Colloquia


Colloquia

We plan to return partly to in-person talks. These talks will be held in the interim lecture hall B 0.32 at MPQ and can additionally be attended online. Some talks remain online only.

2G regulations apply to in-person talks, i.e. every time you wish to participate in person, you will have to prove, e.g. with the CovPass-App, that you are vaccinated or recovered.Whether facemasks have to be worn inside the lecture hall will be communicated in the the e-mail announcement for each talk separately. In any case, you will need a medical facemask in the hallway. Audience not affiliated with MPQ are welcome to join in person as long as they meet 2G criteria.

Details on how to participate online are distributed via the mailing lists [wiss-mpq] and [Mpq-colloquium-stream]. To receive this information, please register using the adjacent link.

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

"Recent experimental and theoretical work demonstrated the power of dissipative dynamics for generating coherent quantum effects. In this context I will present theoretical results on position- and force sensing with precision below the standard quantum limit employing stationary entanglement between an optomechanical system - as the detector - and an atomic ensemble - as a catalyst for back action cancellation. I will also mention other applications of dissipative quantum dynamics in cavity QED for quantum simulations and quantum control." [more]
"Singly-charged ions in traps can form strings at low temperatures, where ordering is due to the interplay of the confining potential and the Coulomb repulsion. At zero temperature the ion string exhibits a structural phase transition to a zigzag structure, tuned by reducing the transverse trap potential or the interparticle distance [1]. The transition is driven by transverse, short wavelength vibrational modes [2]. This is a quantum phase transition, which can be experimentally realized and probed. Indeed, by means of a mapping to the Ising model in a transverse field, the quantum critical point can be determined in terms of the system parameters, finding a finite, measurable deviation from the critical point predicted by the classical theory. A measurement procedure is suggested which can probe the effects of quantum fluctuations at criticality [3]. We then consider the stability and dynamics of an ion chain confined inside a high-finesse optical resonator. When the dipolar transition of the ions strongly couples to one cavity mode, the mechanical effects of light modify the chain properties close to a structural transition. We focus on the linear chain close to the zigzag instability and show that linear and zigzag arrays are bistable for certain strengths of the laser pumping the cavity. For these regimes the chain is cooled into one of the configurations by cavity-enhanced photon scattering. The excitations of these structures mix photonic and vibrational fluctuations, which can be entangled at steady state. These features are signalled by Fano-like resonances in the spectrum of light at the cavity output [4]." [1] "Multiple-shell structures of laser-cooled 24Mg+ ions in a quadrupole storage ring" G. Birkl, S. Kassner, and H. Walther, Nature (London) 357, 310 (1992). [2] "Structural phase transitions in low-dimensional ion crystals", Sh. Fishman, G. De Chiara, T. Calarco, and G. Morigi, Phys. Rev. B 77, 064111 (2008). [3] "Quantum zigzag transition in ion chains", E. Shimshoni, G. Morigi, S. Fishman, Phys. Rev. Lett 106, 010401 (2011). [4] "Structural Transitions of Ion Strings in Quantum Potentials", Cecilia Cormick and Giovanna Morigi, Phys. Rev. Lett. 109, 053003 (2012). [more]
"With the advent of sub-femtosecond ultrashort XUV pulses and of phase-stabilized IR pulses with sub-cycle time resolution, novel pathways have been opened up for studying time-resolved electronic quantum dynamics on the attosecond scale. These experiments pose challenges for theory: How do short pulses interact with matter? Which novel information can be extracted from time-resolved spectroscopies? In this talk, these issues will be addressed with the help of a few examples. One is the notion of time delay as physical observable. We show that the Eisenbud-Wigner-Smith time delay for atomic photoionization can now be determined by streaking or RABBITT with single-digit attosecond precision, however only when long-range interaction and IR field corrections are accounted for. For the more complex systems of a solid surface, time-resolved information on decohering processes such as electron transport or plasmon excitation can be extracted." [more]
"Geometric phases are manifested in many phenomena, like quantum Hall effect, and can characterize the topological properties of Bloch bands. For example in one-dimensional periodic potentials the topological invariant is given by the Zak phase-- the Berry phase acquired during an adiabatic motion of a particle across the Brillouin zone.In this talk I will present two topics, the first one will be about the experimental realization of strong effective magnetic fields in a two-dimensional optical lattice. By use of Raman-assisted atom tunneling we imprint a geometric phase to the atoms which can be interpreted as an Aharonov-Bohm phase. We observe that the effective magnetic field leads to a frustrated ground state and to quantum cyclotron orbits.The second topic will be about the direct measurement of the Zak phase for a dimerized optical lattice, which models polyacetylene. The experimental protocol consists of a combination of Bloch oscillations and Ramsey interferometry. This work establishes a new general approach for probing the topological structure of Bloch bands in optical lattices." [more]
"We present a proposal for a versatile cold-atom-based quantum simulator of relativistic fermionic theories and topological insulators in arbitrary dimensions [1]. The setup consists of a spin-independent optical lattice that traps a collection of hyperfine states of the same alkaline atom, to which the different degrees of freedom of the field theory to be simulated are then mapped. We show that the combination of bi-chromatic optical lattices with Raman transitions can allow the engineering of a spin-dependent tunneling of the atoms between neighboring lattice sites. These assisted-hopping processes can be employed for the quantum simulation of various interesting models, including the realization of different types of relativistic lattice fermions, which can then be exploited to synthesize a majority of phases in the periodic table of topological insulators. An example concerns how to implement Wilson fermions with inverted masses: the atomic gas corresponds to a phase of matter where Maxwell electrodynamics is replaced by axion electrodynamics, i.e. a 3D topological insulator [2]." [more]
Go to Editor View