Creation of Ultracold ${}^{87}\mathrm{Rb}{}^{133}\mathrm{Cs}$ Molecules in the Rovibrational Ground State

We report the creation of a sample of over 1000 ultracold ${}^{87}\mathrm{Rb}{}^{133}\mathrm{Cs}$ molecules in the lowest rovibrational ground state, from an atomic mixture of ${}^{87}\mathrm{Rb}$ and ${}^{133}\mathrm{Cs}$, by magnetoassociation on an interspecies Feshbach resonance followed by stimulated Raman adiabatic passage (STIRAP). We measure the binding energy of the RbCs molecule to be $hc\times 3811.576(1){\mathrm{cm}}^{-1}$ and the $|v^{\prime \prime }=0,J^{\prime \prime }=0⟩$ to $|v^{\prime \prime }=0,J^{\prime \prime }=2⟩$ splitting to be $h\times 2940.09(6)\mathrm{MHz}$. Stark spectroscopy of the rovibrational ground state yields an electric dipole moment of 1.225(3)(8) D, where the values in parentheses are the statistical and systematic uncertainties, respectively. We can access a space-fixed dipole moment of 0.355(2)(4) D, which is substantially higher than in previous work.

Figure 1

Experimental overview. (a) Simplified diagram of key elements of the apparatus. (b) Near-threshold bound states for ${}^{87}\mathrm{Rb}{}^{133}\mathrm{Cs}$ and the magnetoassociation path (solid black line). Stern-Gerlach separation is carried out at 180.487(4) G (closed circle) and STIRAP transfer is carried out at 181.624(1) G (open circle). An avoided crossing at $\sim 181.3\mathrm{G}$ allows transfer between these two states. (c) Potential energy curves for RbCs, indicating the transitions used for STIRAP.

Figure 2

Two-photon spectroscopy of the RbCs vibrational ground state. The molecules remain in the initial near-dissociation state when the Stokes light is on resonance with the $J^{\prime \prime }=0$ and $J^{\prime \prime }=2$ rotational states. The solid lines illustrate Lorentzian fits used to determine the resonance positions. Inset: One-photon loss spectrum for the $|{}^3{\mathrm{\Pi }}_1,v^{\prime }=29,J^{\prime }=1⟩$ state. The pump laser is held on resonance with this transition during the two-photon spectroscopy.

Figure 3

The number of molecules remaining in the Feshbach state $|F⟩$ when both lasers are switched off during the STIRAP sequence. The black solid and red dashed lines show the Feshbach and ground-state populations obtained from the Lindblad model described in the text. Left inset: The final population of Feshbach molecules as a function of Stokes detuning. Right inset: The pump and Stokes beam powers during the STIRAP pulse sequence.

Figure 4

Stark shift of the rovibrational ground state. The solid black line shows the curve fitted to our results from which we extract a permanent electric dipole moment in the molecular frame of 1.225(3) (8) D. The dotted gray lines indicate the upper and lower bounds due to the systematic error in the electric field calculation. Upper inset: Stark shift of the $|{}^3{\mathrm{\Pi }}_1,v^{\prime }=29,J^{\prime }=1⟩$ excited state used for Stark spectroscopy and STIRAP. The behavior is initially linear with a gradient of approximately $500\mathrm{kHz}/(\mathrm{V}{\mathrm{cm}}^{-1})$ up to a field of $\sim 400\mathrm{V}{\mathrm{cm}}^{-1}$. Lower inset: Ground-state electric dipole moment in the laboratory frame as a function of electric field. The gray region indicates the range of electric dipole moments currently accessible in the experiment.