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Depolarisation cooling of an atomic cloud

S. Hensler, Alexander Greiner, J. Stühler, Tilman Pfau

Published May 13, 2005• 12 citations
OpenAlex

Abstract

We propose a cooling scheme based on depolarisation of a polarised cloud of trapped atoms. Similar to adiabatic demagnetisation, we suggest to use the coupling between the internal spin reservoir of the cloud and the external kinetic reservoir via dipolar relaxation to reduce the temperature of the cloud. By optical pumping one can cool the spin reservoir and force the cooling process. In case of a trapped gas of dipolar chromium atoms, we show that this cooling technique can be performed continuously and used to approach the critical phase space density for BEC

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We propose a cooling scheme based on depolarisation of a polarised cloud of trapped atoms. Similar to adiabatic demagnetisation, we suggest to use the coupling between the internal spin reservoir of the cloud and the external kinetic reservoir via dipolar relaxation to reduce the temperature of the cloud. By optical pumping one can cool the spin reservoir and force the cooling process. In case of a trapped gas of dipolar chromium atoms, we show that this cooling technique can be performed continuously and used to approach the critical phase space density for BEC.

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We have studied a general technique for laser cooling a cloud of polarized trapped atoms down to the Doppler temperature. A one-dimensional optical molasses created with polarized light cools the axial motional degree of freedom of the atoms in the trap. Cooling of the radial degrees of freedom can be modeled by reabsorption of scattered photons in the optically dense cloud. We present experimental results for a cloud of chromium atoms in a magnetic trap. A simple model based on rate equations shows quantitative agreement with the experimental results. This scheme allows us to readily prepare a dense cloud of atoms in a magnetic trap with good starting conditions for evaporative cooling.

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We propose a cooling scheme based on depolarisation of a polarised cloud of trapped atoms. Similar to adiabatic demagnetisation, we suggest to use the coupling between the internal spin reservoir of the cloud and the external kinetic reservoir via dipolar relaxation to reduce the temperature of the cloud. By optical pumping one can cool the spin reservoir and force the cooling process. In case of a trapped gas of dipolar chromium atoms, we show that this cooling technique can be performed continuously and used to approach the critical phase space density for BEC
We propose a cooling scheme based on depolarisation of a polarised cloud of trapped atoms. Similar to adiabatic demagnetisation, we suggest to use the coupling between the internal spin reservoir of the cloud and the external kinetic reservoir via dipolar relaxation to reduce the temperature of the cloud. By optical pumping one can cool the spin reservoir and force the cooling process. In case of a trapped gas of dipolar chromium atoms, we show that this cooling technique can be performed continuously and used to approach the critical phase space density for BEC.
Demagnetization cooling relies on spin-orbit coupling that brings motional and spin degrees of freedom into thermal equilibrium. In the case of a gas, one has the advantage that the spin degree of freedom can be cooled very efficiently using optical pumping. We investigate demagnetization cooling of a chromium gas in a deep optical dipole trap over a large temperature range and reach high densities up to $5\ifmmode\times\else\texttimes\fi{}{10}^{19}$ m${}^{\ensuremath{-}3}$. We study the loss mechanism under such extreme conditions and identify excited-state collisions as the main limiting process. We discuss that in some systems demagnetization cooling has a realistic potential of reaching degeneracy by optical cooling only.
Demagnetization cooling utilizes dipolar relaxations that couple the internal degree of freedom (spin) to the external (angular momentum) in order to cool an atomic cloud efficiently. Optical pumping into a dark state constantly recycles the atoms that were thermally excited to higher spin states. The net energy taken away by a single photon is very favorable since the lost energy per atom is the Zeeman energy rather than the recoil energy. As the density of the atomic sample rises the presence of the photons leads to limiting processes. In our previous publication [Volchkov et al. (2014)] we have shown that light-assisted collisions are such an important limiting process. In this paper we suppress light-assisted collisions by detuning the optical pumping light such that the Condon point coincides with the first node of the ground state wave function of two colliding atoms. This leads to an increased cooling efficiency χ ≥ 17 as well as to increased maximum densities of n ≈ 1 · 10(20) m(-3). However, due to the high number of involved molecular states the net suppression is not strong enough to reach quantum degeneracy.
We discuss two experiments that can be performed using a cloud of laser-cooled and trapped atoms, namely Bose-Einstein condensation (BEC) and search for a permanent Electric Dipole Moment (EDM). BEC can be observed in Rb atoms in a magnetic trap by using forced evaporative cooling to continuously lower the temperature below the condensation limit. The cloud is cooled by preferentially ejecting the hottest atoms from a magnetic trap. The magnetic trap is loaded with laser-cooled atoms from a magneto-optic trap. The EDM experiment can be performed with a laser-cooled cloud of Yb atoms. The atoms are spin polarized and the precession of the spin is measured in the presence of a strong electric field applied perpendicular to the spin direction. The use of laser-cooled atoms should greatly enhance the sensitivity of the experiment.
We study the spin distillation of spinor gases of bosonic atoms and find two different mechanisms in [Formula: see text]Cr and [Formula: see text]Na atoms, both of which can cool effectively. The first mechanism involves dipolar scattering into initially unoccupied spin states and cools only above a threshold magnetic field. The second proceeds via equilibrium relaxation of the thermal cloud into empty spin states, reducing its proportion in the initial component. It cools only below a threshold magnetic field. The technique was initially demonstrated experimentally for a chromium dipolar gas (Naylor et al. in Phys Rev Lett 115:243002, 2015), whereas here we develop the concept further and provide an in-depth understanding of the required physics and limitations involved. Through numerical simulations, we reveal the mechanisms involved and demonstrate that the spin distillation cycle can be repeated several times, each time resulting in a significant additional reduction of the thermal atom fraction. Threshold values of magnetic field and predictions for the achievable temperature are also identified.
We have studied a general technique for laser cooling a cloud of polarized trapped atoms down to the Doppler temperature. A one-dimensional optical molasses created with polarized light cools the axial motional degree of freedom of the atoms in the trap. Cooling of the radial degrees of freedom can be modeled by reabsorption of scattered photons in the optically dense cloud. We present experimental results for a cloud of chromium atoms in a magnetic trap. A simple model based on rate equations shows quantitative agreement with the experimental results. This scheme allows us to readily prepare a dense cloud of atoms in a magnetic trap with good starting conditions for evaporative cooling.