Coherent Transfer of Photoassociated Molecules into the Rovibrational Ground State

We report on the direct conversion of laser-cooled ${}^{41}\mathrm{K}$ and ${}^{87}\mathrm{Rb}$ atoms into ultracold ${}^{41}\mathrm{K}{}^{87}\mathrm{Rb}$ molecules in the rovibrational ground state via photoassociation followed by stimulated Raman adiabatic passage. High-resolution spectroscopy based on the coherent transfer revealed the hyperfine structure of weakly bound molecules in an unexplored region. Our results show that a rovibrationally pure sample of ultracold ground-state molecules is achieved via the all-optical association of laser-cooled atoms, opening possibilities to coherently manipulate a wide variety of molecules.

Figure 1

Relevant molecular potential energy curves of KRb. Weakly bound molecules were produced by the photoassociation of laser-cooled ${}^{41}\mathrm{K}$ and ${}^{87}\mathrm{Rb}$ atoms. Molecules in the $v^{\prime \prime }=91$, $J^{\prime \prime }=0$, $F^{\prime \prime }=2$ level of $X{}^1\Sigma ^{+}$ were coupled to the $v^{\prime \prime }=0$, $J^{\prime \prime }=0$ level of $X{}^1\Sigma ^{+}$ by a two-photon transition with wavelengths near $875(L1)$ and $641(L2)\mathrm{nm}$ via the $v^{\prime }=41$, $J^{\prime }=1$ level of $(3){}^1\Sigma ^{+}$. For depletion spectroscopy and SpIDR spectroscopy, two-photon ionization through the $(4){}^1\Sigma ^{+}$ state was used. For the detection of the rovibrational ground-state molecules, a three-photon transition mediated by the $v^{\prime }=6$ level of $(1){}^1\Pi$ was used.

Figure 2

STIRAP transfer from the $v^{\prime \prime }=91$, $J^{\prime \prime }=0$, $F^{\prime \prime }=2$ level to the $v^{\prime \prime }=0$ level of $X{}^1\Sigma ^{+}$. (a) Transfer to the $J^{\prime \prime }=0$ level. The population in the $v^{\prime \prime }=0$ level of $X{}^1\Sigma ^{+}$ is plotted against the frequency of the down transition laser ($L2$). (b) Transfer to the $J^{\prime \prime }=2$ level. The asymmetry observed in the $J^{\prime \prime }=2$ spectrum is expected to be a result of the hyperfine structure induced by the nuclear electric quadrupole moments of ${}^{87}\mathrm{Rb}$. (c) Time evolution of the population in the $v^{\prime \prime }=0$, $J^{\prime \prime }=0$ level of $X{}^1\Sigma ^{+}$ during the multiple STIRAP transfer process. The error bars indicate the standard deviation. (d) Time variation of the intensities of the Raman lasers during the multiple-transfer process. Both intensities are normalized to unity. During the waiting time of $20\mu \mathrm{s}$ set between the transfers, the remaining population that is not transferred by STIRAP is removed by the resonant Raman lasers.

Figure 3

STIRAP spectroscopy of weakly bound molecules (the $v^{\prime \prime }=91$, $J^{\prime \prime }=0$ level of $X{}^1\Sigma ^{+}$). An efficient transfer into the rovibrational ground state enables the precision spectroscopy of the hyperfine structure of photoassociated molecules in an unexplored region with a typical accuracy of 10 kHz. The labeling is based on the total angular momentum including the nuclear spin.

Figure 4

SpIDR spectroscopy of photoassociated molecules. (a) SpIDR spectrum of the $v^{\prime }=41$, $J^{\prime }=1$ level of $(3){}^1\Sigma ^{+}$ taken by exciting molecules in the $v^{\prime \prime }=91$, $J^{\prime \prime }=0$ level of $X{}^1\Sigma ^{+}$. Molecules formed by spontaneous decays from the excited state are counted against the offset frequency of the laser $L1$. The coarse splitting results from the hyperfine structure of the $v^{\prime \prime }=91$, $J^{\prime \prime }=0$ level of $X{}^1\Sigma ^{+}$. The fine splitting in each line implies the existence of hyperfine structure in the excited state. For the two-photon transition, the largest peak ($F^{\prime \prime }=2$) is employed as an initial state. (b) Depletion spectrum. In order to observe any signal, we had to increase the laser intensity by 3 orders of magnitude as compared to (a). Transitions from the $v^{\prime \prime }=91$, $J^{\prime \prime }=0$ level are barely observable since the ion counts are dominated by molecules in the $v^{\prime \prime }=91$, $J^{\prime \prime }=2$ level. (c) Two-photon dark resonance between the $v^{\prime \prime }=91$, $J^{\prime \prime }=0$, $F^{\prime \prime }=2$ level and the $v^{\prime \prime }=0$, $J^{\prime \prime }=0$ level of $X{}^1\Sigma ^{+}$. The down transition laser ($L2$) is scanned while the up transition laser ($L1$) is held on resonance. The dots indicate experimentally obtained data and the solid curve is a fit to the data points. (d) Two-photon dark resonance obtained by scanning the laser $L1$. The laser $L2$ is held on resonance. The blue dashed curve indicates a SpIDR spectrum for the $F^{\prime \prime }=2$ level, while the red solid one indicates a spectrum obtained with the laser $L2$.