Hybrid Systems: Cold Atoms Coupled to Micro Mechanical Oscillators =

Philipp Treutlein

Cris Montoya, J. L. Valencia, Andrew A. Geraci +2 more

J. L. Valencia, Cris Montoya, Gambhir Ranjit +3 more

Cris Montoya, J. L. Valencia, Andrew A. Geraci +2 more

Aline Faber

Hybrid systems combining mechanical oscillators and ultracold atoms provide novel \nopportunities for cooling, detection and quantum control of mechanical motion with \napplications in precision sensing, quantum-level signal transduction and for fundamental \ntests of quantum mechanics. \nIn this thesis I present experiments performed with a hybrid atom-membrane \nsystem, in which the vibrations of a Si_3N_4 membrane in an optical cavity are coupled \nto the motion of laser-cooled atoms in an optical lattice. The interactions are \nmediated by the lattice light over a macroscopic distance and enhanced by the cavity. \nVia the coupling to the cold atoms, the fundamental vibrational mode of the \nmembrane at 2π x 276 kHz is cooled sympathetically from room temperature to \n0.4(2) K, even though the mass of the mechanical oscillator exceeds that of the \natomic ensemble by a factor of 4 x 10^10. In other systems, sympathetic cooling \nof molecules with cold atoms or ions has been limited to mass ratios of up to 90. \nPrevious theoretical work has shown that our coupling mechanism is able to cool the \nmembrane vibration into the ground state and to perform coherent state transfers \nbetween atomic and membrane motion. \nUnder certain experimental conditions, the atom-membrane system shows self-oscillations, \nwhich arise from an effective delay in the backaction of the atoms onto \nthe light. This retardation drives the system into limit-cycle oscillations if the coupling \nis large. I study the dependence of this instability on several system parameters \nand find that a larger atom number and a smaller atom-light detuning make the system \nless stable. Further, the stability of the coupled system in presence of a delay is \ninvestigated theoretically and a modified expression for the sympathetic cooling rate \nis derived. This model allows to fit the measured atom number dependence with a \ndelay of τ = 88(1) ns. Moreover, direct measurements of the atomic backaction onto \nthe lattice light are presented. These show phase lags exceeding 180° in parameter \nregimes where the instability is observed, proving that the retardation arises within \nthe atomic ensemble. Finally, I present the results of numerical simulations, which \nshow that collective atomic effects within the atomic ensemble in an asymmetric \nlattice are able to induce the observed phase lag in the atomic backaction.

David Hunger, Stephan Camerer, Theodor W. Hänsch +8 more

This work investigates different coupling mechanisms between ultracold atoms and mechanical oscillators. In a first experiment, atom-surface force is used to couple the vibrations of a mechanical cantilever to the motion of a Bose-Einstein condensate in a magnetic microtrap on a chip. The atoms are trapped at sub-micrometer distance from the cantilever surface. Coupling is used to read out the cantilever vibrations with the atoms. Coupling via surface forces could be employed to couple atoms to molecular-scale oscillators such as carbon nanotubes. In a second experiment, coupling via a 1D optical lattice that is formed by a laser beam retroreflected from the cantilever tip. The optical lattice serves as a transfer rod which couples vibrations of the cantilever to the atoms and vice versa.