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Physical Review Letters

Optical cooling of atomic hydrogen in a magnetic trap

Irwan D. Setija, H. G. C. Werij, O.J. Luiten, M. W. Reynolds, Tom W. Hijmans, Jook T. M. Walraven

Published Apr 12, 1993• 154 citations
Publisher

Abstract

We present the first experimental demonstration of optical cooling of atomic hydrogen. Two methods are discussed: Doppler cooling, for which we report a minimum achieved temperature of 8 mK, and light-induced evaporation, a new cooling method by which we reached 3 mK.
DOI
10.1103/physrevlett.70.2257

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Atomic hydrogen is an important model system for the investigation of the properties of ultra‐cold gases. We summarize the current experimental situation for magnetically confined hydrogen, putting emphasis on recent optical experiments with Lyman‐α radiation. As the research is aimed at gas‐phase properties, the samples are studied at densities where interatomic collisions are frequent and internal thermal equilibrium may be assumed. Interestingly, although optically thick, the samples still may be cooled with light. We discuss both Doppler cooling and light induced evaporative cooling.
A gas of hydrogen atoms, confined in a static magnetic trap, has been evaporatively cooled to temperatures of a few millikelvin. The initial trap configuration held the gas at 38 mK for as long as 5 h. Evaporative cooling reduced the temperature to 3.0 mK while maintaining the central density at 7.6\ifmmode\times\else\texttimes\fi{}1012{10}^{12}1012 ${\mathrm{cm}}^{\ensuremath{-}3}$. These values were determined by measurement of the rate of electronic spin relaxation and are in agreement with model calculations. Further cooling to 1 mK (inferred from the model) has been achieved. Measurements were made of the efficiency of the evaporative cooling process.
This paper presents an experimental demonstration of light-induced evaporative cooling in a magneto-optical trap. An additional laser is used to interact with atoms at the edge of the atomic cloud in the trap. These atoms get an additional force and evaporated away from the trap by both the magnetic field and laser fields. The remaining atoms have lower kinetic energy and thus are cooled. It reports the measurements on the temperature and atomic number after the evaporative cooling with different parameters including the distance between the laser and the centre of the atomic cloud, the detuning, the intensity. The results show that the light-induced evaporative cooling is a way to generate an ultra-cold atom source.
Atomic hydrogen has been loaded into a 1-K deep magnetic trap through the use of a dilution refrigerator. Evaporative cooling has been used to cool the trapped hydrogen into the submillikelvin temperature regime. These techniques have produced a sample with density of 8 × 1013 cm-3 at a temperature of 100 μK. At this density a transition to a Bose-Einstein condensate is predicted to occur at a temperature of 30 μK. These results have been obtained with a magnetic trap that has been constructed to accommodate upcoming experiments based on laser spectroscopy of the 1S-2S two photon transition in the trapped hydrogen. A frequency doubled narrow linewidth cw dye laser system has been constructed to excite the 1S-2S transition. The excitation will be detected by the observation of electric field-induced Lyman-alpha fluorescence. The optical detection system should yield improved sample density and temperature diagnostic capabilities that will facilitate a closer approach to the quantum degenerate regime. Additionally the cold, confined hydrogen gas offers the possibility of improving the level of resolution of Doppler-free hydrogen 1S-2S laser spectroscopy by several orders of magnitude.
While cold molecule experiments are rapidly moving towards their promised benefits of precision spectroscopy, controllable chemistry, and novel condensed phases, heretofore the field has been greatly limited by a lack of methods to cool and compress chemically diverse species to temperatures below ten millikelvin. While in atomic physics these needs are fulfilled by laser cooling, magneto-optical trapping, and evaporative cooling, until now none of these techniques have been applicable to molecules. In this thesis, two major breakthroughs are reported. The first is the observation of evaporative cooling in magnetically trapped hydroxyl (OH) radicals, which potentially opens a path all the way to Bose-Einstein condensation of dipolar radicals, as well as allowing cold- and ultracold-chemistry studies of fundamental reaction mechanisms. Through the combination of an extremely high gradient magnetic quadrupole trap and the use of the OH Λ-doublet transition to enable highly selective forced evaporation, cooling by an order of magnitude in temperature was achieved and yielded a final temperature no higher than 5mK. The second breakthrough is the successful application of laser cooling and magneto-optical trapping to molecules. Motivated by a proposal in this thesis, laser cooling of molecules is now known to be technically feasible in a select but substantial pool of diatomic molecules. The demonstration of not only Doppler cooling but also two-dimensional magneto-optical trapping in yttrium (II) oxide, YO, is expected to enable rapid growth in the availability of ultracold molecules--just as the invention of the atomic magneto-optical trap stimulated atomic physics twenty-five years ago.