Jannes Heinze

Multiband spectroscopy of ultracold fermions: observation of reduced tunneling in attractive Bose-Fermi mixtures.

We perform a detailed experimental study of the band excitations and tunneling properties of ultracold fermions in optical lattices. Employing a novel multiband spectroscopy for fermionic atoms, we can measure the full band structure and tunneling energy with high accuracy. In an attractive Bose-Fermi mixture we observe a significant reduction of the fermionic tunneling energy, which depends on the relative atom numbers. We attribute this to an interaction-induced increase of the lattice depth due to the self-trapping of the atoms.

Coherent multi-flavour spin dynamics in a fermionic quantum gas

Quantum gases are useful toy models for the study of quantum magnetism. Exquisite control of a spinor gas of fermionic atoms in an optical lattice has now been demonstrated, opening up the exploration of quantum magnetism with high spins.

Giant Spin Oscillations in an Ultracold Fermi Sea

Collective Coherent Spin Dynamics Ultracold gases have shown considerable promise for the quantum simulation of more complicated systems, such as correlated electrons in solids. Usually, researchers use two hyperfine states of the atoms to correspond to the spin up and down states of the electrons; however, these gases typically have a much richer internal state structure. Krauser et al. (p. 157) observed the coherent behavior of a gas of potassium-40 atoms that had 10 accessible internal spin states and that evolved through collisions. The spin state of the system oscillated as a whole, a surprising finding given that the atoms are fermions. Collective behavior in many-body systems is the origin of many fascinating phenomena in nature, ranging from the formation of clouds to magnetic properties of solids. We report on the observation of collective spin dynamics in an ultracold Fermi sea with large spin. As a key result, we observed long-lived and large-amplitude coherent spin oscillations driven by local spin interactions. At ultralow temperatures, Pauli blocking stabilizes the collective behavior, and the Fermi sea behaves as a single entity in spin space. With increasing temperature, we observed a stronger damping associated with particle-hole excitations. Unexpectedly, we found a high-density regime where excited spin configurations are collisionally stabilized. Our results reveal the intriguing interplay between microscopic processes either stimulating or suppressing collective effects in a fermionic many-body system. Long-lived oscillations of the internal state of a trapped fermionic potassium-40 gas occur within a single spatial mode.

Intrinsic photoconductivity of ultracold fermions in optical lattices.

We report on the experimental observation of an analog to a persistent alternating photocurrent in an ultracold gas of fermionic atoms in an optical lattice. The dynamics is induced and sustained by an external harmonic confinement. While particles in the excited band exhibit long-lived oscillations with a momentum-dependent frequency, a strikingly different behavior is observed for holes in the lowest band. An initial fast collapse is followed by subsequent periodic revivals. Both observations are fully explained by mapping the system onto a nonlinear pendulum.

Engineering spin waves in a high-spin ultracold Fermi gas.

We report on the detailed study of multicomponent spin waves in an s=3/2 Fermi gas where the high spin leads to novel tensorial degrees of freedom compared to s=1/2 systems. The excitations of a spin-nematic state are investigated from the linear to the nonlinear regime, where the tensorial character is particularly pronounced. By tuning the initial state we engineer the tensorial spin-wave character, such that the magnitude and the sign of the counterflow spin currents are effectively controlled. A comparison of our data with numerical and analytical results shows good agreement.

The role of mode match in fiber cavities

We study and realize asymmetric fiber-based cavities with optimized mode match to achieve high reflectivity on resonance. This is especially important for mutually coupling two physical systems via light fields, e.g., in quantum hybrid systems. Our detailed theoretical and experimental analysis reveals that on resonance, the interference effect between the directly reflected non-modematched light and the light leaking back out of the cavity can lead to large unexpected losses due to the mode filtering of the incoupling fiber. Strong restrictions for the cavity design result out of this effect and we show that planar-concave cavities are clearly best suited. We validate our analytical model using numerical calculations and demonstrate an experimental realization of an asymmetric fiber Fabry-Pérot cavity with optimized parameters.

Influence of the particle number on the spin dynamics of ultracold atoms

We study the dependency of the quantum spin dynamics on the particle number in a system of ultracold spin-1 atoms within the single-spatial-mode approximation. We find, for all strengths of the spin-dependent interaction, convergence toward the mean-field dynamics in the thermodynamic limit. The convergence is, however, particularly slow when the spin-changing collisional energy and the quadratic Zeeman energy are equal; that is, deviations between quantum and mean-field spin dynamics may be extremely large under these conditions. Our estimates show that quantum corrections to the mean-field dynamics may play a relevant role in experiments with spinor Bose-Einstein condensates. This is especially the case in the regime of few atoms, which may be accessible in optical lattices. Here, spin dynamics is modulated by a beat note at large magnetic fields due to the significant influence of correlated many-body spin states.

Large-amplitude superexchange of high-spin fermions in optical lattices

We show that fermionic high-spin systems with spin-changing collisions allow one to monitor superexchange processes in optical superlattices with large amplitudes and strong spin fluctuations. By investigating the non-equilibrium dynamics, we find a superexchange dominated regime at weak interactions. The underlying mechanism is driven by an emerging tunneling-energy gap in shallow few-well potentials. As a consequence, the interaction-energy gap that is expected to occur only for strong interactions in deep lattices is re-established. By tuning the optical lattice depth, a crossover between two regimes with negligible particle number fluctuations is found: firstly, the common regime with vanishing spin-fluctuations in deep lattices and, secondly, a novel regime with strong spin fluctuations in shallow lattices. We discuss the possible experimental realization with ultracold 40K atoms and observable quantities in double wells and two-dimensional plaquettes.