Engineering Excited-State Interactions at Ultracold Temperatures

Using a recently developed method for precisely controlling collision energy, we observe a dramatic suppression of inelastic collisions between an atom and ion ($\mathrm{Ca}+{\mathrm{Yb}}^{+}$) at low collision energy. This suppression, which is expected to be a universal phenomenon, arises when the spontaneous emission lifetime of the excited state is comparable to or shorter than the collision complex lifetime. We develop a technique to remove this suppression and engineer excited-state interactions. By dressing the system with a strong catalyst laser, a significant fraction of the collision complexes can be excited at a specified atom-ion separation. This technique allows excited-state collisions to be studied, even at ultracold temperature, and provides a general method for engineering ultracold excited-state interactions.

Figure 1

Shuttling in the hybrid atom-ion MOTION trap. (a) Schematic of the MOTION trap. (b) False-color fluorescence image of three shuttled ${\mathrm{Yb}}^{+}$ ions. As the exposure time is greater than the shuttling period, fluorescence from the three ions is concentrated at the positions of the two end points, where the ions spend the most time. (c) Experimental sequence illustrating the shuttling technique. As the ${\mathrm{Yb}}^{+}$ ions are shuttled through the Ca MOT, the 369 nm ${\mathrm{Yb}}^{+}$ cooling beams are extinguished to prepare the ions in the $6s{{}^{2}S}_{1/2}$ state. (d) Measured charge-exchange rate coefficient (with standard errors) for $\mathrm{Ca}({}^1{P}_1)+{\mathrm{Yb}}^{+}({{}^{2}S}_{1/2})$ as a function of collision energy using the shuttling technique. Also shown are rate coefficients from coupled-channels calculations, one with (solid line) and one without (dashed line) the effect of reaction blockading (RB).

Figure 2

Long-range diabatic potential energy curves. (a) Relevant long-range molecular potentials. The two crossings between potentials relevant for charge exchange are indicated with black circular markers. The potential energy zero is located at the $\mathrm{Ca}(4s^2{}^1{S}_0)+{\mathrm{Yb}}^{+}(6s{{}^{2}S}_{1/2})$ dissociation limit. (b) The first pathway corresponds to a collision between an excited $4s4p{}^1{P}_1$ Ca atom with a ground-state $6s{{}^{2}S}_{1/2}{\mathrm{Yb}}^{+}$ ion. The charge-exchange (CE) crossing is shown by a red circle. The vertical wavy line represents spontaneous emission to the ground $\mathrm{Ca}(4s^2{}^1{S}_0)+{\mathrm{Yb}}^{+}(6s{{}^{2}S}_{1/2})$ channel. (c) The second pathway corresponds to a collision between a ground-state $4s^2{}^1{S}_0$ Ca atom with a $6s{{}^{2}S}_{1/2}{\mathrm{Yb}}^{+}$ ion in the presence of a photon of the MOT laser. The dashed blue curve corresponds to the dressed-state potential for this entrance channel. It has an avoided crossing with the excited $\mathrm{Ca}(4s4p{}^1{P}_1)+{\mathrm{Yb}}^{+}(6s{{}^{2}S}_{1/2})$ potential. (d) In the presence of a catalyst laser, the incoming $\mathrm{Ca}(4s^2{}^1{S}_0)+{\mathrm{Yb}}^{+}(6s{{}^{2}S}_{1/2})$ state is coupled to the reactive $\mathrm{Ca}(4s4p{}^1{P}_1)+{\mathrm{Yb}}^{+}(6s{{}^{2}S}_{1/2})$ state at short range, where spontaneous emission is unlikely before reaction.

Figure 3

Dual-isotope technique. False-color fluorescence images of the ${\mathrm{Yb}}^{+}$ ions and the Ca MOT (not to scale), illustrating the dual-isotope method used to measure the low decay rate of ${}^{172}{\mathrm{Yb}}^{+}({{}^{2}S}_{1/2})$. We first trap ${}^{172}{\mathrm{Yb}}^{+}$ and ${}^{174}{\mathrm{Yb}}^{+}$, while laser cooling only ${}^{172}{\mathrm{Yb}}^{+}$ ions (shown in red), while ${}^{174}{\mathrm{Yb}}^{+}$ ions (shown as blue circles) remain dark. We then switch the 369 nm cooling laser frequency to cool ${}^{174}{\mathrm{Yb}}^{+}$ ions (shown in blue), while the ${}^{172}{\mathrm{Yb}}^{+}$ ions (shown as red dashed circles) remain dark. We then overlap the MOT with the laser-cooled ${}^{174}{\mathrm{Yb}}^{+}$ ions as well as the ground-state ${}^{172}{\mathrm{Yb}}^{+}({{}^{2}S}_{1/2})$ ions for a variable amount of time. Finally, we cool and measure the final number of ${}^{172}{\mathrm{Yb}}^{+}$ ions.

Figure 4

Removing suppression with addition of a catalyst laser. Total charge-exchange rate coefficient as a function of catalyst laser (a) frequency and (b) intensity. Plotted alongside experimental data are the results of a coupled-channels calculation and an estimate using the Landau-Zener approximation. For reference, the experimental rate with no catalyst beam is shown. Error bars correspond to the standard error in experimental measurements and error bands include uncertainties from the theoretical simulations and experimental parameters. Horizontal error bars in (a) are smaller than the plot marker. Details are found in the Supplemental Material [37].