PACS. 32.80.Pj – Optical cooling of atoms; trapping. PACS. 33.80.Ps – Optical cooling of molecules; trapping. Abstract. – We describe the statistical mechanics of a new method to produce very cold atoms or molecules. The method results from trapping a gas in a potential well, and sweeping through the well a semi-permeable barrier, one that allows particles to leave but not to return. If the sweep is sufficiently slow, all the particles trapped in the well compress into an arbitrarily cold gas. We derive analytical expressions for the velocity distribution of particles in the cold gas, and compare these results with numerical simulations. Introduction. – Evaporative cooling was originally suggested as a means to achieve Bose-Einstein condensation in atomic hydrogen [1–3]. Its application to magnetically trapped alkali atoms [4,5] culminated in the first observation of Bose–Einstein condensation in atomic vapors [6–8]. Since then it has been the essential process by which to obtain degenerate quantum gases. Nevertheless it has shortcomings. The main two are
PACS. 32.80.Pj – Optical cooling of atoms; trapping. PACS. 33.80.Ps – Optical cooling of molecules; trapping. Abstract. – We describe the statistical mechanics of a new method to produce very cold atoms or molecules. The method results from trapping a gas in a potential well, and sweeping through the well a semi-permeable barrier, one that allows particles to leave but not to return. If the sweep is sufficiently slow, all the particles trapped in the well compress into an arbitrarily cold gas. We derive analytical expressions for the velocity distribution of particles in the cold gas, and compare these results with numerical simulations. Introduction. – Evaporative cooling was originally suggested as a means to achieve Bose-Einstein condensation in atomic hydrogen [1–3]. Its application to magnetically trapped alkali atoms [4,5] culminated in the first observation of Bose–Einstein condensation in atomic vapors [6–8]. Since then it has been the essential process by which to obtain degenerate quantum gases. Nevertheless it has shortcomings. The main two are
We describe the statistical mechanics of a new method to produce very cold atoms or molecules. The method results from trapping a gas in a potential well, and sweeping through the well a semi-permeable barrier, one that allows particles to leave but not to return. If the sweep is sufficiently slow, all the particles trapped in the well compress into a very cold gas. We derive analytical expressions for the velocity distribution of particles in the cold gas, compare these results with numerical simulations, and discuss limitations on reduction in energy due to imperfections of the barrier.
Slowing down atoms using laser cooling, together with techniques like magnetic trapping and evaporative cooling, has led to numerous exciting results over the last decade [1], with the realization of Bose-Einstein condensation as the most spectacular achievement [2]. Laser-cooling schemes are applicable to atoms since closed multilevel systems can be realized and many consecutive absorptionemission cycles can be induced. This leads to a significant momentum transfer from the photons of the laser to the atom, resulting in a change of the velocity of the atom. There currently is a great interest in producing cold molecules for the study of cold molecule-molecule collisions, molecular quantum collective effects, Doppler-free spectroscopy, frequency standards, as well as many other applications already realized for atoms [3]. Slowing down of neutral molecules, a prerequisite for being able to trap them, has proven to be considerably more difficult to be achieved, however. The above-mentioned laser-cooling scheme cannot be applied to slow down molecules as no simple closed-level systems can be found; the unavoidable off-resonant radiative decay of molecules from the excited state to other vibrational levels in the electronic ground state hampers efficient momentum transfer [4]. Once molecules are present at sufficiently low kinetic energies, below a fraction of a cm21, they can be trapped in various ways. Trapping of neutral molecules in inhomogeneous magnetic [3] and electric fields [5] and trapping based on the use of evanescent fields close to the dielectricvacuum interface have been proposed [6]. It has also been suggested that extremely high laser powers might be used to induce a dipole moment in polarizable molecules, leading to trapping of these molecules in the laser beam [7]. Other methods to produce slow molecules have been put forward and have recently been successfully applied to produce the first samples of trapped molecules. By injecting molecules in a cold buffer gas, sufficiently low kinetic energies of the injected molecules can be obtained after thermalization that they can subsequently be trapped in an inhomogeneous magnetic field. This has recently been demonstrated for CaH injected in 3He buffer gas at temperatures of around 0.3 K [8]. Associative collisions between ultracold trapped atoms [9] have proven to be
In this paper we first focus on the methods developed to control the position and the velocity of atoms, taking advantage of the radiative forces exerted on atoms placed in a laser beam. Temperatures in the range of μK can be reached for dilute atomic clouds trapped under vacuum in a very small region of space. The application to fountain clocks based on cold cesium atoms is presented. We then describe the characterization and the main features of Bose‐Einstein condensates, a new state of matter of purely quantum origin, which can be obtained by subsequent evaporative cooling. The methods in use for cooling molecules are considered, in particular the collision processes or the photoassociation of cold atoms. The possibility of changing interactions between ultracold particles is also explained and photoassociation is illustrated by the recent experiments of our group dealing with metastable helium atoms.
Slowing, cooling, and trapping molecules One of the major achievements in physics during the last two decades of the twentieth century was the ability to cool and trap atoms [1]. Laser cooling can produce previously inconceivable atomic temperatures of less than 1 μK. Atoms with sufficiently low kinetic energy can be trapped in magnetic and optical traps and thus studied for a prolonged time, thereby improving the resolution of spectroscopy by many orders of magnitude as well as reducing Doppler broadening. Furthermore, besides the external degrees of freedom, the internal degrees of freedom of the atoms can also be manipulated, leading to full control of the atoms. One of the holy grails of physics, the achievement of Bose–Einstein condensation in a dilute gas, was achieved mainly because of laser cooling and trapping of atoms. One of the natural extensions of laser cooling and trapping of atoms is the application of the techniques to molecules. Molecules offer a much richer internal structure than atoms, and thus provide extended possibilities to experimentalists. However, atomic laser cooling is based on the repetitive scattering of light and requires a closed, or at least nearly closed, two-level system. Because of the complicated internal structure of molecules, a closed two-level system cannot usually be found because they have many different electronic, vibrational, and rotational states and in general can decay to many different states after absorption of light (see Chap. 16). This requires a complete rethinking of the schemes that are used to cool atoms. Still, because of the extended possibilities cold molecules can offer with respect to cold atoms, there have been a large number of techniques developed to slow, cool, and trap them. These techniques can be divided into two classes, namely direct and indirect cooling. In the first case, molecules at cryogenic or room temperature are directly slowed and cooled. Although many different slowing techniques are used, this chapter focuses on Stark slowing of molecules since it was the first technique developed. Apart from the Stark shift of the molecules, the Zeeman and light shift of molecules can also be used to slow them. The idea behind Stark slowing is to subject the moving molecules to a continuously increasing electric field so that their kinetic energy is converted into potential energy.
Laser radiations are used to slow down atoms by the process o f momentum transfer.This leads to reducing the temperature to microkclvin region.Gas phase atoms are trapped by using magnetic fields.The recent advances have led to the realization of the dream of physicists o f confining the atoms and reducing their velocities to the limit imposed by quantum mechanics.A number of new experiments arc possible with the cooled and trapped atoms and ions that would be useful to solve many problems of theoretical physics. 1 unher cooling by the process o f evaporative technique has led to the observation of Bose-Hinslcin Condensation predicted by Einstein and Rose nearly seventy-five years ago.A brief review of the method of laser cooling, magnetic trapping and evaporative cooling methods used for obtaining ultracold atoms are discussed It is possible to obtain temperature in the nanokelvin region without using cryogenic methods thus simplifying the experimental methods to a great extent.
While cold molecule experiments are rapidly moving towards their promised benefits of precision spectroscopy, controllable chemistry, and novel condensed phases, heretofore the field has been greatly limited by a lack of methods to cool and compress chemically diverse species to temperatures below ten millikelvin. While in atomic physics these needs are fulfilled by laser cooling, magneto-optical trapping, and evaporative cooling, until now none of these techniques have been applicable to molecules. In this thesis, two major breakthroughs are reported. The first is the observation of evaporative cooling in magnetically trapped hydroxyl (OH) radicals, which potentially opens a path all the way to Bose-Einstein condensation of dipolar radicals, as well as allowing cold- and ultracold-chemistry studies of fundamental reaction mechanisms. Through the combination of an extremely high gradient magnetic quadrupole trap and the use of the OH Λ-doublet transition to enable highly selective forced evaporation, cooling by an order of magnitude in temperature was achieved and yielded a final temperature no higher than 5mK. The second breakthrough is the successful application of laser cooling and magneto-optical trapping to molecules. Motivated by a proposal in this thesis, laser cooling of molecules is now known to be technically feasible in a select but substantial pool of diatomic molecules. The demonstration of not only Doppler cooling but also two-dimensional magneto-optical trapping in yttrium (II) oxide, YO, is expected to enable rapid growth in the availability of ultracold molecules--just as the invention of the atomic magneto-optical trap stimulated atomic physics twenty-five years ago.