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Massachusetts Inst. of Tech. Report

Forces on neutral atoms due to electromagnetic fields

David E. Pritchard

Published Oct 19, 1984• 0 citations
OpenAlex

Abstract

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.

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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.
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.
Abstract : The program is to slow, trap and cool neutral atoms. Laser light is used to slow and cool atoms and magnetic forces are used to trap them. Ultimately we hope to be able to trap atoms for days and cool them to temperatures on the order of .000001 K in addition to the interesting scientific and technical challenges involved in trapping and cooling atoms, we hope to open several new lines of investigation with these cooled atoms. Cold trapped atoms constitute a good system for studying collective phenomena such as Bose condensation, low energy - Na and Na - Na, and coherent optical effects (the deBrogile wavelength can exceed the optical wavelength). They also offer opportunities for performing ultra-high resolution spectroscopy and are extremely promising candidates for a new generation of frequency standards.
We have constructed a Zeemanshift Optical Trap (ZOT) for neutral cesium atoms within a sealed cryostat cooled to 4.2K. The low residual background pressure of these temperatures, plus low velocities and masses of the background molecules (mostly He and H{sub 2}), leads to an exceptionally long holding time for trapped Cs atoms in the ZOT. In our initial attempts we measured trap holding times {approximately}5-10 minutes, but increasing this to several hours should be possible. Our latest experimental results will be presented. Long trap holding times may be useful for achieving a Bose condensed gas via evaporative cooling, since thermal relaxation times are fairly long and background gas collisions are therefore important. We are also beginning experiments to trap atoms with superconducting coils inside the cryostat. Our initial aim is to construct microscopic neutral atom traps and other structures with large magnetic field gradients (as high as 10{sup 8} G/cm), where the {triangledown}({rvec {mu}}{center_dot}{rvec B}) force is much greater than has been realized in previous trapping geometries.
The trapping of charged elementary particles and ions by various electromagnetic field configurations is a well established art. Last month (page 17) we told of a lone electron that was confined for almost a year in the vacuum of a Penning trap, yielding a superbly delicate observation of lowenergy relativistic hysteresis. But the trapping of electrically neutral particles—atoms in particular—is quite another matter. In the absence of the simple Lorentz force acting on a charged particle, one has to reach for a much weaker, second-order handle—the atom's magnetic or induced electric dipole moment. A variety of ingenious magnetic and laser traps have been proposed for atoms, but they all suffer a severe limitation: The weak dipole interaction with the trapping field yields only very shallow trapping potential wells, generally incapable of holding an atom moving much faster than a man can run—about 10 meters/second.
We propose several ideas for neutral atom traps which utilize spontaneous light forces produced by static laser beams. Although the “optical Earnshaw theorem”1 would seem to prohibit such traps, the theorem does not apply if the proportionality constant between force and light intensity is made to vary with position. We suggest possible ways to accomplish this: Zeeman tuning the atom’s resonant frequency by applying a changing magnetic field; varying the quantization axis of the system via a magnetic field cork-screw; utilizing optical pumping effects which arise from matrix element imbalances between hyperfine sublevels in certain atoms. These traps are true in the sense that they provide a potential energy minimum in three dimensions, as well as dissipative cooling. Compared with gradient force light traps, these traps are many times deeper, a million times larger, and can be operated cw.