We explore a situation where the van der Waals long-range atom-surface interaction is repulsive. This repulsion originates in a resonant coupling between a virtual emission at 12.15 mm of a Cs ‫ء‬ ͑6D 3͞2 ͒ atom and a virtual excitation of a surface polariton in sapphire. The experimental evidence is based upon the analysis of the spectroscopic response of Cs* in the near-infrared range with a technique that probes a distance range ϳ100 nm away from the sapphire surface. We also demonstrate the critical dependence of atom-surface forces on the sapphire crystal orientation. PACS numbers: 34.50.Dy, 12.20.Fv, 71.36. + c The van der Waals (vW) force between neutral polarizable systems [1,2] represents a universal interaction of paramount importance in numerous fields of physics, chemistry, and biology. Its attractive character is essential for the cohesion of many chemical or biological systems. vW attraction between atomic systems and metallic or dielectric bodies is also a fundamental property in cavity quantum electrodynamics (QED) [3], and its main characteristics have been experimentally studied by means of mechanical approaches (atomic beam deflection produced by surfaces [4,5], energy threshold in atomic mirrors [6]), or, in our group, spectroscopic approaches (spectral monitoring of surface-induced atomic level shifts [7-10]).In the nonretarded regime, the vW interaction between an atom and a surface originates in the quantum fluctuations of the atomic dipole: The fluctuating dipole polarizes the surface, and induces a dipole image instantaneously correlated with the atomic dipole. This near-field image is responsible for the attractive character of the vW interaction, which scales in 1͞z 3 (z is the atom-surface distance) [1].Is it possible to turn this near-field attraction into a repulsion? The answer is yes, if a resonant coupling between the fluctuating atomic dipole and a surface excitation can occur [11][12][13]. For a dispersive dielectric medium with a complex permittivity´͑v͒, the well-known electrostatic image coefficient S ͑´2 1͒͑͞´1 1͒ (with 0 # S # 1) has to be generalized to a complex frequency-dependent surface response [13]:whose resonances ("surface polaritons" [14]) are determined by the poles of S͑v͒.Consider an excited atom for which one of the dipoleallowed deexcitation channels (at frequency v a ) is near resonant with a surface polariton. Because of nonradiative, virtual coupling between the atom and surface (atomic decay followed by surface excitation), the surface response at frequency v a is magnified, implying a dielectric image coefficient, 2Re͓S͑v a ͔͒, possibly larger than one-the value obtained for an ideal reflector. Because of the resonant enhancement, S is complex and the image dipole is dephased as well. When this image is phase reversed, the near-field vW attraction is turned into a near-field vW repulsion, also scaling in 1͞z 3 . This is one among the rare situations of a long-range, state-selective repulsion exerted by a cavity wall on an atom, at distances spanning f...

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We have studied relative-intensity fluctuations for a variable set of orthogonal elliptic polarization components of a linearly polarized laser beam traversing a resonant 87 Rb vapor cell. Significant polarization squeezing at the threshold level (-3dB) required for the implementation of several continuous variables quantum protocols was observed. The extreme simplicity of the setup, based on standard polarization components, makes it particularly convenient for quantum information applications.PACS numbers: 42.50. Lc,42.50.Ct,32.80.Qk In recent years, large attention has been given to the use of continuous variables for quantum information processing. A foreseen goal is the distribution of entanglement between distant nodes. For this, quantum correlated light beams are to interact with separate atomic systems in order to build quantum mechanical correlations between them [1,2].A particular kind of quantum correlation between two light beams occurs when the intensity difference between them has fluctuations smaller than the standard quantum limit (SQL), that is smaller than the fluctuations of the intensity difference of two coherent states of the same intensity. The two beams are said to present relative-intensity squeezing (RIS). RIS has been generated through different nonlinear optics techniques. One of the most successful is parametric down conversion in a nonlinear χ (2) crystal. Up to 9.7 dB RIS has been obtained with this method [3]. Such experiments require a relatively elaborate and expensive setup. The resulting light beams are spectrally broad and usually far detuned from convenient alkali atoms D transitions. An alternative approach has considered the use of four-wave mixing in atomic samples [4][5][6][7].Reduced relative-intensity fluctuations have been observed between light beams of different frequency. However, RIS can also occur between orthogonal polarization components of a single light beam. In such case, the field is said to be polarization squeezed [8,9] and the noise reduction is described in terms of squeezing of the fluctuations of one of the Stokes operators:Here a x , a y are the field destruction operators for the orthogonal linear polarizations x and y. Polarization squeezing has been produced via propagation in optical fibers [10,11], through the combination on a polarizing beam splitter of two quadrature squeezed light beams [12] and through the interaction of linearly polarized light with cold atoms inside an optical cavity [9,13].It has been recently demonstrated that the single passage of a linearly polarized pump beam through a few-cmlong atomic vapor cell results in squeezing of the polarization orthogonal to that of the pump (vacuum squeezing) [14][15][16][17] as a consequence of the nonlinear optics mechanism known as polarization self-rotation (PSR) [18][19][20]. Vacuum squeezing via PSR has been observed for the D1 [15][16][17] and D2 [14] transitions using 87 Rb vapor. As noted in [9], the existence of polarization squeezing can be inferred from these results.In this ...

Nonlinear magneto-optical (NMO) resonances occurring for near-zero magnetic field are studied in Rb vapor using light-noise spectroscopy. With a balanced detection polarimeter, we observe high contrast variations of the noise power (at fixed analysis frequency) carried by diode laser light resonant with the 5S 1/2 (F = 2) → 5P 1/2 (F = 1) transition of 87 Rb and transmitted through a rubidium vapor cell, as a function of magnetic field B. A symmetric resonance doublet of anticorrelated noise is observed for orthogonal polarizations around B = 0 as a manifestation of ground state coherence. We also observe sideband noise resonances when the magnetic field produces an atomic Larmor precession at a frequency corresponding to one half of the analysis frequency. The resonances on the light fluctuations are the consequence of phase to amplitude noise conversion owing to nonlinear coherence effects in the response of the atomic medium to the fluctuating field. A theoretical model (derived from linearized Bloch equations) is presented that reproduces the main qualitative features of the experimental signals under simple assumptions.

The explanation presented in [Taichenachev et al, Phys. Rev. A 61, 011802 (2000)] according to which the electromagnetically induced absorption (EIA) resonances observed in degenerate two level systems are due to coherence transfer from the excited to the ground state is experimentally tested in a Hanle type experiment observing the parametric resonance on the D1 line of 87 Rb. While EIA occurs in the F = 1 → F ′ = 2 transition in a cell containing only Rb vapor, collisions with a buffer gas (30 torr of N e) cause the sign reversal of this resonance as a consequence of collisional decoherence of the excited state. A theoretical model in good qualitative agreement with the experimental results is presented.

We report on the first spectroscopic observation of the rotational Doppler shift associated with light beams carrying orbital angular momentum. The effect is evidenced as the broadening of a Hanle/EIT coherence resonance on Rb vapor when the two incident Laguerre-Gaussian laser beams have opposite topological charges. The observations closely agree with theoretical predictions.PACS numbers: 42.50. Gy, 42.15.Dp, 32.70.Jz Light beams with twisted wavefronts, as is the case for the Laguerre-Gaussian (LG) modes, are known to carry orbital angular momentum (OAM) along their propagation direction. The properties of such modes have attracted considerable attention in recent years and a wide range of applications were suggested [1,2]. LG beams were used for atom trapping and cooling [3,4,5] and special attention was put on the application of LG beams for the exchange of orbital angular momentum between light and Bose-Einstein condensates [6,7,8,9]. Some of us have demonstrated that OAM can be recorded in the position dependent population and coherence of a cold atom sample [10], and transferred between internal atomic states. In addition, the generation of new fields with OAM via non-linear wave mixing in coherently prepared cold atoms was observed [11]. Quite recently, the use of photons in LG modes, was suggested for quantum information processing. The state of such a photon lies in a multi-dimensional Hilbert space, describing the total (intrinsic plus orbital) angular momentum, in which quantum computation with improved efficiency should be possible [12]. Entanglement between pairs of photons in modes with OAM was recently reported [13].The interaction of a moving atom with a LG field raises the fundamental question of the Doppler effect [14]. As an atom moves across the helicoidal wavefronts of the LG mode, it experiences, in addition to the usual Doppler shift related to the velocity in the light propagation direction (and a small shift associated to radial motion in a curved wavefront), a most intriguing frequency shift, the so called rotational Doppler effect (RDE) associated to the azimuthal velocity. To date, the RDE has only been observed interferometrically. The RDE results in frequency shift when the light beam is rotated around its propagation axis [15]. Such shift was observed in [16,17] using millimeter-waves. In the optical domain, a frequency shift in the field generated by a rotating plate was observed in [18]. A different approach, used in [19], relates the RDE to the asymmetric interferometric spatial pattern occurring in the superposition of a Gaussian and a LG modes. In this work we present the first experimental demonstration of the RDE arising directly from the interaction of LG light beams with an atomic sample.Laguerre-Gaussian modes are usually identified with two integer numbers: l and p [20]. The topological charge l corresponds to the phase variation (in units of 2π) of the field along a loop encircling the optical axis. The integer p + 1 corresponds to the number of maxima of the field...

We have numerically solved the Heisenberg-Langevin equations describing the propagation of quantized fields through an optically thick sample of atoms. Two orthogonal polarization components are considered for the field and the complete Zeeman sublevel structure of the atomic transition is taken into account. Quantum fluctuations of atomic operators are included through appropriate Langevin forces. We have considered an incident field in a linearly polarized coherent state (driving field) and vacuum in the perpendicular polarization and calculated the noise spectra of the amplitude and phase quadratures of the output field for two orthogonal polarizations. We analyze different configurations depending on the total angular momentum of the ground and excited atomic states. We examine the generation of squeezing for the driving field polarization component and vacuum squeezing of the orthogonal polarization. Entanglement of orthogonally polarized modes is predicted. Noise spectral features specific of (Zeeman) multi-level configurations are identified.

We present the first study of light induced atom desorption (LIAD) of an alkali atom (Rb) in porous alumina. We observe the variation due to LIAD of the rubidium density in a vapor cell as a function of illumination time, intensity and wavelength. The simple and regular structure of the alumina pores allows a description of the atomic diffusion in the porous medium in which the diffusion constant only depends on the known pore geometry and the atomic sticking time to the pore wall. A simple one-dimensional theoretical model is presented which reproduces the essential features of the observed signals. Fitting of the model to the experimental data gives access to the diffusion constant and consequently the atom-wall sticking time and its dependence on light intensity and wavelength. The non-monotonic dependence of the LIAD yield on the illumination light frequency is indicative of the existence of Rb clusters in the porous medium