John L. Bohn

Creation of ultracold molecules from a Fermi gas of atoms

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 or a magnetic field Feshbach resonance. 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. Here we report the creation and quantitative characterization of ultracold 40K2 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.

Quantum-State Controlled Chemical Reactions of Ultracold Potassium-Rubidium Molecules

Colliding in the Cold Chemical reactions occur through molecular collisions, which, in turn, are governed by the distributions of energy in each colliding partner. What happens when molecules are cooled so that they no longer have sufficient energy to collide? Ospelkaus et al. (p. 853; see the Perspective by Hutson) explored this question by preparing a laser-cooled sample of potassium rubidium (KRb) diatomics with barely any residual energy in any form (translational, rotational, vibrational, or electronic). By monitoring heat release over time, evidence was gathered for exothermic atom exchange reactivity through quantum mechanical tunneling. As predicted by theory, these reactions were exquisitely sensitive to the molecular states, with rates changing by orders of magnitude on varying minor factors such as nuclear spin orientation. Reactions mediated by quantum mechanical tunneling are observed, even in a sample of molecules cooled almost to a standstill. How does a chemical reaction proceed at ultralow temperatures? Can simple quantum mechanical rules such as quantum statistics, single partial-wave scattering, and quantum threshold laws provide a clear understanding of the molecular reactivity under a vanishing collision energy? Starting with an optically trapped near–quantum-degenerate gas of polar 40K87Rb molecules prepared in their absolute ground state, we report experimental evidence for exothermic atom-exchange chemical reactions. When these fermionic molecules were prepared in a single quantum state at a temperature of a few hundred nanokelvin, we observed p-wave–dominated quantum threshold collisions arising from tunneling through an angular momentum barrier followed by a short-range chemical reaction with a probability near unity. When these molecules were prepared in two different internal states or when molecules and atoms were brought together, the reaction rates were enhanced by a factor of 10 to 100 as a result of s-wave scattering, which does not have a centrifugal barrier. The measured rates agree with predicted universal loss rates related to the two-body van der Waals length.

Dipolar collisions of polar molecules in the quantum regime

Ultracold polar molecules offer the possibility of exploring quantum gases with interparticle interactions that are strong, long-range and spatially anisotropic. This is in stark contrast to the much studied dilute gases of ultracold atoms, which have isotropic and extremely short-range (or ‘contact’) interactions. Furthermore, the large electric dipole moment of polar molecules can be tuned using an external electric field; this has a range of applications such as the control of ultracold chemical reactions, the design of a platform for quantum information processing and the realization of novel quantum many-body systems. Despite intense experimental efforts aimed at observing the influence of dipoles on ultracold molecules, only recently have sufficiently high densities been achieved. Here we report the experimental observation of dipolar collisions in an ultracold molecular gas prepared close to quantum degeneracy. For modest values of an applied electric field, we observe a pronounced increase in the loss rate of fermionic potassium–rubidium molecules due to ultracold chemical reactions. We find that the loss rate has a steep power-law dependence on the induced electric dipole moment, and we show that this dependence can be understood in a relatively simple model based on quantum threshold laws for the scattering of fermionic polar molecules. In addition, we directly observe the spatial anisotropy of the dipolar interaction through measurements of the thermodynamics of the dipolar gas. These results demonstrate how the long-range dipolar interaction can be used for electric-field control of chemical reaction rates in an ultracold gas of polar molecules. Furthermore, the large loss rates in an applied electric field suggest that creating a long-lived ensemble of ultracold polar molecules may require confinement in a two-dimensional trap geometry to suppress the influence of the attractive, ‘head-to-tail’, dipolar interactions.

Tuning p-wave interactions in an ultracold Fermi gas of atoms.

We have measured a p-wave Feshbach resonance in a single-component, ultracold Fermi gas of 40K atoms. We have used this resonance to enhance the normally suppressed p-wave collision cross section to values larger than the background s-wave cross section between 40K atoms in different spin states. In addition to the modification of two-body elastic processes, the resonance dramatically enhances three-body inelastic collisional loss.

Resonant control of elastic collisions in an optically trapped Fermi gas of atoms

We have loaded an ultracold gas of fermionic atoms into a far-off resonance optical dipole trap and precisely controlled the spin composition of the trapped gas. We have measured a magnetic-field Feshbach resonance between atoms in the two lowest energy spin states, /9/2,-9/2> and /9/2,-7/2>. The resonance peaks at a magnetic field of 201.5+/-1.4 G and has a width of 8.0+/-1.1 G. Using this resonance, we have changed the elastic collision cross section in the gas by nearly 3 orders of magnitude.

Observation of heteronuclear feshbach resonances in a mixture of bosons and fermions

Three magnetic-field induced heteronuclear Feshbach resonances were identified in collisions between bosonic 87Rb and fermionic 40K atoms in their absolute ground states. Strong inelastic loss from an optically trapped mixture was observed at the resonance positions of 492, 512, and 543+/-2 G. The magnetic-field locations of these resonances place a tight constraint on the triplet and singlet cross-species scattering lengths, yielding (-281+/-15)a(0) and (-54+/-12)a(0), respectively. The width of the loss feature at 543 G is 3.7+/-1.5 G wide; this broad Feshbach resonance should enable experimental control of the interspecies interactions.

Collisions near threshold in atomic and molecular physics

We review topics of current interest in the physics of electronic, atomic and molecular scattering in the vicinity of thresholds. Starting from phase space arguments, we discuss the modifications of the Wigner law that are required to deal with scattering by Coulomb, dipolar and dispersion potentials, as well as aspects of threshold behaviour observed in ultracold atomic collisions. We employ the tools of quantum defect and semiclassical theories to bring out the rich variety of threshold behaviours. The discussion is then turned to recent progress in understanding threshold behaviour of many-body break-ups into both charged and neutral species, including both Wannier double ionization and three-body recombination in ultracold gases. We emphasize the dominant role that hyperspherical coordinate methods have played in understanding these problems. We assess the effects of external fields on scattering, and the corresponding modification of phase space that alters the Wigner law. Threshold laws in low dimensions and examples of their applications to specific collision processes are discussed.

Controlling the Hyperfine State of Rovibronic Ground-State Polar Molecules

We report the preparation of a rovibronic ground-state molecular quantum gas in a single hyperfine state and, in particular, the absolute lowest quantum state. This addresses the last internal degree of freedom remaining after the recent production of a near quantum degenerate gas of molecules in their rovibronic ground state, and provides a crucial step towards full control over molecular quantum gases. We demonstrate a scheme that is general for bialkali polar molecules and allows the preparation of molecules in a single hyperfine state or in an arbitrary coherent superposition of hyperfine states. The scheme relies on electric-dipole, two-photon microwave transitions through rotationally excited states and makes use of electric nuclear quadrupole interactions to transfer molecular population between different hyperfine states.

Multiplet structure of Feshbach resonances in nonzero partial waves

We report a unique feature of magnetic-field Feshbach resonances in which atoms collide with nonzero orbital angular momentum. p-wave (l=1) Feshbach resonances are split into two components depending on the magnitude of the resonant states projection of orbital angular momentum onto the field axis. This splitting is due to the magnetic dipole-dipole interaction between the atoms and it offers a means to tune anisotropic interactions of an ultracold gas of atoms. Furthermore this splitting in the p-wave Feshbach resonance has been experimentally observed and is reported. A parametrization of the p-wave resonance in terms of an effective-range expansion is given.

Magnetoelectrostatic Trapping of Ground State OH Molecules

We report magnetic confinement of neutral, ground state OH at a density of approximately 3 x 10(3) cm(-3) and temperature of approximately 30 mK. An adjustable electric field sufficiently large to polarize the OH is superimposed on the trap in various geometries, making an overall potential arising from both Zeeman and Stark effects. An effective molecular Hamiltonian is constructed, with Monte Carlo simulations accurately modeling the observed single-molecule dynamics in various trap configurations. Magnetic trapping of cold polar molecules under adjustable electric fields may enable study of low energy dipolar interactions.