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A New Method for the Development of Frequency Standards for the Optical Wavelength Range of 243nm

E. K. Izrailov, В. Ф. Ежов

Published Oct 1, 2005• 0 citations
Publisher

Abstract

A new method and devices for cooling of atomic hydrogen up to the values less than 100 µK aimed for the development of the optical frequency standard with significantly improved parameters (Δν/ν£1 x 10-16) is discussed. The method exploits the unique properties of atomic hydrogen such as atomic hydrogen does not condense at temperatures as low as 20µK and can not be heated by IR radiation in the absence of atom-wall collisions. Therefore, the most efficient and well-known gas cooling technique can be employed, namely, the adiabatic expansion of the volume occupied by the gas. This approach is used in a cryogenic gas expansion machine. It is suggested to use the adiabatic expansion of the volume of the magnetic atomic trap containing atomic gaseous hydrogen for embodiment of this idea.

DOI
10.4028/www.scientific.net/kem.295-296.195

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A new method and devices for cooling of atomic hydrogen up to the values less than 100 µK aimed for the development of the optical frequency standard with significantly improved parameters (Δν/ν£1 x 10-16) is discussed. The method exploits the unique properties of atomic hydrogen such as atomic hydrogen does not condense at temperatures as low as 20µK and can not be heated by IR radiation in the absence of atom-wall collisions. Therefore, the most efficient and well-known gas cooling technique can be employed, namely, the adiabatic expansion of the volume occupied by the gas. This approach is used in a cryogenic gas expansion machine. It is suggested to use the adiabatic expansion of the volume of the magnetic atomic trap containing atomic gaseous hydrogen for embodiment of this idea.
A new method for cooling atomic hydrogen down to 100 µK or below is suggested. The method exploits the unique properties of atomic hydrogen, which are the following: atomic hydrogen does not condense at temperatures as low as 20 µK and cannot be heated by IR radiation in the absence of atom-wall collisions. Therefore, the most efficient and well-known gas cooling technique can be employed, namely, the adiabatic expansion of the volume occupied by the gas (this approach is used in a gas-expansion machine). It is suggested to adiabatically expand the volume of a magnetic trap containing atomic hydrogen.
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
A quantitative discussion of several interesting and attractive properties of magneto-optical traps is presented. Calculations are given for the trap depth, sample size, and intrinsic cooling capability of this especially simple example of a radiative force trap. The discussion is directed toward the cooling and confinement of metastable 23S neutral helium, and a new loading procedure suitable for this atom is proposed. It is planned to operate the trap and to cool atoms in it with 1.08-μm light from solid-state, single-mode lasers pumped by efficient diode lasers. Because of the dissipative nature of the radiative force, a trapped sample of atoms may be magnetically compressed without increasing its millikelvin optical temperature, so that a subsequent adiabatic expansion can cool it to the nanokelvin temperature range.
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.
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.