oscillator is not, in general, the cavity frequency, because it results from a compromise between the cavity and the material (atom, crystal) frequencies: this phenomenon is known as frequency pulling. We study in this paper the mechanisms leading to frequency pulling in oscillators with photorefractive gain (OPG), and compare them with those of lasers. We find that because the pulling in OPG does not depend explicitly on the empty cavity mode frequency, behaviors different from those observed in lasers are obtained. In particular, for a given set of parameters, the frequency pulling has a constant sign, so that the emission frequency is always larger (or smaller) than the maximum gain frequency. This leads to the counterintuitive effect that, in some conditions, the emitted intensity could decrease as the losses are decreased.

We present a scheme for controlling the decoherence of a linear superposition of two coherent states with opposite phases in a high-$Q$ microwave cavity, based on the injection of appropriately prepared “probe” and “feedback” Rydberg atoms, improving the one presented by D. Vitali et al. [Phys. Rev. Lett. $79$, 2442 (1997)]. In the present scheme, the information transmission from the probe to the feedback atom is directly mediated by a second auxiliary cavity. The detection efficiency for the probe atom is no longer a critical parameter, and the decoherence time of the superposition state can be significantly increased using presently available technology.

A systematic semiclassical expansion of the hydrogen problem about the classical Kepler problem is shown to yield remarkably accurate results. Ad hoc changes of the centrifugal term, such as in the WKB approximation and semiclassical quantization of hydrogen, where the factor $l(l+1\mathrm{})\mathrm{}$ is replaced by ${\mathrm{}(l+1/2\mathrm{})\mathrm{}}^2$, are avoided. Expanding systematically in powers of ħ, the semiclassical energy levels are shown to be exact to first order in $ħ$ with all higher-order contributions vanishing. The wave functions and dipole matrix elements are also discussed.

In this paper a set of generalized coherent states for the d-dimensional Coulomb problem in coordinate representation are constructed. A coordinate transformation in hyperspherical space is used that maps the d-dimensional Coulomb problem into the D-dimensional harmonic oscillator and the generalized coherent states for the d-dimensional Coulomb problem are then obtained. This exactly soluble model can provide an adequate means for a quantum coherency description of the Coulomb problem in arbitrary dimensions, specifically in the special case of the hydrogen atom, in many theoretical and applied related fields such as in coherent scattering.

A peculiar class of weakly bound states is discussed. These states are made up of three entities (A, B, and C) which have the following property: A does not form bound states with B or C separately but there is a BC bound state and an ABC bound state. The term Tango state is suggested to distinguish this class from other weakly bound systems. Two of the mechanisms for forming the ABC Tango state are discussed. The following systems are predicted to form bound states: ${\mathrm{H}}_3^{\mathrm{-}},$ ${\mathrm{HeH}}^{\mathrm{-}},$ ${\mathrm{NeH}}^{\mathrm{-}},$ ${\mathrm{ArH}}^{\mathrm{-}},\dots$ and ${\mathrm{Mg}}_2^{\mathrm{-}};$ quantum chemistry calculations very strongly indicate that ${\mathrm{H}}_3^{\mathrm{-}}$ and ${\mathrm{HeH}}^{\mathrm{-}}$ form bound states. For the first four species, any exchange of D for H will also be a Tango state.

The lifetime of the metastable ${\mathrm{Be}}^{-}{(2s2p}^2{}^4{P}_{3/2})$ state is measured using an electrostatic linear ion trap which stores keV-ion beams. Trapping using electrostatic fields avoids the complication of magnetic-field-induced mixing effects which can interfere with the measurement of the spontaneous decay. The result is found to be $42.07\pm 0.12\mu \mathrm{s},$ which is a factor of 40 better in accuracy than the previous result determined in a heavy-ion storage ring. The measured lifetime is found to be 30% longer than the most recent theoretical value.

A systematic scheme for improving the variational wave functions and corresponding energy levels for two simple one-particle systems is presented. A family of independent wave functions are generated in a systematic fashion about a variational parameter value. The eigenstates are then obtained by diagonalizing the Hamiltonian matrix within the basis and optimized with respect to the variational parameter. This method has been applied to two quantum systems: the quartic anharmonic oscillator and an electron in a uniform electric field. The results for the ground state energies show a convergence comparable to or faster than the conventional Lanczos method. Results are also compared with those from other variational methods.

A simple formulation for calculating the magnetic field external to an extended nonpermeable conducting body due to thermal current fluctuations within the body is developed, and is applied to a recent experimental search for the atomic electric-dipole moment (EDM) of ${}^{199}\mathrm{Hg}.$ It is shown that the thermal fluctuation field is only slightly smaller in magnitude than other noise sources in that experiment. The formulation is extended to permeable bodies, and the implications for general EDM experiments are discussed.

Tong and Chu [Phys. Rev. A 58, R2656 (1998)] recently suggested a scheme of how to produce circularly polarized high harmonics. It is compared to an alternative scheme, and some characteristic differences between the two are discussed.

In a recent article [Phys. Rev. A 57, 1572 (1998)] Caticha has concluded that “nonlinear variants of quantum mechanics are inconsistent.” In this paper we identify which assumptions made in that article lead to this conclusion.