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Physical Review Letters

Long Atomic Coherence Times in an Optical Dipole Trap

Nir Davidson, Heun Jin Lee, Charles S. Adams, Mark A. Kasevich, Steven Chu

Published Feb 20, 1995• 171 citations
Publisher

Abstract

Sodium atoms have been stored in a blue-detuned dipole trap based on sheets of argon ion laser light which support against gravity. In this trap, the atoms spend most of their time in free fall, resulting in a large reduction in the perturbation of the atomic levels due to the trapping potential. This reduction enabled us to probe the ground state hyperfine splitting with a measurement time of 4 s, yielding a linewidth of 0.125 Hz and a Ramsey fringe contrast of 43%. The coherence time was $\ensuremath{\sim}$300 times longer than achieved in a red-detuned Nd:YAG laser dipole trap with comparable depth.

DOI
10.1103/physrevlett.74.1311

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Dipole Trapping, Cooling in Traps, and Long Coherence Times

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Long spin relaxation times in a single-beam blue-detuned optical trap

Roee Ozeri, Lev Khaykovich, Nir Davidson

A scheme to trap atoms in a blue-detuned optical dipole trap, formed with a single laser beam and a holographic phase plate, is demonstrated. ${10}^{5}$ rubidium atoms are trapped for (1/e lifetime) \ensuremath{\sim}300 msec, at temperatures of \ensuremath{\sim}24 \ensuremath{\mu}K and a density of $\ensuremath{\sim}7\ifmmode\times\else\texttimes\fi{}{10}^{11}{\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}\mathrm{s}/\mathrm{c}\mathrm{m}}^{3},$ for a trapping beam detuning of 0.1--30 nm. The time for a trapped atom to scatter one photon off the trapping beam is measured and found to be linear in the trapping laser detuning. At a detuning of 0.5 nm from resonance this time was measured to be \ensuremath{\sim}100 msec, indicating that the atoms are exposed on average only to \ensuremath{\sim}1/700 of the maximal light intensity in the trap. The use of a single laser beam allows for simple dynamical changes of the potential and large adiabatic compressions, while keeping the atoms mainly in the dark.

Physical Review A•Mar 1, 1999•128 citations
Sodium atoms have been stored in a blue-detuned dipole trap based on sheets of argon ion laser light which support against gravity. In this trap, the atoms spend most of their time in free fall, resulting in a large reduction in the perturbation of the atomic levels due to the trapping potential. This reduction enabled us to probe the ground state hyperfine splitting with a measurement time of 4 s, yielding a linewidth of 0.125 Hz and a Ramsey fringe contrast of 43%. The coherence time was \ensuremath∼\ensuremath{\sim}\ensuremath∼300 times longer than achieved in a red-detuned Nd:YAG laser dipole trap with comparable depth.
Conditions for trapping sodium atoms in far detuned dipole traps are explored with the goal of maximizing phase space density and coherence times. With red detuned traps, we have been able to achieve densities as high as 2 ×1012 atoms/cm3 at temperatures on the order of (11ħk)2/2M. We also report Raman cooling in a red de‐tuned trap to a one dimensional “temperature” of 0.7(ħk)2/2M. With a blue detuned trap, we have been able to observe Ramsey interference fringes between the ground states of sodium for over 4 seconds. Finally, we report an atom interferometer based on adiabatic transfer of ground state populations.
Blue-detuned dipole traps and their ability to preserve atomic coherences are interesting for precision measurement applications. In this paper, we present experimental studies on the differential ac Stark shift of the ground-state hyperfine splitting in 87{}^{87}87Rb atoms confined in a dynamic blue-detuned dipole trap. We systematically study the power and detuning effects on the Rabi resonance frequency (differential ac Stark shift) and its linewidth (coherence) and find that their performance is compatible with future parity violation experiments in Fr.
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.
The interaction of light with atoms and molecules is of fundamental interest in many branches of science. In atomic physics, this interaction can be used to cool and spatially confine (trap) atoms. These traps can be used as the starting point for other experiments, but the dynamics of the cooling and trapping processes is itself of interest. In order to better understand the physics of trapping atoms in an optical dipole force trap, we have conducted a series of spectroscopic measurements of ultracold rubidium atoms in such a trap. The trap was created at the focus of a Nd:YAG laser beam with wavelength 1064nm and nearly Gaussian spatial mode. For rubidium, the trap light is red-detuned and the atoms are confined to the highest intensity in the beam. In order to probe the atoms confined in the trap, we have performed spectroscopy on the 5S1/2 → 5P3/2 transition. Because the polarizability of the ground and excited states is not the same at the trap wavelength, the spectra exhibit both a shift and inhomogeneous broadening. We have investigated the spectra for both linear and circular polarized traps. We also studied the application of a second laser to couple the excited state (5P3/2) to another higher excited state (5D5/2) to reduce the inhomogeneous broadening of the 5S1/2 → 5P 3/2 transition. Finally, two-photon spectroscopy was performed on atoms in the dipole force trap and compared to spectra taken in a magneto optical trap (MOT). Autler-Townes splitting was clearly observed in the MOT and appears to have been observed in the dipole force trap also.
A scheme to trap atoms in a blue-detuned optical dipole trap, formed with a single laser beam and a holographic phase plate, is demonstrated. 105{10}^{5}105 rubidium atoms are trapped for (1/e lifetime) \ensuremath{\sim}300 msec, at temperatures of \ensuremath{\sim}24 \ensuremath{\mu}K and a density of $\ensuremath{\sim}7\ifmmode\times\else\texttimes\fi{}{10}^{11}{\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}\mathrm{s}/\mathrm{c}\mathrm{m}}^{3},$ for a trapping beam detuning of 0.1--30 nm. The time for a trapped atom to scatter one photon off the trapping beam is measured and found to be linear in the trapping laser detuning. At a detuning of 0.5 nm from resonance this time was measured to be \ensuremath{\sim}100 msec, indicating that the atoms are exposed on average only to \ensuremath{\sim}1/700 of the maximal light intensity in the trap. The use of a single laser beam allows for simple dynamical changes of the potential and large adiabatic compressions, while keeping the atoms mainly in the dark.