Cooling and Trapping neutral particles or ions using laser beam becomes a new research field inPhysics. This field was started in 1970’s by Ashkin who demonstrated that a focused laser beam ondielectric particles in a glass cell can attract the particles to the focal area because the potenstialenergy there is lower. Cooling which means to reduce the average thermal velocity of atoms orparticles and trapping which means controlling the positions of the atoms can be performed using twokinds of forces from Laser beam i.e. scattering force and gradient force. A scattering force is resultedfrom the laser beam because the laser beam contains fotons which have momentum. The gradientforce appears because of the gradient of intensity around focal area. One of famous atom traps isMagneto-Optical Trap (MOT). MOT uses Doppler effect for cooling and Zeeman effect for trappingWith this technique, cold atoms with temperature in the order of mikro Kelvin can be produced. Coldatom can be used for many applications such as to make Bose Einstein Condensate (BEC), atominterfermetry and atomic clocks.
Cooling and Trapping neutral particles or ions using laser beam becomes a new research field inPhysics. This field was started in 1970’s by Ashkin who demonstrated that a focused laser beam ondielectric particles in a glass cell can attract the particles to the focal area because the potenstialenergy there is lower. Cooling which means to reduce the average thermal velocity of atoms orparticles and trapping which means controlling the positions of the atoms can be performed using twokinds of forces from Laser beam i.e. scattering force and gradient force. A scattering force is resultedfrom the laser beam because the laser beam contains fotons which have momentum. The gradientforce appears because of the gradient of intensity around focal area. One of famous atom traps isMagneto-Optical Trap (MOT). MOT uses Doppler effect for cooling and Zeeman effect for trappingWith this technique, cold atoms with temperature in the order of mikro Kelvin can be produced. Coldatom can be used for many applications such as to make Bose Einstein Condensate (BEC), atominterfermetry and atomic clocks.
The advent of the laser opened the way to the development of powerful new methods for manipulating and cooling atoms which were exploited in the realization of Bose–Einstein condensation in alkali atom vapours. 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’. In other experiments the MOT is filled by transferring atoms from a second MOT where atoms are captured directly from the vapour.
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 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.
Of the many applications of lasers, the one with possibly the most dramatic impact on fundamental physics is laser cooling and trapping: the Nobel Prize for physics was awarded for laser cooling in 1997. Laser cooling allows one to obtain samples of atoms essentially at rest, with temperatures in the micro-Kelvin region being routine. At such low temperatures, it becomes easy to trap or move atoms with modest electromagnetic forces and a whole new class of experiments previously confined to the realm of fantasy becomes possible. The high brightness of masers and lasers makes it possible to access a regime of atomic excitation for which Einstein’s rate equations are inadequate. Three-dimensional optical molasses using counter-propagating laser beams along three mutually orthogonal directions was first observed in 1985 by Chu et al. The Sisyphus mechanism works well for atoms that are already cold and has a smaller capture velocity than conventional Doppler cooling.
Collisions between ultracold neutral atoms have for some time been the subject of investigation, initially with hydrogen and more recently with laser cooled alkali atoms. Advances in laser cooling and trapping of neutral atoms in a Magneto-Optic Trap (MOT) have made cold atoms available as the starting point for many laser cooled atomic physics investigations. The most spectacularly successful of these, the observation of Bose-Einstein Condensation (BEC) in a dilute ultra-cold spin polarized atomic vapor, has accelerated the study of cold collisions. Experimental and theoretical studies of BEC and the long range interaction between cold alkali atoms is at the boundary of atomic and low temperature physics. Such studies have been difficult and would not have been possible without the development and advancement of laser cooling and trapping of neutral atoms. By contrast, ion-atom interactions at low temperature, also very difficult to study prior to modern day laser cooling, have remained largely unexplored. But now, many laboratories worldwide have almost routine access to cold neutral atoms. The combined technologies of ion trapping, together with laser cooling of neutrals has made these studies experimentally feasible and several very important, novel applications might come out of such investigations . This paper is an investigation of ion-atom interactions in the cold and ultra-cold temperature regime. Some of the collisional ion-atom interactions present at room temperature are very much reduced in the low temperature regime. Reaction rates for charge transfer between unlike atoms, A + B(+) approaches A(+) + B, are expected to fall rapidly with temperature, approximately as T(sup 5/2). Thus, cold mixtures of atoms and ions are expected to coexist for very long times, unlike room temperature mixtures of the same ion-atom combination. Thus, it seems feasible to cool ions via collisions with laser cooled atoms. Many of the conventional collisional interactions, exploited as a useful tool at room temperature and higher, are greatly enhanced at low energy. For example, collisional spin transfer from one species of polarized atoms to another has long been a useful method for polarizing a sample of atoms where no other means was available. Because optical pumping cannot be used to polarize the nuclear spin of Xe-129 or He-3 (for use in nmr imaging of the lungs), the nuclear spins are polarized via collisions with an optically pumped Rb vapor in a cell containing both gases. In another case, a spin polarized thermal Cs beam was used to polarize the hyperfine states of trapped He(+)-3 ions in order to measure their hyperfine clock transition frequency. The absence of an x-ray light source to optically pump the ground state of the He(+)-3 ion necessitated this alternative state preparation. Similarly, Cd(+) and Sr(+) ions were spin-oriented via collisions in a cell with optically pumped Rb vapor. Resonant RF spin changing transitions in the ground state of the ions were detected by changes in the Rb resonance light absorption. Because cold collision spin exchange rates scale with temperature as T(sup -1/2) this technique is expected to be a far more powerful tool than the room temperature counterpart. This factor of 100 or more enhancement in spin exchange reaction rates at low temperatures is the basis for a novel trapped ion clock where laser cooled neutrals will cool, state select and monitor the ion clock transition. The advantage over conventional direct laser cooling of trapped ions is that the very expensive and cumbersome UV laser light sources, required to excite the ionic cooling transition, are effectively replaced by simple diode lasers.