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Journal of the Optical Society of America B

Optical cooling of atomic hydrogen in a magnetic trap

Tom W. Hijmans, O.J. Luiten, Irwan D. Setija, Jook T. M. Walraven

Published Nov 1, 1989• 52 citations
Publisher

Abstract

We describe the prospects for optical cooling of magnetically trapped atomic hydrogen. We analyze the performance of an optical system currently under development in our laboratory and present calculations for the optical cooling rate. We conclude that by using optical techniques hydrogen can be cooled to below 10 mK while the density is simultaneously boosted to approximately 1014 cm−3. The same system can be used for thermometry down to temperatures well into the microkelvin regime.

DOI
10.1364/josab.6.002235

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We describe the prospects for optical cooling of magnetically trapped atomic hydrogen. We analyze the performance of an optical system currently under development in our laboratory and present calculations for the optical cooling rate. We conclude that by using optical techniques hydrogen can be cooled to below 10 mK while the density is simultaneously boosted to approximately 1014 cm−3. The same system can be used for thermometry down to temperatures well into the microkelvin regime.
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.
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.
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.
We consider the prospect of using ultracold hydrogen atoms for sympathetic cooling of fluorine atoms to microkelvin temperatures. We carry out quantum-mechanical calculations on collisions between cold F and H atoms in magnetically trappable states and show that the ratio of elastic to inelastic cross sections remains high across a wide range of temperatures and magnetic fields. For F atoms initially in the spin-stretched state (2P3/2{}^{2}{P}_{3/2}2P3/2​, f=mf=+2f={m}_{f}=+2f=mf​=+2), sympathetic cooling appears likely to succeed from starting temperatures around 1 K or even higher. This occurs because inelastic collisions are suppressed by ppp-wave and ddd-wave barriers that are 600 mK and 3.2 K high, respectively. In combination with recent results on H+NH\mathrm{H}+\mathrm{NH}H+NH and H+OH\mathrm{H}+\mathrm{OH}H+OH collisions [M. L. Gonz\'alez-Mart\'{\i}nez and J. M. Hutson, Phys. Rev. Lett. 111, 203004 (2013)], this establishes ultracold H atoms as a very promising and versatile coolant for atoms and molecules that cannot be laser-cooled.
Because of its simple structure, the hydrogen atom is often used as a testbed for quantum electrodynamics. Spectroscopy of trapped atomic samples can greatly improve the accuracy of these tests. Trapping atomic hydrogen in an optical dipole trap or an optical lattice has never been achieved. Only trapping in magnetic fields that lead to large Zeeman shifts has been demonstrated. Standard techniques of atomic physics are difficult to apply to atomic hydrogen. The small mass of the atom and the large photon energy of the 1-2 cooling transition significantly complicate Doppler cooling. This proposal introduces a photon recoil-assisted loading scheme that uses these properties to our advantage to load atomic hydrogen into an optical dipole trap without laser cooling. The magic wavelength (515 nm) for the 1-2 clock transition (1.3-Hz natural linewidth) is easily accessible with current laser technology. Since the 1-2 clock transition can be driven Doppler free, we do not require a very low temperature. Besides improving spectroscopy for fundamental science, such a system can also be used as a “computable” atomic clock that may one day justify the redefinition of the SI second in terms of the Rydberg constant.