Pierre Pillet

Observation of collective excitation of two individual atoms in the Rydberg blockade regime

When two single Rydberg atoms—those with electrons in highly excited states—interact, one can be used to control the quantum state of the other. Two independent experiments demonstrate such ‘Rydberg blockade’, an effect that might make long-range quantum gates between neutral atoms possible.

Optical pumping and vibrational cooling of molecules.

The methods producing cold molecules from cold atoms tend to leave molecular ensembles with substantial residual internal energy. For instance, cesium molecules initially formed via photoassociation of cold cesium atoms are in several vibrational levels ν of the electronic ground state. We applied a broadband femtosecond laser that redistributes the vibrational population in the ground state via a few electronic excitation/spontaneous emission cycles. The laser pulses are shaped to remove the excitation frequency band of the ν = 0 level, preventing re-excitation from that state. We observed a fast and efficient accumulation (∼70% of the initially detected molecules) in the lowest vibrational level, ν = 0, of the singlet electronic state. The validity of this incoherent depopulation pumping method is very general and opens exciting prospects for laser cooling and manipulation of molecules.

Formation of ultracold molecules (T≤200 μK) via photoassociation in a gas of laser-cooled atoms

Publisher Summary This chapter discusses the formation of ultracold molecules—that is, at temperatures well below 1 mK, the photoassociation scheme in a sample of laser-cooled ultracold atoms is currently the best route, yielding a few millions of molecules at a translational temperature in the microkelvin range, which were successfully trapped. In photoassociation experiments, a pair of ultracold ground-state atoms absorbs a photon slightly (∼ a fraction of cm -1 to 1000 cm -1 ) red-detuned relative to the resonance line. Because of the ultracold conditions, the width of the statistical distribution of the kinetic energy E c is markedly reduced. For a gas of atoms at thermal equilibrium with a temperature T, this width is of the order of k B T. It corresponds to ∼200 cm -1 at room temperature (300 K), yielding the well-known appearance of diffuse spectra, but only to ∼ 2 MHz or 7 × 10 –5 cm -1 , at 100 μK yielding well-resolved spectral lines, similar to bound–bound transitions. Besides, only a few partial waves have to be considered in the description of the initial continuum state, so the rotational structure is most often limited to J

Atom-molecule collisions in an optically trapped gas.

Cold inelastic collisions between confined cesium (Cs) atoms and Cs2 molecules are investigated inside a CO2 laser dipole trap. Inelastic atom-molecule collisions can be observed and measured with a rate coefficient of approximately 2.6 x 10(-11) cm3 s(-1), mainly independent of the molecular rovibrational state populated. Lifetimes of purely atomic and molecular samples are essentially limited by rest gas collisions. The pure molecular trap lifetime ranges 0.3-1 s, 4 times smaller than the atomic one, as is also observed in a pure magnetic trap. We give an estimation of the inelastic molecule-molecule collision rate to be approximately 10(-11) cm3 s(-1).

Electric-field induced dipole blockade with Rydberg atoms.

High resolution laser Stark excitation of np (60<n<85) Rydberg states of ultracold cesium atoms shows an efficient blockade of the excitation attributed to long-range dipole-dipole interaction. The dipole blockade effect is observed as a quenching of the Rydberg excitation depending on the value of the dipole moment induced by the external electric field. Effects of ions which could match the dipole blockade effect are discussed in detail but are ruled out for our experimental conditions. Analytic and Monte Carlo simulations of the excitation of an ensemble of interacting Rydberg atoms agree with the experiments and indicate a major role of the nearest neighboring Rydberg atom.

Experimental versus theoretical rates for photoassociation and for formation of ultracold molecules

We report on the spectra of the long-range attractive molecular states of the Cs/sub 2/ dimer below the dissociation limits 6s+6p/sub 1/2/ and 6s+6p/sub 3/2/ which are accessible by molecular photoassociation of cold Cs atoms. For the states O/sub u//sup +/ and O/sub g//sup -/(6s+6p/sub 3/2/), we have performed trap-loss measurements, which are in good agreement with the theoretical calculations, performed in a perturbative approach. For the O/sub g//sup -/ and 1/sub u/(6s+6p/sub 3/2/) states, we observe, after spontaneous decay of the electronically excited molecules, the formation of translationally cold molecules. A rate of formation of cold molecules of the order of /spl sim/10/sup 6/ molecules per second is obtained in the case of the state O/sub g//sup

Photoassociation in a gas of cold alkali atoms: I. Perturbative quantum approach

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Observation of a Resonant Four-Body Interaction in Cold Cesium Rydberg Atoms

We develop a quantum-mechanical approach to the photoassociation phenomenon in a sample of cold atoms initially at thermal equilibrium with temperature T. We calculate the rate of formation of molecules in the weak field regime using a perturbative approach. Analytical expressions are obtained for the overlap between the initial and final molecular wavefunctions, and are shown to agree with numerical calculations. The photoassociation rate is found to decrease with increasing atomic mass m, from Li to Cs. Also considered is the heating of the atomic sample through spontaneous photodissociation.

Efficient formation of deeply bound ultracold molecules probed by broadband detection

Cold Rydberg atoms subject to long-range dipole-dipole interactions represent a particularly interesting system for exploring few-body interactions and probing the transition from 2-body physics to the many-body regime. In this work we report the direct observation of a resonant 4-body Rydberg interaction. We exploit the occurrence of an accidental quasicoincidence of a 2-body and a 4-body resonant Stark-tuned Förster process in cesium to observe a resonant energy transfer requiring the simultaneous interaction of at least four neighboring atoms. These results are relevant for the implementation of quantum gates with Rydberg atoms and for further studies of many-body physics.

Coherent excitation of a single atom to a Rydberg state

SUMMARY Using a non-selective broadband detection scheme we discovered an efficient mechanism of formation of ultracold Cs