Microscopic control over polar molecules with tunable interactions enables the realization of distinct quantum phenomena. Using an electric field gradient, we demonstrated layer-resolved state preparation and imaging of ultracold potassium-rubidium molecules confined to two-dimensional planes in an optical lattice. The rotational coherence was maximized by rotating the electric field relative to the light polarization for state-insensitive trapping. Spatially separated molecules in adjacent layers interact through dipolar spin exchange of rotational angular momentum; by adjusting these interactions, we regulated the local chemical reaction rate. The resonance width of the exchange process vastly exceeded the dipolar interaction energy, an effect attributed to thermal energy. This work realized precise control of interacting molecules, enabling electric field microscopy on subwavelength scales and allowing access to unexplored physics in two-dimensional systems.
Microscopic control over polar molecules with tunable interactions enables the realization of distinct quantum phenomena. Using an electric field gradient, we demonstrated layer-resolved state preparation and imaging of ultracold potassium-rubidium molecules confined to two-dimensional planes in an optical lattice. The rotational coherence was maximized by rotating the electric field relative to the light polarization for state-insensitive trapping. Spatially separated molecules in adjacent layers interact through dipolar spin exchange of rotational angular momentum; by adjusting these interactions, we regulated the local chemical reaction rate. The resonance width of the exchange process vastly exceeded the dipolar interaction energy, an effect attributed to thermal energy. This work realized precise control of interacting molecules, enabling electric field microscopy on subwavelength scales and allowing access to unexplored physics in two-dimensional systems.
Full control of molecular interactions, including reactive losses, would open new frontiers in quantum science. We demonstrate extreme tunability of ultracold chemical reaction rates by inducing resonant dipolar interactions by means of an external electric field. We prepared fermionic potassium-rubidium molecules in their first excited rotational state and observed a modulation of the chemical reaction rate by three orders of magnitude as we tuned the electric field strength by a few percent across resonance. In a quasi-two-dimensional geometry, we accurately determined the contributions from the three dominant angular momentum projections of the collisions. Using the resonant features, we shielded the molecules from loss and suppressed the reaction rate by an order of magnitude below the background value, thereby realizing a long-lived sample of polar molecules in large electric fields.
Abstract : With the production of polar molecules in the quantum regime, long-range dipolar interactions are expected to facilitate understanding of strongly interacting many-body quantum systems and to realize lattice spin models for exploring quantum magnetism. In ordinary atomic systems, where contact interactions require wavefunction overlap, effective spin interactions on a lattice can be mediated by tunnelling, through a process referred to as superexchange; however, the coupling is relatively weak and is limited to nearest-neighbour interactions. In contrast, dipolar interactions exist even in the absence of tunnelling and extend beyond nearest neighbours. This allows coherent spin dynamics to persist even for gases with relatively high entropy and low lattice filling. Measured effects of dipolar interactions in ultracold molecular gases have been limited to the modification of inelastic collisions and chemical reactions. Here we use dipolar interactions of polar molecules pinned in a three-dimensional optical lattice to realize a lattice spin model. Spin is encoded in rotational states of molecules that are prepared and probed by microwaves. Resonant exchange of rotational angular momentum between two molecules realizes a spin-exchange interaction. The dipolar interactions are apparent in the evolution of the spin coherence, which shows oscillations in addition to an overall decay of the coherence. The frequency of these oscillations, the strong dependence of the spin coherence time on the lattice filling factor and the effect of a multipulse sequence designed to reverse dynamics due to two-body exchange interactions all provide evidence of dipolar interactions. Furthermore, we demonstrate the suppression of loss in weak lattices due to a continuous quantum Zeno mechanism. Measurements of these tunnelling-induced losses allow us to determine the lattice filling factor independently.
The creation of a gas of ultracold polar molecules with a high phase space density brings new possibilities beyond experiments with ultracold atomic gases. In particular, long-range, anisotropic, and tunable dipole-dipole interactions open the way for novel quantum gases, with applications including strongly correlated many-body systems, and ultracold chemistry. This thesis will present the final steps to complete control over both internal and external degrees of freedom of the molecule which allows us to control, and even completely suppress, the chemical reactions between molecules. First, the control over internal states has been achieved through coherent state transfer to the ro-vibronic ground state and coherent manipulations of the hyperfine and rotational states with microwave radiation. Second, external degrees of freedom are controlled by loading the gas into an optical lattice. With the molecules loaded into a one-dimensional lattice, the orientation of the molecular collisions is controlled by manipulating both internal (hyperfine states) and external (motional states in the direction of tight confinement) degrees of freedom. Most striking is that by preparing the molecules all in the lowest band of the lattice in the same internal state, the molecular collisions can only occur in a side-by-side" orientation, where the chemical reaction rate is suppressed by the repulsive dipole-dipole interactions. The chemical reaction can be suppressed completely by further constraining the motion in the trap in a strong 3D lattice. Here we see lifetimes longer than 20 s, limited by o-resonant light scattering. Finally, the ac polarizability of the molecules is explored and controlled. The different rotational states of the molecule have different polarizabilities and will experience a different trapping force in both the optical dipole trap or lattice. We show that there is a magic angle" between the quantization axis and the polarization of the trapping laser at which the polarizabilities of two different rotational states can be matched, eliminating dephasing and allowing for coherent manipulations between rotational states.
Ultracold Chemistry
Because reactive collisions limit the lifetime of ultracold molecular ensembles, controlling chemical reactivity at ultralow temperatures has been a long-standing goal. Using large electric fields that trigger resonant dipolar interactions between potassium-rubidium molecules trapped in a quasi–two-dimensional geometry, Matsuda et al. report suppression of the reactive loss rate in the vicinity of the dipolar-mediated resonances by up to an order of magnitude below the background value. The proposed shielding mechanism is general and is expected to be effective in three-dimensional geometry. It could also be used for creating long-lived quantum molecular gases of other polar molecules under strong electric fields.
Science , this issue p. [1324][1]
[1]: /lookup/doi/10.1126/science.abe7370
Abstract The control of molecules is key to the investigation of quantum phases, in which rich degrees of freedom can be used to encode information and strong interactions can be precisely tuned1. Inelastic losses in molecular collisions2–5, however, have greatly hampered the engineering of low-entropy molecular systems6. So far, the only quantum degenerate gas of molecules has been created via association of two highly degenerate atomic gases7, 8. Here we use an external electric field along with optical lattice confinement to create a two-dimensional Fermi gas of spin-polarized potassium–rubidium (KRb) polar molecules, in which elastic, tunable dipolar interactions dominate over all inelastic processes. Direct thermalization among the molecules in the trap leads to efficient dipolar evaporative cooling, yielding a rapid increase in phase-space density. At the onset of quantum degeneracy, we observe the effects of Fermi statistics on the thermodynamics of the molecular gas. These results demonstrate a general strategy for achieving quantum degeneracy in dipolar molecular gases in which strong, long-range and anisotropic dipolar interactions can drive the emergence of exotic many-body phases, such as interlayer pairing and p-wave superfluidity.
Ultracold polar molecules, because of their long-range, spatially anisotropic interactions, are a new quantum system in which to study novel many-body phenomena. In our lab, we have produced the first quantum gas of 40K87Rb polar molecules. These molecules were found to undergo exothermic chemical reactions, and this led to interesting studies of chemistry near absolute zero. By creating the molecules at individual sites of a 3D optical lattice, we completely suppress these chemical reactions, and the polar molecule gas becomes stable and lives for tens of seconds. This thesis documents our efforts to explore coherent, many-body phenomena resulting from long-range dipolar interactions in the lattice. By encoding a spin-1=2 system in the rotational states of the molecules, we were able to realize spin-exchange interactions based on a spin Hamiltonian, which is one of the first steps in studying quantum magnetism with polar molecules. While this study was the first realization of such coherent dipolar interactions with polar molecules in a lattice, its full potential was limited by the low lattice filling fractions. Using our ability to exquisitely control the initial atomic gas mixture, we loaded a Mott insulator of Rb and a band insulator of K into the lattice. This quantum synthesis approach led to significantly higher molecular filling fractions and represents the first fully connected system of polar molecules in an optical lattice. This low-entropy quantum gas of polar molecules opens the door to interesting quantum simulations, which should be attainable in the next generation of the experiment.