Laser-cooled Rubidium Research Articles

Determination of the Newtonian Gravitational Constant Using Atom Interferometry

We present a new measurement of the Newtonian gravitational constant G based on cold-atom interferometry. Freely falling samples of laser-cooled rubidium atoms are used in a gravity gradiometer to probe the field generated by nearby source masses. In addition to its potential sensitivity, this method is intriguing as gravity is explored by a quantum system. We report a value of G = 6.667 x 10(-11) m(3) kg(-1) s(-2), estimating a statistical uncertainty of +/-0.011 x 10(-11) m(3) kg(-1) s(-2) and a systematic uncertainty of +/-0.003 x 10(-11) m(3) kg(-1) s(-2). The long-term stability of the instrument and the signal-to-noise ratio demonstrated here open interesting perspectives for pushing the measurement accuracy below the 100 ppm level.

Transfer of trapped atoms between two optical tweezer potentials

Trapped, laser-cooled rubidium atoms are transferred between two strongly focused, horizontal, orthogonally intersecting laser beams. The transfer efficiency is studied as a function of the vertical distance between the beam axes. Optimum transfer is found when the distance equals the beam waist radius. Numerical simulations reproduce well the experimental results.

Experiments on the reflection of cold atoms from magnetic thin films

We report the results from a series of experiments in which ferromagnetic thin films were used as atom mirrors for laser-cooled rubidium atoms released from a magneto-optical trap. The thin films were made of cobalt and lanthanum calcium manganite (LCMO) with thicknesses between 20 and 300 nm. The magnetic domains in these thin films have a periodic structure where the spatial period is of the order of the thickness of the film, and the field decays exponentially above the film over a length scale comparable to the domain size. Thus, the neutral atoms reflect off these films from distances comparable to the thickness of the film, resulting in modification of the reflectivity due to the competition between the repulsive magnetic force and the attractive short-range forces such as van der Waals and Casimir forces. The smoothness of the atom mirror is also modified due to the proximity of the magnetic domains. The reflectivity is sensitive to the domain structure and size, which can be modified in LCMO by applying a modest external magnetic field. In this paper, we discuss the evaluation of the thin films as magnetic mirrors for atom optics, and the measurement of the van der Waals force with an accuracy of about 15%, using cobalt thin films. We also discuss some preliminary results on the temperature-dependent reflectivity for atoms near the ferromagnetic transition at 250 K in the LCMO film, and on the domain dynamics and relaxation.

Nonlinear lensing mechanisms in a cloud of cold atoms

We present an experimental study of nonlinear lensing of near-resonant light by a cloud of laser-cooled rubidium atoms, specifically aimed at understanding the role of the interaction time between the light and the atomic vapor. We identify four different nonlinear mechanisms, each associated with a different time constant: electronic nonlinearity, Zeeman optical pumping, hyperfine optical pumping and radiation pressure. Our observations can be quite accurately reproduced using a simple rate equation model which allows for a straightforward discussion of the various effects. The results are important for planning more refined experiments on transverse nonlinear optics and self-organization in samples of cold atoms.

Radiation trapping in a cold atomic gas

We experimentally study radiation trapping of near-resonant light in a cloud of laser-cooled rubidium atoms. Unlike in most previous studies, dealing with hot vapors, collisional broadening is here negligible and Doppler broadening due to the residual atomic velocity is narrower than the homogeneous broadening. This is an interesting new regime, at the boundary between coherent and incoherent radiation transport. We analyze in detail our low-temperature data (quasi-elastic regime) and then provide some experimental evidence for Doppler-based frequency redistribution. The data are compared with an analytical model valid for coherent transport and a Monte Carlo simulation including the Doppler effect.

Coherent population transfer of ground-state atoms into Rydberg states

The technique of stimulated Raman adiabatic passage (STIRAP) is used to excite laser-cooled rubidium atoms from the $^{85}\mathrm{Rb}\phantom{\rule{0.3em}{0ex}}5{S}_{1∕2}$ ground state to the $44{D}_{5∕2}$ Rydberg state through the $5{P}_{3∕2}$ state. The utilized double-STIRAP scheme allows for an accurate determination of the absolute excitation efficiency. Experimental data are compared with results of a detailed theoretical model, and good agreement is found. It is concluded that at the center of the excitation region an excitation efficiency of 70% is achieved.

Multiple scattering of light in a resonant medium

We investigate some signatures of multiple scattering of light in a vapor of laser-cooled rubidium atoms. This sample presents several unusual properties when compared to classical samples i.e. white paper or suspensions of dielectric particles: a very strong resonance, an internal structure of the atomic scatterer, and a spherically symmetric geometry with a quasi-Gaussian density profile. We study how the light frequency modifies static quantities such as the coherent transmission, the diffuse transmission and reflection, and the coherent backscattering cone. The experimental data are compared to a Monte Carlo simulation including all experimental parameters.

Two-species cold atomic beam

We generate a bright atomic beam containing laser-cooled rubidium and cesium, and we use this beam to load a mixed-species ultrahigh-vacuum (UHV) magneto-optical trap. We have characterized our two-species atomic beam over a range of operating conditions, and we obtain similar atom fluxes for each species. Within the UHV trap, interspecies inelastic collisions are observed in the form of enhanced decay rates of a given species in the presence of a second trapped species. We analyze the trap decays to obtain a loss rate due to heteronuclear cold collisions, and we compare our result to similar measurements in vapor-cell traps [Phys. Rev. A 63, 033406 (2001)].

Coherent backscattering of light by an inhomogeneous cloud of cold atoms

When a quasiresonant laser beam illuminates an optically thick cloud of laser-cooled rubidium atoms, the average diffuse intensity reflected off the sample is enhanced in a narrow angular range around the direction of exact backscattering. This phenomenon is known as coherent backscattering (CBS). By detuning the laser from resonance, we are able to modify the light scattering mean-free path inside the sample and we record accordingly the variations of the CBS cone shape. We then compare the experimental data with theoretical calculations and Monte Carlo simulations including the effect of the light polarization and of the internal structure of the atoms. We confirm that the internal structure strongly affects the enhancement factor of the cone and we show that the unusual shape of the atomic medium---approximately a spherically-symmetric, Gaussian density profile---strongly affects the width and shape of the cone.

Hanle effect in coherent backscattering.

We study the shape of the coherent-backscattering (CBS) cone obtained when resonant light illuminates a thick cloud of laser-cooled rubidium atoms in the presence of a homogenous magnetic field. We observe new magnetic field-dependent anisotropies in the CBS signal. We show that the observed behavior is due to the modification of the atomic-radiation pattern by the magnetic field (Hanle effect in the excited state).

Modelling a ratchet with cold atoms in an optical lattice

In this paper, we present an experimental study of a model ratchet consisting of laser-cooled rubidium atoms localized in an asymmetric periodic potential. This potential is obtained from a near-resonant light field and an additional static magnetic field. We show that there is a net motion of atoms with a mean velocity on the order of a few cm per second, and we investigate the role of the asymmetry of the potential and of the dissipation in the atomic motion. A link with the stochastic resonance is proposed.

Observation of transient gain without population inversion in a laser-cooled rubidiumΛsystem

We have observed clear Rabi oscillations of a weak probe in a strongly driven three-level lambda system in laser-cooled rubidium for the first time. When the coupling field is non-adiabatically switched on using a Pockels cell, transient probe gain without population inversion is obtained in the presence of uncoupled absorptions. Our results are supported by three-state computations.

Superresolution of pulsed multiphoton Raman transitions.

We have investigated higher order multiphoton Raman resonances with two pulsed optical frequencies. Multiphoton transfer with up to 50 photons is observed with milliwatts of laser power. We demonstrate that the spectral width of the multiphoton resonances can be far below the Fourier transform linewidth of the driving optical pulses. The functional dependence of the transition linewidth on the number of exchanged photons is found to vary with the pulse shape. Our experiment is performed with laser-cooled rubidium atoms confined in a CO2-laser optical dipole trap.

Large Faraday rotation of resonant light in a cold atomic cloud

We experimentally studied the Faraday rotation of resonant light in an optically thick cloud of laser-cooled rubidium atoms. Measurements yield a large Verdet constant in the range of ${10}^{5}$\ifmmode^\circ\else\textdegree\fi{}/T mm and a maximal polarization rotation of 150\ifmmode^\circ\else\textdegree\fi{}. A complete analysis of the polarization state of the transmitted light was necessary to account for the role of the probe laser's spectrum.

Observations of a doubly driven V system probed to a fourth level in laser-cooled rubidium

Observations of a doubly driven V system probed to a fourth level in an N configuration are reported. A dressed-state analysis is also presented. The expected three-peak spectrum is explored in a cold rubidium sample in a magneto-optic trap. Good agreement is found between the dressed-state theory and the experimental spectra once light shifts and uncoupled absorptions in the rubidium system are taken into account.

FM spectroscopy in fluorescence in laser-cooled rubidium

A novel variation of the technique of two-photon frequency modulation (FM) spectroscopy has been developed, which allows signals to be obtained from small (∼10 5 atoms) and dilute (∼10 10 cm −3) samples – in this case laser-cooled rubidium atoms. This was possible by detecting and demodulating the fluorescence signal rather than the transmission through the sample. Data are presented demonstrating the dependence of the signal on phase and frequency of the imposed modulation. Initially the atoms were held in a magneto-optic trap but, in order to eliminate line broadening due to the presence of the trapping fields, data were taken with the atoms in free fall. A similar signal, slightly shifted and broadened by the trapping fields, was obtained while the trap was on and used for long-term stabilisation of the probe laser.

Zeeman-coherence-induced transparency and gain without inversion in laser-cooled rubidium

We report experiments demonstrating the effects of optical pumping, coherent population trapping and light polarisation on electromagnetically induced transparency in simple Λ-type configurations of rubidium 87 where the relevant coherences are within the three Zeeman substates of the F=1 component of the 5S 1/2 ground level. Inversionless gain is also demonstrated. The sample is cooled to below 1 mK in a magneto-optic trap operating without repumping light to give a working population of F=1 ground state atoms. The experimental results are compared with master equation computations of a closed seven-substate-model representing the ground level ( F=1) and the excited levels F′=0 and F′=1.

CO2-laser optical lattice with cold rubidium atoms

Laser-cooled rubidium atoms have been trapped in a nondissipative optical lattice with a large lattice constant. The lattice potential is generated by the ac Stark shift in an infrared standing wave near 10.6 $\ensuremath{\mu}$m. The atoms are confined in steep potential wells with a periodicity of several micrometers. By modulating the potential depth we parametrically excite the atoms and measure their vibrational frequencies.

Ion-recoil energy measurement in photoionization of laser-cooled rubidium

We report a measurement of the ion recoil energy in photoionization of laser-cooled atoms. ${}^{87}\mathrm{Rb},$ cooled in a magneto-optical trap, is two-photon ionized by a pulsed laser. The recoil energy is determined by time-of-flight in a weak electric field. A resolution in the range of 1 \ensuremath{\mu}eV is demonstrated. Refinement of the technique promises resolution in the range of several neV to be feasible.

Novel Optical Trap of Atoms with a Doughnut Beam

We have constructed a novel optical trap for neutral atoms by using a Laguerre-Gaussian (doughnut) beam whose frequency is blue detuned to the atomic transition. Laser-cooled rubidium atoms are trapped in the dark core of the doughnut beam with the help of two additional laser beams which limit the atomic motion along the optical axis. About ${10}^{8}$ atoms are initially loaded into the trap, and the lifetime is 150 ms. Because the atoms are confined at a point in a weak radiation field in the absence of any external field, ideal circumstances are provided for precision measurements. The trap opens the way to a simple technique for atom manipulation, including Bose-Einstein condensation of gaseous atoms.