Abstract A novel laser cooling mechanism based on many-body effects is presented. The method can be applied to the cooling of a large class of atoms and molecules at a higher density than commonly excepted by existing methods. The cooling mechanism relies on the collective encounter of particles and light. Stochastic events between the particles and photons, as well as a collective effect, give rise to energy transfer between these media. Such a mechanism relies on multiple light–matter encounters, therefore requiring a sufficient particle density, ρ ∼ 10 14 cm −3 . This is an advantage for experiments where a high phase space density is required. A second tuning laser can be added, increasing the applicability to many types of atoms and molecules. This tuning laser changes the inter-particle potential by inducing an AC Stark effect. As a result, the required trapping density can be reduced to ρ ∼ 10 6 cm −3 . Simulations of phase space distributions were performed, comparing different particle densities, trap potentials and light field intensity profiles. The modelling shows efficient cooling rates up to 10 2 K s −1 for a dense ensemble of 87 Rb atoms, and cooling rates of up to 6 · 10 2 K s −1 when adding an additional tuning source.
Abstract A novel laser cooling mechanism based on many-body effects is presented. The method can be applied to the cooling of a large class of atoms and molecules at a higher density than commonly excepted by existing methods. The cooling mechanism relies on the collective encounter of particles and light. Stochastic events between the particles and photons, as well as a collective effect, give rise to energy transfer between these media. Such a mechanism relies on multiple light–matter encounters, therefore requiring a sufficient particle density, ρ ∼ 10 14 cm −3 . This is an advantage for experiments where a high phase space density is required. A second tuning laser can be added, increasing the applicability to many types of atoms and molecules. This tuning laser changes the inter-particle potential by inducing an AC Stark effect. As a result, the required trapping density can be reduced to ρ ∼ 10 6 cm −3 . Simulations of phase space distributions were performed, comparing different particle densities, trap potentials and light field intensity profiles. The modelling shows efficient cooling rates up to 10 2 K s −1 for a dense ensemble of 87 Rb atoms, and cooling rates of up to 6 · 10 2 K s −1 when adding an additional tuning source.
We present a simple and general optical cooling method based on 3D degenerate Raman sideband cooling with adiabatic release that goes significantly beyond the density and temperature limitations of optical molasses. In 10 ms we cool a sample of 3×108 cesium atoms to a temperature of 330 nK at a density of 1.1×1011 cm−3, which corresponds to a phase-space density nλdB3=1/500.We further propose to cool atoms or molecules inside an optical cavity that enhances the scattering of blue-detuned photons and show that the dissipative mechanism can be viewed as a cavity-induced generalized Doppler cooling. Since the cooling depends on the atom’s internal level structure only through the photon scattering rate, cavity Doppler cooling is applicable to particles that do not possess a closed optical transition, which may allow one to extend laser cooling to a greater class of atoms or molecules. Large samples are cooled at the same rate as single atoms if the effect of one atom on the cavity resonance frequency is small. We also show how to achieve 3D cooling with a single optical cavity.
We propose and demonstrate a novel laser cooling mechanism applicable to particles with narrow-linewidth optical transitions. By sweeping the frequency of counter-propagating laser beams in a sawtooth manner, we cause adiabatic transfer back and forth between the ground state and a long-lived optically excited state. The time-ordering of these adiabatic transfers is determined by Doppler shifts, which ensures that the associated photon recoils are in the opposite direction to the particle's motion. This ultimately leads to a robust cooling mechanism capable of exerting large forces via a weak transition and with reduced reliance on spontaneous emission. We present a simple intuitive model for the resulting frictional force, and directly demonstrate its efficacy for increasing the total phase-space density of an atomic ensemble. We rely on both simulation and experimental studies using the 7.5 kHz linewidth 1 S 0 to 3 P 1 transition in 88 Sr. The reduced reliance on spontaneous emission may allow this adiabatic sweep method to be a useful tool for cooling particles that lack closed cycling transitions, such as molecules.
We show that an ensemble of two-level atoms commonly coupled to a single, driven, damped high-Q cavity mode could be cooled and trapped via the correlated dynamics of cavity field and the atomic motion. For sufficiently large detuning between the atoms and the field, spontaneous emission plays no role in the dynamics, and the cooling scheme can be applied to all particles with a sufficient optical dipole moment. The time scale of this collective cooling process increases with the particle number and can be optimized by suitably tailoring the pump field amplitude and frequency. The key properties of the underlying mechanism are discussed using a linearized model, where one can derive a closed set of equations for certain ensemble averages and the field. The results are then compared with N-particle dynamical simulations of the full equations.
In this paper, we identify a quantum coherence with the ability to induce collective dynamics in an atomic gas, when a cooling laser is applied. When present, the mean phonon number per particle changes at a rate, which is proportional to the total number of particles in the system. We then propose to use this effect to cool atoms in a cyclic two-stage process to very low temperatures. During each displacement stage, the atoms move freely inside an asymmetric trapping potential, thereby accumulating away from its centre. During each cooling stage, a cooling laser is applied, which returns the atoms to the centre of the trap, while decreasing their mean phonon number. Alternating both stages is shown to transfer the atoms to a final temperature, which vanishes in the infinitely-many particle limit.
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.
Different from atoms, molecules have unique properties, and play an important role in the research of atomic, molecular and optical physics. Cold molecules have important applications in science and have been studied for more than 20 years. But traditional methods, such as the Stark decelerator, have hit a bottleneck: it is hard to increase the phase space density of molecules. Extending the direct laser-cooling technique to new molecular species has recently been a hot topic and also a big challenge. In this review paper, on one hand, we make a brief review to recent progresses on the direct laser cooling of polar molecules. On the other hand, a demonstration on the feasibility of laser cooling BaF molecule has been experimentally illustrated, including the analysis on the molecular energy levels, measurements of the high-resolution spectroscopy, efficient pre-cooling and state preparation via buffer-gas cooling and detailed investigations on the molcule-light interactions. All these results not only pave the way for future laser-cooling and -trapping experiments, but also serve as a reference for the laser-cooling explorations on new molecular species.