Primary vacuometer based on an ultracold gas in a shallow optical dipole trap

Abstract

A calibration-free primary vacuometer based on an ultracold atomic gas in a shallow far-off-resonance optical dipole trap is proposed and demonstrated. The pressure is obtained by measuring the loss of trapped atoms which is caused by collisions with the ambient gas of the vacuum chamber. The loss is related to the ambient-gas pressure via a theoretical model based on first principles. The model is applicable owing to elimination of a number of systematic effects which otherwise preclude or complicate construction of a first-principle model. These systematics include loss unrelated to collisions with the ambient gas as well as loss dependance on the number and energy of trapped atoms. In the demonstrated vacuometer, the atom-number decay is exponential with the rate proportional to the pressure, where the proportionality coefficient is expressed via the gas composition and van der Waals coefficients C6. Whenever the gas composition is unknown, the systematic error is typically well below that of the hot-cathode ionization gauge. The vacuometer is implemented using a gas of ultracold lithium-6, which is the optimal working body for such a vacuometer. The lowest measured pressure, Pa, is limited by the vacuum in the apparatus, while the dominant error source of 4% is due to uncertainty in the C6 value and may be improved. Comparison with reading of a hot-cathode ionization gauge is also shown.