Ultracold Atoms Coupled to Ultracold Nano-mechanical Oscillators
Final Report Abstract
This project aimed at realizing a hybrid quantum system comprised of cold or ultracold atoms and a cryogenically precooled nano-mechanical oscillator. Within the funding period of this DFG grant a complex experimental setup devoted to this ambitious task was completed. The low frequency nanomechanical oscillator is configured in a so-called membrane-in-the-middle setup, which we realize with an all-fiber cavity to minimize any kind of optical apertures in the ultra-high vacuum 3He-4He dilution refrigerator that houses the whole optomechanical unit. We routinely operate this device at a base temperature of T = 500 mK as an ideal starting point for further cooling and coupling experiments. Furthermore, we completed a separate apparatus to reliable produce very large laser-cooled ensembles and Bose-Einstein condensates of 87Rb atoms to be coupled to the nano-mechanical oscillator via an optical fiber link. In the funding period we studied in detail prospects to further cool the optomechanical oscillator and characterized the hybrid coupling between laser cooled atoms and a commercial high-stress Si3N4 membrane oscillator. Due to the requirements dictated by a preferably large hybrid coupling, we operate the MiM system at moderate cavity finesse, which excludes optomechanical sideband cooling and demands for active feedback cooling to further reduce the mechanical mode occupation. We succeeded in cooling the membrane oscillator to a mode occupation of n=16. After careful analysis of the cooling results we devised new nano-mechanical trampoline oscillators, which are much better suited for our envisioned hybrid experiments. With these new devices we recently cooled down to a mode occupation of n=3 with room for further improvement so that we are confident of reaching the quantum mechanical ground state with active feedback cooling very soon. In a second series of experiments we investigated the hybrid coupling mechanism through sympathetic cooling experiments, where energy is removed from the nano-mechanical oscillator by coupling it to laser cooled atoms. We could successfully cool the membrane oscillator to T = 20 mK coupling it to atoms in optical molasses as well as in a magneto optical trap through a blue detuned optical coupling lattice. The hybrid cooperativity we find in this way looks very promising for reaching the strong coupling regime by simply exchanging the membrane by a trampoline oscillator and switching to a fiber cavity with a ten times larger finesse, both of which we have already fabricated and characterized. First ongoing experiments in this configuration look very promising. Interestingly for red lattice detuning, moderate pump asymmetry and dense atomic samples we observe a hybrid instability that prevents further sympathetic cooling and instead drives the nano-mechanical oscillator into limit cycle oscillations. Finally, we have prepared to implement other coupling schemes addressing internal degrees of freedom of the atoms to the nano-mechanical oscillator. State independent optical traps and lattices for the atoms have been set up and characterized and spin-state preparation-, manipulation- and detection schemes successfully implemented. Altogether we believe, that the results obtained within this DFG project pave the way to the first realization of a true hybrid quantum system comprised of a ground state nano-mechanical oscillator and a Bose-Einstein condensate in the near future.
Publications
- “The role of mode match in fiber cavities”, Review of Scientific Instruments 87 (2016)
A. Bick, C. Staarmann, P. Christoph, O. Hellmig, J. Heinze, K. Sengstock and C. Becker
(See online at https://doi.org/10.1063/1.4939046) - “A millikelvin all-fiber cavity optomechanical apparatus for merging with ultra-cold atoms in a hybrid quantum system”, Review of Scientific Instruments 88 (2), 023115 (2017)
H. Zhong, G. Fläschner, A. Schwarz, R. Wiesendanger, P. Christoph, T. Wagner, A. Bick, C. Staarmann, B. Abeln, K. Sengstock and C. Becker
(See online at https://doi.org/10.1063/1.4976497) - “Combined feedback and sympathetic cooling of a mechanical oscillator coupled to ultracold atoms”, New Journal of Physics 20, 093020 (2018)
P. Christoph, T. Wagner, H. Zhong, R. Wiesendanger, K. Sengstock, A. Schwarz and C. Becker
(See online at https://doi.org/10.1088/1367-2630/aadf20)
