Measurements of Diffusion Coefficients for Rubidium Atoms in Inert Gas Mixtures Using Coherent Scattering From Optically Pumped Population Gratings

Abstract

We present comprehensive determinations of the diffusion coefficients for rubidium atoms in six commonly used buffer gases using a newly developed coherent transient technique. The experiments are carried out by establishing a spatially periodic rubidium population grating using two laser beams intersecting at an angle of a few milliradians. The grating decays exponentially in time due to diffusive motion induced by momentum-changing elastic collisions with buffer gas atoms. The decay can be monitored over a large dynamic range using a heterodyne detection system that records the coherently scattered light from the grating. We are able to distinguish the contribution of diffusion from other collisional processes by measuring the characteristic dependence of the decay rate on the angle between excitation beams. These experiments are carried out in a non-magnetic atomic vapour cell manifold that allows magnetic fields and magnetic field gradients to be cancelled so that rubidium atoms can be manipulated in targeted internal ground states in the presence of different inert gases that can be maintained at pressures ranging from a few hundred pascals to one atmosphere. Our measurements agree with theoretical calculations of diffusion coefficients after reconciling key systematic effects, and this agreement appears to resolve both the widespread scatter in the values of diffusion coefficients using other techniques obtained over several decades and their disagreement with theory. Our measurements lay the groundwork for the development of a quantum pressure sensor that will rely on the intrinsic properties of atoms to calibrate commercial pressure gauges and impact emerging quantum technologies such as magnetometry, spin polarized imaging, and quantum memory that rely on accurate knowledge of diffusion coefficients. We also describe preliminary, comparative studies of a traditional population magnetometer and a unique coherence magnetometer developed by our group, which led to the development of the technique for measuring diffusion. All our experiments were carried out using a low-cost, home-built diode laser system. We present a detailed characterization of this system, which has supported wide-ranging experiments in precision metrology such as optical tweezers-based determination of micro-particle masses, measurements of atomic lifetimes, and atom interferometric measurements of velocity and gravitational acceleration.