14 research outputs found
Controlling and Detecting Spin Correlations of Ultracold Atoms in Optical lattices
We report on the controlled creation of a valence bond state of delocalized
effective-spin singlet and triplet dimers by means of a bichromatic optical
superlattice. We demonstrate a coherent coupling between the singlet and
triplet states and show how the superlattice can be employed to measure the
singlet-fraction employing a spin blockade effect. Our method provides a
reliable way to detect and control nearest-neighbor spin correlations in
many-body systems of ultracold atoms. Being able to measure these correlations
is an important ingredient to study quantum magnetism in optical lattices. We
furthermore employ a SWAP operation between atoms being part of different
triplets, thus effectively increasing their bond-length. Such SWAP operation
provides an important step towards the massively parallel creation of a
multi-particle entangled state in the lattice.Comment: 6 pages, 4 figure
Experimental realization of strong effective magnetic fields in an optical lattice
We use Raman-assisted tunneling in an optical superlattice to generate large
tunable effective magnetic fields for ultracold atoms. When hopping in the
lattice, the accumulated phase shift by an atom is equivalent to the
Aharonov-Bohm phase of a charged particle exposed to a staggered magnetic field
of large magnitude, on the order of one flux quantum per plaquette. We study
the ground state of this system and observe that the frustration induced by the
magnetic field can lead to a degenerate ground state for non-interacting
particles. We provide a measurement of the local phase acquired from
Raman-induced tunneling, demonstrating time-reversal symmetry breaking of the
underlying Hamiltonian. Furthermore, the quantum cyclotron orbit of single
atoms in the lattice exposed to the magnetic field is directly revealed.Comment: 6 pages, 5 figure
Controlling Correlated Tunneling and Superexchange Interactions with AC-Driven Optical Lattices
The dynamical control of tunneling processes of single particles plays a
major role in science ranging from Shapiro steps in Josephson junctions to the
control of chemical reactions via light in molecules. Here we show how such
control can be extended to the regime of strongly interacting particles.
Through a weak modulation of a biased tunnel contact, we have been able to
coherently control single particle and correlated two-particle hopping
processes. We have furthermore been able to extend this control to
superexchange spin interactions in the presence of a magnetic-field gradient.
We show how such photon assisted superexchange processes constitute a novel
approach to realize arbitrary XXZ spin models in ultracold quantum gases, where
transverse and Ising type spin couplings can be fully controlled in magnitude
and sign.Comment: 10 pages, 9 figure
Quantum Spin Dynamics of Mode-Squeezed Luttinger Liquids in Two-Component Atomic Gases
We report on the observation of the phase dynamics of interacting
one-dimensional ultracold bosonic gases with two internal degrees of freedom.
By controlling the non-linear atomic interactions close to a Feshbach resonance
we are able to induce a phase diffusive many-body spin dynamics. We monitor
this dynamical evolution by Ramsey interferometry, supplemented by a novel,
many-body echo technique. We find that the time evolution of the system is well
described by a Luttinger liquid initially prepared in a multimode squeezed
state. Our approach allows us to probe the non-equilibrium evolution of
one-dimensional many-body quantum systems.Comment: 4 pages, 3 figures Updated version, minor change
Many-body Landau-Zener dynamics in coupled 1D Bose liquids
The Landau-Zener model of a quantum mechanical two-level system driven with a
linearly time dependent detuning has served over decades as a textbook paradigm
of quantum dynamics. In their seminal work [L. D. Landau, Physik. Z. Sowjet. 2,
46 (1932); C. Zener, Proc. Royal Soc. London 137, 696 (1932)], Landau and Zener
derived a non-perturbative prediction for the transition probability between
two states, which often serves as a reference point for the analysis of more
complex systems. A particularly intriguing question is whether that framework
can be extended to describe many-body quantum dynamics. Here we report an
experimental and theoretical study of a system of ultracold atoms, offering a
direct many-body generalization of the Landau-Zener problem. In a system of
pairwise tunnel-coupled 1D Bose liquids we show how tuning the correlations of
the 1D gases, the tunnel coupling between the tubes and the inter-tube
interactions strongly modify the original Landau-Zener picture. The results are
explained using a mean-field description of the inter-tube condensate
wave-function, coupled to the low-energy phonons of the 1D Bose liquid.Comment: 13 pages, 10 figures
Quantum dynamics of a single, mobile spin impurity
Quantum magnetism describes the properties of many materials such as transition metal oxides and cuprate superconductors. One of its elementary processes is the propagation of spin excitations. Here we study the quantum dynamics of a deterministically created spin-impurity atom, as it propagates in a one-dimensional lattice system. We probe the full spatial probability distribution of the impurity at different times using single-site-resolved imaging of bosonic atoms in an optical lattice. In the Mott-insulating regime, a post-selection of the data allows to reduce the effect of temperature, giving access to a space- and time-resolved measurement of the quantum-coherent propagation of a magnetic excitation in the Heisenberg model. Extending the study to the bath's superfluid regime, we determine quantitatively how the bath strongly affects the motion of the impurity. The experimental data shows a remarkable agreement with theoretical predictions allowing us to determine the effect of temperature on the coherence and velocity of impurity motion. Our results pave the way for a new approach to study quantum magnetism, mobile impurities in quantum fluids, and polarons in lattice systems
Femtosecond up-conversion technique for probing the charge transfer in a P3HT : PCBM blend via photoluminescence quenching
Abstract We report on an experimental study of the charge transfer dynamics in a P3HT : PCBM blend by means of a femtosecond fluorescence up-conversion technique. Using two-photon excitation we probe the exciton dynamics in P3HT and a P3HT : PCBM blend with a weight ratio of 1 : 1 at excitation densities of up to 6 × 10 18 cm −3 . In both samples we find strongly nonexponential decay traces compatible with (i) diffusion-limited exciton-exciton annihilation and (ii) diffusion-limited donor-acceptor charge transfer in the polymer blend. Additionally, our results indicate that in the P3HT : PCBM blend about 50% of the photogenerated excitons undergo a prompt charge transfer process on a time scale of about 150 fs. Our study shows that fluorescence spectroscopy with femtosecond time resolution is a powerful technique for probing ultrafast charge transfer processes in solar cell materials