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Dieter Meschedes Forschungsgruppe
Home AMO-Physikkolloquien
  • Eschrig

  • Invited speaker: Matthias Eschrig
    Affiliation: Royal Holloway University, London
    Title: New Avenues toward Complex Pairing States

    Time and room: 17:15, lecture hall IAP
    Abstract: Current challenges of quantum many-body theory include correlated super-conductivity, topological materials, non-equilibrium phenomena, and complexity at surfaces and interfaces. In particular, the physics near interfaces and surfaces has attracted attention, as it can lead to phases of matter that might not be realized in bulk materials. The increase in functionality due to spin-triplet pairing states in devices has led to the new field of superconducting spintronics.
    Of particular interest are effects based on the presence of gemetric phases and on topological stability. Whereas the former open new avenues toward spin control, the latter have recently attracted strong interest due to their robustness with respect to surface disorder. Spin triplet pairing states in bulk materials can be realized in superfluid 3He, in some correlated super-conductors, or in non-centrosymmetric superconductors. Spin-orbit locking in non-centrosymmetric materials may lead to topological superconductivity giving rise to topologically protected surface Andreev states.
    I will present self-consistent calculations for topological superconductivity in non-centrosymmetric materials, and for superfluid order in 3He in the presence of spin-active surfaces.

  • Alexander Pukhov


  • Invited speaker: Alexander Pukhov
    Affiliation: Universität Düsseldorf
    Title:  Laser Absorption in Plasmas: From Nano-Targets to Near-QED Regime

    Time and room: 17:15, lecture hall IAP
    Abstract: As the laser technology continues its spectacular development, ever higher field intensities and power levels become accessible in laboratories. The ELI project opens new horizons for laser applications in ultra-bright sources of short wavelength radiation. At the same time, the laser pulse quality – like the contrast ratio – is greatly improved so that fine structured targets maintain their structure till the main pulse arrival. This opens new and unexpected possibilities for laser-plasma engineering towards new physics.
    In the talk, we consider laser pulse interaction with nano- and micro-structured targets like nano-grass [1-4] in the intensity range 1018-1020 W/cm2. At intensities higher than 1022 W/cm2, the radiation damping force becomes important and can exceed the Lorentz force acting on an electron [2]. The g-ray emission is then the major channel of laser energy absorption [3,4].
    1.     MA Purvis, VN Shlyaptsev, R Hollinger, C Bargsten, A Pukhov, et al. Nature Photonics 7, 796 (2013)
    2.     Vural Kaymak, Alexander Pukhov, Vyacheslav N. Shlyaptsev, Jorge J. Rocca, “Nano-scale ultra-dense Z-pinches formation from laser-irradiated nanowire arrays”, Physical Review Letters 117 (3), 035004 (2016)
    3.     R Hollinger, C Bargsten, VN Shlyaptsev, V Kaymak, A Pukhov et al., “Efficient picosecond x-ray pulse generation from plasmas in the radiation dominated regime”, Optica 4 (11), 1344-1349 (2017)
    4.     JJ Rocca, V Shlyaptsev, R Hollinger, C Bargsten, A Pukhov, V Kaymak, et al. “Compact ultra-intense lasers and nanostructures open a path to extreme pressures” LASER FOCUS WORLD 53 (5), 21-26 (2017)
    5.     LL Ji, A Pukhov, IY Kostyukov, BF Shen, K Akli Physical review letters 112, 145003 (2014)
    6.     LL Ji, A Pukhov, EN Nerush, IY Kostyukov, BF Shen, KU Akli Physics of Plasmas 21, 023109 (2014)
    7.     Longqing Yi, Alexander Pukhov, Phuc Luu-Thanh, and Baifei Shen, Phys. Rev. Lett. 116, 115001 (2016)

  • Dagmar Bruss

  • Invited speaker: Dagmar Bruss
    Affiliation: Universität Düsseldorf
    Title: Quantum Cryptography With Many Users

    Time and room: 17:15, lecture hall IAP
    Abstract: The laws of quantum mechanics allow for quantum cryptography, i.e. the distribution of a secret random key between two parties. When generalising this idea to the situation where more than two parties want to establish a common secret key, one can use certain multipartite entangled states as a resource. In the security analysis for this multi-user scheme some intricate new features arise and will be discussed. Finally, it is shown that our protocol for multipartite quantum cryptography offers a speed-up in certain quantum networks with bottlenecks.

  • Jan Gieseler

  • Special Colloquium
    Invited speaker:
    Jan Gieseler
    Affiliation: Harvard University, Cambridge (USA)
    Title: Quantum Nanomechanics - From Levitation to Many-Body Spin-Spin Interactions

    Time and room: 13:15, lecture hall IAP
    Abstract: Nitrogen vacancy (NV) centers are promising candidates for quantum computation, with room temperature optical spin read-out and initialization, microwave manipulability, and weak coupling to the environment resulting in long spin coherence times. The major
    outstanding challenge involves engineering coherent interactions between the spin states of spatially separated NV centers. To address this challenge, we are working towards the experimental realization of mechanical spin transducers.
    The spin transducer consists of a magnetic mechanical resonator in proximity of the NV centers. Consequently, the magnetic field at the NV location depends on the resonator motion. On the other hand, spin flips of the electronic spin of the NV center exert a force on the resonator. Hence, the spin-resonator interaction can be used to mediate an effective spin-spin interaction between two distant NV centers that are coupled to the same mechanical mode. This principle is in close analogy to trapped ions that interact via a common mechanical mode and which have already demonstrated high fidelity quantum gates. To maximize the coherent spin-resonator coupling it is required to employ a low
    mass, high quality mechanical resonator, NV centers with very long spin coherence times, strong magnetic field gradients, and to combine them while preserving the excellent properties of the individual components. To date, we have successfully fabricated doubly-clamped
    silicon nitride mechanical resonators and fabricated nano-magnets on top of them while maintaining a high-quality factor (Q>105). In addition, the resonators are integrated close to a bulk diamond sample to access bulk NV centers with long coherence times and to maximize the spin resonator coupling. In a second approach, we start with a levitated micromagnet and aim at using its degrees of freedom to couple to the NV-center spin. The absence of any support structure gives a large magnetic moment to mass ratio, which is favorable for large couplings, and can give rise to low mechanical damping.
    In this talk, I report on our experimental progress towards achieving a coherent coupling of the motion of these resonators with the electronic spin states of individual NV centers under cryogenic conditions. Such a system is expected to provide a scalable platform for mediating effective interactions between isolated spin qubits and to enable the preparation of non-classical states of motion of a macroscopic object.

  • Christophe Salomon

  • Invited speaker: Christophe Salomon
    Affiliation: CNRS, Ecole Normale Supérieure, Paris
    Title: Single-Qubit Operations and Two-Qubit Entanglement with Individually Controlled Neutral Atoms

    Time and room: 17:15, lecture hall IAP
    Abstract: We report on the production and study of a mixture of Bose and Fermi superfluids.
    Such a mixture has long been sought in liquid helium where superfluidity was achieved separately in bosonic 4He and fermionic 3He. However due to strong interactions between isotopes, phase separation occurs when the 3He concentration exceeds 6%, which, so far, has prevented reaching simultaneous superfluidity for both species.
    Using dilute quantum gases where interactions can be tuned, we have produced a Bose-Fermi mixture where both species are superfluid [1]. By exciting center of mass oscillations of the mixture we probe the collective dynamics of the system. Coherent energy exchange between the Bose and Fermi gas is observed with very small damping below a certain critical velocity. We compare this critical velocity for superfluid counterflow to a recent theoretical prediction [2,3]. Raising the temperature of the system slightly above the superfluid transition reveals an unexpected phase-locking of the oscillations induced by dissipation. Finally the lifetime of the Bose-Fermi mixture is governed by a very simple formula involving the fermionic two-body contact [4].

    1. Igor Ferrier-Barbut, Marion Delehaye, Sebastien Laurent, Andrew T. Grier, Matthieu Pierce, Benno S. Rem, Frédéric Chevy, Christophe Salomon, A Mixture of Bose and Fermi Superfluids, Science 345, 1035, (2014)
    2. Y. Castin, I. Ferrier-Barbut, and C. Salomon, The Landau critical velocity for a particle in a Fermi superfluid, Comptes Rendus Physique, 16, 241 (2015).
    3. M. Delehaye, S. Laurent, I. Ferrier-Barbut, S. Jin, F. Chevy, and C. Salomon, Critical Velocity and Dissipation of an ultracold Bose-Fermi Counterflow, Phys. Rev. Lett., 115, 265303 (2015).
    4. S. Laurent, M. Pierce, M. Delehaye, T. Yefsah, F. Chevy, C. Salomon, Connecting few-body inelastic decay to quantum correlations in a many-body system : a weakly coupled impurity in a resonant Fermi gas, Phys. Rev. Lett., 118, 103403 (2017)