Mike Tarbutt

Ben Sauer

Recent advances in the production of ultracold samples of dipolar molecules [1] makes it feasible to build versatile quantum simulators with single molecules as the fundamental entities. Such simulators may be used to investigate problems intractable to conventional methods using even the most powerful computers. The large dipole moments of polar molecules may in principle be tuned on demand and various arbitrary interaction potentials created to mimic a wide range of quantum many body systems [2]. To demonstrate the fundamental building block of a quantum simulator we are designing an experiment to trap a single CaF molecule in an optical tweezer, cool it to its motional ground state and demonstrate full quantum state control.

 As a first step towards this goal we will experimentally investigate sympathetic cooling of CaF using a sample of ultracold Rb atoms as the coolant. We will make an atomic and molecular magneto optical trap (MOT) and overlap these to enforce collisions between the atoms and molecules. Simulations show that after a stage of evaporative cooling of Rb atoms the phase space density of molecules may be significantly increased [3]. This is important for loading an optical tweezer as the loading rate (especially for filling multiple traps) increases with phase-space density.

The experimental sequence for trapping a single molecule will begin by capturing CaF in a MOT after an initial stage of 1-D slowing and cooling. The molecules can then be cooled to 50 uK in an optical molasses, or to even lower temperatures if sympathetic cooling works as anticipated. The molecules will then be transferred to a magnetic trap and transported to a science chamber where the tweezer experiment will take place. Here, an optical molasses will provide with a dissipative mechanism for the molecule to cool into the optical trap and will make the molecule bright to our imaging system.

To create the optical tweezer itself we will employ techniques developed for trapping single atoms [4]. A high numerical aperture aspherical lens inside the vacuum chamber will be used to both focus the far off-resonant trapping light and collect the fluorescence of the trapped molecule. An imaging system capable of registering small numbers of photons will be used to detect the presence of a trapped molecule and trigger the single particle experiments.

[1] Truppe, S., et al. "Molecules cooled below the Doppler limit." Nature Physics 13.12 (2017):1173.

[2] Micheli, Andrea, G. K. Brennen, and PeterZoller. "A toolbox for lattice-spin models with polar molecules." Nature Physics 2.5 (2006):341.

[3] Lim, Jongseok, et al. "Modeling sympathetic cooling of molecules by ultracold atoms." Physical Review A 92.5 (2015): 053419.

[4] Nogrette, Florence, et al. "Single-atom trapping in holographic 2D arrays of microtraps with arbitrary geometries." Physical Review X 4.2 (2014):021034.