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Quantum technologies

Dieter Meschede's research group
Home AMO physics colloquia
  • Hendrik Bluhm

  • Invited speaker: Hendrik Bluhm
    Affiliation: RWTH Aachen
    Title: Semiconductor Spin Qubits - From Proof of Principle Demonstrations to High Fidelity Quantum Gates  

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

    Qubits based on electron spins trapped in fabricated semiconductor nanostructures such as gate controlled quantum dots or individual donors are considered promising candidates for scalable solid state quantum computing, which promises an exponential speedup for certain computational tasks.
    Starting from an outline of the development of key concepts, I will give a (selective) overview of the current state of the art of GaAs and Si based spin qubit devices. Key results include single shot readout, high fidelity single-qubit manipulation, and first demonstrations of two-qubit gates. One particular focus of my talk will be effects from the hyperfine interaction of nuclear spins with the electron spin qubit, which is a source of strong decoherence when present and unavoidable in GaAs devices. The intricate physics emerging from hyperfine coupling is now rather well understood and we have developed effective methods to reduce the associated dephasing and achieve high fidelity qubit control.

  • Yoichi Ando

  • Invited speaker: Yoichi Ando
    Affiliation: Universität zu Köln

    Title: Topological Insulators and Superconductors  

    Time and room: exceptionally:  17:30 h, lecture hall IAP

    Topological insulators and superconductors are new quantum states of matter that are characterized by nontrivial topological structures of the Hilbert space [1]. Recently, they attract a lot of attention because of the appearance of exotic quasiparticles such as spin-momentum-locked Dirac fermions or Majorana fermions on their surfaces, which hold promise for various novel applications [2]. In this talk, I will introduce the basics of those materials and present some of the key contributions we have made in this new frontier.
    [1] Y. Ando, Topological Insulator Materials, J. Phys. Soc. Jpn. 81, 102001 (2013).
    [2] Y. Ando and L. Fu, Topological Crystalline Insulators and Topological Superconductors: From Concepts to Materials, Annu. Rev. Condens. Mater Phys. 6, 361 (2015).

  • Florian Marquardt

  • Invited speaker: Florian Marquardt
    Affiliation: Universität Erlangen-Nürnberg
    Title: Light, Sound, and Topology 

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

    In this talk, I will first give a brief introduction to the field of cavity optomechanics, where one couples radiation fields
    to the motion of mechanical resonators. I will then explain how optomechanical interactions can be exploited to modify the transport
    of phonons and photons in two-dimensional arrays of coupled optical and vibrational modes. These can e.g. be implemented in photonic crystal slabs.
    Engineering the light field wave front, it is possible to generate a topologically nontrivial bandstructure via the optomechanical interaction.
    This gives rise to transport of sound waves along chiral edge channels that are robust against disorder. In the last part of the talk, I will
    indicate how one can even use a purely geometrical nanoscale design for chiral sound wave transport in a pseudo-magnetic field.

    "Topological Phases of Sound and Light"
    Vittorio Peano, Christian Brendel, Michael Schmidt, and Florian Marquardt, Phys. Rev. X 5, 031011 (2015)

    "Pseudomagnetic fields for sound at the nanoscale"
    Christian Brendel, Vittorio Peano, Oskar Painter, and Florian Marquardt, arXiv:1607.04321 (2016)

  • Per Delsing

  • Invited speaker: Per Delsing
    Affiliation: Chalmers University of Technology, Göteborg
    Title:  Quantum Optics with Microwaves and Superconducting Circuits 

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

    Recently it has become possible to do quantum optics experiments, where propagating microwaves interact with artificial atoms in the form of superconducting circuits [1]. In our case, the artificial atoms are made from transmon qubits, where we utilize also the higher levels of the transmon. In this colloquium I will discuss several such experiments.In the first set of experiments, we embed a transmon artificial atom in an open transmission line. When a weak coherent state is on resonance with the atom, we observe extinction of >99% in the forward propagating field. Addressing the higher levels, it is possible to observe the Autler-Towns splitting, and the Mollow triplet. Using the Autler-Towns splitting we demonstrate how photons can be routed efficiently and fast on-chip [2]. By applying a control tone, we also observe a giant cross-Kerr effect [4]. Furthermore we study the statistics of the reflected and transmitted radiation and we demonstrate antibunching in the reflected field and superbunching of the transmitted field.
    In a second set of experiments, we embed a transmon at a distance from the end of an open transmission line, which acts as a mirror[5]. By tuning the wavelength of the atom, we effectively change the normalized distance between atom and mirror, allowing us to effectively move the atom from a node to an antinode of the vacuum fluctuations. We probe the strength of vacuum fluctuations by measuring spontaneous emission rate of the atom.
    [1] I.-C. Hoi et al. New Journal of Physics 15, 025011 (2013)
    [2] I.-C. Hoi et al. Physical Review Letters 107, 073601 (2011)
    [3] I.-C. Hoi et al. Physical Review Letters 108, 263601 (2012)
    [4] I.-C. Hoi et al. Physical Review Letters 111, 053601 (2013)
    [5] I.-C. Hoi et al. Nature Physics, 11, 1045 (2015)


  • Jörg Schmiedmayer

  • Invited speaker: Jörg Schmiedmayer
    Affiliation: Vienna Center for Quantum Science and Technology (VCQ), Atominstitut, TU-Wien

    Title: Does an isolated quantum system relax?

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

    The evolution of an isolated quantum system is unitary.  This is simple to probe for small systems consisting of few non-interacting particles.  But what happens if the system becomes large and its constituents interact? In general one will not be able to follow the evolution of the complex many body eigenstates. Ultra cold quantum gases are an ideal system to probe these aspects of many body quantum physics and the related quantum fields. Our pet systems are one-dimensional Bose-gases. Interfering two systems allows studying coherence between the two quantum fields and the full distribution functions and correlation functions give detailed insight into the many body states and their non-equilibrium evolution.  In our experiments we study how the coherence created between the two isolated one-dimensional quantum gases by coherent splitting slowly degrades by coupling to the many internal degrees of freedom available [1]. We find that a one-dimensional quantum system relaxes to a pre-thermalisatized quasi steady state [2] which emerges through a light cone like spreading of ’de-coherence’ [3]. The pre-thermalized state is described by a generalized Gibbs ensemble [5]. Finally we investigate two distinct ways for subsequent evolution away from the pre-thermalized state. One proceeds by further de-phasing, the other by higher order phonon scattering processes.  In both cases the final state is indistinguishable from a thermally relaxed state.  We conjecture that our experiments points to a universal way through which relaxation in isolated many body quantum systems proceeds if the low energy dynamics is dominated by long lived excitations (quasi particles).

    Supported by the Wittgenstein Prize, the Austrian Science Foundation (FWF) SFB FoQuS: F40-P10 and the EU through the ERC-AdG QuantumRelax
    [1] S. Hofferberth et al. Nature, 449, 324 (2007).
    [2] M. Gring et al., Science, 337, 1318 (2012); D. Adu Smith et al. NJP, 15, 075011 (2013).
    [3] T. Langen et al., Nature Physics, 9, 640–643 (2013).4] T. Langen et al., Science 348 207-211 (2015).

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