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Journal of the Optical Society of America B

Laser cooling of magnetically trapped neutral atoms

Kristian Helmerson, Alex Martin, David E. Pritchard

Published Nov 1, 1992• 42 citations
Publisher

Abstract

We demonstrate laser Doppler cooling of magnetically trapped neutral atoms. Following the application of one-dimensional optical molasses to a sample of magnetically trapped Na atoms, a substantial reduction has been observed in the linewidths of both laser absorption and rf resonance spectra, indicating cooling of the sample to ∼2 mK. A simple model of the one-dimensional Doppler cooling process of the atoms in the trap is presented and compared with the experimentally observed results.

DOI
10.1364/josab.9.001988

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We demonstrate laser Doppler cooling of magnetically trapped neutral atoms. Following the application of one-dimensional optical molasses to a sample of magnetically trapped Na atoms, a substantial reduction has been observed in the linewidths of both laser absorption and rf resonance spectra, indicating cooling of the sample to ∼2 mK. A simple model of the one-dimensional Doppler cooling process of the atoms in the trap is presented and compared with the experimentally observed results.
We have studied a general technique for laser cooling a cloud of polarized trapped atoms down to the Doppler temperature. A one-dimensional optical molasses created with polarized light cools the axial motional degree of freedom of the atoms in the trap. Cooling of the radial degrees of freedom can be modeled by reabsorption of scattered photons in the optically dense cloud. We present experimental results for a cloud of chromium atoms in a magnetic trap. A simple model based on rate equations shows quantitative agreement with the experimental results. This scheme allows us to readily prepare a dense cloud of atoms in a magnetic trap with good starting conditions for evaporative cooling.
We present a simple review of the basic physical processes allowing one to control, with laser beams, the velocity and the position of neutral atoms. The control of the velocity corresponds to a cooling of atoms, that is, a reduction of the atomic velocity spread around a given value. The control of the position means a trapping of atoms in real space. The best present performances will be given, in terms of the lowest temperatures and the highest densities. The corresponding highest quantum degeneracy will also be estimated. It is imposed by fundamental limits, which will be described briefly. We also give the general trends in this field of research and outline the new directions which look promising for observing quantum statistical effects in laser cooled atomic samples, but which are for the moment restricted by unsolved problems. Introducing the Simple Schemes The radiative forces acting on atoms in a light field can be split into two parts, a reactive one and a dissipative one. The dissipative force (radiation pressure), which basically involves scattering processes, is velocity dependent. We will see that this dependence leads to the Doppler cooling scheme and to the concept of optical molasses, and we will give the corresponding minimal achievable temperature. The dissipative force can be made position dependent, through a gradient of the magnetic field, so that the atoms are also trapped in the so-called magneto-optical trap.
Laser cooled and trapped atoms have been the workhorse of atomic physics for the past four decades. The predominant method has been the highly versatile Magneto-Optical Trap. We describe an alternative laser trap involving a simple geometry of collimated laser beams that provides both a velocity and position dependent restoring force such that a dense cloud of cold atoms is formed. This technique produces similar atom number (>106>10^6>106) and density (101010^{10}1010\,atoms/cm3^{3}3) to the Magneto-Optical Trap, albeit with \emph{no magnetic field}. The beam geometry is compatible with conventional sub-Doppler cooling techniques, allowing the trapped cloud to be cooled to <10 μ< 10~μ<10 μK. We demonstrate the validity and robustness of the trap by capturing 87^{87}87Rb atoms directly from the background vapor and provide a theoretical discussion of the underlying principles. This trap has many unique properties that make it highly suitable for quantum sensing, timing, and computing applications as well as a new tool in fundamental science and metrology.
We present a scheme for laser cooling applicable to an extremely dilute sample of magnetically trapped antihydrogen atoms ( ). Exploiting and controlling the dynamical coupling between the 's motional degrees of freedom in a magnetic trap, three-dimensional cooling can be achieved from Doppler cooling in one dimension using the 1s 1/2 − 2p 3/2 transition. The lack of three-dimensional access to the trapped and the nearly separable nature of the trapping potential leads to difficulties in cooling. Using realistic models for the spatial variation of the magnetic fields, we find that it should be possible to cool the 's to ∼20 mK even with these constraints.
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.