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.
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.
Of the many application 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 [1]. The staggering progress achieved by laser cooling is reflected by considering a thought experiment that begins ‘take an atom. . . ’. Such an experiment was impossible to realize in practice because room temperature atoms move at supersonic speeds and, as atoms are neutral, there was no easy way to grab hold of them. Laser cooling allows one to obtain samples of atoms essentially at rest, with temperatures in the microKelvin region being routine. At such low temperatures, it becomes easy to trap or move atoms with modest electromagnetic forces [2, 3] and a whole new class of experiments previously confined to the realm of fantasy becomes possible. For example, one can throw atoms upwards and use precision spectroscopy to measure time or gravity with unprecedented accuracy. One can place atoms at a desired location on a surface creating a novel lithographic technique or place an atom just above the surface to create a qubit in a quantum information processor. One can take many atoms and cool them further by evaporation to form a Bose-Einstein condensate [4] or degenerate Fermi gas. This last development has propelled atomic and optical physics right into the heart of condensed matter physics and resulted in the award of the Nobel Prize in 2001.
The past few years have seen remarkable developments in the ability to control atomic motion by optical techniques. Among the key tools that have been developed for this purpose are the techniques of laser cooling, the significance of which was recognized by the award of the 1997 Nobel Prize in Physics to Steven Chu, Claude Cohen-Tannoudji, and William Phillips. These techniques have made it possible to cool atoms to temperatures lower than have been produced in any other physical system - around 10(-8) degrees Kelvin above absolute zero - and to confine those in traps for extended periods of observation.
When the Nobel Prize for Physics was awarded to Claude Cohen-Tannoudji, William D. Philipps and Steven Chu in 1997 for the development of methods for cooling and trapping atoms using lasers, this was the reward for two decades of investigations not only touching upon the fundamental aspects of the light–matter interaction, but also leading to a range of applications. On the theoretical level, the advent of this field of research in the late 1970s stimulated the development of various theoretical methods for describing the radiative forces by which light affects the motion of atoms. On the experimental level, judicious use of the radiative forces exerted by lasers made it possible to obtain a drastic reduction of atomic velocities – in other words, to cool an atomic vapour. These advances were characterized by a remarkable cross-fertilization of theoretical and experimental innovations. It was not long before applications began to appear in the field of high-resolution spectroscopy, since ultracold, i.e. slow, atoms can be observed for longer periods, and this allows greater accuracy when measuring atomic resonance frequencies than could be obtained for atoms at room temperature. Based on such resonances, the most accurate clocks in the world use laser-cooled atoms or ions. Atom interferometry is another application of laser manipulation of atoms, which can be applied to measure inertial effects, due for example to the rotation of the Earth or the motion of a vehicle, with an accuracy that already exceeds traditional methods.
Laser cooling and trapping are central to modern atomic physics. It has been roughly three decades since laser cooling techniques produced ultracold atoms, leading to rapid advances in a vast array of fields and a number of Nobel prizes. Prior to the work presented in this thesis, laser cooling had not yet been extended to molecules because of their complex internal structure. However, this complexity makes molecules potentially useful for a wide range of applications. The first direct laser cooling of a molecule and further results we present here provide a new route to ultracold temperatures for molecules. In particular, these methods bridge the gap between ultracold temperatures and the approximately 1 kelvin temperatures attainable with directly cooled molecules (e.g. with cryogenic buffer gas cooling or decelerated supersonic beams). Using the carefully chosen molecule strontium monofluoride (SrF), decays to unwanted vibrational states are suppressed. Driving a transition with rotational quantum number R=1 to an excited state with R'=0 eliminates decays to unwanted rotational states. The dark ground-state Zeeman sublevels present in this specific scheme are remixed via a static magnetic field. Using three lasers for this scheme, a given molecule should undergo an average of approximately 100,000 photon absorption/emission cycles before being lost via unwanted decays. This number of cycles should be sufficient to load a magneto-optical trap (MOT) of molecules. In this thesis, we demonstrate transverse cooling of an SrF beam, in both Doppler and a Sisyphus-type cooling regimes. We also realize longitudinal slowing of an SrF beam. Finally, we detail current progress towards trapping SrF in a MOT. Ultimately, this technique should enable the production of large samples of molecules at ultracold temperatures for molecules chemically distinct from competing methods.
Forty years ago, twenty years after the discovery of the laser, physicists were developing laser cooling methods for ions trapped in electromagnetic fields.From the 1980s onwards, these techniques were refined and extended to atoms, thanks to the audacity and inventiveness of a generation of pioneering researchers.Actually, it was necessary to succeed simultaneously in trapping and cooling samples of atomic gases in a vacuum at a distance from any wall.Spectacular results followed and extraordinarily low temperatures were quickly reached, very close to absolute zero.The field of "cold atoms" was born, rewarded by successive Nobel prizes, first of which was awarded in 1997 to William D. Phillips, Steven Chu and Claude Cohen-Tannoudji, last in 2022 to Alain Aspect.Gaseous samples of a few thousand to a few billion atoms can be prepared at a few millionths of a degree above absolute zero, which means that the particles move at extremely low speed, of the order of centimetres per second.At these extreme temperatures, the behaviour of matter changes and its properties can only be described using quantum mechanics and the wave properties of particles.New physical phenomena have been discovered and innovations have followed the theoretical and experimental progress of the research.Initially imagined as a wonderful method for perfecting atomic physics, cold atoms have gradually proved to be powerful tools for research in cross-cutting fields of physics, such as condensed matter and even high-energy physics.These atoms are now referred to as 'quantum gases' at such low temperatures that their collective behaviour is modified by the laws of quantum mechanics.
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.