Niels Syassen

Cavity cooling of a single atom

All conventional methods to laser-cool atoms rely on repeated cycles of optical pumping and spontaneous emission of a photon by the atom. Spontaneous emission in a random direction provides the dissipative mechanism required to remove entropy from the atom. However, alternative cooling methods have been proposed for a single atom strongly coupled to a high-finesse cavity; the role of spontaneous emission is replaced by the escape of a photon from the cavity. Application of such cooling schemes would improve the performance of atom–cavity systems for quantum information processing. Furthermore, as cavity cooling does not rely on spontaneous emission, it can be applied to systems that cannot be laser-cooled by conventional methods; these include molecules (which do not have a closed transition) and collective excitations of Bose condensates, which are destroyed by randomly directed recoil kicks. Here we demonstrate cavity cooling of single rubidium atoms stored in an intracavity dipole trap. The cooling mechanism results in extended storage times and improved localization of atoms. We estimate that the observed cooling rate is at least five times larger than that produced by free-space cooling methods, for comparable excitation of the atom.

Normal-mode spectroscopy of a single bound atom-cavity system

The energy-level structure of a single atom strongly coupled to the mode of a high-finesse optical cavity is investigated. The atom is stored in an intracavity dipole trap and cavity cooling is used to compensate for inevitable heating. Two well-resolved normal modes are observed both in the cavity transmission and the trap lifetime. The experiment is in good agreement with a Monte Carlo simulation, demonstrating our ability to localize the atom to within lambda/10 at a cavity antinode.

Strong Dissipation Inhibits Losses and Induces Correlations in Cold Molecular Gases

Atomic quantum gases in the strong-correlation regime offer unique possibilities to explore a variety of many-body quantum phenomena. Reaching this regime has usually required both strong elastic and weak inelastic interactions because the latter produce losses. We show that strong inelastic collisions can actually inhibit particle losses and drive a system into a strongly correlated regime. Studying the dynamics of ultracold molecules in an optical lattice confined to one dimension, we show that the particle loss rate is reduced by a factor of 10. Adding a lattice along the one dimension increases the reduction to a factor of 2000. Our results open the possibility to observe exotic quantum many-body phenomena with systems that suffer from strong inelastic collisions.

Preparation of a quantum state with one molecule at each site of an optical lattice

Ultracold gases in optical lattices are of great interest, because these systems bear great potential for applications in quantum simulations and quantum information processing, in particular when using particles with a long-range dipole–dipole interaction, such as polar molecules1,2,3,4,5. Here we show the preparation of a quantum state with exactly one molecule at each site of an optical lattice. The molecules are produced from an atomic Mott insulator6 with a density profile chosen such that the central region of the gas contains two atoms per lattice site. A Feshbach resonance is used to associate the atom pairs to molecules7,8,9,10,11,12,13,14. The remaining atoms can be removed with blast light13,15. The technique does not rely on the molecule–molecule interaction properties and is therefore applicable to many systems.

Dissipation-induced hard-core boson gas in an optical lattice

We present a theoretical investigation of a lattice Tonks-Girardeau gas that is created by inelastic, instead of elastic interactions. An analytical calculation shows that in the limit of strong two-body losses, the dynamics of the system is effectively that of a hard-core boson gas. We also derive an analytic expression for the effective loss rate. We find good agreement between these analytical results and results from a rigorous numerical calculation. The hard- core character of the particles is visible both in a reduced effective loss rate and in the momentum distribution of the gas.

Lieb-Liniger model of a dissipation-induced Tonks-Girardeau gas

We show that strong inelastic interactions between bosons in one dimension create a Tonks-Girardeau gas, much as in the case of elastic interactions. We derive a Markovian master equation that describes the loss caused by the inelastic collisions. This yields a loss rate equation and a dissipative Lieb-Liniger model for short times. We obtain an analytic expression for the pair correlation function in the limit of strong dissipation. Numerical calculations show how a diverging dissipation strength leads to a vanishing of the actual loss rate and renders an additional elastic part of the interaction irrelevant.

Feshbach spectroscopy of a shape resonance

We present a new spectroscopic technique for investigating shape resonances based on the association and dissociation of ultracold molecules using a Feshbach resonance.

Collisional decay of 87Rb Feshbach molecules at 1005.8 G

We present measurements of the loss-rate coefficients K{sub am} and K{sub mm} caused by inelastic atom-molecule and molecule-molecule collisions. A thermal cloud of atomic {sup 87}Rb is prepared in an optical dipole trap. A magnetic field is ramped across the Feshbach resonance at 1007.4 G. This associates atom pairs to molecules. A measurement of the molecule loss at 1005.8 G yields K{sub am}=2x10{sup -10} cm{sup 3}/s. Additionally, the atoms can be removed with blast light. In this case, the measured molecule loss yields K{sub mm}=3x10{sup -10} cm{sup 3}/s.

Atom-molecule Rabi oscillations in a Mott insulator.

We observe large-amplitude Rabi oscillations between an atomic and a molecular state near a Feshbach resonance. The experiment uses 87Rb in an optical lattice and a Feshbach resonance near 414 G. The frequency and amplitude of the oscillations depend on the magnetic field in a way that is well described by a two-level model. The observed density dependence of the oscillation frequency agrees with theoretical expectations. We confirmed that the state produced after a half-cycle contains exactly one molecule at each lattice site. In addition, we show that, for energies in a gap of the lattice band structure, the molecules cannot dissociate.

Collisional relaxation of Feshbach molecules and three-body recombination in 87Rb Bose-Einstein condensates

We predict the resonance-enhanced magnetic field dependence of atom-dimer relaxation and three-body recombination rates in a {sup 87}Rb Bose-Einstein condensate close to 1007 G. Our exact treatments of three-particle scattering explicitly include the dependence of the interactions on the atomic Zeeman levels. The Feshbach resonance distorts the entire diatomic energy spectrum, causing interferences in both loss phenomena. Our two independent experiments confirm the predicted recombination loss over a range of rate constants that spans four orders of magnitude.