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

  • The colloquium series will resume at the beginning of the next term in April/October.



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

If you wish to view the live stream of the MPQ colloquium, please use the link to subscribe to the corresponding mailing list. Detailed instructions will be sent to all subscribers.


"Optical flux lattices for ultracold atomic gases."

"One of the most important techniques in the ultracold atom toolbox is the optical lattice: a periodic scalar potential formed from standing waves of light. Optical lattices are central to the use of atomic gases as quantum simulators, and allow the exploration of strong-correlation phenomena related to condensed matter systems. In this talk, I shall describe how simple laser configurations can give rise to a new kind of optical lattice -- a so-called "optical flux lattice" -- in which optically dressed atoms experience a periodic effective magnetic flux with high mean density. Optical flux lattices have narrow energy bands with nonzero Chern numbers, analogous to the Landau levels of a charged particle in a uniform magnetic field. These lattices will greatly facilitate the achievement of the quantum Hall regime for ultracold atomic gases." [more]

"Ultracold ensembles of molecules near quantum degeneracy."

"Dipolar quantum gas systems at ultralow temperatures are expected to exhibit novel many-body quantum phases as a result of the long-range and anisotropic dipole-dipole interaction. For our Rb-Cs mixture experiment the focus is on the creation of a bosonic quantum gas of polar ground-state RbCs molecules using Feshbach association and subsequent stimulated adiabatic Raman transfer (STIRAP). We have created a high phase-space density sample of ultracold RbCs Feshbach molecules from an ultracold mixture of Rb and Cs and have performed high-resolution molecular spectroscopy using the Feshbach molecules and have found intermediate electronically excited levels suitable for RbCs ground-state transfer. We have measured the binding energy of the RbCs rovibrational ground state in two-photon spectroscopy and have performed STIRAP experiments with transfer efficiencies of up to 90%. We have implemented an optical lattice with the ultimate aim to create a Mott-insulator state having precisely one atom of each species at each lattice site to improve the creation efficiency for the Feshbach molecules and the STIRAP transfer efficiency. Presently, we switch on the lattice after Feshbach molecule creation to localize the molecules and to prevent collisions. To improve the STIRAP efficiency we have set up ultra-stable optical resonators to which we lock the transfer lasers to reduce laser phase noise. Finally, we will give an update on our endeavor to create a BEC of ground-state Cs dimers." [more]

Cloaking magnetic fields

Recent advances like transformation optics have opened new possibilities of controlling electromagnetic fields, including cloaking devices that may render an object invisible to incident electromagnetic radiation. These cloaks have been theoretically presented but their practical implementation in microwaves, infrared or even visible light are actually not exact cloaks but only reduced versions (e. g. with only reduced scattering and some shadow). Here we present how in the case of static magnetic fields one can design an exact cloak using simply a superconductor-ferromagnetic bilayer, which makes it a unique case of an exact and feasible cloak. An experimental realization of the magnetic cloak is presented. [more]

"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]

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