Dr. Stephan Dürr
Stephan Dürr

Telefon: +49 89 3 29 05 - 291
Raum: A 2.22
Prof. Dr. Thomas Udem
Thomas Udem

Telefon: +49 89 3 29 05 - 282 // -257
Raum: D 0.21 // D 0.39

kommende Kolloquien



Die Gastvorträge im Rahmen des MPQ-Kolloquiums finden von April bis Juli sowie von Oktober bis Januar jeweils dienstags um 14:30 Uhr statt. Verantaltungsort ist der Herbert-Walther-Hörsaal im Foyer des Max-Planck-Instituts für Quantenoptik.

Ansprechpartner für die wissenschaftliche Organisation:

Dr. Stephan Dürr und Dr. Thomas Udem

Wenn Sie einen Vortrag im Livestream verfolgen möchten, ist es nötig, dass Sie sich in eine entsprechende Mailing Liste eintragen. Daraufhin erhalten Sie Instruktionen zum Empfang des Livestreams.


"Modeling and characterizing chiral topological states in two-dimensional lattices".

"Chiral topological states are exotic quantum phases of matter in two dimensions with broken time-reversal symmetry (e.g. fractional quantum Hall states). These states support topological ground state degeneracy, stable gapless edge excitations, and quasiparticles carrying fractional charges and fractional statistics. In this talk, I will discuss the bulk-edge correspondence as a powerful tool for modeling and characterizing chiral topological phases in 2D lattices. In the first part, I will describe how chiral topological wave functions and their parent Hamiltonians can be constructed by using the chiral correlators of the edge conformal field theory. In the second part, I will introduce a new quantity named as momentum polarization, which allows extracting topological spin and chiral central charge from the ground-state wave functions and provides an efficient approach to identify 2D chiral topological states from finite-size numerics." [mehr]

"Non-equilibrium quantum dynamics of spin impurities using ultracold atoms."

"Ultracold-atom experiments are suitable to study out-of-equilibrium dynamics of quantum many-body systems. Here I will present the quantum dynamics of deterministically and locally created mobile spin impurities in the one-dimensional bosonic atoms in optical lattices. First, we investigate the dynamics of a single-spin impurity. In the Mott-insulating regime, the coherent propagation of a magnetic excitation, or a magnon, in the Heisenberg model can be observed. Extending the study to the superfluid regime of the bath, we quantitatively determine how the bath affects the motion of the impurity, showing evidence of polaronic behavior. Second, we observe bound states of two magnons in a Heisenberg chain by tracking their dynamics. Such bound states were pointed out theoretically by H. Bethe in 1931, and our novel microscopic study of quantum magnets can directly confirm their existence." [mehr]

"New tools for precision metrology: Spatial Coherent Control and Ramsey Comb Spectroscopy."

"Direct frequency comb spectroscopy (DFCS) is a powerful application of frequency combs that is increasing in importance rapidly. In my talk I will discuss two new methods related to DFCS that we developed in our pursuit of precision spectroscopy of the ground states of helium and the helium-ion. The first method is spatial coherent control with comb laser pulses, which enables complex pattern formation and background elimination with two-photon DFCS. Secondly, I will discuss Ramsey-comb spectroscopy, which combines high pulse energies at the mJ level with the accuracy and resolution of frequency combs. It is based on a new laser system that can selectively and phase coherently amplify two frequency comb pulses at widely different time delays. The high pulse energy provides straightforward access to multi-photon transitions and nonlinear wavelength conversion, while the original frequency comb laser accuracy and resolution is fully recovered from a series of Ramsey-like measurements. The special properties of this approach, such as a cancelation of optical light-shift effects, are demonstrated by improving the accuracy of several weak two-photon transitions in atomic rubidium and cesium up to thirty times." [mehr]

"Superfluid atom circuits".

"We have created a superfluid atom circuit using a toroidal Bose-Einstein Condensate. Just as a current in a superconducting circuit will flow forever, if a current is created in our superfluid circuit, the flow will not decay as long as the current is below a critical value. A repulsive optical barrier across one side of the torus creates the tunable weak link in the condensate circuit and can be used to control the current around the loop. By rotating the weak link at low rotation rates, we have observed phase slips between well-defined persistent current states. This behavior is analogous to that of a weak link in a superconducting loop. A feature of our system is the ability to dynamically vary the weak link, which in turn varies the critical current, a feature that is difficult to implement in superconducting circuits. For higher rotation rates, we observe a transition to a regime where vortices penetrate the bulk of the condensate. These results demonstrate an important step toward realizing an atomic SQUID analog." [mehr]

"Quantum Theory of the Classical."

"I will describe three insights into the transition from quantum to classical. I will start with (i) a minimalist (decoherence-free) derivation of preferred states. Such pointer states define events (e.g., measurement outcomes) without appealing to Born's rule . Probabilities and; (ii) Born’s rule can be then derived from the symmetries of entangled quantum states. Derivation of Born’s rule will be the focus of my presentation. With probabilities at hand one can analyze information flows from the system to the environment in course of decoherence. They explain how (iii) robust “classical reality” arises from the quantum substrate by accounting for objective existence of pointer states of quantum systems through redundancy of their records in the environment. Taken together, and in the right order, these three advances elucidate quantum origins of the classical." *W. H. Zurek, Nature Physics 5, 181-188 (2009) [mehr]

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