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Canadian Journal of Physics

A new path to ultracold hydrogen

Robert deCarvalho, Nathan Brahms, Bonna K. Newman, John M. Doyle, Daniel Kleppner, Thomas J. Greytak

Published Apr 1, 2005• 13 citations
Publisher

Abstract

Ultracold hydrogen offers unique possibilities for precision spectroscopy, studies of atomic interactions, and the creation of quantum fluids containing mixtures of hydrogen and deuterium. Current techniques for trapping and cooling hydrogen have produced large condensates with N ∼ 109 atoms, but suffer from a variety of experimental limitations. Among these are the slow evaporative cooling rate due to the small H–H elastic-scattering cross section, the need for a superfluid helium film in the initial thermalization process, a geometry that severely limits detection efficiency, and the inability to trap deuterium. We are constructing a new apparatus based on buffer-gas cooling that will overcome these problems. To accelerate evaporative cooling, the thermalization rate is increased by simultaneously loading lithium and hydrogen into a 4.2 T anti-Helmholtz trap. Lithium accelerates evaporative cooling because the Li–H elastic-scattering cross section is ∼1200 times larger than that of H–H. Hydrogen and lithium will be produced by laser ablation of solid LiH in a 3He buffer gas held at temperatures of ∼350 mK. Because no wall collisions are needed for initial thermalization, ablating a solid sample of LiD should enable studies of deuterium. The apparatus is in the final stages of construction. We report on results of initial tests and discuss the new scientific opportunities made possible by this approach.PACS No.: 32.80.Pj

DOI
10.1139/p05-013

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Ultracold hydrogen offers unique possibilities for precision spectroscopy, studies of atomic interactions, and the creation of quantum fluids containing mixtures of hydrogen and deuterium. Current techniques for trapping and cooling hydrogen have produced large condensates with N ∼ 109 atoms, but suffer from a variety of experimental limitations. Among these are the slow evaporative cooling rate due to the small H–H elastic-scattering cross section, the need for a superfluid helium film in the initial thermalization process, a geometry that severely limits detection efficiency, and the inability to trap deuterium. We are constructing a new apparatus based on buffer-gas cooling that will overcome these problems. To accelerate evaporative cooling, the thermalization rate is increased by simultaneously loading lithium and hydrogen into a 4.2 T anti-Helmholtz trap. Lithium accelerates evaporative cooling because the Li–H elastic-scattering cross section is ∼1200 times larger than that of H–H. Hydrogen and lithium will be produced by laser ablation of solid LiH in a 3He buffer gas held at temperatures of ∼350 mK. Because no wall collisions are needed for initial thermalization, ablating a solid sample of LiD should enable studies of deuterium. The apparatus is in the final stages of construction. We report on results of initial tests and discuss the new scientific opportunities made possible by this approach.PACS No.: 32.80.Pj
Experimental efforts to produce a Bose Einstein condensed (BEC) gas of spin-polarized atomic hydrogen have been intensely pursued since 1980. Initial efforts studied hydrogen in contact with superfluid helium surfaces. In order to avoid the limiting wall recombination a static magnetic trap was developed which utilizes magnetic barriers to isolate atoms from the van der Wals walls. In this approach traps can be filled to about 10{sup 14} atoms/cm{sup 3} for which the critical temperature is in the region of 20-30 {mu}K. However, approaching BEC conditions, a new instability dominates the behavior. The authors have developed a new type of electromagnetic trap, a microwave (mw) trap in which the trapped atoms are in the ground spin-state so that at low temperature relaxation to a barrier state is strongly suppressed and BEC should be attainable. The shallow mw trap has recently been demonstrated experimentally for the cesium atom. Because the mw trap is so shallow it is necessary to precool the atoms for loading. The authors are constructing a hybrid trap in which the atoms are cooled in a static trap and then transferred to a mw trap where they can be further evaporatively cooled in an effort to achieve BEC.more » This trap should have density limits in the region of 10{sup 16} to 10{sup 17}/cm{sup 3} due to three-body recombination. A second, promising approach is in a 2-D geometry for which in finite sized systems BEC is allowed. Hydrogen atoms are confined to a superfluid helium surface by the absorption potential to form a 2-D gas. Although recombination is an important process on helium surfaces, recently the authors have shown that only a very small fraction of the recombination energy is coupled into the surface. With an appropriate geometry the recombination energy can be removed from the sample region to suppress heating to a negligible level. Furthermore in 2-D a magnetic field can have a maximum so that atoms can be confined to a very small area.« less
We consider the possibility of enhanced cooling of hydrogen atoms by a buffer gas of alkali-metal atoms Na, K, Rb, and Cs. Ultracold elastic collision cross sections for the Na-H and Rb-H purely spin-polarized pairs are found to be 640 and 860 times larger than that for the H-H pair, respectively. From an analysis of the techniques of production of ultracold sodium and rubidium samples, it seems feasible that the critical conditions for Bose-Einstein condensation of hydrogen could be achieved already at the stage of optical cooling of the sodium or rubidium buffer gas.
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.
Laser cooling and atom trapping techniques have enabled atomic physicists to attain temperatures on the order of 100 nK. In this regime, a trapped gas must be described within the framework of quantum statistical mechanics. In 1995, Bose-Einstein condensation (BEC) was achieved for the first time in such a system by three independent groups. The success of attaining BEC has encouraged research in quantum degenerate Fermi gases (DFG). One of the most exciting opportunities for a DFG is the prospect for Cooper pairing in a two-component gas. The experimental techniques for obtaining degenerate gases, as well as the mechanism for Cooper pairing, hinges on understanding and characterizing ultracold collisional interactions.\nThis thesis is a twofold study in this exciting field. The first section is an in-depth treatment of collisions in atomic lithium. Lithium has two isotopes, 6Li which is a fermion and 7Li which is a boson. This makes it an ideal candidate for studying quantum degenerate gases. In a series of experiments using a magneto-optical trap, we have spectroscopically measured the ground and excited state molecular potentials for both isotopes. From this work we have been able to obtain the most accurate potentials for lithium to date. These precise measurements have enabled us to extract the most accurate value to date for the 2P atomic radiative lifetime of lithium. Furthermore, we have developed a coupled-channel treatment of two-body collisions which has enabled us to calculate the scattering amplitude, elastic cross section, and rate of spin-exchange decay for any ground state collision in lithium as a function of temperature and magnetic field.\nArmed with this knowledge, we have made progress on the design and construction of an electromagnetic trap for lithium. This trap will enable the study of a degenerate Fermi gas of 6Li as well as the possibility of observing a Cooper pairing transition. The trap performance has been characterized for each isotope and current work is focussed on cooling lithium down to degeneracy. Measurements of spin-exchange decay have been made and compare well with the collisional theory that has been developed.
A cold atomic gas formed by two different species represents an intriguing system for a deeper understanding of atom-atom interactions at ultralow temperatures. We present experiments on a mixture of atomic lithium and cesium which are of particular interest regarding the formation of heteronuclear molecules on the one hand, and the prospects for sympathetic cooling of atomic gases through mutual thermalization on the other hand. A first series of experiments on interaction in presence of a near-resonant light field is performed in a two-species magneto-optical trap. The collisional properties of the lithium-cesium mixture are investigated through detailed analysis of trap-loss processes induced by the trap light. Photoassociation in an additional near-resonant laser field yields high-resolution spectra of the excited Cs_2 dimers, but shows no unambiguous indication of LiCs molecule formation. A second series of experiments on pure ground-state collisional properties utilizes an optical dipole trap formed by light that is detuned extremely far below atomic resonance (quasi-electrostatic trap). Storage times of many minutes are achieved in a particularly simple and versatile setup for both atomic species. Cooling of cesium through evaporation and thermalization by elastic collisions is observed. The evolution of temperature and particle number is compared with model simulations of evaporative cooling. Direct laser cooling of trapped cesium in the absolute energetic ground state is demonstrated. Homonuclear spin-changing collisions of ground-state cesium and lithium atoms are analyzed, and first evidence for pure ground-state collisions between atoms of different species is found. Based on the current achievements, prospects for future experiments are discussed. (orig.)
We propose experiments with atomic hydrogen gas at ultra-low temperatures T < 100 μ K when the thermal energy of atoms is comparable with the changes of their potential energy in the Earth gravity field. At these conditions we suggest implementing a gravitational spectroscopy for studies of quantum properties of ultra-cold atomic hydrogen and its interactions with matter and gravity, similar to experiments with ultra-cold neutrons (Nesvizhevsky et al. Nature 415 , 297 2002 ). A magnetic trap used for reaching the Bose-Einstein Condensation (Fried et al. Phys. Rev. Lett. 81 , 3811 1998 ) can be used for cooling a large number of H atoms below 1 mK. Evaporative cooling over the trap barrier allows effective cooling of the vertical degree of freedom of the trapped atoms. Releasing these ultra-slow atoms from the trap onto the cold surface of superfluid helium will allow studies of quantum bounces and stationary gravitational states of H atoms in the potential well created by this surface and the field of Earth gravity. Experimental study of properties of gravitational quantum states of hydrogen and quantum reflection of ultracold hydrogen from surface would be of major importance for designing similar experiments with antihydrogen, which are currently prepared in CERN.