Creation of Ultracold ${\mathrm{Sr}}_2$ Molecules in the Electronic Ground State

We report on the creation of ultracold ${}^{84}\mathrm{Sr}_2$ molecules in the electronic ground state. The molecules are formed from atom pairs on sites of an optical lattice using stimulated Raman adiabatic passage (STIRAP). We achieve a transfer efficiency of 30% and obtain $4\times {10}^4$ molecules with full control over the external and internal quantum state. STIRAP is performed near the narrow ${}^{1}S_0\mathrm{\text{−}}{}^{3}P_1$ intercombination transition, using a vibrational level of the $1(0_u^{+})$ potential as an intermediate state. In preparation of our molecule association scheme, we have determined the binding energies of the last vibrational levels of the $1(0_u^{+})$, $1(1_u)$ excited-state and the $X{}^1\Sigma _g^{+}$ ground-state potentials. Our work overcomes the previous limitation of STIRAP schemes to systems with magnetic Feshbach resonances, thereby establishing a route that is applicable to many systems beyond alkali-metal dimers.

Figure 1

Molecular potentials and vibrational levels of ${}^{84}\mathrm{Sr}_2$ involved in STIRAP. The initial state $|a⟩$, an atom pair in the ground state of an optical lattice well, and the final molecular state $|m⟩$ are coupled by laser fields $L_1$ and $L_2$ to the excited state $|e⟩$ with Rabi frequencies ${\Omega }_1$ and ${\Omega }_2$, respectively. The parameter $\Delta$ is the detuning of $L_1$ from the ${}^{1}S_0\mathrm{\text{−}}{}^{3}P_1$ transition, and $\Gamma$ is the decay rate of $|e⟩$. The insets show the last vibrational levels of the molecular potentials and the wave functions of states $|m⟩$ and $|e⟩$. For comparison, the wave function of atomic state $|a⟩$ (not shown) has its classical turning point at a radius of $800a_0$, where $a_0$ is the Bohr radius. The potentials are taken from Refs. 39, 40, and the wave functions are calculated by using the WKB approximation. The energies of states $|m⟩$ and $|e⟩$ are not to scale in the main figure.

Figure 2

Two-color PA spectra near state $|e⟩$ for two intensities of $L_2$. (a) For high intensity ($20\mathrm{W}/{\mathrm{cm}}^2$) the spectrum shows an Autler-Townes splitting. (b) For low intensity ($80\mathrm{mW}/{\mathrm{cm}}^2$) a narrow dark resonance is visible. For both spectra, the sample was illuminated by $L_1$ for 100 ms with an intensity of $7\mathrm{mW}/{\mathrm{cm}}^2$ at varying detuning $\Delta$ from the ${}^{1}S_0\mathrm{\text{−}}{}^{3}P_1$ transition. The lines are fits according to a three-mode model [32].

Figure 3

Time evolution of STIRAP transfers from atom pairs to ${\mathrm{Sr}}_2$ molecules and back. (a) Intensities of $L_1$, $L_2$, and cleaning laser $C$, normalized to one. (b),(c) Atom number evolution. For these measurements, $L_1$ and $L_2$ are abruptly switched off at a given point in time, and the atom number is recorded on an absorption image after 10 ms free expansion. Note the scaling applied to data taken during the first $100\mu \mathrm{s}$ (triangles). The starting point for the time evolution shown in (b) is a Mott insulator, whereas the starting point for (c) is a sample for which 80% of the atoms occupy lattice sites in pairs.

Figure 4

Quasimomentum distribution of repulsively bound pairs. (a) Average of 20 absorption images recorded 10 ms after release of the atoms from the lattice. (b) Integral of the distribution along $y$.