08:45  09:00

Roderich Moessner
(MPIPKS)
Workshop opening

09:00  09:45

Ignacio Cirac
(MPI of Quantum Optics)
Tensor Networks: A quantum information perspective to manybody physics
The theory of entanglement offers a new perspective to view manybody quantum systems. In particular, systems in thermal equilibrium and with local interactions contain very little entanglement, which allows us to describe them efficiently, circumventing the exponential growth of parameters with the system size. Tensor Networks offer such a description, where few simple tensors contain all the information about all physical properties. In this talk I will review some of the latest results on entanglement and tensor networks, and explain some of their connections to quantum computing, condensed matter, and highenergy physics.

10:00  10:30

Friedemann Reinhard
(Technische Universität München)
Magnetic resonance at the nanoscale  quantum sensors for quantum matter?
Magnetic resonance methods are currently undergoing a major boost in sensitivity and resolution: novel sensors [1,2,3], such as the NitrogenVacancy center in diamond, have enabled spectroscopy and imaging on samples as small as a single biomolecule. I will present past work to demonstrate feasibility of this technique [4,5] and our ongoing efforts to push it to a 3D imaging method with atomic resolution. I will speculate on applications in exotic solid state phases, hoping to stimulate discussion.
[1] C. Degen et al., PNAS 106, 1313 (2009)
[2] D. Vasyukov et al., Nat Nano 8, 639 (2013)
[3] J. Taylor, P. Cappellaro et al., Nature Physics 4, 810 (2008)
[4] H. J. Mamin et al., Science 339, 557 (2013)
[5] T. Staudacher et al., Science 339, 561 (2013)

10:45  11:15

Coffee

11:15  11:35

Malte Kremser
(Technische Universität München)
Threestage decoherence dynamics of an electron spin qubit in an optically active quantum dot
A. Bechtold (1), M. Kremser (1), T. Simmet (1), P. L. Ardelt (1), D. Rauch (1), F. Li (2), N. A. Sinitsyn (2), K. Müller (1) and J. J. Finley (1)
(1) Walter Schottky Institut, Am Coulombwall 4, 85748 Garching, Germany
(2) Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
The control of solidstate qubits for quantum information processing requires a detailed understanding of the coupling mechanisms responsible for decoherence. For electron spins in quantum dots (QDs), considerable progress has been achieved in this field for qubit dynamics in strong external magnetic fields; however, decoherence at very low magnetic fields remains puzzling when the magnitude of the Zeeman energy becomes comparable with intrinsic couplings. Phenomenological models of decoherence currently recognize two basic types of spin relaxation; fast ensemble dephasing due to the coherent precession of spin qubits around nearly static but randomly distributed hyperfine fields (∼2ns) and a much slower process (>1μs) of irreversible monotonic relaxation of the spin qubit polarization due to nuclear spin coflips with the central spin or due to other complex manybody interaction effects [1]. In this contribution, we demonstrate experimentally and theoretically that not only two but three distinct stages of decoherence can be identified in the relaxation of a QD electron spin qubit. Measurements and simulations of the spin projection without an external field clearly reveal an additional decoherence stage at intermediate timescales [2]. The additional stage corresponds to the effect of coherent dephasing processes that occur in the nuclear spin bath itself induced by quadrupolar coupling of nuclear spins to strain driven electric field gradients, which leads to a relatively fast but incomplete nonmonotonic relaxation of the central spin polarization at intermediate (∼750ns) timescales. In addition, our measurement technique opens up an alternative route to characterize both the ensemble and the quantum decoherence, T2* and T2, of a spin qubit by studying the higher order time correlators of the spin, using repeated projective measurements [3].
[1] I. Merkulov et al. Phys. Rev. B 65, 205309 (2002)
[2] A. Bechtold et al., Nature Physics 11, 1005–1008 (2015)
[3] A. Bechtold et al., Phys. Rev. Lett. 117, 027402 (2016)
(Topic can alternatively be presented as poster)

11:45  12:05

Alexander Schnell
(MPIPKS)
Inducing Bose condensation with a hot needle
A quantum system exchanging energy with a thermal bath will assume an equilibrium state that is completely determined by the bath temperature. In contrast, if the system is driven out of equilibrium, it will assume a nonequilibrium steady state that does depend on the very details of the coupling to its environment. This offers great freedom to tailor the properties of a system by bath engineering and can also give rise to counter intuitive effects. We consider a onedimensional Bose gas immersed in a cold bath. We show that Bose condensation can be induced in a system of finite extent by coupling it additionally to a "hot needle" (a second, spatially localized bath that is much hotter than the first one).
We observe this for the ideal gas as well as in the presence of weak interactions. For the ideal gas a simple description of this open system is given by the BornMarkov approximation. Within this framework, the bath induces quantum jumps between energy eigenstates. Taking into account temperaturedependent dissipation for the interacting gas is challenging. Already on the level of a simple mean field approximation, it requires the diagonalization of the mean field Hamiltonian in every step of the time integration. We propose and test a scheme to circumvent this problem by treating the systembath coupling semiclassically.

12:15  12:35

Stepan Timr
(Institute of Organic Chemistry and Biochemistry of CAS)
Combination of quantum and classical calculations helps to design membrane fluorescent probes
Fluorescence microscopy has become an invaluable source of our knowledge about how living cells work, with techniques based on nonlinear light absorption showing enormous potential to visualize structures and processes inside individual cells or entire living tissues. The success of these techniques, however, critically depends on the availability of suitable fluorescent probes and on the accurate description of their optical properties in the complex cellular environment. In this talk, I will demonstrate that quantum calculations of nonlinear absorption anisotropies coupled with molecular dynamics simulations can provide a basis for the interpretation of experiments in terms of structural information. Moreover, I will discuss how the combined efforts of molecular modeling and twophoton polarization microscopy can lead to the development of novel probes of cellular processes, such as genetically encoded indicators of neuronal activity that would offer a realtime insight into the function of the brain.

12:45  14:15

Lunch

14:15  14:45

Andreas Rost
(MPI for Solid State Research)
Scanning Tunnelling Microscopy on Dirac and Weyl semimetals
3D Dirac and Weyl semimetals have been a key playground of condensed matter physics in recent years. Their fascination lies both with new insights gained into the role of topological phenomena in condensed matter systems as well as the promise for applications. A main focus of our research is the exploration of such materials through control of band structure and magnetism. Examples are the stabilisation of topological crystalline insulator states through the introduction of gaps and Weyl fermion excitations through time reversal symmetry breaking in magnetic materials.
We employ a range of techniques to study these phases including transport, magnetotorque, scanning tunnelling microscopy and neutron scattering. I will give a brief overview of these activities before particularly focusing on the specific role spectroscopic imaging scanning tunnelling microscopy can play in the study of these materials. A key focus will be on the unique contributions of this technique to the study of both the surface and bulk properties of Dirac and Weyl semimetals at low temperatures and high magnetic fields.

15:00  15:20

Veronika Sunko
(MPI for Chemical Physics of Solids)
Surface states of transitionmetal delafossite oxides
Delafossite oxides have recently attracted considerable attention because of their fascinating transport properties [1, 2]. As well as having extremely long low temperature mean free paths, PdCoO$_2$ and $PtCoO_{2}$ are the most conductive oxides known at room temperature, with resistivities comparable to those of silver, copper or gold [3, 4]. This high conductivity is attributed to a single broad band crossing the Fermi level [4]. However, due to the polarity of the structure the electronic properties at the crystal surfaces can be very different to those of the bulk [5, 6]. Here we use angle resolved photoemission (ARPES) to show that the CoO$_2$ terminated surfaces of $(Pd,Pt)CoO_{2}$ indeed host a set of states which do not appear in the bulk, with much higher masses and apparently stronger interactions. Comparing ARPES with density functional theory (DFT) and model tightbinding calculations, we investigate the origin of these states, paying special attention to the role of the spinorbit coupling.
[1] Moll et al., Science 351 (2016) 6277
[2] Kikugawa et al., Nature Commun. 7 (2016) 10903 [3] Hicks et al., Phys. Rev. Lett. 109 (2012) 116401 [4] Kushwaha et al., Science Adv. 1 (2015) e1500692 [5] Kim et al., Phys. Rev. B 80 (2009) 035116
[6] Noh et al., Phys. Rev. Lett. 102 (2009) 256404

15:30  15:50

Daniil TolouiMantadakis
(MPI for Solid State Research)
Magnetic response of spinorbit coupled delectrons in nonspherical potentials
The Hubbard model and its multiorbital extensions is one of the most prominent microscopic starting points to understand properties of ground state and low energy excitations. Its explicit formulation in terms of single particle operators and the effective Coulomb interaction crucially relies on approximations which are based on an assumed hierarchy of energy scales: while in 3d transition metal compounds crystal field effects dominate and yield good quantum numbers, in 4f rareearth compounds (e.g. heavy fermion systems like $CeX_2Si_2$) strong spinorbit coupling renders total angular momentum to be a much better single particle basis. However, a clear hierarchy is not always
present and when we move towards the 4d and 5d compounds we can hardly define any good quantum numbers at all. Here, we present a dynamical meanfield study of a generic three band model including both crystal field and spinorbit coupling op
erators on equal footing, using the fully SU(2) symmetric Coulomb interaction. We show the results of the twoparticle uniform magnetic response which is, other than single particle spectra quantities, much more sensitive to effects of electronic correlations, and we compare them with the exact atomic limit ones.

16:00  16:30

Coffee

16:30  17:30

CONQUA17 Colloquium
Chair: Jens H. Bardarson (MPIPKS)
Vincenzo Vitelli (Leiden University)
Topological sound and odd viscosity in chiral active matter
Active materials are composed of interacting particles individually powered by motors. In this talk, we focus on chiral active materials that violate parity and time reversal symmetry. First, we show how to generate topological sound in fluids of selfpropelled particles exhibiting a spontaneous chiral active flow under confinement. These topological sound modes propagate unidirectionally, without backscattering, along either sample edges or domain walls and despite overdamped particle dynamics. Next, we discuss an exotic transport coefficient characteristic of quantum Hall fluids, called odd viscosity, which controls the hydrodynamics of classical fluids composed of active rotors. This odd viscosity couples pressure to vorticity leading to transverse flow in piston compression experiments. We envision that such transverse response may be exploited to design selfassembled hydraulic cranks that convert between linear and rotational motion in microscopic machines powered by active rotors fluids.

18:00  19:00

Supper

19:00

Poster Preview
