Colloquia and Seminars

Our series of Colloquium Talks takes place from October till January and from April till July, on Tuesdays, at 2:30 p.m., at MPQ’s Herbert Walther Lecture Hall.

If you wish to view the live stream of the MPQ colloquium, please use the following link to subscribe to the corresponding mailing list. Detailed instructions will be sent to all subscribers.
08.04.2014 14:30
A Physicist´s View of the German „Energiewende”
Professor Christoph Buchal
Forschungszentrum Jülich

"This talk discusses the ideas and goals of the Energiewende, which aims at successively decarbonizing all of the german energy supply systems, but suffers from patchwork, wild exaggerations, cost explosions,  and true pitfalls due to the construction of the European Climate Program.


After a general overview we have a closer look at the electrical power generation and distribution, which is strongly affected by the Energiewende. We evaluate the consequences  of the intermittent power production by wind turbines and photovoltaic  panels, the dynamics of the present conventional power plants  and the need and potential of present or future storage systems.  The role of hydrogen and synthetic fuels as a form of chemical energy storage is discussed."





15.04.2014 14:30
Visualization of atomic and electronic motion in 4D
Double Feature!
Dr. Peter Baum
Ludwig-Maximilians-Universität München, Lehrstuhl für Experimentalphysik - Laserphysik

"On a fundamental level, chemical reactions and condensed-matter transformations are defined by the motion of atoms and electrons from initial to final conformations, typically along a complex reaction path involving ultrasmall and ultrafast dimensions. Here we report on our progress towards a full visualization of structural dynamics in space and time. After laser excitation, ultrashort electron pulses at 30-100 keV are diffracted with time delays and provide a pump-probe sequence of structural snapshots with atomic resolution. We solve the most essential problem for time resolution, Coulomb repulsion, by using single-electron pulses in combination with a microwave compressor. The so achieved 12-fs electron pulses (rms) are among the shortest worldwide and now provide access to the fastest phonons or molecular modes with atomic resolution. In order to further advance towards the regime of purely electronic motion, we apply the microwave compressor’s time-dependent fields for reshaping the single-electron phase space from the temporal into the energetic domain. The achievable pulse durations are shorter than optical light cycles, promising direct diffraction access to electronic motion with a resolution of picometers and attoseconds. We report our first proof-of-principle results and reflect on what discoveries we may expect to see."



15.04.2014 14:30
Single-photon switch based on Rydberg Blockade
Double Feature!
Simon Baur
Max Planck Institut für Quantenoptik, Abt. Quantendynamik

"All-optical switching is a technique in which a gate light pulse changes the transmission of a target light pulse without the detour via electronic signal processing. We take this to the quantum regime, where the incoming gate light pulse contains only one photon on average. The gate pulse is stored as a Rydberg excitation in an ultracold atomic gas using electromagnetically induced transparency. Rydberg blockade suppresses the transmission of the subsequent target pulse. Finally, the stored gate photon can be retrieved. A retrieved photon heralds successful storage. The corresponding postselected subensemble shows an extinction of 0.05. Recent improvements of our experiment made it possible to observe a gain of 20. The single-photon switch offers many interesting perspectives ranging from quantum communication to quantum information processing."




22.04.2014 14:30
Entangled ions in an optical cavity
Dr. Tracy Northup
Universität Innsbruck

"Optical cavities provide a coherent interface between light and matter that can be used to link remote quantum systems.  With such an interface, quantum information can be mapped from a single atom onto a photon for long-distance transport, and an atom can be entangled with a cavity photon as a resource for teleportation.  However, in a future quantum network, it would be advantageous for each cavity to contain multiple atoms.  These atoms could be used for local quantum information processing, error correction between network nodes, and improved quantum memories, among other tasks.

I will describe the coupling of two calcium ions to the mode of a high-finesse optical cavity.  When both ions are coupled with near-maximum strength to the cavity, we entangle the ions with one another, heralded by the measurement of two orthogonally polarized photons.  Applications of entangled ions in a cavity will be discussed, in the context of both quantum information tasks and the investigation of open quantum systems.  In particular, I will present recent measurements of enhanced quantum state transfer from a superradiant two-ion state."





29.04.2014 13:30
Breaking dogmas with ultracold erbium atoms
Prof. Francesca Ferlaino
Universität Innsbruck, Institut für Experimentalphysik and Zentrum für Quantenphysik

"Non-alkali-metal atoms have recently proved to be fascinating systems to explore novel lands in ultracold quantum physics. Here, we present recent results with ultracold dipolar gases of erbium atoms. As a consequence of the strong dipole-dipole interaction and of the large anisotropy in the dispersion potential, Er shows a spectacularly high number of Fano-Feshbach resonances both in the fermionic and bosonic isotopes. The complex Er scattering behavior escapes to traditional scattering models and requires novel approaches based on statistical analysis. Following the powerful toolset provided by Random-Matrix theory, we elucidate the chaotic nature of the scattering. Finally, we report on the first degenerate Fermi gas of Er, which is realized by direct cooling of identical fermions based on dipole-dipole interaction."




13.05.2014 14:30
Infinite-dimensional-matrix product states, topology, and criticality
Double Feature!
Dr. Anne Ersbak Bang Nielsen
Max Planck Institut für Quantenoptik, Abt. Theorie

13.05.2014 14:30
Precision spectroscopy of atomic hydrogen for a new determination of the Rydberg Constant and the proton charge radius
Double Feature!
Axel Beyer
Max Planck Institut für Quantenoptik, Abt. Laser-Spektroskopie

"The comparison between experimental values of transition frequencies in atomic hydrogen, the most simple atomic system, and the corresponding theoretical predictions provides stringent tests of bound state QED calculations. For more than a decade, this comparison has been limited by insufficient knowledge about the size of the proton, strictly speaking its r.m.s. charge radius. In 2010, a value for the proton size has been extracted from laser spectroscopy of muonic hydrogen which is ten times more accurate than any previous determination. However, this value deviates from the value found by precision spectroscopy of regular hydrogen by four combined standard deviations. An even larger inconsistency of 7σ is obtained when including electron-proton scattering data. The muonic hydrogen value has been confirmed and improved in 2013 while the source of the discrepancy, referred to as the ‘proton size puzzle’, remains unclear.

In this talk, we report on a new precision spectroscopy experiment, aiming to shed light on the regular hydrogen part of the puzzle: In contrast to previous high resolution experiments probing transition frequencies between the meta-stable 2S state and a higher lying nL state (n = 3, 4, 6, 8, 12, L = S, P, D), our measurement of the 2S – 4P transition frequency is the first experiment being performed on a cryogenic beam of hydrogen atoms in the 2S state. We will discuss how this helps to efficiently suppresses leading systematic effects of previous measurements and present preliminary results obtained so far."





20.05.2014 14:30

Professor Zoran Hadzibabic
University of Cambridge, UK

03.06.2014 14:30

Professor Fedor Jelezko
Universität Ulm

10.06.2014 14:30

Professor Seth Lloyd
Massachusetts Institute of Technology

17.06.2014 14:30
Design of a superconducting quantum computer
Professor John Martinis
University of California

"Superconducting quantum computing is now at an important crossroad, where “proof of concept” experiments involving small numbers of qubits can be transitioned to more challenging and systematic approaches that could actually lead to building a quantum computer.  Our optimism is based on two recent developments: a new hardware architecture for error detection based on “surface codes”, and recent improvements in the coherence of superconducting qubits.  I will explain how the surface code is a major advance for quantum computing, as it allows one to use qubits with realistic fidelities, and has a connection architecture that is compatible with integrated circuit technology.  Additionally, the surface code allows quantum error detection to be understood using simple principles.  I will also discuss how the hardware characteristics of superconducting qubits map into this architecture, and review recent results that show gate errors can be reduced to below that needed for the error detection threshold."





26.06.2014 14:30

Thursday Colloquium!
Professor Barbara Terhal
Rheinisch-Westfälische Technische Hochschule Aachen

01.07.2014 14:30
Nonlinear optics with ultra-broadband oscillators
Professor Uwe Morgner
Universität Hannover

08.07.2014 14:30
Controlling charge and spin faster than a cycle of light
Professor Rupert Huber
Universität Regensburg

"The physical properties of condensed matter are often caused by ultrafast phenomena involving low-energy elementary excitations, such as lattice vibrations, spin pre­ces­sion, plasmons, or superconducting energy gaps. Phase-locked few-cycle pulses in the terahertz and mid-infrared spectral domain in combination with field-sensitive detectors have been able to monitor these excitations directly with a time resolution faster than an oscillation period of the carrier wave. Next-generation high-intensity terahertz sources have now opened the exciting possibility to explore condensed matter in an unprecedented regime of atomically strong electric and magnetic terahertz biasing. We show that terahertz fields of up to 11 GV/m can accelerate electrons in bulk semiconductors to perform complete Bloch oscillation cycles within half an oscillation period of the infrared drive field. The concomitant magnetic field component allows us to control spins in magnetically ordered solids on the sub-cycle scale. Our experiments challenge latest quantum theories and spark hope for superfast electronics and magnetic storage devices at multi-terahertz clock rates."