We produce ultracold dense trapped samples of ^{87}Rb^{133}Cs molecules in their rovibrational ground state, with full nuclear hyperfine state control, by stimulated Raman adiabatic passage (STIRAP) with efficiencies of 90%. We observe the onset of hyperfine-changing collisions when the magnetic field is ramped so that the molecules are no longer in the hyperfine ground state. A strong quadratic shift of the transition frequencies as a function of applied electric field shows the strongly dipolar character of the RbCs ground-state molecule. Our results open up the prospect of realizing stable bosonic dipolar quantum gases with ultracold molecules.

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Ultracold dense samples of dipolar RbCs molecules in the rovibrational and hyperfine ground state.

1. Unconventional Conditions at Ultracold Temperatures 4949 1.

Ultracold molecules under control

Cooling atoms to ultralow temperatures has produced a wealth of opportunities in fundamental physics, precision metrology, and quantum science. The more recent application of sophisticated cooling techniques to molecules, which has been more challenging to implement owing to the complexity of molecular structures, has now opened the door to the longstanding goal of precisely controlling molecular internal and external degrees of freedom and the resulting interaction processes. This line of research can leverage fundamental insights into how molecules interact and evolve to enable the control of reaction chemistry and the design and realization of a range of advanced quantum materials.

http://science.sciencemag.org/content/sci/357/6355/1002.full.pdf

Cold molecules: Progress in quantum engineering of chemistry and quantum matter.

The angular momentum of molecules, or, equivalently, their rotation in three-dimensional space, is ideally suited for quantum control. Molecular angular momentum is naturally quantized, time evolution is governed by a well-known Hamiltonian with only a few accurately known parameters, and transitions between rotational levels can be driven by external fields from various parts of the electromagnetic spectrum. Control over the rotational motion can be exerted in one-, two-, and many-body scenarios, thereby allowing one to probe Anderson localization, target stereoselectivity of bimolecular reactions, or encode quantum information to name just a few examples. The corresponding approaches to quantum control are pursued within separate, and typically disjoint, subfields of physics, including ultrafast science, cold collisions, ultracold gases, quantum information science, and condensed-matter physics. It is the purpose of this review to present the various control phenomena, which all rely on the same underlying physics, within a unified framework. To this end, recall the Hamiltonian for free rotations, assuming the rigid rotor approximation to be valid, and summarize the different ways for a rotor to interact with external electromagnetic fields. These interactions can be exploited for control—from achieving alignment, orientation, or laser cooling in a one-body framework, steering bimolecular collisions, or realizing a quantum computer or quantum simulator in the many-body setting.

Quantum control of molecular rotation

Experimental realization of a quantum degenerate gas of molecules would provide access to a wide range of phenomena in molecular and quantum sciences. However, the very complexity that makes ultracold molecules so enticing has made reaching degeneracy an outstanding experimental challenge over the past decade. We now report the production of a degenerate Fermi gas of ultracold polar molecules of potassium-rubidium. Through coherent adiabatic association in a deeply degenerate mixture of a rubidium Bose-Einstein condensate and a potassium Fermi gas, we produce molecules at temperatures below 0.3 times the Fermi temperature. We explore the properties of this reactive gas and demonstrate how degeneracy suppresses chemical reactions, making a long-lived degenerate gas of polar molecules a reality.

/pdf/a-degenerate-fermi-gas-of-polar-molecules-o26bmyd9g1.pdf

A degenerate Fermi gas of polar molecules.

We have studied interspecies scattering in an ultracold mixture of ${}^{87}$Rb and ${}^{133}$Cs atoms, both in their lowest-energy spin states. The three-body loss signatures of 30 incoming $s$- and $p$-wave magnetic Feshbach resonances over the range 0 to 667 G have been cataloged. Magnetic field modulation spectroscopy was used to observe molecular states bound by up to 2.5 $\text{MHz}\ifmmode\times\else\texttimes\fi{}h$. We have created RbCs Feshbach molecules using two of the resonances. Magnetic moment spectroscopy along the magnetoassociation pathway from 197 to 182 G gives results consistent with the observed and calculated dependence of the binding energy on magnetic field strength. We have set up a coupled-channel model of the interaction and have used direct least-squares fitting to refine its parameters to fit the experimental results from the Feshbach molecules, in addition to the Feshbach resonance positions and the spectroscopic results for deeply bound levels. The final model gives a good description of all the experimental results and predicts a large resonance near 790 G, which may be useful for tuning the interspecies scattering properties. Quantum numbers and vibrational wave functions from the model can also be used to choose optimal initial states of Feshbach molecules for their transfer to the rovibronic ground state using stimulated Raman adiabatic passage.

Towards the production of ultracold ground-state RbCs molecules: Feshbach resonances, weakly bound states, and the coupled-channel model

We have performed high-resolution two-photon dark-state spectroscopy of an ultracold gas of $^{87}\mathrm{Rb}$${}_{2}$ molecules in the ${a}^{3}{\ensuremath{\Sigma}}_{u}^{+}$ state at a magnetic field of about 1000 G. The vibrational ladder as well as the hyperfine and low-lying rotational structure are mapped out. Energy shifts in the spectrum are observed due to singlet-triplet mixing at binding energies as deep as a few hundred $\text{GHz}\ifmmode\times\else\texttimes\fi{}h$. This information, together with data from other sources, is used to optimize the potentials of the ${a}^{3}{\ensuremath{\Sigma}}_{u}^{+}$ and ${X}^{1}{\ensuremath{\Sigma}}_{g}^{+}$ states in a coupled-channel model. We find that the hyperfine structure depends weakly on the vibrational level. This provides a possible explanation for inaccuracies in recent Feshbach resonance calculations.

Hyperfine, rotational, and vibrational structure of the a 3 Σ u + state of Rb 87 2

We demonstrate a generally applicable technique for mixing two-species quantum degenerate bosonic samples in the presence of an optical lattice, and we employ it to produce low-entropy samples of ultracold $^{87}\mathrm{Rb}^{133}\mathrm{Cs}$ Feshbach molecules with a lattice filling fraction exceeding 30%. Starting from two spatially separated Bose-Einstein condensates of Rb and Cs atoms, Rb-Cs atom pairs are efficiently produced by using the superfluid-to-Mott insulator quantum phase transition twice, first for the Cs sample, then for the Rb sample, after nulling the Rb-Cs interaction at a Feshbach resonance's zero crossing. We form molecules out of atom pairs and characterize the mixing process in terms of sample overlap and mixing speed. The dense and ultracold sample of more than 5000 RbCs molecules is an ideal starting point for experiments in the context of quantum many-body physics with long-range dipolar interactions.

Quantum Engineering of a Low-Entropy Gas of Heteronuclear Bosonic Molecules in an Optical Lattice.

We present the results of an experimental and theoretical study of the electronically excited $(1){}^{3}{\ensuremath{\Sigma}}_{\mathrm{g}}^{+}$ state of $^{87}\mathrm{Rb}$${}_{2}$ molecules. The vibrational energies are measured for deeply bound states from the bottom up to ${v}^{\ensuremath{'}}=15$ using laser spectroscopy of ultracold Rb${}_{2}$ Feshbach molecules. The spectrum of each vibrational state is dominated by a 47-GHz splitting into ${0}_{\mathrm{g}}^{\ensuremath{-}}$ and ${1}_{\mathrm{g}}$ components caused mainly by a strong second-order spin-orbit interaction. Our spectroscopy fully resolves the rotational, hyperfine, and Zeeman structure of the spectrum. We are able to describe this structure to the first order using a simplified effective Hamiltonian.

Tetsu Takekoshi

Papers

Towards the production of ultracold ground-state RbCs molecules: Feshbach resonances, weakly bound states, and the coupled-channel model

Hyperfine, rotational, and vibrational structure of the a 3 Σ u + state of Rb 87 2

Quantum Engineering of a Low-Entropy Gas of Heteronuclear Bosonic Molecules in an Optical Lattice.

Hyperfine, rotational, and Zeeman structure of the lowest vibrational levels of the Rb 87 2 (1) 3 Σ g + state