Control of dipolar collisions in the quantum regime

The preparation of ultracold polar molecular gases close to quantum degeneracy opens novel research prospects ranging from dipolar quantum many-body physics to ultracold chemistry. With a near quantum degenerate gas of fermionic 40K87Rb polar molecules, this thesis presents studies on dipolar collision and chemical reaction dynamics, exhibiting long range interactions and spatial anisotropy. With full control over the internal quantum state of the molecules, we show how quantum statistics of the molecule determines the rate of chemical reactivity in the limit of vanishing collisional energy. Manipulating the interac-tion potential between indistinguishable polar molecules by means of control over the dipole moment of the molecules, we study the dramatic influence of the dipolar interaction on the chemical reaction rate. In particular, we show that the chemical reaction rate increases steeply with the dipole moment following a characteristic power law. This power law reflects the long-range character of the dipole-dipole interaction. Studies on thermodynamics in the molecular quantum gas reveal the anisotropic properties of the dipolar interaction. Finally, combining control over the molecular dipole moment and the dimensionality of the spatial confinement, we suppress inelastic collisions between polar molecules by up to two orders of magnitude. The suppression of inelastic collisions is achieved by changing the geometry of the confinement from three-dimensional to two-dimensional optical trapping. With the combination of a sufficiently tight 2D confinement and Fermi statistics of the molecules, two polar molecules approach each other only in a “side-by-side” collision, where the in-elastic collisions are suppressed by the repulsive dipole-dipole interaction. This suppression requires quantum state control of internal (electronic, vibrational, rotational and hyperfine states) and external (harmonic oscillator levels of the optical lattice) degrees of freedom of the molecules. This is a fundamental advance in stabilizing a polar molecular gas for future applications in quantum many-body systems.
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Department of Physics
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University of Colorado Boulder
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