Ultracold Heteronuclear Mixture of Ground and Excited State Atoms

We report on the realization of an ultracold mixture of lithium atoms in the ground state and ytterbium atoms in an excited metastable (${}^3{P}_2$) state. Such a mixture can support broad magnetic Feshbach resonances which may be utilized for the production of ultracold molecules with an electronic spin degree of freedom, as well as novel Efimov trimers. We investigate the interaction properties of the mixture in the presence of an external magnetic field and find an upper limit for the background interspecies two-body inelastic decay coefficient of $K_2^{\prime }<3\times {10}^{-12}{\mathrm{cm}}^3/\mathrm{s}$ for the ${}^3{P}_2$ $m_J=-1$ substate. We calculate the dynamic polarizabilities of the $\mathrm{Yb}({}^3{P}_2)$ magnetic substates for a range of wavelengths, and find good agreement with our measurements at 1064 nm. Our calculations also allow the identification of magic frequencies where Yb ground and metastable states are identically trapped and the determination of the interspecies van der Waals coefficients.

Figure 1

${\mathrm{Yb}}^{*}$ preparation and detection scheme. (a) Low-lying energy levels of ytterbium with solid (dashed) lines indicating the relevant laser excitation (spontaneous decay) processes. The natural linewidths of the short-lived $\{{}^1{P}_1,{}^3{P}_1,{}^3{D}_2,{}^3{S}_1\}$ excited states are $2\pi \times \{28,0.18,0.35,12\}\mathrm{MHz}$, respectively. (b) Top-down view of experimental setup. Two counterpropagating 404 nm excitation beams (blue arrows) are overlapped with the ODT (brown). Laser beams at 649 and 770 nm (red, brown arrows) pump atoms back to the ground state prior to absorption imaging using the 399 nm beam (purple arrow).

Figure 2

${\mathrm{Yb}}^{*}$state preparation. (a) A particular magnetic substate of ${}^3{D}_2$ can be resolved via Zeeman splitting and selectively excited with 404 nm light. This state will subsequently spontaneously decay to substates in the ${}^3{P}_2$ manifold according to the indicated branching ratios. Radio frequency transitions within the ${}^3{P}_2$ manifold are spectroscopically resolved due to the state-dependent Stark shifts in the 1064 nm ODT. (b)–(f) Absorption images of different ${}^3{P}_2$ spin compositions after Stern-Gerlach separation. The images correspond to (b) transfer via the ${}^3{D}_2$ $m_J=-1$ substate; (c) transfer via the ${}^3{D}_2$ $m_J=-2$ substate; (d) transfer via ${}^3{D}_2$ $m_J=-2$ substate with an applied in-trap gradient to obtain a pure sample of ${}^3{P}_2$ $m_J=-1$ atoms; (e) and (f) are the same as (d), followed by rf $\pi$-pulse transfer to $m_J=-2$ and $m_J=0$, respectively, of the ${}^3{P}_2$ manifold. The rf pulses in (e) and (f) are $300\mu \mathrm{s}$ long with frequencies close to the Zeeman splitting of 26 MHz but separated by 430 kHz due to the state-dependent Stark shift.

Figure 3

Calculated dynamic polarizability of ${\mathrm{Yb}}^{*}$ as a function of frequency of light polarized parallel to the quantization axis. Green, red, and blue solid lines correspond to the $|m_J|=0$, 1, and 2 substates, respectively, and the dashed line to the ground state. The open circle indicates measurements of the ac polarizability at 532 nm [24]. Our polarizability ratio measurements at 1064 nm, scaled to the calculated ${}^1{S}_0$ value, are shown as solid circles in inset.

Figure 4

Number and temperature (inset) evolution of ${\mathrm{Yb}}^{*}$ and Li in the ODT at an external magnetic field of 12 G. Lines show fits based on a model including two-body inelastic effects (see text).