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Annual Meeting Optical Society of America

Continuous stopping and trapping of neutral atoms

Vanderlei Salvador Bagnato, Gregory P. Lafyatis, Alexander G. Martin, Kristian Helmerson, David E. Pritchard

Published Jan 1, 1987• 0 citations
Publisher

Abstract

We have continuously stopped thermal sodium atoms with laser light and loaded them into a 0.1-K deep superconducting magnetic trap. The trap is formed by a local minimum of the magnetic field produced by two superconducting coils and a radially confining octupole field. The atoms are slowed and stopped in two stages: The atoms are first slowed to ~200 m/s using a counterpropagating resonant laser beam and tapered magnetic field to compensate for the changing Doppler shift as the atoms are slowed.1 A second laser beam and tapered magnetic field, actually located within the trap, are used to stop the atoms in the trap. This continuous loading process has allowed us to accumulate up to 1 × 109 trapped atoms, and trapping times of several minutes have been achieved. Our trap has the feature of having a uniform magnetic field at its bottom, opening up the possibility of precision spectroscopy of the trapped atoms. The fluorescence from the trapped atoms has been studied, and they have been cooled to ~1 mK using 1-D optical molasses.

DOI
10.1364/oam.1987.wu6

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•Jan 1, 2002•33 citations
We have continuously stopped thermal sodium atoms with laser light and loaded them into a 0.1-K deep superconducting magnetic trap. The trap is formed by a local minimum of the magnetic field produced by two superconducting coils and a radially confining octupole field. The atoms are slowed and stopped in two stages: The atoms are first slowed to ~200 m/s using a counterpropagating resonant laser beam and tapered magnetic field to compensate for the changing Doppler shift as the atoms are slowed.1 A second laser beam and tapered magnetic field, actually located within the trap, are used to stop the atoms in the trap. This continuous loading process has allowed us to accumulate up to 1 × 109 trapped atoms, and trapping times of several minutes have been achieved. Our trap has the feature of having a uniform magnetic field at its bottom, opening up the possibility of precision spectroscopy of the trapped atoms. The fluorescence from the trapped atoms has been studied, and they have been cooled to ~1 mK using 1-D optical molasses.
Neutral sodium atoms have been continuously loaded into a 0.1-K-deep superconducting magnetic trap with laser light used to slow and stop them. At least 1 \ifmmode\times\else\texttimes\fi{} 109{10}^{9}109 atoms were trapped with a decay time of 2\textonehalf{} min. The fluorescence of the trapped atoms was studied as a function of time; possible loss mechanisms from the trap are discussed.
A superconducting neutral trap has been constructed and preliminary tests of it have begun. The tap consists of superconducting solenoids to permit slowing and axial confinement and a superconducting octopole magnet for radial confinement in the trap. The trap includes several smaller winding for manipulating trapped atoms. The trap itself is about 40 cm long and has a diameter of 7 cm. It is about 1 Kelvin deep. Na atoms from an atomic beam are to be slowed and then stopped in the center of the trap with counterpropagating laser beams. Initial testing of the magnets, cryogenic vacuum apparatus, and atom source has been completed. A program to optically trap atoms has been initiated. This is based on the recent discovery of exceptions to the optical Earnshaw theorem. Several possible traps using only spontaneous forces have been suggested and detailed modelling of these traps is underway. An apparatus to trap atoms based on these new ideas is being conducted.
A thermal atomic Na beam is opposed by a resonantly tuned laser beam. The radiation pressure decelerates and compresses the thermal velocity distribution, bringing the average velocity to zero and reducing the width from ~1000 m/s to a few tens of m/s or less.1 The density of stopped atoms is > 106/cm3 within a bandwidth of a few m/ s. These densities and velocities are suitable for trapping by a number of proposed optical and magnetic traps. Using these stopped atoms, we have demonstrated the first electromagnetic trapping of neutral atoms. The trap, a magnetic quadrupole formed by two separated, coaxial, opposed current loops, confines the atoms by virtue of the force exerted on the Na magnetic dipole moment by the inhomogeneous magnetic field. The time constant for exponential decay of the atomic population in the trap is 0.83(7) s and is limited mainly by collisions with fast background gas atoms. The maximum velocity of atoms contained in the trap is 3.5 m/s, corresponding to a maximum energy of 17 mK.
The advent of the laser opened the way to the development of powerful methods for producing and manipulating cold atomic gases. To set the stage we describe a typical experiment, which is shown schematically in Fig. 4.1. A beam of sodium atoms emerges from an oven at a temperature of about 600 K, corresponding to a speed of about 800 m s −1 , and is then passed through a so-called Zeeman slower, in which the velocity of the atoms is reduced to about 30 m s −1 , corresponding to a temperature of about 1 K. In the Zeeman slower, a laser beam propagates in the direction opposite that of the atomic beam, and the radiation force produced by absorption of photons retards the atoms. Due to the Doppler effect, the frequency of the atomic transition in the laboratory frame is not generally constant, since the atomic velocity varies. However, by applying an inhomogeneous magnetic field designed so that the Doppler and Zeeman effects cancel, the frequency of the transition in the rest frame of the atom may be held fixed. On emerging from the Zeeman slower the atoms are slow enough to be captured by a magneto-optical trap (MOT), where they are further cooled by interactions with laser light to temperatures of order 100 μK. Another way of compensating for the changing Doppler shift is to increase the laser frequency in time, which is referred to as ‘chirping’.
We demonstrate experimentally the continuous and pulsed loading of a slow and cold atomic beam into a magnetic guide. The slow beam is produced using a vapor loaded laser trap, which ensures two-dimensional magneto-optical trapping, as well as cooling by a moving molasses along the third direction. It provides a continuous flux larger than 10910^9109 atoms/s with an adjustable mean velocity ranging from 0.3 to 3 m/s, and with longitudinal and transverse temperatures smaller than 100μ100 \mu100μK. Up to 31083 10^83108 atoms/s are injected into the magnetic guide and subsequently guided over a distance of 40 cm.