Near-Resonant Light Scattering by a Subwavelength Ensemble of Identical Atoms.

We study theoretically the scattering of light by an ensemble of N resonant atoms in a subwavelength volume. We consider the low intensity regime so that each atom responds linearly to the field. While N noninteracting atoms would scatter N^{2} more than a single atom, we find that N interacting atoms scatter less than a single atom near resonance. In addition, the scattered power presents strong fluctuations, either from one realization to another or when varying the excitation frequency. We analyze this counterintuitive behavior in terms of collective modes resulting from the light-induced dipole-dipole interactions. We find that for small samples and sufficiently large atom number, their properties are governed only by their volume.

Optical Transmission of an Atomic Vapor in the Mesoscopic Regime.

By measuring the transmission of near-resonant light through an atomic vapor confined in a nanocell we demonstrate a mesoscopic optical response arising from the nonlocality induced by the motion of atoms with a phase coherence length larger than the cell thickness. Whereas conventional dispersion theory-where the local atomic response is simply convolved by the Maxwell-Boltzmann velocity distribution-is unable to reproduce the measured spectra, a model including a nonlocal, size-dependent susceptibility is found to be in excellent agreement with the measurements. This result improves our understanding of light-matter interaction in the mesoscopic regime and has implications for applications where mesoscopic effects may degrade or enhance the performance of miniaturized atomic sensors.

Fabrication and characterization of super-polished wedged borosilicate nano-cells.

We report on the fabrication of an all-glass vapor cell with a thickness varying linearly between (exactly) 0 and ∼1 µm. The cell is made in Borofloat glass that allows state-of-the-art super polish roughness, a full optical bonding assembling and easy filling with alkali vapors. We detail the challenging manufacture steps and present experimental spectra resulting from fluorescence and transmission spectroscopy of the cesium D1 line. The very small surface roughness of 1 Å rms is promising to investigate the atom-surface interaction or to minimize parasite stray light.

Collective Lamb Shift of a Nanoscale Atomic Vapor Layer within a Sapphire Cavity.

We measure the near-resonant transmission of light through a dense medium of potassium vapor confined in a cell with nanometer thickness in order to investigate the origin and validity of the collective Lamb shift. A complete model including the multiple reflections in the nanocell reproduces accurately the observed line shape. It allows the extraction of a density-dependent shift and width of the bulk atomic medium resonance, deconvolved from the cavity effect. We observe an additional, unexpected dependence of the shift with the thickness of the medium. This extra dependence demands further experimental and theoretical investigations.

Coherent Scattering of Near-Resonant Light by a Dense Microscopic Cold Atomic Cloud.

We measure the coherent scattering of light by a cloud of laser-cooled atoms with a size comparable to the wavelength of light. By interfering a laser beam tuned near an atomic resonance with the field scattered by the atoms, we observe a resonance with a redshift, a broadening, and a saturation of the extinction for increasing atom numbers. We attribute these features to enhanced light-induced dipole-dipole interactions in a cold, dense atomic ensemble that result in a failure of standard predictions such as the "cooperative Lamb shift". The description of the atomic cloud by a mean-field model based on the Lorentz-Lorenz formula that ignores scattering events where light is scattered recurrently by the same atom and by a microscopic discrete dipole model that incorporates these effects lead to progressively closer agreement with the observations, despite remaining differences.

Optical Resonance Shifts in the Fluorescence of Thermal and Cold Atomic Gases.

We show that the resonance shifts in the fluorescence of a cold gas of rubidium atoms substantially differ from those of thermal atomic ensembles that obey the standard continuous medium electrodynamics. The analysis is based on large-scale microscopic numerical simulations and experimental measurements of the resonance shifts in a steady-state response in light propagation.

Observation of suppression of light scattering induced by dipole-dipole interactions in a cold-atom ensemble.

We study the emergence of collective scattering in the presence of dipole-dipole interactions when we illuminate a cold cloud of rubidium atoms with a near-resonant and weak intensity laser. The size of the atomic sample is comparable to the wavelength of light. When we gradually increase the number of atoms from 1 to ~450, we observe a broadening of the line, a small redshift and, consistently with these, a strong suppression of the scattered light with respect to the noninteracting atom case. We compare our data to numerical simulations of the optical response, which include the internal level structure of the atoms.

Direct measurement of the Wigner time delay for the scattering of light by a single atom.

We have implemented the Gedanken experiment of an individual atom scattering a wave packet of near-resonant light, and measured the associated Wigner time delay as a function of the frequency of the light. In our apparatus, the atom behaves as a two-level system and we have found delays as large as 42 ns at resonance, limited by the lifetime of the excited state. This delay is an important parameter in the problem of collective near-resonant scattering by an ensemble of interacting particles, which is encountered in many areas of physics.

Free-space lossless state detection of a single trapped atom.

We demonstrate the lossless state-selective detection of a single rubidium 87 atom trapped in an optical tweezer. This detection is analogous to the one used on trapped ions. After preparation in either a dark or a bright state, we probe the atom internal state by sending laser light that couples an excited state to the bright state only. The laser-induced fluorescence is collected by a high numerical aperture lens. The single-shot fidelity of the detection is 98.6±0.2% and is presently limited by the dark count noise of the detector. The simplicity of this method opens new perspectives in view of applications to quantum manipulations of neutral atoms.

Controlled single-photon emission from a single trapped two-level atom.

By illuminating an individual rubidium atom stored in a tight optical tweezer with short resonant light pulses, we created an efficient triggered source of single photons with a well-defined polarization. The measured intensity correlation of the emitted light pulses exhibits almost perfect antibunching. Such a source of high-rate, fully controlled single-photon pulses has many potential applications for quantum information processing.

Search for variations of fundamental constants using atomic fountain clocks.

Over five years, we have compared the hyperfine frequencies of 133Cs and 87Rb atoms in their electronic ground state using several laser-cooled 133Cs and 87Rb atomic fountains with an accuracy of approximately 10(-15). These measurements set a stringent upper bound to a possible fractional time variation of the ratio between the two frequencies: d/dt ln([(nu(Rb))/(nu(Cs))]=(0.2+/-7.0)x 10(-16) yr(-1) (1sigma uncertainty). The same limit applies to a possible variation of the quantity (mu(Rb)/mu(Cs))alpha(-0.44), which involves the ratio of nuclear magnetic moments and the fine structure constant.

Controlling the cold collision shift in high precision atomic interferometry.

We present a new method based on a transfer of population by adiabatic passage that allows one to prepare cold atomic samples with a well-defined ratio of atomic density and atom number. This method is used to perform a measurement of the cold collision frequency shift in a laser cooled cesium clock at the percent level, which makes the evaluation of the cesium fountain accuracy at the 10(-16) level realistic. With improvements, the adiabatic passage would allow measurements of density-dependent phase shifts at the 10(-3) level in high precision experiments.

Cold collision frequency shifts in a 87Rb atomic fountain.

We present measurements of cavity frequency pulling and collisional frequency shifts in a 87Rb fountain with a frequency resolution of 3x10(-16). Agreement with theory is found for the cavity pulling and the measured collisional shifts. The clock shift is found at least 50 times smaller than in 133Cs.

(87)Rb versus (133)Cs in cold atom fountains: a comparison.

We describe the operation of a laser cooled (87)Rb frequency standard and present a new measurement of the (87)Rb ground state hyperfine frequency with a relative accuracy of 2.4x10(-15), by comparison with a Cs fountain primary standard. The measured frequency is 6 834 682 610.904 333(17) Hz. An evaluation of the frequency shift induced by cold collisions gives Deltanu/nu(Rb)=(-7.2+/-20)x10(-24) n , where n is the average atomic density in cm(-3). With our present 1 sigma uncertainty of 10(-15), this measurement is still compatible with 0 and about 300 times smaller than for (133)Cs. We also report a test of a possible variation of the fine structure constant at the level of 2.7x10(-14) yr(-1), comparing Rb and Cs cold atom fountains.

Interrogation oscillator noise rejection in the comparison of atomic fountains.

The frequency stability of an atomic fountain clock can be limited by the phase noise of the interrogation oscillator via the "Dick effect." In this paper we demonstrate the rejection of the phase fluctuations of the interrogation oscillator by the synchronization of atomic fountains. A reduction by a factor of 16 in the Allan standard deviation of the relative frequency difference between two fountains has been obtained.