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Physical Review A

Stochastic cooling of atoms using lasers

M. G. Raizen, James Kevin Koga, Bala Sundaram, Yasuaki Kishimoto, Hiroshi Takuma, T. Tajima

Published Dec 1, 1998• 58 citations
Publisher

Abstract

We propose a method to laser-cool atoms based on stochastic cooling, first developed at CERN to cool antiprotons. Fluctuations in the momentum distribution will be detected in a pump-probe configuration with far-detuned lasers, and the appropriate correction kick will be accomplished with optical dipole potentials. Each stage of an iterative cooling process will involve measurement and feedback, with phase space remixing in between. We discuss possible applications of this method to magnetically trapped atoms and molecules.

DOI
10.1103/physreva.58.4757

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We propose a method to laser-cool atoms based on stochastic cooling, first developed at CERN to cool antiprotons. Fluctuations in the momentum distribution will be detected in a pump-probe configuration with far-detuned lasers, and the appropriate correction kick will be accomplished with optical dipole potentials. Each stage of an iterative cooling process will involve measurement and feedback, with phase space remixing in between. We discuss possible applications of this method to magnetically trapped atoms and molecules.
Summary form only given. Atoms in an optical lattice, though very slow, perform a slight random motion that reflects the non-zero temperature of the trapped atoms. We are proposing a way of reducing such random motion using real-time feedback. The method is related to the stochastic cooling used to transversely cool particle beams. Another stochastic cooling method for laser-cooled atoms has been proposed in "Stochastic cooling of atoms using lasers," see Phys. Rev. A, vol. 58, p. 4757 (1998). Our scheme involves a measurement of the coherent redistribution of photons between different laser beams of the optical lattice.
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.
The development of an experimental apparatus to produce Bose-Einstein condensates (BECs) of ⁸⁷Rb atoms and their application to magnetometry are discussed. Optical detection of atomic Larmor precession is a widely explored method for high-sensitivity measurements of magnetic fields. In this context, short laser/atom interaction time, atomic thermal diffusion and decoherence effects are among the main limitations. In this thesis, we overcome such problems by using spin-polarised ⁸⁷Rb ultra-cold atoms as the sensing element. After the atoms are polarised, a resonant pulse of radio-frequency excites Larmor precession, which is sensitive to external magnetic fields. By measuring the perturbations of the radio-frequency induced spin precession, information on the magnetic fields of interest. This is achieved by monitoring the polarisation plane’s rotation of a linearly polarised resonant laser probe. In the first part of this thesis, the building and optimisation of a laser-cooling set up to obtain a BEC in a hybrid trap is reported. In order to achieve the Phase Space Density (PSD) required for BEC, several different stages of trapping and cooling are necessary. Each phase has been implemented and optimised. The first step consists in the magneto-optical trap (MOT). Here a velocity dependent damping force and a spatially dependent confining force give the largest changes in PSD. Then atoms are loaded into a hybrid trap obtained by overlapping a quadrupole magnetic potential and a far detuned optical crossed dipole trap. The final stage for the condensation consists of forced evaporative cooling, both via magnetic and optical evaporation. In the second part of the thesis, a general overview of the principles of optical atomic magnetometry is provided and the advantages of using ultra-cold atoms with respect to conventional thermal vapours are discussed. The implementation, operation and a preliminary characterisation of the ultra-cold atom magnetometer are described along with the preliminary results collected. Finally, a plan for future improvements of its sensitivity is presented.
This thesis examines two complimentary methods for the laser cooling of a magnetically guided ultra-cold atom beam. If combined, these methods could serve as a starting point for high-through put and possibly even continuous production of Bose-Einstein condensates. First, a mechanism is outlined to harvest ultra cold atoms from a magnetically guided atom beam into an optical dipole trap. A continuous loading scheme is described that dissipates the directed kinetic energy of a captured atom via deceleration by a magnetic potential barrier followed by optical pumping to the energetically lowest Zeeman sublevel. The application of this scheme to the transfer of ultra cold chromium atoms from a magnetically guided atom beam into a deep optical dipole trap is investigated via numerical simulations of the loading process. Based on the results of the theoretical studies the feasibility and the efficiency of our loading scheme, including the realisation of a suitable magnetic field configuration, are analysed. Second, experiments were conducted on the transverse laser cooling of a magnetically guided beam of ultra cold chromium atoms. Radial compression by a tapering of the guide is employed to adiabatically heat the beam. Inside the tapered section heat is extracted from the atom beam by a two-dimensional optical molasses perpendicular to it, resulting in a significant increase of atomic phase space density. A magnetic offset field is applied to prevent optical pumping to untrapped states. Our results demonstrate that by a suitable choice of the magnetic offset field, the cooling beam intensity and detuning, atom losses and longitudinal heating can be avoided. Final temperatures below 65 µK have been achieved, corresponding to an increase of phase space density in the guided beam by more than a factor of 30.
The interaction of laser radiation with matter is widely used to cool atomic and molecular gases to extremely low temperatures. In connection with this, a considerable number of works on quantum optics have recently been devoted to laser cooling of matter. The principle of Doppler cooling for neutral atoms was proposed by T. Nanch and A. Shavlov, and for trapped ions by D. Wineland and H. Dehmelt. The chapter shows the scheme of the principle of Doppler cooling. The atomic beam is slowed by transmission of a pulse that occurs at the time when the atom absorbs a quantum of light. When laser cooling of atoms in a certain volume, a problem arises with optical pumping. The scheme of cooling an atomic beam by laser radiation in a magnetic field, called the Zeeman slowing, is most often used to obtain slow atomic beams.
We propose to use the magnetic-dipole transition at coupling two ground state fine-structure components of thulium (Tm), as a 'clock transition' in optical clocks. We have demonstrated first stage laser cooling of Tm atoms down to a temperature of 25 K using a strong transition at and we have also shown preliminary results for second stage cooling using a weaker transition at (natural linewidth γ G = 350 kHz). Laser cooled atoms have been trapped in an optical dipole trap and in a 1D optical lattice operating near 532 nm.