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International Laser Science Conference

Improved collection powers of an optical Maxwell demon

Alan L. Migdall, H. Metcalf, William D. Phillips, P. L. Gould

Published Jan 1, 1986• 0 citations
Publisher

Abstract

We present an improvement of our previously proposed optical Maxwell demon1 that captures and cools Na atoms in a gas cell. The intersection of three pairs of orthogonal counterpropagating light beams tuned below resonance provides strong viscous forces on atoms.2 In a vapor cell, it is the ability of atoms to enter this intersection from the outside that allows our demon to collect a high density of cold atoms. The original scheme suffers a reduced collection rate because the 3-D damping region is largely surrounded by a region of 1-D damping produced by only a single pair of beams. This deficiency can be removed by several methods, one of which involves optical pumping. Sodium requires two frequencies to prevent optical pumping and maintain the damping force. If two pairs of beams contain one frequency and the third pair contains the other frequency, the effect of the 1-D damping is reduced while the region of 3-D damping is largely unaffected. Thus, an atom can enter the intersection undeflected by a region containing only a single pair of beams. We present computer analysis and experimental progress.

DOI
10.1364/ils.1986.wf5

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arXiv:0802.1585•Physical Review Letters•Jun 20, 2008•93 citations
We present an improvement of our previously proposed optical Maxwell demon1 that captures and cools Na atoms in a gas cell. The intersection of three pairs of orthogonal counterpropagating light beams tuned below resonance provides strong viscous forces on atoms.2 In a vapor cell, it is the ability of atoms to enter this intersection from the outside that allows our demon to collect a high density of cold atoms. The original scheme suffers a reduced collection rate because the 3-D damping region is largely surrounded by a region of 1-D damping produced by only a single pair of beams. This deficiency can be removed by several methods, one of which involves optical pumping. Sodium requires two frequencies to prevent optical pumping and maintain the damping force. If two pairs of beams contain one frequency and the third pair contains the other frequency, the effect of the 1-D damping is reduced while the region of 3-D damping is largely unaffected. Thus, an atom can enter the intersection undeflected by a region containing only a single pair of beams. We present computer analysis and experimental progress.
We propose an optical Maxwell demon which efficiently captures, concentrates, and damps the kinetic motion of sodium atoms in a gas cell. A bath of radiation tuned just below resonance can prove to be a very viscous medium for atoms. We propose to use this effect to produce and concentrate very slow atoms in a vapor cell. Our bath of radiation consists of three orthogonally intersecting pairs of counterpropagating laser beams. Each beam is tuned about half of the natural linewidth below the resonance transition frequency. (Because of the hyperfine splitting of the Na ground state, two separate frequencies are used.) Atoms which enter the intersection region with velocities less than ~15 m/s (for 1-cm diam beams) are quickly damped to ~0.5 m/s. Thereafter, the velocity is randomized in a characteristic time of ~10 μs. The atoms will then execute diffusive motion in this photon bath, escaping only after many seconds. Since the effusion time for atoms to enter the intersecting beam demon is much shorter than the diffusion time out of it, the demon acts to collect and concentrate the slow atoms into a cold, dense, central gas superposed on the hot, rarified, background gas.
We implemented an optical one-way potential barrier that allows ultracold 87Rb^{87}\text{R}\text{b}87Rb atoms to transmit through when incident on one side of the barrier but reflect from the other. This asymmetric barrier is a realization of Maxwell's demon, which can be employed to produce phase-space compression and has implications for cooling atoms and molecules not amenable to standard laser-cooling techniques. The barrier comprises two focused Gaussian laser beams that intersect the focus of a far-off-resonant single-beam optical dipole trap that holds the atoms. The main barrier beam presents a state-dependent potential to incident atoms, while the repumping barrier beam optically pumps atoms to a trapped state. We investigated the robustness of the barrier asymmetry to changes in the barrier-beam separation, the initial atomic potential energy, the intensity of the second beam, and the detuning of the first beam. We performed simulations of the atomic dynamics in the presence of the barrier, showing that the initial three-dimensional momentum distribution plays a significant role, and that light-assisted collisions are likely the dominant loss mechanism. We also carefully examined the relationship to Maxwell's demon and explicitly accounted for the apparent decrease in entropy for our particular system.
The general features of optical molasses,1 continuously loaded with slow sodium atoms, are described. Three orthogonal pairs of counterpropagating laser beams, tuned slightly below resonance, capture and viscously confine the slow atoms which are produced continuously by a Zeeman tuned laser-cooled atomic beam.2 The continuous loading of this molasses region is further enhanced by using optical depumping techniques (optically pumping the atoms out of the cooling cycle with another laser) to extract more slow atoms from the beam than are otherwise available. Results from simple analytic theories and Monte Carlo simulations of the molasses are compared to the experimental findings. In particular, we look at the molasses lifetime as a function of beam imbalance and laser frequency. Although some features of the molasses can be explained (in particular at low intensity) many discrepancies are noted. One most obvious of these is the relative insensitivity of the lifetime to intensity imbalance, the discrepancy being at least an order of magnitude in added rate.
Summary form only given. Atoms without ground state splitting, e.g. alkaline earths, are of special interest for atom interferometry and frequency standards due to their small sensitivity to external fields and for BEC and the study of cold collisions due to the simplified theoretical interpretation of their interaction. We demonstrate Maxwell's-demon cooling, a novel method which allows to further cool alkaline earth atoms in one dimension. It consists of three consecutive steps which are repeated several times. First, from a cloud of cooled atoms released from a standard magneto-optical trap a small part of the velocity distribution is selected by excitation on a narrow transition (intercombination transition). Second, these atoms are optically pumped to another long lived state. Third, the ground state velocity distribution is restored by turning on the optical molasses on the strong transition used for the magneto-optical trapping. In addition to Maxwell's thought experiment, where a demon separates the cold atoms from the hot ones, in the novel scheme the hot atoms are furthermore cooled again to the original 3 mK. By repeating these steps slow atoms are accumulated in the excited state. The velocity distribution of the atoms is measured after the cold atoms are decayed to the ground state by recording the Doppler broadening of the intercombination transition.
We investigate the loading of a two-color vapor-cell magneto-optical trap for sodium atoms as a function of several operational parameters of the trap. We use two laser frequencies acting in different spatial regions to obtain either enhancement or inhibition of the capture mechanism. We demonstrate that the best enhancement is achieved when the trap operates on a combination of atomic transitions, known as type I tuning, by using a second laser beam to capture atoms through a different combination of transitions (type II tuning).
We demonstrate an asymmetric optical potential barrier for ultracold 87Rb atoms using laser light tuned near the D2 optical transition. Such a one-way barrier, where atoms incident on one side are transmitted but reflected from the other, is a realization of Maxwell's demon and has important implications for cooling atoms and molecules not amenable to standard laser-cooling techniques. In our experiment, atoms are confined to a far-detuned dipole trap consisting of a single focused Gaussian beam, which is divided near the focus by the barrier. The one-way barrier consists of two focused laser beams oriented almost normal to the dipole-trap axis. The first beam is tuned to present either a potential well or barrier, depending on the state of the incident atoms. On the reflecting side of the barrier, the second beam optically pumps the atoms to the reflecting (barrier) state, thus producing the asymmetry.