Invited speaker: Dr. Janos K. Asbóth
Affiliation: Wigner Research Centre for Physics of the Hungarian Academy of Sciences, Budapest
Title: The Hofstadter butterfly takes flight in quantum walks
Time and room: 17:15 h, lecture hall IAP
The Hofstadter butterfly [1] is the intricate self-similar structure of subgaps that the single energy band of a charged particle hopping on a 2-dimensional lattice develops as a magnetic field perpendicular to the lattice is turned on. It has inspired physicists for 40 years (e.g., an important role in the exploration of the quantum Hall effect), and now enjoys a renewed interest as experiments might finally be close to observing it. In case the charged particle has several internal states, the spectrum as a function of magnetic field is a multiband Hofstadter butterfly, where each energy band develops a set of subgaps. We show that besides developing subgaps, the bands of topologically nontrivial (e.g., Chern) insulators must also flow in energy as the magnetic field is tuned, because eigenstates flow across topological gaps at a steady rate. We thus connect the global topology of multiband Hofstadter butterflies, i.e., the pattern in which bands flow into each other, with the topological invariants of the underlying lattice Hamiltonians. Our results also apply to quantum walks, and other periodically driven systems, where we obtain a simple formula for the Rudner topological invariant [2], which has potential to be directly measured.
Invited speaker: Prof. Dieter Suter
Affiliation: TU Dortmund
Title: Quantum Computers: Promise, Problems and Possible Solutions
Time and room: 17:15 h, lecture hall IAP
The "digital revolution" that is transforming our lives and our economy is based on the ubiquity of information-processing devices whose processing power increased exponentially, following Moore's law. As this trend is approaching fundamental physical limits, new directions are explored for even more powerful computational devices based on quantum mechanical systems. Such devices can solve problems that will remain out of reach for conventional computers. The main difficulty for their implementation is the fragility of information stored in coherent superpositions of quantum mechanical eigenstates. This talk will highlight some aspects of the potential offered by quantum computers, as well as the difficulties that must be overcome to realise this potential. Our current work concentrates on finding solutions for some of these problems.
Invited speaker: Dr. Philipp Strack
Affiliation: Universität zu Köln
Title: Interacting and Emergent Photons in Quantum Optics and Condensed Matter
Time and room: 17:15 h, lecture hall IAP
Photons, that is, the quanta of light fields, typically interact only weakly. In our research at the interface of quantum optics and condensed matter [1], we explore theoretically the behavior of quantum materials in which quantum light couples strongly and coherently to non-linear matter components. The matter, in turn, can mediate photon-photon interactions. Examples include dense optical media in optical cavities or fractionalized spins in quantum magnets, which couple to emergent gauge fields. In this talk, will describe our work on many-body physics across these areas and try to elucidate the differences and commonalities between the photons of quantum optics versus the emergent photons in strongly correlated condensed matter materials.
[1] http://www.thp.uni-koeln.de/~strack/
Invited speaker: Prof. Sven Höfling
Affiliation: Universität Würzburg; University of St Andrews, UK
Special Colloquium
Title: Polariton Lasers
Time and room: 16:15 h, lecture hall IAP
More than a century after the introduction of incandescent lighting and half a century after the realization of semiconductor lasers, semiconductor light sources are continuing to revolutionize applications and having a paramount impact on our everyday life. The creativity of quantum and photonics engineers and material scientists results in semiconductor light emitting diodes and semiconductor lasers with unprecedented characteristics, including ever better efficiency or brightness and ultra-wide wavelength coverage. In this talk, after a summary of general semiconductor laser research undertaken in our group, I will focus on the description of a novel kind of coherent light emitter based on a semiconductor microcavity with embedded quantum wells. In contrast to conventional lasers, this sort of device, termed polariton laser, relies not on stimulated emission of photons but on stimulated scattering of bosonic quasiparticles, the polaritons. These devices have lower thresholds than conventional lasers, and I will describe the physics underlying these devices and routes towards possible practical applications.
Invited speaker: Dr. Michael Johanning
Affiliation: Universität Siegen
Title: Scalable Quantum Information Processing in ion traps using MAGIC
Time and room: 17:15 h, lecture hall IAP
Ion traps are one of the promising platforms for quantum information processing and many proof of principle experiments have been realized with laser cooled trapped ions. A recent development is the application of inhomogeneous magnetic fields and microwave radiation for selective coherent manipulation and conditional dynamics. This allows for long coherence times and high fidelity one and two qubit gates, as demonstrated recently.
In this talk, I will focus on the storage, manipulation and readout of qubits encoded in the spin degree of freedom which can interact, upon application of an additional magnetic field gradient, by a magnetic gradient induced coupling (MAGIC). I will illustrate the concepts using results obtained with macroscopic and micro-structured traps, and compare to atomic qubits manipulated by optical means.