We present a comprehensive mathematical model and experimental measurements for the evolution of a trapped-particle ensemble driven by collisions with a room-temperature background vapor. The model accommodates any trap geometry, confining potential, initial trapped distribution, and other experimental details; it only depends on the probability distribution function
Pt(E) for the collision-induced energy transfer to the trapped ensemble. We describe how to find
Pt(E) using quantum scattering calculations and how it can be approximated using quantum diffractive universality. We then compare our model to experimental measurements of a
87Rb ensemble energy evolution exposed to a room-temperature background gas of Ar by means of a single parameter fit for the total collision rate $\mathrm{\ensuremath{\Gamma}}$. We extracted a collision rate of $\mathrm{\ensuremath{\Gamma}}=0.649(2)\phantom{\rule{4pt}{0ex}}{\text{s}}^{\ensuremath{-}1}$. We further refine our analysis by using monotonic Gaussian process regression to smooth the experimental data, which extracts a collision rate of $\mathrm{\ensuremath{\Gamma}}=0.646(1)\phantom{\rule{4pt}{0ex}}{\text{s}}^{\ensuremath{-}1}$. This is compared to a value of 0.67(1) ${\text{s}}^{\ensuremath{-}1}$ found by the commonly used method of zero-trap depth extrapolation, a 3.5% correction that is a result of our model fully taking ensemble loss and heating into account. Finally, we report a fivefold increase in the precision of our collision rate extraction from the experimental data and a tenfold increase in the precision of our collision rate extraction from the smoothed experimental data.
We present a method for determining the depth of an atomic or molecular trap of any type. This method relies on a measurement of the trap loss rate induced by collisions with background gas particles. Given a fixed gas composition, the loss rate uniquely determines the trap depth. Because of the ``soft'' long-range nature of the van der Waals interaction, these collisions transfer kinetic energy to trapped particles across a broad range of energy scales, from room temperature to the microkelvin energy scale. The resulting loss rate therefore exhibits a significant variation over an enormous range of trap depths, making this technique a powerful diagnostic with a large dynamic range. We present trap depth measurements of a Rb magneto-optical trap using this method and a different technique that relies on measurements of loss rates during optical excitation of colliding atoms to a repulsive molecular state. The main advantage of the method presented here is its large dynamic range and applicability to traps of any type requiring only knowledge of the background gas density and the interaction potential between the trapped and background gas particles.
We derive a simple formula for the heating rate that arises from quantum-diffractive background gas collisions in atom traps. This result appears to explain the residual heating rates reported for recent experiments with a Cs vapor-loaded, far-detuned optical trap at $\ensuremath{\simeq}{10}^{\ensuremath{-}9}$ Torr [Phys. Rev. Lett. 81, 5768 (1998)]. Diffractive collisions may determine the minimum heating rates achievable in shallow all-optical or magnetic atom traps operating at low temperature and high density.
This thesis describes an investigation and application of the loss of laser cooled atoms from a trap induced by background collisions. The loss rate constant depends on the density of background gas and the velocity averaged collisional loss cross section due to collisions. The velocity averaged collisional loss cross section can be calculated and its dependence on trap depth was verified using a magneto-optical trap. This verification involved measurements of the loss rate constant for a quadrupole magnetic and a magneto-optical trap and measurement of the density of Ar background gas using a residual gas analyzer. The second part of the thesis focuses on an application of these measurements of the loss rate constant to measure the pressure of the background gas. The experimental progress to date on the atom pressure sensor is provided.