Cooling and trapping of atoms with light provides us with samples far colder and denser than possible by any other means and enables us to study and control these systems in exquisite detail [1]. We can not only learn about these systems, but can devise experiments of such precision that we test the underlying physics. It is unfortunate therefore that the most effective techniques, Doppler cooling and the Magneto Optical Trap, rely on a simple energy level structure found only in a handful of elements. Finally, I will present an approach in which a particle can be cooled using the dipole, rather than the scattering, force. The dipole force is conservative, but by allowing the potential to vary in time, one can change the energy of a trapped particle. Specifically, if the potential depends on the position of the particle with some time delay, we can create a friction-like force [5, 6], and we are currently building an experiment to demonstrate this effect. This technique is not restricted to atoms with a simple structure, but requires only that the particle be polarisable; in principle, therefore, it can be extended without significant change to direct optical cooling of molecules.
Cooling and trapping of atoms with light provides us with samples far colder and denser than possible by any other means and enables us to study and control these systems in exquisite detail [1]. We can not only learn about these systems, but can devise experiments of such precision that we test the underlying physics. It is unfortunate therefore that the most effective techniques, Doppler cooling and the Magneto Optical Trap, rely on a simple energy level structure found only in a handful of elements. Finally, I will present an approach in which a particle can be cooled using the dipole, rather than the scattering, force. The dipole force is conservative, but by allowing the potential to vary in time, one can change the energy of a trapped particle. Specifically, if the potential depends on the position of the particle with some time delay, we can create a friction-like force [5, 6], and we are currently building an experiment to demonstrate this effect. This technique is not restricted to atoms with a simple structure, but requires only that the particle be polarisable; in principle, therefore, it can be extended without significant change to direct optical cooling of molecules.
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.
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.
The dipole force, which avoids the closed cycle of pumping and spontaneous emission that renders laser cooling unsuitable for molecules, is conservative: without dissipation, particles entering a trap retain the energy to escape. Fortunately, dissipation need not involve spontaneous emission if it instead results from the decoherence or decay of the optical trapping field that is coupled to the particle. To enhance the weak attraction of an atom to its reflection, cavity-mediated cooling recycles light through multiple reflections, amplifying the force and the retardation - a process related to the mechanical amplification in a near confocal cavity [5]. Resonant cavities and structures can increase the retardation, but not the intensity, even when the atoms or particles lie outside them - e.g. when the cavity is the narrow-band coating of a single mirror. Plasmon resonances of a structured metal can enhance both the delay and the optical intensity. Such processes, possible with arrays of micromirrors, resemble speckle field cooling [6], except that spontaneous emission is again replaced by decay of the optical field, and offer a new class of cooling mechanisms in which weak cooling is extended over a broad array, rather than concentrated at the centre of a single external cavity.
Abstract Many areas of physics—precision measurements, quantum information, and physical chemistry, to name a few—are starting to benefit from the enormous advantages offered by cold and ultracold polar molecules. Molecules have more states, more interactions, and more chemical properties compared to atoms, which make them exciting to study but difficult to tame. In particular, the powerful techniques of atomic laser cooling cannot be naïvely applied to molecules due to their complicated structure. Developments over the past few years have made directly laser cooled and trapped molecules a reality, and now much effort is focused on making these samples larger, denser, and colder—an important step to realizing many of their exciting applications. A careful experimental and numerical study by Truppe et al (2017 New J. Phys. 19 022001 ) demonstrates a significant improvement and advance in understanding of one of the most limiting steps in laser cooling and trapping of molecules—slowing them from a molecular beam to a near-standstill, with small enough kinetic energy that they can be loaded into a trap.
Microscopically controlled neutral atoms in optical tweezers and lattices have led to exciting advances in the study of quantum information and quantum many-body systems. The light shifts of atomic levels from the trapping potential in these systems can result in detrimental effects such as fluctuating dipole force heating, inhomogeneous detunings, and inhibition of laser cooling, which limits the atomic species that can be manipulated. In particular, these light shifts can be large enough to prevent loading into optical tweezers directly from a magneto-optical trap. We implement a general solution to these limitations by loading, as well as cooling and imaging the atoms with temporally alternating beams, and present an analysis of the role of heating and required cooling for single atom tweezer loading. Because this technique does not depend on any specific spectral properties, it should enable the optical tweezer platform to be extended to nearly any atomic or molecular species that can be laser cooled and optically trapped.
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.