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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.

Ultracold polar molecules possess long-range, anisotropic and tunable dipolar interactions, providing opportunities to probe quantum phenomena that are inaccessible with existing cold gas platforms. However, experimental progress has been hindered by the dominance of two-body loss over elastic interactions, which prevents efficient evaporative cooling. Although recent work has demonstrated controlled interactions by confining molecules to a two-dimensional geometry, a general approach for tuning molecular interactions in a three-dimensional stable system has been lacking. Here we demonstrate tunable elastic dipolar interactions in a bulk gas of ultracold 40K87Rb molecules in three dimensions, facilitated by an electric field-induced shielding resonance that suppresses the reactive loss by a factor of 30. This improvement in the ratio of elastic to inelastic collisions enables direct thermalization. The thermalization rate depends on the angle between the collisional axis and the dipole orientation controlled by an external electric field, a direct manifestation of the anisotropic dipolar interaction. We achieve evaporative cooling mediated by the dipolar interactions in three dimensions. This work demonstrates full control of a long-lived bulk quantum gas system with tunable long-range interactions, paving the way for the study of collective quantum many-body physics.Realizing the potential of dipolar molecular gases to explore quantum physics needs elastic, tunable interactions and low temperatures. This is now possible due to advances in control that suppress molecular losses and enable efficient cooling.

Ultracold collisions of the polyatomic species CaOH are considered, in internal states where the collisions should be dominated by long-range dipole-dipole interactions. The computed rate constants suggest that evaporative cooling can be quite efficient for these species, provided they start at temperatures achievable by laser cooling. The rate constants are shown to become more favorable for evaporative cooling as the electric field increases. Moreover, long-range dimer states (CaOH) 2 * are predicated to occur, having lifetimes on the order of microseconds.

Evaporative cooling of molecules has not been achieved so far, owing to unfavourable collision properties and trap losses; microwave-forced evaporative cooling of hydroxyl molecules loaded in a magnetic quadrupole trap is now reported. Ultracool OH molecules approach quantum regime Evaporative cooling is the process that makes a cup of steaming hot coffee grow cold: the temperature of a substance is decreased by the removal of particles with energies much greater than the average total energy per particle. In the form of forced evaporative cooling of magnetically trapped atoms, it is used to produce Bose–Einstein condensates and other ultracold states of matter in which the quantum regime rules. Ultracold quantum gases of molecules—as opposed to atoms—may have even richer physics, but evaporative cooling of molecules has not been achieved so far because of unfavourable collision properties and trap losses. The paper reports microwave-forced evaporative cooling of hydroxyl (OH) molecules loaded in a magnetic quadrupole trap. This unexpected result occurs because of a long-range, repulsive interaction in the OH system that prevents short-range inelastic losses. Much colder temperatures are expected to be reachable, which may enable a large number of molecular species—including chemically interesting ones—to enter the quantum regime. Atomic physics was revolutionized by the development of forced evaporative cooling, which led directly to the observation of Bose–Einstein condensation 1 , 2 , quantum-degenerate Fermi gases 3 and ultracold optical lattice simulations of condensed-matter phenomena 4 . More recently, substantial progress has been made in the production of cold molecular gases 5 . Their permanent electric dipole moment is expected to generate systems with varied and controllable phases 6 , 7 , 8 , dynamics 9 , 10 , 11 and chemistry 12 , 13 , 14 . However, although advances have been made 15 in both direct cooling and cold-association techniques, evaporative cooling has not been achieved so far. This is due to unfavourable ratios of elastic to inelastic scattering 13 and impractically slow thermalization rates in the available trapped species. Here we report the observation of microwave-forced evaporative cooling of neutral hydroxyl (OH • ) molecules loaded from a Stark-decelerated beam into an extremely high-gradient magnetic quadrupole trap. We demonstrate cooling by at least one order of magnitude in temperature, and a corresponding increase in phase-space density by three orders of magnitude, limited only by the low-temperature sensitivity of our spectroscopic thermometry technique. With evaporative cooling and a sufficiently large initial population, much colder temperatures are possible; even a quantum-degenerate gas of this dipolar radical (or anything else it can sympathetically cool) may be within reach.

Following the realization of Bose–Einstein condensates in atomic gases, an experimental challenge is the production of molecular gases in the quantum regime. A promising approach is to create the molecular gas directly from an ultracold atomic gas; for example, bosonic atoms in a Bose-Einstein condensate have been coupled to electronic ground-state molecules through photoassociation 1 or a magnetic field Feshbach resonance 2 . The availability of atomic Fermi gases offers the prospect of coupling fermionic atoms to bosonic molecules, thus altering the quantum statistics of the system. Such a coupling would be closely related to the pairing mechanism in a fermionic superfluid, predicted to occur near a Feshbach resonance 3 , 4 . Here we report the creation and quantitative characterization of ultracold 40 K 2 molecules. Starting with a quantum degenerate Fermi gas of atoms at a temperature of less than 150 nK, we scan the system over a Feshbach resonance to create adiabatically more than 250,000 trapped molecules; these can be converted back to atoms by reversing the scan. The small binding energy of the molecules is controlled by detuning the magnetic field away from the Feshbach resonance, and can be varied over a wide range. We directly detect these weakly bound molecules through their radio-frequency photodissociation spectra; these probe the molecular wavefunction, and yield binding energies that are consistent with theory.

An ultracold gas of erbium atoms is shown to have many scattering resonances whose quantum fluctuations exhibit chaotic behaviour resulting from the anisotropy of the atoms’ interactions. Scattering complexities The ability to tune the interactions between atoms or molecules cooled to ultracold temperatures provides a powerful test-bed for realizing and exploring exotic states of matter. For the case of simple atoms, the scattering interactions between these cold particles are well understood; less clear is what happens when the constituent particles are more complex. Albert Frisch and colleagues have now entered this uncharted territory using magnetic lanthanide atoms, where they observe the first signatures of chaotic behaviour in the interactions between ultracold atoms. Atomic and molecular samples reduced to temperatures below one microkelvin, yet still in the gas phase, afford unprecedented energy resolution in probing and manipulating the interactions between their constituent particles. As a result of this resolution, atoms can be made to scatter resonantly on demand, through the precise control of a magnetic field 1 . For simple atoms, such as alkalis, scattering resonances are extremely well characterized 2 . However, ultracold physics is now poised to enter a new regime, where much more complex species can be cooled and studied, including magnetic lanthanide atoms and even molecules. For molecules, it has been speculated 3 , 4 that a dense set of resonances in ultracold collision cross-sections will probably exhibit essentially random fluctuations, much as the observed energy spectra of nuclear scattering do 5 . According to the Bohigas–Giannoni–Schmit conjecture, such fluctuations would imply chaotic dynamics of the underlying classical motion driving the collision 6 , 7 , 8 . This would necessitate new ways of looking at the fundamental interactions in ultracold atomic and molecular systems, as well as perhaps new chaos-driven states of ultracold matter. Here we describe the experimental demonstration that random spectra are indeed found at ultralow temperatures. In the experiment, an ultracold gas of erbium atoms is shown to exhibit many Fano–Feshbach resonances, of the order of three per gauss for bosons. Analysis of their statistics verifies that their distribution of nearest-neighbour spacings is what one would expect from random matrix theory 9 . The density and statistics of these resonances are explained by fully quantum mechanical scattering calculations that locate their origin in the anisotropy of the atoms’ potential energy surface. Our results therefore reveal chaotic behaviour in the native interaction between ultracold atoms.

A six-dimensional potential energy surface is constructed for the spin-polarized triplet state of CaF-CaF by \\textit{ab initio} calculations at the CCSD(T) level of theory, followed by Gaussian process interpolation. The potential is utilized to calculate the density of states for this bi alkaline-earth-halogen system where we find the value 0.038 \\(\\mu\\)K\\(^{-1}\\), implying a mean resonance spacing of 26 \\(\\mu\\)K in the collision complex. This value implies an associated Rice-Ramsperger-Kassel-Marcus lifetime of 18 \\(\\mu\\)s, thus predicting long-lived complexes in collisions at ultracold temperatures.

We apply a hyperspherical formulation to a trapped Bose-Einstein condensate with dipolar and contact interactions. Central to this approach is a general correspondence between K-harmonic hyperspherical methods and a suitable Gaussian ansatz to the Gross-Pitaevskii equation, regardless of the form of the interparticle potential. This correspondence allows one to obtain hyperspherical potential energies for a wide variety of physical problems. In the case of the dipolar Bose-Einstein condensate, this motivates the inclusion of a beyond-mean field term within the hyperspherical picture, which allows us to describe the energies and wavefunctions of excitations of self-bound dipolar droplets outside of the mean-field limit.

A full six-dimensional Born-Oppenheimer singlet potential energy surface is constructed for the reaction CaF + CaF \\(\\rightarrow\\) CaF\\(_2\\) + Ca using a multireference configuration interaction (MRCI) electronic structure calculation. The {\\it ab initio} data thus calculated are interpolated by Gaussian process (GP) regression. The four-body potential energy surface features one \\(D_{2h}\\) global minimum and one \\(C_s\\) local minimum, connected by a barrierless transition state that lends insight to the reaction mechanism. This surface is intended to be of use in understanding ultracold chemistry of CaF molecules.