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Absolute temperature, the fundamental temperature scale in thermodynamics, is usually bound to be positive. Under special conditions, however, negative temperatures - where high-energy states are more occupied than low-energy states - are also possible. So far, such states have been demonstrated in localized systems with finite, discrete spectra. Here, we were able to prepare a negative temperature state for motional degrees of freedom. By tailoring the Bose-Hubbard Hamiltonian we created an attractively interacting ensemble of ultracold bosons at negative temperature that is stable against collapse for arbitrary atom numbers. The quasi-momentum distribution develops sharp peaks at the upper band edge, revealing thermal equilibrium and bosonic coherence over several lattice sites. Negative temperatures imply negative pressures and open up new parameter regimes for cold atoms, enabling fundamentally new many-body states and counterintuitive effects such as Carnot engines above unity efficiency., 5 pages, 4 figures + supplementary material

We describe a chip-based magnetic conveyor that transports ultracold atoms with high positioning accuracy over long distances, into an interaction region which is well separated from the magneto-optical trap and gives good optical access to the atoms. The conveyor can work in two different modes, with or without external bias field. The transport potential is generated by a two-layer conductor pattern, enabling a significantly smoother transport than our earlier single-layer conveyor. This is confirmed by numerical field calculations, using an optimization procedure that minimizes shape deformations as well as deviations from the linear transport path. We experimentally demonstrate the use of this conveyor in the mode with external bias field, transporting a cloud of cold atoms over a linear distance of 6 cm and a total distance of 24 cm. We also describe an on-chip quadrupole trap that can be rotated by π/2. This trap is used to remove design constraints on the orientation of the laser beams in the surface magneto-optical trap. The long-distance conveyor is a versatile tool for experiments with trapped cold atoms, and can achieve sub-micrometric positioning precision. Possible applications of this tool are discussed.

Ultracold bosonic and fermionic quantum gases in optical lattices can be used as versatile model systems for the investigation of fundamental condensed matter physics hamiltonians. We report on two novel detection techniques, which have allowed us to reveal e.g. the in trap density profile of strongly correlated bosonic quantum gases in the lattice. Thereby we have been able to observe the formation of spatial shell structures in the density of the trapped atom cloud across the superfluid to Mott insulator transition. In addition we use quantum noise correlations in expanding bosonic and fermionic atom clouds released from the lattice to observe quantum statistical Hanbury‐Brown and Twiss type hunching and antibunching of the atoms and show that this can be used to reveal the ordering of the particles in the lattice.

We review our experiments on quantum information processing with neutral atoms in optical lattices and magnetic microtraps. Atoms in an optical lattice in the Mott insulator regime serve as a large qubit register. A spin-dependent lattice is used to split and delocalize the atomic wave functions in a controlled and coherent way over a defined number of lattice sites. This is used to experimentally demonstrate a massively parallel quantum gate array, which allows the creation of a highly entangled many-body cluster state through coherent collisions between atoms on neighbouring lattice sites. In magnetic microtraps on an atom chip, we demonstrate coherent manipulation of atomic qubit states and measure coherence lifetimes exceeding one second at micron-distance from the chip surface. We show that microwave near-fields on the chip can be used to create state-dependent potentials for the implementation of a quantum controlled phase gate with these robust qubit states. For single atom detection and preparation, we have developed high finesse fiber Fabry-Perot cavities and integrated them on the atom chip. We present an experiment in which we detected a very small number of cold atoms magnetically trapped in the cavity using the atom chip., Comment: 15 pages, 11 figures, corrected author list

We demonstrate quantum control over both internal and external quantum degrees of freedom in a high number of identical ``chemical reactions,'' carried out in an array of microtraps in a 3D optical lattice. Starting from a Mott insulating phase of an ultracold atomic quantum gas, we use two-photon Raman transitions to create molecules on lattice sites occupied by two atoms. In the atom-molecule conversion process, we can control both the internal rovibronic and external center of mass quantum state of the molecules. The lattice isolates the microscopic chemical reactions from each other, thereby allowing photoassociation spectra without collisional broadening even at high densities of up to $2\ifmmode\times\else\texttimes\fi{}{10}^{15}\text{ }{\mathrm{c}\mathrm{m}}^{\ensuremath{-}3}$.

By loading Bose-Einstein condensates into a three dimensional optical lattice potential we are able to demonstrate several new intriguing regimes in the physics of ultracold atoms. For example by changing the lattice potential depth we have been able to induce a quantum phase transition from a superfluid to a Mott insulating ground state of the system. Furthermore, by rapidly isolating the different potential wells from each other the collapse and revival of the matter wave field of a BoseEinstein condensate has been observed.

We demonstrate the controlled coherent transport and splitting of atomic wave packets in spin-dependent optical lattice potentials. Such experiments open intriguing possibilities for quantum state engineering of many body states. After first preparing localized atomic wave functions in an optical lattice through a Mott insulating phase, we place each atom in a superposition of two internal spin states. Then state selective optical potentials are used to split the wave function of a single atom and transport the corresponding wave packets in two opposite directions. Coherence between the wave packets of an atom delocalized over up to 7 lattice sites is demonstrated., 4 pages, 6 figures

A quantum phase transition from a superfluid to a Mott insulating ground state was observed in a Bose–Einstein condensate stored in a three-dimensional optical lattice potential. With this experiment a new field of physics with ultracold atomic quantum gases is entered. Now interactions between atoms dominate the behavior of the many-body system, such that it cannot be described by the usual theories for weakly interacting Bose gases anymore.