The manipulation of light pulse from subluminal to superluminal propagation in a degenerate two-level Cs atomic system

We experimentally demonstrate the propagation of light pulse from subluminal to superluminal light based on quantum coherence in a degenerate two-level atomic system in a Cs vapor cell.It is shown that the group velocity of light pulse can be switched from subluminal to superluminal propagation via changing the coupling field from a traveling wave to a standing wave,while can also be continuously manipulated by varying the intensity of two waves superposed to form a standing wave.The observed maximum delay and advance times are about 0.45 and 0.54μs,corresponding to the group velocity of g=168km/s and g=138 km/s,respectively.This investigation may have the practical applications of devices for optical tunable delay lines,optical switching and optical buffering.     

Dispersive Properties of tunnelling—induced transparency in an asymmetric double quantum well

We have investigated the dispersive properties of tunnelling-induced transparency in asymmetric double quantum well structures where two excited states are coupled by resonant tunnelling through a thin barrier in a three-level system of electronic subbands. The intersubband transitions exhibit high dispersion at zero absorption, which leads to the slow light velocity in this medium as compared with that in vacuum (c=3×10⁸). The group velocity in a specific GaAs/AlGaAs sample is calculated to be v_g=c/4.30. This structure can be used to compensate for the dispersion and energy loss in fibre optical communications.     

Optical analogue of the Aharonov-Bohm effect using anisotropic media

We show that in the context of paraxial optics, which can be analyzed through a wave equation similar to the non-relativistic Schrödinger equation of quantum mechanics but replacing time t by spatial coordinate z, the existence of a vector potential A_⊥ mimicking the magnetic vector potential in quantum mechanics is allowed by specific gauge symmetries of the optical field in a medium with anisotropic refractive index. In this way, we use Feynman?s path integral to demonstrate an optical analogue of the quantum-mechanical Aharonov-Bohm effect, encouraging the search for another optical systems with analogies with more complex quantum field theories.     

Effects of the velocity dependence of the collision frequency on the Dicke line narrowing

The effect of the velocity (v) dependence of the transport collision frequency ν_trv on the Dicke line narrowing is analyzed in terms of the strong-collision model generalized to velocity-dependent collision frequencies (the so-called kangaroo model). This effect has been found to depend on the mass ratio of the resonance (M) and buffer (M _b) particles, β = M _b/M: it is at a minimum for β ? 1 and reaches a maximum for β ? 3. A power-law particle interaction potential, U(r) ∝ r ^?n , is used as an example to show that, compared to ν_trv (v) = const (n = 4), the line narrows if ν_trv (v) decreases with increasing v (n < 4) and broadens if ν _trv (v) increases with v(n > 4). At β ? 3, the line width can increase [compared to ν_trv (v) = const] by 5 and 12% for the potentials with n = 6 and n ? 10, respectively; for the potentials with n = 1 (Coulomb potential) and n = 3, it can decrease by more than half and 6%, respectively. The line profile I(Ω) has been found to be weakly sensitive to ν_trv (v) at some detuning Ω_c of the radiation frequency Ω. Dicke line narrowing is used as an example to analyze the collisional transport of nonequilibrium in the resonance-particle velocity distribution in a laser field. The transport effect is numerically shown to be weak. This allows simpler approximate one-dimensional quantum kinetic equations to be used instead of the three-dimensional ones to solve spectroscopic problems in which it is important to take into account the velocity dependence of the collision frequency when the phase memory is preserved during collisions.     

On the energy transport velocity in a dissipative medium

An exact definition of the group velocity v _g is proposed for a wave process with arbitrary dispersion relation ω = ω′(k) + iω″(k). For the monochromatic approximation, a limit expression v _g (k) is obtained. A condition under which v _g (k) takes the form of the Kuzelev–Rukhadze expression [1] dω′(k)/dk is found. In the general case, it appears that v _g (k) is defined not only by the dispersion relation ω(k), but also by other elements of the initial problem. As applied to the dissipative medium, it is shown that v _g (k) defines the field energy transfer velocity, and this velocity does not exceed thee light speed in vacuum. An expression for the energy transfer velocity is also obtained for the case where the dispersion relation is given in the form k = k′(ω) + ik″(ω) which corresponds to the boundary problem.     

ZN Gauge theories on a lattice and quantum memory

In the present paper we shall study (2+1)-dimensional Z_N gauge theories on a lattice. It is shown that the gauge theories have two phases, one is a Higgs phase and the other is a confinement phase. We investigate low-energy excitation modes in the Higgs phase and clarify relationship between the Z_N gauge theories and Kitaev’s model for quantum memory and quantum computations. Then we study effects of random gauge couplings (RGC) which are identified with noise and errors in quantum computations by Kitaev’s model. By using a duality transformation, it is shown that time-independent RGC give no significant effects on the phase structure and the stability of quantum memory and computations. Then by using the replica methods, we study Z_N gauge theories with time-dependent RGC and show that nontrivial phase transitions occur by the RGC.     

On the energy losses of slow ions in an interacting degenerate electron gas

Energy losses of slow ions with velocitiesv?v _F (v _F is the Fermi velocity) in an interacting degenerate electron gas are calculated on the basis of the dielectric theory of Singwi. It is shown that the local field effects taken into account in this theory lead to an increase of the energy losses as compared with the Lindhard stopping power by a factor which can reach a few tens of percent for some compressed ICF plasmas. Corresponding factors for simple metals are as much as 2–3 and are in agreement with experiment.     

Effects of superficial gas velocity on process dynamics in bioreactors

Present work analyzes the flow hydrodynamics and mass transfer mechanisms in double Rushton and CD-6 impeller on wide range (0.0075–0.25 m/s) of superficial gas velocity (v _g) in a gas-liquid phase bioreactor by employing computational fluid dynamics (CFD) technique. The volume averaged velocity magnitude and dissipation rate are found higher with increasing superficial gas velocity. Higher relative power draw (P _g/P ₀) is predicted in CD-6 than the Rushton impeller but no significant difference in volume averaged mass transfer coefficient (k _L a) observed between these two types of impeller. The ratio of power draw with mass transfer coefficient has been found higher in CD-6 impeller (25–50 %) than the Rushton impeller.     

Coupling to a microdisk cavity containing a three-level quantum-dot with two orthogonal modes

The dynamics of a composite system containing two orthogonal degenerate whispering-gallery cavity modes coupling to a quantum dot (QD) is presented by a full quantum approach. The energy levels of the quantum dot are modeled as a V-type three-level system, which consist of the ground state, right- and left-polarized excitons. The counterclockwise mode a and the clockwise mode b are coupled with the transitions corresponding to the right- and left-polarized excitons with coupling rates g_R and g_L, respectively. An exact solution is proposed in a real-space approach. We majorly discuss the effects of the backscattering rate β on the spectra of the transmission and reflection in a strong coupling regime. A new insight is that one can overcome the excitons' fine structure splitting of a real QD with appropriate backscattering rate β by fine designing the cavity, which would be possible for applications to produce the degenerate entangled photon pairs in a real QD system.     

Microwave spectrum of γ-valerolactone

An Ar⁺ laser at 4579 Å is used to excite the B¹Π_u-X¹Σ_g⁺ fluorescence of the ⁶Li⁷Li molecule in a crossed heat pipe oven. The spectrum in the region 4400–6300 Å is recorded photoelectrically with an emphasis on the observation of higher vibrational levels in the ground state close to the dissociation limit. P and R doublets corresponding to v″ ≤ 26 originating from the (v′ = 13, J′ = 19) level are observed and identified using mass-reduced quantum numbers. Two additional ⁶Li⁷Li series, known from earlier work of Velasco, Ottinger, and Zare, are also analyzed. These data are used to construct effective Rydberg-Klein-Rees (RKR) potentials for specific J″ (=9, 19) quantum numbers of the ground state and from them the true (rotationless) potential energy curve (for X¹Σ_g⁺) is derived. This extends the previously known curve of Li₂ from 4.28 to 5.18 Å (outer turning point); this turning point corresponds to an energy which is approximately 88% of the dissociation energy, which is estimated here to be 8516 ± 18 cm^?1.     

Drag of ballistic electrons by an ion beam

Drag of electrons of a one-dimensional ballistic nanowire by a nearby one-dimensional beam of ions is considered. We assume that the ion beam is represented by an ensemble of heavy ions of the same velocity V. The ratio of the drag current to the primary current carried by the ion beam is calculated. The drag current turns out to be a nonmonotonic function of velocity V. It has a sharp maximum for V near v _nF/2, where n is the number of the uppermost electron miniband (channel) taking part in conduction and v _nF is the corresponding Fermi velocity. This means that the phenomenon of ion beam drag can be used for investigation of the electron spectra of ballistic nanostructures. We note that whereas observation of the Coulomb drag between two parallel quantum wires may in general be complicated by phenomena such as tunneling and phonon drag, the Coulomb drag of electrons of a one-dimensional ballistic nanowire by an ion beam is free of such spurious effects.     

New observations and analyses of the laser excited fluorescence of the A1Σu+-X1Σg+ bands of the Na2 molecule

A medium power (～50 mW, 6328 Å) HeNe laser is used to excite the A¹Σ_u⁺-X¹Σ_g⁺ fluorescence of the Na₂ molecule in a crossed heat pipe oven. The spectrum in the region 5800–8500 Å is recorded both photographically (3.4 M Ebert) and photoelectrically (GaAs detector) with an emphasis on accurate relative intensities and on the observation of higher vibrational levels in the ground state close to the dissociation limit. P and R doublets in four series originating from (v′ = 14, J′ = 45), (v′ = 16, J′ = 17), (v′ = 22, J′ = 86), and (v′ = 25, J′ = 87) levels are observed and identified. The first two series, known from earlier work, are extended further to longer wavelengths to include 13 to 17 additional ground-state vibrational levels. The latter two series are observed for the first time. They originate from higher J′ levels and span a wide range of v″ levels (0 ≤ v″ ≤ 48). Effective RKR potentials for specific J″ (= 17, 45, 86, and 87) quantum numbers of the ground state are constructed and from them the true (rotationless) potential energy curve (for X¹Σ_g⁺) is derived which (a) reproduces the RKR curve previously given by Kusch and Hessel and (b) extends the curve from 5.77 to 7.26 Å (outer turning point). The dissociation energy D_e is estimated from these data to be 6022 ± 21 cm^?1.     

Influence of the group-velocity on the pulse propagation in 1D silicon photonic crystal waveguides

We present a detailed analysis of the influence of the group velocity (GV) on the dynamics of optical pulses upon their propagation in one-dimensional photonic crystal waveguides (PhCW). The theoretical model used in our analysis incorporates the linear optical properties of the PhCW (GV dispersion and optical losses), free-carrier (FC) effects (FC dispersion and FC-induced optical losses) and nonlinear optical effects (Kerr nonlinearity and two-photon absorption). Our analysis shows that, unlike the case of uniform waveguides, the GV of the pulse, dispersion coefficients, and the waveguide nonlinear coefficient are periodic functions with respect to the propagation distance. We also demonstrate that linear and nonlinear effects depend on the group velocity, v _g , as v_g^-1v_{g}^{-1} and v_g^-2v_{g}^{-2}, respectively.     

Probabilities of rovibronic transitions in the I 1Π g − , J 1Δ g − →C 1Π ± u systems of bands of the deuterium molecule

Probabilities of spontaneous rovibronic transitions I ¹Π _g ^? , v′ J′, J ¹Δ _g ^? , v′, J′→C ¹Π _u ^± , v″, J″ of the D₂ molecule (for vibrational and rotational quantum numbers v′=v″=0–3 and J′=1–9, J″=J′±1) have been obtained for the first time. They were determined using (1) the previously proposed nonadiabatic model, which takes into account the electron-rotational interaction of the upper levels; (2) the coefficients of expansion of wave functions of perturbed states in the Born-Oppenheimer basis, which were found from the experimental data on rovibronic terms; and (3) semiempirical b initio data on electronic transition dipole moments of the 3dπ¹Π_g→2pπ¹Π_u and 3dπδ¹Δ_g→2pπ¹Π_u transitions. The dependences of the transition probabilities on J′ for the same bands of both hydrogen isotopomers H₂ and D₂ were found to be identical. They represent monotone functions for R and P branches and functions with a maximum (minimum) for Q branches. The ratios of transition probabilities of different isotopomers for different branches of the same systems of bands and for the same branches of different systems of bands were found to be correlated. The semiempirical values obtained in the paper agree with the experimental values within the limits of the errors of their determination. The nonempirical values of transition probabilities agree with the experiment considerably worse.     

Relative probabilities of spontaneous transitions in v″ progressions of the G1Σ g + , v′→B 1Σ u + , v″ bands of the H2 molecule

The probabilities of spontaneous transitions in v″ progressions of the G ¹Σ _g ⁺ →B ¹Σ _u ⁺ bands of the H₂ molecule (the 3D→2B electronic transition in notations of G.H. Dieke) are, for the first time, experimentally studied. The line strength ratios were measured for 78 G ¹Σ _g ⁺ , v′, J′→B ¹Σ _u ⁺ , v″, J″ electronic-vibrational-rotational spectral lines having a common upper level but belonging to different bands of v″ progressions (the vibrational branching coefficients). For this purpose, the intensities of lines of the P and R branches, emitted by a low-pressure plasma and corresponding to different values of the rotational (J′=0–11) and vibrational (v′=0–3 and v″=0–7) quantum numbers, were used. It was found that the changes in the vibrational branching coefficients with variation of v′ and v″ are significant (up to a factor of 20). For most bands studied, the dependences of the vibrational branching coefficients on the rotational quantum number J′ of an upper level are rather weak and do not exceed 30%. It was established that the difference between the experimental values of ratios of the vibronic transition probabilities (summed over J″) and the results of calculation in the adiabatic approximation strongly depends on v′, reaching a factor of 25 for a transition from the v′=2 level. At the same time, the discrepancy between the experimental data and the results of nonadiabatic ab initio calculations lies between 1.0 and 2.3.     

Geometry of multidimensional universes

LetG be a compact group of transformation (global symmetry group) of a manifoldE (multidimensional universe) with all orbits of the same type (one stratum). We studyG invariant metrics onE and show that there is one-to-one correspondence between those metrics and triples (g _μv,A _μ ^ä ,h _αβ), whereg _μv is a (pseudo-) Riemannian metric on the space of orbits (space-time),A _μ ^ä is a Yang-Mills field for the gauge groupN|H, whereN is the normalizer of the isotropy groupH inG, andh _αβ are certain scalar fields characterizing geometry of the orbits (internal spaces). The scalar curvature ofE is expressed in terms of the component fields onM. Examples and model building recipes are also given. The results generalize those of non-abelian Kaluza-Klein theories to the case where internal spaces are not necessarily group manifolds.     

Capture of ultracold neutrons in thin absorbing films

The dependence of the ultracold neutron capture cross section in targets with a thickness smaller than the neutron wavelength is calculated in the time-dependent quantum theory. It is shown that, for low velocities of neutrons, their capture cross section σ_c～v, i.e., tends to zero as the neutron velocity v tends to zero.     

Comparison of semiempirical and ab initio absolute probabilities of rovibronic transitions for the I 1Π g − , J 1Δ g − →C 1Π u ± system of bands of the H2 molecule

The absolute values of probabilities of the I ¹Π _g ^? , v′, J′; J ¹ Δ _g ^? , v′, J′→C ¹Π _u ^± , v″, J″ spontaneous transitions in the H₂ molecule (for the vibrational and rotational quantum numbers v′=v″=0–3, J′=1–6, and J″=J′, J′ ±1) are calculated by using ab initio and semiempirical data on the dipole moments of the 3dπ ¹Π_g, 3dδ¹Δ_g→2pπ¹Π_u electronic transitions. In both cases, the calculations are performed both in the adiabatic approximation and with an allowance for the nonadiabatic effect of electronic-rotational interaction. The coefficients of expansion of the wave functions of perturbed rovibronic states in the Born-Oppenheimer basis functions used in the calculations were obtained in the approximation of pure precession from experimental values of the terms. It was found that the values of transition probabilities based on the ab initio calculations systematically exceed the corresponding semiempirical data by a factor of 1.2–1.9 for the I ¹Π_g→C ¹Π _u ^± transition and by a factor of 1.4–1.6 for the J ¹Δ _g ^? →C ¹Π _u ^± transition. It was established that the difference between the ab initio and semiempirical values of electronic transition moments virtually has no effect on the dependence of the transition probabilities on the vibrational quantum numbers. The discrepancies between the results of adiabatic and nonadiabatic calculations are significant and reach two orders of magnitude, which is indicative of the important role of perturbations in the probabilities of the transitions considered.