Dr. Stephan Dürr
Stephan Dürr
Group Leader
Phone: +49 89 3 29 05 - 291
Room: A 2.22
Prof. Dr. Thomas Udem
Thomas Udem
Phone: +49 89 3 29 05 - 282 // -257
Room: D 0.21 // D 0.39

next colloquium



Our series of Colloquium Talks takes place from October till January and from April till July, on Tuesdays, at 2:30 p.m..

Attention! Due to the recontstruction of the foyer at the MPQ talks will take place at the interims Lecuture Hall in Room B 0.32.

Scientific organization of the talks: Dr. Stephan Dürr and Dr. Thomas Udem


"Superconducting Quantum Circuits: Ultra-strong Light-Matter Interaction and Path Entanglement of Continuous-variable Quantum Microwaves”

"Superconducting nanocircuits behave in many aspects similar to natural atoms. Despite the fact that these so-called artificial atoms are huge compared to their natural counterparts, they have a discrete level structure and exhibit properties unique to the world of quantum mechanics. In the simplest case, these artificial atoms form quantum two-level systems, also called quantum bits. We have realized superconducting flux quantum bits where the quantum two-level system is formed by symmetric and anti-symmetric superposition states of persistent currents circulat-ing clock- and anticlockwise in a superconducting loop [1]. Coupling these flux qubits to on-chip superconducting microwave resonators gives rise to the prospering field of superconducting circuit quantum electrodynamics (circuit-QED), which allows us to study the fundamental inter-action between artificial solid-state atoms and single microwave photons as the basis for com-municating quantum information. We discuss the realization of circuit-QED systems operating in the ultra-strong coupling regime, where the atom-cavity coupling rate reaches a considerable fraction of the atom transition frequency [2]. We also address quantum state tomography of propagating microwaves using a novel dual path detection scheme [3]. We have used this scheme to demonstrate for the first time frequency degenerate path entanglement of continu-ous-variable propagating quantum microwave signals. To this end, we entangle two spatially separate modes of the same frequency using a hybrid ring beam splitter and detect the entan-glement by means of cross-correlation techniques. The input fields of the beam splitter are squeezed vacuum and vacuum, respectively, and the correlations are evaluated up to the fourth moments in amplitude."This work is supported by the German Research Foundation via SFB 631 and the German Excellence Initia-tive via the Nanosystems Initiative Munich (NIM).[1] T. Niemczyk et al., Supercond. Sci. Techn. 22, 034009 (2009); F. Deppe et al., PRB 76, 214503 (2007).[2] T. Niemczyk et al., Nat. Phys. 6, 772-776 (2010); F. Deppe et al., Nat. Phys. 4, 686 (2008); T. Niemczyk et al., arXiv:1107.0810v1.[3] E. Menzel et al., Phys. Rev. Lett. 105, 100401 (2010); M. Mariantoni et al., Phys. Rev. Lett. 105, 133601 (2010). [more]

"The complexity of learning quantum states (with applications to face recognition)."

"The complete characterization of a quantum system by physical measurements seems to be a conceptually simple task and is routinely carried out experimentally. It is thus all the more surprising that many fundamental questions pertaining to this procedure remain unanswered. (And, what is more, lead to highly non-trivial mathematical problems). A prime example is determining the sample complexity of quantum state estimation: under realistic conditions, how many experimental runs does one need in order to obtain an estimate for an unknown quantum state with acceptable error bars? Simple answers based on asymptotic statistics turn out to be highly inaccurate (in fact, way too pessimistic). I will report recent progress on this and related problems. It is both based on, and has contributed to, new developments in classical statistics and machine learning theory. I will mention proposals for tasks as varied as face recognition and prediction of online behavior which have been influenced by methods from quantum state tomography." [more]

"An elementary quantum network of single atoms in optical cavities."

"Quantum networks form the basis of distributed quantum computing architectures and quantum communication. Single atoms in optical cavities are ideally suited as universal quantum network nodes capable of sending, storing and retrieving quantum information. We demonstrate this by presenting an elementary version of a quantum network based on two identical nodes in remote, independent laboratories. The dynamic control of coherent dark states allows for the reversible exchange of quantum information by exchange of a single photon." [more]

"A quantum information approach to statistical mechanics."

"I will show how one can apply quantum information tools to study various problems in statistical mechanics. We focus on classical spin models, which are toy models used in a variety of fields such as magnetism or quantum gravity, and we tackle them from three different angles. First, we show how the partition function of a class of widely different classical spin models (models in different dimensions, different types of many-body interactions, different symmetries, etc) can be mapped to the partition function a single model. Second, we give efficient quantum algorithms to estimate the partition function of various classical spin models, such as the Ising or Potts model. Finally, we outline the possibility of applying quantum information tools to a model of discrete quantum gravity called causal dynamical triangulation." [more]

"Localization of ultra-cold bosons in a 3D laser speckle disordered potential."

"We have observed 3D localization of ultra-cold atoms of a Bose Einstein Condensate, suspended against gravity, and released in a 3D optical disordered potential with short correlation lengths in all directions. Fluorescence imaging of the expanding cloud yields density profiles composed of a steady localized part and a diffusive part. A phenomenological analysis of the data allows us to determine the localized fraction and the diffusion coefficients of the diffusing part. I will present and discuss these results, in the context of Anderson localization." [more]

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