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Quantum technologies

Dieter Meschede's research group
Home AMO physics colloquia
  • Nikolay Vitanov

  • Invited speaker: Prof. Nikolay Vitanov
    Affiliation: Universität Sofia
    Title: High-fidelity quantum control and quantum information processing with composite pulses

    Time and room:  17:15 h, lecture hall IAP
    Abstract: The technique of composite pulses has been used for a long time in nuclear magnetic resonance and, since recently, in quantum optics and quantum information. This technique replaces the single pulse used traditionally for driving a two-state quantum transition by a sequence of pulses with suitably chosen phases, which are used as control parameters for shaping the excitation profile in a desired manner. Composite pulses produce unitary operations, which combine very high fidelity with robustness to parameter variations. We have developed a pool of composite pulses by using a novel SU(2) approach to design recipes for construction of single-qubit operations, including broadband, narrowband and passband pulses, universal composite pulses, composite adiabatic passage and composite STIRAP, some of which have already been demonstrated in experiments with doped solids. We have also designed efficient and robust composite techniques for construction of highly entangled states, e.g. Dicke and NOON states, and multi-qubit gates, e.g. C-phase, Toffoli, and generally CN-phase gates.

  • Isabelle Staude

  • Invited speaker: Dr. Isabelle Staude
    Affiliation: Universität Jena
    Title: Tailoring Light Fields with Resonant Dielectric Nanosurfaces

    Time and room:  17:15 h, lecture hall IAP


    High-refractive-index dielectric nanoresonators and their assemblies show complex and sometimes unexpected interactions with light, including optically-induced magnetic response, directional scattering, Fano resonances, and strong near-field enhancements. Using the capabilities of modern nanotechnology, these interactions can be tuned by the size, shape, material composition, and arrangement of the nanoresonators. In addition, dielectric nanoresonators exhibit very low absorption losses at optical frequencies. Based on these unique optical properties, high-index dielectric nanoresonators represent versatile building blocks of resonant nanosurfaces with tailored linear and nonlinear optical properties. This talk will review our recent advances in light wave control with dielectric nanosurfaces using silicon nanodisks as nanoresonators. It will focus on nanosurfaces designed to impose a spatially variant phase shift onto an incident light field, thereby providing control over its wave front. Based on the simultaneous excitation of electric and magnetic dipole resonances, the nanoresonators can be tailored to emulate the behavior of the forward-propagating elementary wavelets known from Huygens’ principle. This concept allows for the realization of nanosurfaces with near-unity transmittance efficiency, full phase coverage, and a polarization insensitive response. Various examples of wavefront control will be discussed, including beam shaping and holographic imaging, both of which we have experimentally demonstrated with high efficiency at telecom frequencies.

  • Jens Eisert

  • Invited speaker: Prof. Jens Eisert
    Affiliation: Freie Universität Berlin
    Title: Taming The Non-Equilibrium

    Time and room:  17:15 h, lecture hall IAP
    Abstract: Complex quantum systems out of equilibrium are at the basis of a number of the most intriguing puzzles in physics. This talk will be concerned with recent progress on understanding how quantum many-body systems out of equilibrium eventually come to rest and thermalise. The first part of the talk will highlight theoretical progress on this question, taking in several ways a quantum information view - employing ideas of Lieb-Robinson bounds, quantum central limit theorems and of concentration of measure. These findings will be complemented by experimental work with ultra-cold atoms in optical lattices, in setups constituting dynamical "quantum simulators", allowing to probe physical questions that are not only out of reach for state-of-the-art numerical techniques based on matrix-product states, but also relate to classically computationally hard problems.

  • David DiVincenzo

  • Invited speaker: Prof. David DiVincenzo
    Affiliation: RWTH Aachen, FZ Jülich
    Title: Engineering the Quantum Computer: A Case Study of the Circulator

    Time and room:  17:15 h, lecture hall IAP
    Abstract: The Faraday-effect circulator was invented in the 1950's, based on some fundamental theoretical insights about the role of nonreciprocity in transmission systems. These Faraday devices are used successfully at both optical and at microwave frequencies; the latter have a unique and essential role in making solid-state quantum computing work. Also in the 1950's, microwave circulators based on a very different phenomenon, the Hall effect, were also considered. It was "proved" then that a Hall bar cannot make a good gyrator (a close cousin to the circulator). This proof is flawed, and we have shown that good gyrators are possible for Hall angle -> 90 degrees (aka "quantum Hall") if the device is contacted capacitively. We predict that the resulting Hall circulator can be much more miniaturized than the Faraday kind, and I will show some preliminary experimental efforts in this direction. I will discuss the relation of this device functionality to the physics of chiral edge magnetoplasmons in the Hall conductor.

  • Thilo Kopp

  • Invited speaker: Prof. Thilo Kopp
    Affiliation: Universität Augsburg
    Title: Superconductivity With Rashba Spin-Orbit Coupling And Magnetic Field:
    A Route To Topological Superconductivity

    Time and room:  17:15 h, lecture hall IAP
    Abstract: A two-dimensional s-wave superconductor in a magnetic field with a sufficiently strong Rashba spin-orbit coupling is a candidate system for a topological superconductor. Typically, the required magnetic field to convert the superconductor into a topologically non-trivial state is however by far larger than the upper critical field, which excludes its realization. This problem is overcome by rotating the magnetic field into the superconducting plane. The character of the superconducting state changes with the strength and the orientation of the magnetic field. A topological state indeed extends to an in-plane field orientation. Mapping the spin texture in momentum space reveals a meron-like structure. In analogy to skyrmion patterns, the momentum-space spin texture translates into an integer number which offers an alternative to reflect the topological character of the superconducting state. A possible realization of the topological s-wave superconductor at LaAlO3/SrTiO3 interfaces is examined.

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