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arXiv:quant-ph/0606018 · Journal of Modern Optics

Transfer of trapped atoms between two optical tweezer potentials

M. Schulz, H. Crepaz, F. Schmidt-Kaler, J. Eschner, R. Blatt

Published Jul 20, 2007• 27 citations
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Abstract

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.

DOI
10.1080/09500340600861740

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Acta Physica Sinica•Jan 1, 2012•1 citation
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.
This thesis reports on the design, setup, and operation of a new atom trap apparatus, which is developed to confine few rubidium atoms in ultrahigh vacuum by means of taylored and dynamically variable light potentials (dipole traps), thus making the atoms available for spectroscopy and controlled manipulations. To maintain low background pressure, atoms of a vapour cell are transferred into a cold atomic beam by laser cooling techniques, and accumulated by a magneto-optic trap (MOT) in a separate part of the vacuum system. The laser cooled atoms are then transferred into dipole traps made of focused far-off-resonant laser fields in single or crossed-beam geometry, which are superimposed with the center of the MOT. Gaussian as well as hollow Laguerre-Gaussian (LG01) beam profiles are used with red-detuned and blue-detuned light. Microfabricated dielectric phase objects allow the efficient and robust mode conversion of Gaussian into Laguerre-Gaussian laser beams. Trap geometries can easily be changed due to the highly flexible experimental setup. The dipole trap laser beams have powers up to several hundred milliwatts and are focused to around 10 m. At a detuning of several tens of nanometers from the atomic resonance, the resulting traps are a few millikelvin deep, and the trapped atoms exhibit motional frequencies near 30 kHz, trap light scattering rates of a few hundred photons per second per atom, and storage times of several seconds. The number of dipole-trapped atoms ranges from more than ten thousand to below ten. The dipole-trapped atoms are detected either by a photon counting system with efficient straylight discrimination, or by recapture into the MOT, which is imaged onto a sensitive photodiode and a CCD-camera. Due to the strong AC-Stark shift imposed by the high-intensity trapping light, energy-selective resonant excitation and detection of the stored atoms is possible. The measured energy distribution is consistent with a harmonic potential shape and allows for the determination of temperatures and heating rates. From initial measurements, the thermal energy is found to be about 10 % of the trap depth. Using a crossed-beam geometry with red-detuned laser light, efficient transfer of atoms between the beams is observed. Optimum transfer occurs when the two beams cross at a radial offset, which can qualitatively be understood when the particle energy and geometrical properties of the two-beam trapping potential are considered. Numerical simulations reproduce the general features of the measured transfer efficiency vs. radial beam offset. This experiment is a first step towards dynamically variable potentials. Atoms have been radially confined in a blue-detuned hollow beam, and their decay time has been determined. This configuration is currently extended to a three-dimensionally confining blue-detuned dipole trap. For advanced laser cooling, state manipulation and spectroscopy of trapped atoms, a system of two diode lasers has been set up which are phase-coherently locked with a difference frequency near 6.834 GHz, to drive Raman transitions between the hyperfine-split ground states of 87Rb atoms. Resonances of such a transition have been recorded in a buffer gas loaded rubidium vapour cell with linewidths below 100 Hz.
We theoretically study the loading of cold atoms into a crossed optical dipole trap (ODT). We compared the loading efficiency dependent on the trap geometry for Nd:YAG and CO2 laser ODTs. By crossing two Nd:YAG laser beams at a small angle, such as 10°, we can shape the geometry of the effective potential in the intersection region and achieve comparable performance to that of a single-beam CO2 laser ODT. We also calculate the optimal experimental parameters for these two laser wavelengths for laser power per beam ranging from 5 W to 100 W. We further experimentally studied the loading of laser-cooled Rb87 atoms into a small-angle crossed Nd:YAG ODT and found that the experimental data agreed with the simulation results.
We design an experiment to obtain a Bose-Einstein Condensate (BEC) using 87Rb atoms. Taking advantage of this setup, we develop a simple and efficient method to transfer atoms from a first Magneto- Optical-Trap(MOT) to a second one. Our system consists of two glass vacuum cells horizontally connected with a MOT aligned at the center of each cell. Once rubidium atoms are loaded into the first MOT, the pushig beam forces them to move forward to the second MOT. We use a near-resonant weak laser beam to make the atoms reach the second cell. MOT1 and MOT2 both run in retroreflected-three beam MOT configuration.
We model the efficiency of loading atoms of various species into a one-dimensional optical lattice from a cold ensemble, taking into account the initial cloud temperature and size, the lattice laser properties affecting the trapping potential, and the atomic parameters. Stochastic sampling and dynamical evolution are used to simulate the transfer, leading to estimates of transfer efficiency for varying trap depth and profile. Tracing the motion of the atoms also enables the evaluation of the equilibrium temperature and site occupancy in the lattice. The simulation compares favorably against a number of experimental results and is used to compute an optimum lattice-waist-to-cloud-radius ratio for a given optical power.
The preparation and manipulation of single neutral atoms in optical dipole traps have important applications in quantum simulation and information. For this purpose, a single neutral atom, trapped in a static optical dipole trap which is formed by a strongly focused red-detuned far-off-resonance laser, can be transferred to a movable optical dipole trap when the movable trap crosses the static trap and the transfer efficiency can reach about 94%, meanwhile this transferred atom could be located at given position in the focal plane. This experimental result has potential applications in realizing entanglement of two individual neutral atoms in an optical dipole trap array.