Inducing Transport in a Dissipation-Free Lattice with Super Bloch Oscillations

Particles in a perfect lattice potential perform Bloch oscillations when subject to a constant force, leading to localization and preventing conductivity. For a weakly-interacting Bose-Einstein condensate (BEC) of Cs atoms, we observe giant center-of-mass oscillations in position space with a displacement across hundreds of lattice sites when we add a periodic modulation to the force near the Bloch frequency. We study the dependence of these "super" Bloch oscillations on lattice depth, modulation amplitude, and modulation frequency and show that they provide a means to induce linear transport in a dissipation-free lattice. Surprisingly, we find that, for an interacting quantum system, super Bloch oscillations strongly suppress the appearance of dynamical instabilities and, for our parameters, increase the phase-coherence time by more than a factor of hundred.Comment: 4 pages, 5 figure

Confinement-Induced Resonances in Low-Dimensional Quantum Systems

We report on the observation of confinement-induced resonances in strongly interacting quantum-gas systems with tunable interactions for one- and two-dimensional geometry. Atom-atom scattering is substantially modified when the s-wave scattering length approaches the length scale associated with the tight transversal confinement, leading to characteristic loss and heating signatures. Upon introducing an anisotropy for the transversal confinement we observe a splitting of the confinement-induced resonance. With increasing anisotropy additional resonances appear. In the limit of a two-dimensional system we find that one resonance persists.Comment: 4 pages, 4 figure

Active stabilization of kilogauss magnetic fields to the ppm level for magnetoassociation on ultranarrow Feshbach resonances

Feshbach association of ultracold molecules using narrow resonances requires exquisite control of the applied magnetic field. Here we present a magnetic field control system to deliver magnetic fields of over 1000\,G with ppm-level precision integrated into an ultracold-atom experimental setup. We combine a battery-powered current-stabilized power supply with active feedback stabilization of the magnetic field using fluxgate magnetic field sensors. As a real-world test we perform microwave spectroscopy of ultracold Rb atoms and demonstrate an upper limit on our magnetic field stability of 2.4(3)\,mG at 1050~G [2.3(3)\,ppm relative] as determined from the spectral feature.Comment: 8 pages, 4 figure

Ultracold Dense Samples of Dipolar RbCs Molecules in the Rovibrational and Hyperfine Ground State

We produce ultracold dense trapped samples of Rb87Cs133 molecules in their rovibrational ground state, with full nuclear hyperfine state control, by stimulated Raman adiabatic passage (STIRAP) with efficiencies of 90%. We observe the onset of hyperfine-changing collisions when the magnetic field is ramped so that the molecules are no longer in the hyperfine ground state. A strong quadratic shift of the transition frequencies as a function of applied electric field shows the strongly dipolar character of the RbCs ground-state molecule. Our results open up the prospect of realizing stable bosonic dipolar quantum gases with ultracold molecules

Molecular spectroscopy for ground-state transfer of ultracold RbCs molecules

We perform one- and two-photon high resolution spectroscopy on ultracold samples of RbCs Feshbach molecules with the aim to identify a suitable route for efficient ground-state transfer in the quantum-gas regime to produce quantum gases of dipolar RbCs ground-state molecules. One-photon loss spectroscopy allows us to probe deeply bound rovibrational levels of the mixed excited (A1{\Sigma}+ - b3{\Pi}0) 0+ molecular states. Two-photon dark state spectroscopy connects the initial Feshbach state to the rovibronic ground state. We determine the binding energy of the lowest rovibrational level |v"=0,J"=0> of the X1{\Sigma}+ ground state to be DX 0 = 3811.5755(16) 1/cm, a 300-fold improvement in accuracy with respect to previous data. We are now in the position to perform stimulated two-photon Raman transfer to the rovibronic ground state.Comment: Submitted to PCCP themed issue: Physics and Chemistry of Cold Molecule

Pinning quantum phase transition for a Luttinger liquid of strongly interacting bosons

One of the most remarkable results of quantum mechanics is the fact that many-body quantum systems may exhibit phase transitions even at zero temperature. Quantum fluctuations, deeply rooted in Heisenberg's uncertainty principle, and not thermal fluctuations, drive the system from one phase to another. Typically, the relative strength of two competing terms in the system's Hamiltonian is changed across a finite critical value. A well-known example is the Mott-Hubbard quantum phase transition from a superfluid to an insulating phase, which has been observed for weakly interacting bosonic atomic gases. However, for strongly interacting quantum systems confined to lower-dimensional geometry a novel type of quantum phase transition may be induced for which an arbitrarily weak perturbation to the Hamiltonian is sufficient to drive the transition. Here, for a one-dimensional (1D) quantum gas of bosonic caesium atoms with tunable interactions, we observe the commensurate-incommensurate quantum phase transition from a superfluid Luttinger liquid to a Mott-insulator. For sufficiently strong interactions, the transition is induced by adding an arbitrarily weak optical lattice commensurate with the atomic granularity, which leads to immediate pinning of the atoms. We map out the phase diagram and find that our measurements in the strongly interacting regime agree well with a quantum field description based on the exactly solvable sine-Gordon model. We trace the phase boundary all the way to the weakly interacting regime where we find good agreement with the predictions of the 1D Bose-Hubbard model. Our results open up the experimental study of quantum phase transitions, criticality, and transport phenomena beyond Hubbard-type models in the context of ultracold gases

A general quantum-engineering technique for efcient production of ultracold dipolar molecules

In dieser Arbeit berichte ich über eine universell einsetzbare Methode zur Erzeugung eines Ensembles mit niedriger Entropie aus heteronuklearen Molekülen. Die Zielsetzung ist die quantenphysikalische Konstruktion eines ausgeprägt dipolaren Quantengases mit einem einzelnen Teilchen an jedem Gitterplatz. Zuerst steht die überarbeitete, zeitgleiche Produktion zweier quantenentarteter Ensembles von 87 Rb und 133 Cs unter Berücksichtigung der folgenden Manipulationsschritte im Vordergrund. Danach laden wir die beiden räumlich getrennten Bose-Einstein Kondensate aus Rb und Cs kontrolliert in ein optisches Gitter, sodass wir einen Cs Mott Isolator mit genau einem Cs Atom pro Gitterplatz erschaffen um Teilchenverluste zu verhindern. Die Gittertiefe wird so hochgefahren, dass sich zeitglich ein Rb Superuid im Gitter ausbilden kann, was fundamental für den nächsten Schritt ist, da in diesem das superuide Rb mit einer beweglichen Dipolfalle in Richtung dem Cs Mott Isolator bewegt wird und schließlich mit diesem zur vollständigen Überlagerung gebracht wird. Dazu wird auch eine magnetische Feshbach Resonanz eingesetzt um die interspezies Wechselwirkung auszuschalten und die Überlagerung überhaupt zu ermöglichen. Durch die außerdem gegebene repulsive intraspezies Wechselwirkung bei Rb is es möglich genau ein Rb-Cs Atompaar pro Gitterplatz zu erzeugen, sodass eine adiabatische Magnetfeldrampe angewendet werden kann um aus den Rb-Cs Paaren RbCs Moleküle zu erzeugen. Anschließender kohärenter adiabatischer Transfer durch stimulated Raman adiabatic passage (STIRAP) ermöglicht den Transfer der schwach gebundenen Moleküle in den rovibronischen Grundzustand. Mittels eines elektrischen Feldes kann das Ensemble polarisiert und die Stärke der langreichweitigen Dipol-Dipol Wechselwirkung eingestellt werden. Diese Arbeit konnte erfolgreich ein bisher unerreichtes Gitterfüllungsverhältnis mit Molekülen von über 30% nachweisen. Es wird erwartet, dass dieser Wert durch einige Verbesserungen, die ebenfalls in dieser Arbeit beschrieben werden, weiter gesteigert werden kann. Die hier vorgestellte Methode ermöglicht die efziente Erzeugnung eines Systems mit einstellbarer, langreichweitiger Wechselwirkung, welches sich perfekt für Quantensimulationsexperimente, Tests von Naturkonstanten und Untersuchung neuartiger Materiezuständen eignet.This thesis reports on an universal method for highly efcient production of a low entropy ensemble of heteronuclear molecules. The goal is to efciently "quantum-engineer" a strongly dipolar quantum gas in an optical lattice with unity lling. First, the simultaneous production of two quantum degenerate samples of 87 Rb and 133 Cs is revisited in view of the subsequent manipulation steps. Then we load the two spatially separated Bose- Einstein condensates of Rb and Cs into an optical lattice and we create a Mott insulator of Cs with exactly one atom per lattice site to suppress loss. The lattice depth is adjusted to simultaneously support a superuid of Rb which is of fundamental importance because in the next step we move Rb towards the Cs Mott insulator with the movable Rb trap. A magnetic Feshbach resonance is used to tune the interspecies interactions to zero and to merge both atomic samples. In combination with the repulsive Rb-Rb interactions, Rb-Cs precursor pairs can form on each individual lattice site. An adiabatic magnetic-eld ramp associates the paired atoms to RbCs molecules. Coherent adiabatic transfer by stimulated Raman adiabatic passage (STIRAP) can be applied to subsequently transfer the weakly-bound molecules to the rovibronic ground state. An electrical eld can be used to polarize the ensemble and to tune the long-range dipole-dipole interactions. This work succeeded in producing an outstanding lling fraction of more than 30% of molecules in an optical lattice. This value can be pushed much higher by several improvements, which will be discussed in this work. The method developed in this thesis provides a system with tunable long-range interactions, perfectly suited for quantum-simulation experiments, tests of fundamental constants and investigations of novel states of matter.by Lukas K. ReichsöllnerZusammenfassung in deutscher SpracheUniversität Innsbruck, Dissertation, 2017OeBB(VLID)226874

Fast and Selective Post-Polymerization Modification of Conjugated Polymers Using Dimethyldioxirane

Modification of functional groups attached to conjugated polymer backbones can drastically alter the material properties. Oxidation of electron-donating thioalkyl substituents to electron-withdrawing sulfoxides or sulfones is a particularly effective modification. However, so far, this reaction has not been studied for the modification of conjugated polymers used in organic electronics. Crucial questions regarding selectivity and reaction time waited to be addressed. Here, we show that the reaction is highly selective and complete within just a few minutes when using dimethyldioxirane (DMDO) for the oxidation of thioalkyl substituents attached to the well investigated conjugated polymers poly(9-(1-octylnonyl)carbazole-alt-4,7-dithienylbenzothiadiazole) (PCDTBT) and poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT). The selectivity was confirmed by comparison with polymers obtained from pre-oxidized monomers and by control experiments using related polymers without thioalkyl substituents. Using DMDO, the oxidation yields acetone as the only side-product, which reduces the work-up to mere evaporation of solvents and excessive reagent. Our results show that this oxidation is an exciting method for the preparation of electron-deficient conjugated polymers. It may even allow the preparation of electron acceptors for solar cells directly from the electron donors