Session J2: Focus Session: Cold Molecules

Fast single-laser optical cooling of molecular rotations

Although ion-ion sympathetic cooling is now commonly used to reduce speeds of trapped molecular ions to milliKelvin temperatures, cooling of the decoupled internal molecular modes remains a challenge. Here, we demonstrate that a single spectrally filtered femtosecond laser, tuned to the electronic A-X transition of trapped AlH$^+$, can efficiently cool rotations from room temperature to the ground state. Since this single-laser technique simultaneously drives rotationally de-exciting electronic excitations from all initially populated states, each parity can be cooled as quickly as 100 $\mu$s. In our current implementation, full population collection into the $N=0$ positive-parity level relies on vibrational relaxation on the timescale of 100 ms. Since this technique cools by step-wise rotational pumping, it is most readily applicable to ``alkali-like" molecular species, such as AlH$^+$, with semi-closed cycling transitions. Although we currently use a destructive two-photon state readout technique, these species are also promising candidates for single-molecule fluorescence detection. The ability to quickly cool molecular rotations is expected to be important for precision spectroscopy experiments, quantum information processing applications, and for ultracold chemistry studies.   

Quantum logic scheme for state detection of molecular ions

Quantum state detection of a single molecular ion can be applied towards precision measurement and controlled studies of cold chemistry. Such detection can be performed using quantum logic techniques, where the internal state of the molecular ion is mapped onto the state of a co-trapped atomic ion by coupling both of them to the common motional modes. The rotational state of the molecular ion can therefore be read out using the fluorescence of the atomic ion. Further, we can use the Zeeman splitting as a signature of different rotational levels due to their different magnetic g-factors. We will report implementation of this scheme using two atomic ions as a proof-of-principle experiment.   

Trapping of CaF via Optical Loading and Magnetic Slowing

General methods for delivering cold, chemically diverse molecules in large quantities could impact research in quantum simulation, cold controlled chemistry, and particle physics. We report the demonstration of a general method for trapping magnetic molecules using few photon scattering in combination with magnetic slowing. Starting from a two-stage helium (He) buffer gas cell, calcium monofluoride (CaF) molecules with an initial velocity of $v_{f}\sim30$ m/s are slowed as they enter a 800 mK deep superconducting magnetic trap region. Irreversible trap loading is achieved using two optical pumping stages, where two scattered photons remove molecular potential energy and entropy. CaF molecules in the $X^{2}\Sigma^{+}(v=0,\, N=1)$ state are observed in a trap with a lifetime exceeding 500 ms, which is limited by collisions with the background He gas. In future applications, co-loading of atoms with molecules seems straightforward using this method. We will also report the experimental progress towards co-trapping of calcium monohydride (CaH) molecules with lithium (Li) atoms for studying Li-CaH collisions, which should allow us to explore the possibility of sympathetic cooling of molecules and investigation of cold controlled chemistry.   

Sympathetic cooling of molecules with laser-cooled atoms

Cooling molecules through collisions with laser-cooled atoms is an attractive route to ultracold, ground state molecules. The technique is simple, applicable to a wide class of molecules, and does not require molecule specific laser systems. Particularly suited to this technique are charged molecules, which can be trapped indefinitely, even at room temperature, and undergo strong, short-ranged collisions with ultracold atoms. In this talk, I will focus on recent efforts to use the combination of a magneto-optical trap (MOT) and an ion trap, dubbed the MOTion trap, to produce cold, ground state diatomic charged molecules. The low-energy internal structure of these diatomic molecules, e.g. the electric dipole moment and vibrational, rotational, and $\Omega$-doublet levels, presents a host of opportunities for advances in quantum simulation, precision measurement, cold chemistry, and quantum information. Excitingly, recent proof-of-principle experiments have demonstrated that the MOTion trap is extremely efficient at cooling the vibrational motion of molecular ions.   

Continuous all-optical deceleration of molecular beams

A significant impediment to generating ultracold molecules is slowing a molecular beam to velocities where the molecules can be cooled and trapped. We report on progress toward addressing this issue with a general optical deceleration technique for molecular and atomic beams. We propose addressing the molecular beam with a pump and dump pulse sequence from a mode-locked laser. The pump pulse counter-propagates with respect to the beam and drives the molecules to the excited state. The dump pulse co-propagates and stimulates emission, driving the molecules back to the ground state. This cycle transfers 2$\hbar $k of momentum and can generate very large optical forces, not limited by the spontaneous emission lifetime of the molecule or atom. Importantly, avoiding spontaneous emission limits the branching to dark states. This technique can later be augmented with cooling and trapping. We are working towards demonstrating this optical force by accelerating a cold atomic sample.   

Permanent magnetic trap for improved OH evaporation efficiency

Evaporation of the hydroxyl radical (OH) has previously been demonstrated in our experiment, suggesting exciting prospects for further cooling and phase space density increase. Thus far evaporation efficiency has been limited by collision rate and vacuum lifetime, prompting an investigation of possible improvements. Monte Carlo simulations of a newly designed permanent magnetic trap with a factor of two steeper gradient and a more favorable loading geometry show an order of magnitude increase in initial density and in collision rate. We will report on the efficiency of OH evaporation in this improved trap.   

Analytical study of level crossings in the Stark-Zeeman spectra of ground state OH

The ground electronic, vibrational and rotational state of the OH molecule is currently of interest as it can be manipulated by electric and magnetic fields for experimental studies in ultracold chemistry and quantum degeneracy. Based on our recent exact solution of the corresponding effective Stark-Zeeman Hamiltonian, we present an analytical study of the crossings and avoided crossings in the spectrum. These features are relevant to non-adiabatic transitions, conical intersections and Berry phases. Specifically, for an avoided crossing employed in the recent evaporative cooling of OH, we compare our exact results to those derived earlier from perturbation theory.