Y. Sortais

Field Emission Tip as a Nanometer Source of Free Electron Femtosecond Pulses

We report a source of free electron pulses based on a field emission tip irradiated by a low-power femtosecond laser. The electron pulses are shorter than 70 fs and originate from a tip with an emission area diameter down to 2 nm. Depending on the operating regime we observe either photofield emission or optical field emission with up to 200 electrons per pulse at a repetition rate of 1 GHz. This pulsed electron emitter, triggered by a femtosecond oscillator, could serve as an efficient source for time-resolved electron interferometry, for time-resolved nanometric imaging and for synchrotrons.

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.

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.

Diffraction-limited optics for single-atom manipulation

We present an optical system designed to capture and observe a single neutral atom in an optical dipole trap, created by focusing a laser beam using a large-numerical-aperture (NA= 0.5) aspheric lens. We experimentally evaluate the performance of the optical system and show that it is diffraction limited over a broad spectral range (~200 nm) with a large transverse field (±25 µm) The optical tweezer created at the focal point of the lens is able to trap single atoms of 87 Rb and to detect them individually with a large collection efficiency. We measure the oscillation frequency of the atom in the dipole trap and use this value as an independent deter-mination of the waist of the optical tweezer. Finally, we produce with the same lens two dipole traps separated by 2.2 µm and show that the imaging system can resolve the two atoms.

Extreme localization of electrons in space and time

Electron emission from sharp metal tips can take place on sub-femtosecond time scales if the emission is driven by few cycle femtosecond laser pulses. Here we outline the experimental prerequisites in detail, discuss emission regimes and relate them to recent experiments in the gas phase (attosecond physics). We present a process that leads to single atom tip emitters that are stable under laser illumination and conclude with a discussion of how to achieve short electron pulses at a target.

Cold-Atom Clocks on Earth and in Space

We present recent progress on microwave clocks that make use of lasercooled atoms. With an ultra-stable cryogenic sapphire oscillator as interrogation oscillator, a cesium fountain operates at the quantum projection noise limit. With 6 × 105 detected atoms, the relative frequency stability is 4 × 10−14 τ −1/2, where τ is the integration time in seconds. This stability is comparable to that of hydrogen masers. At τ = 2 × 104 s, the measured stability reaches 6 × 10−16. A 87Rb fountain has also been constructed and the 87Rb ground-state hyperfine energy has been compared to the Cs primary standard with a relative accuracy of 2.5 × 10−15. The 87Rb collisional shift is found to be at least 30 times below that of cesium. We also describe a transportable cesium fountain, which will be used for frequency comparisons with an accuracy of 10−15 or below. Finally, we present the details of a space mission for a cesium standard which has been selected by the European Space Agency (ESA) to fly on the International Space Station in 2003.

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.

Light-assisted collisions between a few cold atoms in a microscopic dipole trap

We study light-assisted collisions in an ensemble containing a small number (∼3) of cold 87 Rb atoms trapped in a microscopic dipole trap. Using our ability to operate with one atom exactly in the trap, we measure the one-body heating rate associated with a near-resonant laser excitation, and we use this measurement to extract the two-body loss rate associated with light-assisted collisions when a few atoms are present in the trap. Our measurements indicate that the two-body loss rate can reach surprisingly large values β > 10 −8 cm 3 s −1 and varies rapidly with the trap depth and the parameters of the excitation light.

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.