Magnetic Trapping of an Ultracold Gas of Polar Molecules

We demonstrate the efficient transfer of molecules from a magneto-optical trap into a conservative magnetic quadrupole trap. Our scheme begins with a blue-detuned optical molasses to cool SrF molecules to $\approx 50\mu \mathrm{K}$. Next, we optically pump the molecules into a strongly trapped sublevel. This two-step process reliably transfers $\approx 40\%$ of the molecules initially trapped in the magneto-optical trap into a single quantum state in the magnetic trap. Once loaded, the molecule cloud is compressed by increasing the magnetic field gradient. We observe a magnetic trap lifetime of over 1 s. This opens a promising new path to study ultracold molecular collisions, and potentially to produce quantum-degenerate molecular gases via sympathetic cooling with co-trapped atoms.

Figure 1

Sub-Doppler cooling: (a) Molecular temperature after molasses vs molasses laser intensity. The $0\mathrm{mW}/{\mathrm{cm}}^2$ point indicates the initial cloud temperature. (b) Molecular temperature after molasses vs east-west shim coil $B$ field. Temperature dependence on north-south and up-down shim coil $B$ fields is similar (see Supplemental Material [46]).

Figure 2

Molecular state distribution in the MQT: (a) Microwave spectroscopy in the compressed (red circles) and low-gradient (black squares) MQT shows trap loss at frequencies corresponding to the $|1;3/2;1⟩$ and $|1;3/2;2⟩$ states. In the low-gradient case, the depth of the central loss feature indicates that $\approx 75\%$ of trapped molecules are in $|1;3/2;2;m=+2⟩$. Dashed (dotted) lines mark the field-free frequencies of microwave transitions to $|0;1/2;1⟩$ (to $|0;1/2;0⟩$), as illustrated in plot (b). Error bars show the standard error of multiple measurements. Lines between points are a guide to the eye. (b) Breit-Rabi diagram for $X{{}^2\mathrm{\Sigma }}^{+}$ $|N=1;J=1/2,3/2⟩$ and $|N=0,J=1/2⟩$. Vertical arrows mark the six available microwave transitions, aligned below the relevant frequencies in plot (a). Red shaded regions show the energy spread for molecules in each trapped state, for a Boltzmann distribution at $260\mu \mathrm{K}$.

Figure 3

Lifetime of molecules in the compressed MQT. Inset: MOT LIF signal vs in-vacuum shutter open duration $\mathrm{\Delta }t$. Setting $\mathrm{\Delta }t\ge 17\mathrm{ms}$ (dashed line) yields optimal loading of the MOT. Main plot: with $\mathrm{\Delta }t=17\mathrm{ms}$, we measure ${\tau }_{\mathrm{MQT}}=1.21(9)\mathrm{s}$. Error bars show the standard error of multiple measurements; line is an exponential fit.