## Colloquium 25.10.2012; 09:00; Room D 326

### Dr. Ryan Bowler

NIST (National Institute of Standards and Technolog), Boulder USA

Diabatic Ion Transport in a Multi-zone Trap Array

The building blocks required for scalable quantum information processing using trapped atomic ions include qubits with long coherence times, a universal logic gate set, motional state initialization for entangling gates, and information transport. We report progress on the speed-up of ion transport to a time scale comparable to logic gates. Our experiments are carried out in a linear multi-zone trap array in which ions are transported to spatially distinct locations spanning up to 370 mm. An arbitrary waveform generator with a high update rate enables us to smoothly alter the trapping potential on time scales much smaller than the those corresponding to the ions’ motional oscillation period (typically on the order of 0.1 – 1 ms). When transporting ions in a timescale regime comparable to the motional oscillation period, we observe that excitations of coherent states of motion play an important role. Nevertheless the ion returns to near the ground state of motion at the end of the transport for suitable transport parameters. We also investigate the optimization of separation of ion chains at fast time scales.

## Colloquium 25.10.2012; 14:00-15:30; Room D 326

### Dr. Axel Griesmaier

Universität Stuttgart, Physikalisches Institut

Stabilization, de-stabilization and collapse of dipolar Bose-Einstein condensates in 1D lattices

All effects connected with the formation of ordered states in dipolar quantum gases appear close to the border between stability and instability of the trapped gas. These features are mediated by the interplay between short-range and long-range, isotropic and anisotropic interactions and the trap. Due to the anisotropy of dipole-dipole interaction, the stability properties are in fact strongly affected by the confining potential. We have experimentally studied these stability conditions for various scenarios of different trapping geometries and interaction strengths with a dipolar 52Cr Bose-Einstein condensate, especially in the relevant case of periodic 1D lattice potentials where we observe that inter-sites effects play an important role. A 1D lattice, in contrast to a purely contact interacting gas, induces a crossover from a dipolar de-stabilization to a dipolar stabilized regime with increasing lattice depth. In a deep lattice, a dipolar condensate can be stabilized even at large negative scattering length in the interaction-dominated regime. As an important consequence, a dipolar condensate can be stable in trap but immediately collapse as soon as the external confinement is removed. This characteristic feature of strongly dipolar BECs makes the usual mapping from time-of flight measurements on the momentum distribution highly non-trivial. Together with these recent experiments, I will also report on our progress towards the generation of even more dipolar quantum gases of Dysprosium atoms, the species with the highest magnetic moment in the periodic table of elements and on our recent efforts to use the inelastic part of dipole-dipole interaction between atoms to cool dipolar atoms to degeneracy by de-magnetization.

## Colloquium 29.10.2012; 14:00-15:30 h; Room D 326

### Dr. Alessandro Zenesini

Universität Insbruck

"Universal" vs "Non-Universal". The Cs case

A powerful approach to study an ultracold collision between an atom and a diatomic molecule is based on the assumption that interactions are long-range and universal. In this framework, the short-range details of the inter-particle potential and of the atomic and molecular wave-functions are negligible. Because atom-dimer system is also the simplest and non-trivial paradigm of three interacting particles, the physics is tightly related to the problem of universality in three-body bound states.Despite recent results on the understanding of universal phenomena in three-body collisions, many open questions remain on the atom-dimer system.

In our lab, we have investigated the collisional properties of ultracold mixtures of cesium halo dimers and atoms in the high magnetic field region. In particular we observed a new atom-dimer loss resonance at a value of scattering length different from the one observed in the low magnetic field region. These results suggest that, when different Feshbach resonances are involved, universality in atom-dimer collision is not preserved, in clear contrast with observations in a purely atomic process. Furthermore we confirmed that in cesium, measurements do not show an avalanche loss feature similar to what observed in experiments with other atomic species.

## Colloquium 01.11.2012; 14:00-15:30; Room D 326

### Dr. Nathan Lemke

Pysikalisch-Technische Bundesanstalt

Optical lattice clock with spin-1/2 ytterbium atoms

An optical lattice clock probes a spectrally narrow electronic transition in an ensemble of optically trapped, laser-cooled atoms, for use as a time and frequency standard. To date, several lattice clocks have been demonstrated with superior stability and accuracy compared to primary frequency standards based on microwave transitions. Yet, the question of which atomic system (including the element and isotope) will ultimately perform best as a lattice clock remains unsettled. In this talk, I will describe some key features of an optical lattice clock using a spin-1/2 isotope of the ytterbium atom. The frequency stability of the Yb clock is highlighted by resolving an ultra-narrow clock spectrum with a full-width at half-maximum of 1 Hz, corresponding to a quality factor Q = 5e14. Moreover, this system can be highly accurate, which is demonstrated by characterizing the Yb clock frequency at the 3e-16 level of fractional uncertainty, with further progress toward a ten-fold improvement also presented. To reach this low level of uncertainty required careful consideration of important systematic effects, including the identification of the Stark-canceling "magic" wavelength, a precise determination of the static polarizability of the clock transition, and the measurement and control of atom-atom collisions. These measurements were performed at NIST in Boulder, USA.

## Colloquium 08.11.2012; 14:00-15:30; Room D 326

### Prof. Dr. Arno Rauschenbeutel

Technische Universität Wien, Atominstitut

Trapping and Interfacing Cold Neutral Atoms Using Optical Nanofibers

We have recently demonstrated a new experimental platform for trapping and optically interfacing laser-cooled cesium atoms [1]. The scheme uses a two-color evanescent field surrounding an optical nanofiber to localize the atoms in a one-dimensional optical lattice 200 nm above the nanofiber surface. At the same time, the atoms are efficiently interrogated with light which is sent through the nanofiber. Remarkably, an ensemble of 2000 trapped atoms yields an optical depth of up to 32, equivalent to 1.6 % absorbance per atom. Moreover, when dispersively interfacing the atoms, we observe ~ 1 mrad phase shift per atom at a detuning of six times the natural linewidth [2]. Our technique provides unprecedented ease of access for the coherent optical manipulation of trapped neutral atoms and opens the route towards the direct integration of atomic ensembles into fiber networks, an important prerequisite for large scale quantum communication. Moreover, our nanofiber trap is ideally suited to the realization of hybrid quantum systems combining atoms with solid state quantum devices. Finally, the use of nanofibers for atom trapping allows one to straightforwardly realize interesting trapping geometries which are not easily accessible with freely propagating laser beams.

[1] E. Vetsch et al., Phys. Rev. Lett. 104, 203603 (2010).

[2] S. T. Dawkins et al., Phys. Rev. Lett. 107, 243601 (2011).

## Colloquium 22.11.2012; 14:00-15:30; Room D 326

### Prof. Dr. Philipp Treutlein

University of Basel, Department of Physics

Quantum metrology with a scanning probe atom interferometer

Atom interferometers are extremely precise measurement devices for quantities such as time, inertial forces, and electromagnetic fields. When operated with an ensemble of uncorrelated (non-entangled) particles, interferometers are fundamentally limited by shot noise, giving rise to the standard quantum limit (SQL) of interferometric measurement. State-of-the-art devices operate at this limit. Recent proof-of-principle experiments have shown that the SQL can be overcome using many-particle entangled states in the interferometer. Such quantum metrology can potentially lead to significant improvements in interferometer sensitivity. At the same time, it provides new insights into the elusive nature of many-particle entanglement. I will discuss the physics behind the standard quantum limit and how it can be overcome using entangled states. In a recent experiment, we have realized an atom interferometer operating with an uncertainty of 4.0 dB below the SQL. Our interferometer employs entangled atoms in a spin-squeezed Bose-Einstein condensate and maintains performance below the SQL for Ramsey interrogation times up to 20 ms. Quantum-state tomography is used to characterize the interferometer input state, revealing a depth of entanglement of more than 40 particles. Using an atom chip, we spatially scan the atoms over tens of micrometers while maintaining sub-SQL operation. We use this scanning capability to perform a spatially resolved measurement of microwave fields from an integrated circuit. These techniques are promising for high-resolution imaging of electromagnetic fields near solid-state microstructures.

References:

R. Schmied and P. Treutlein, New J. Phys. 13, 065019 (2011).

M. F. Riedel, P. Böhi, Yun Li, T. W. Hänsch, A. Sinatra, and P. Treutlein, Nature 464, 1170 (2010).

P. Böhi, M. F. Riedel, T. W. Hänsch, and P. Treutlein, Appl. Phys. Lett. 97, 051101 (2010).

## Colloquium 06.12.2012; 14:00-15:30; Room D 326

### Prof. Dr. Michael Fleischhauer

Universität Kaiserslautern, Fachbereich Physik

"Optically driven Rydberg gases and Rydberg polaritons: Many-body dynamics in open systems"

Photons interacting with Rydberg gases in a two-photon coupling scheme can be described in terms of slow-light polaritons. They are subject to a strong and non-local interaction mediated by a van-der Waals coupling between excited Rydberg atoms.

I will present and discuss an effective many-body model for these Rydberg polaritons characterized by a power-law interaction for large separations and a dissipative blockade phenomenon for small interparticle distances. The latter effect is essential for explaining recent experiments on light propagation in Rydberg gases. The long-range interaction can moreover give rize to the formation of quasi-crystalline structures of photons. The latter process will be discussed in a one-dimensional system in terms of a Luttinger-liquid model, where the relevant parameter are obtained by DMRG simulations. Furthermore an alternative approach to create orderd structures of Rydberg excitations by steady-state optical pumping is presented. The quantum correlations in the steady state are calculated by open-system DMRG simulations and discussed in terms of an analytically solvable effective model.

## Colloquium 13.12.2012; 14:00-15:30; Room D 326

### Dr. Andrea Bertoldi

Institut d´Optique, Graduate School

Feedback control of atomic coherent spin states

Coherence is the essential resource of interferometry, which brings matterwave based sensors to extreme sensitivities for the measurement of gravity, inertial forces, magnetic fields, and time. A state of maximal coherence is obtained when all atoms of an ensemble occupy the same pure single particle state, forming a coherent spin state. The coherence of a quantum state can be destroyed by incoherent interactions with the environment, but also by undetermined coherent processes, because of the loss of information about the atomic state. In this second case, the processes can be reversed - at least partially - by non destructively measuring and correcting their effect. Unlike partially projective measurements in quantum feedback control, weak measurements can yield precise values for collective observables, while setting only a slight back-action on the particles. We feedback controlled the internal states of atomic ensembles and their protection against collective noise. Weak measurement with negligible projection and coherent microwave manipulations are used to protect the superposition state induced on a optically trapped rubidium sample. The efficiency of the feedback is studied for a simple binary noise model and characterized in terms of the trade-off between information retrieval and destructivity for the optical probe. I will present more complex feedback scenarios which provide a way towards novel atom interferometry schemes using repeated measurements and feedback to boost the sensitivity.

## Colloquium 17.01.2013; 14:00-15:30; Room D 326

### Prof. Dr. Matthias Weidemüller

Universität Heidelberg, Physikalisches Institut

PDF dokument (not available)

Rydberg blockade, slow light and interacting dark-state polaritons

Interfacing light and matter at the quantum level is at the heart of modern atomic and optical physics and is a unifying theme of many diverse areas of research. A prototypical realization is electromagnetically induced transparency (EIT), whereby quantum interference gives rise to long-lived hybrid states of atoms and photons called dark-state polaritons. In my talk I will give a general introduction into the field of ultracold Rydberg gases, with special emphasis on recent developments towards nonlinear quantum optics and the observation of strong interactions between dark-state polaritons in an ultracold atomic gas involving highly excited (Rydberg) states.

By combining optical imaging with counting of individual Rydberg excitations we probe both aspects of this atom-light system. Extreme Rydberg-Rydberg interactions give rise to a polariton blockade, which is revealed by a strongly nonlinear optical response of the atomic gas. For our system the polaritons are almost entirely matter-like allowing us to directly measure the statistical distribution of polaritons in the gas. For increasing densities we observe a clear transition from Poissonian to sub-Poissonian statistics, indicating the emergence of spatial and temporal correlations between polaritons. These experiments, which can be thought of as Rydberg dressing of photons, show that it is possible to control the statistics of light fields, and could form the basis for new types of long-range interacting quantum fluids.

Work performed in collaboration with Christoph Hofmann, Georg Günter, Hanna Schempp, Martin Robert-de-Saint-Vincent and Shannon Whitlock.