# Theory Seminar

New Rules (since May 2012), Old rules

Contact: johannes.kofler@mpq.mpg.de

 Date Seminars 2015   (Go to Seminars 2014, Seminars 2013, Seminars 2012, and earlier therein) 14.01.2015 Macrorealism Lucas Clemente Macrorealism — the world view that physical properties of macroscopic objects exist independent of measurements and are not influenced by them — has recently been a focus of both theoretical and experimental work in quantum physics. As experiments get closer to showing quantum superpositions of macroscopically distinct states, it becomes interesting to look at conditions for macrorealism beyond the well-known Leggett-Garg inequalitites. In this talk I will discuss a condition called no-signaling in time, that, in the right combination, can serve not only as a necessary, but also as a sufficient condition. We will show how to apply these conditions to physical experiments, and construct a definition for the classicality of quantum measurements and Hamiltonians. The talk's slides can be found at https://clemente.io/macrorealism. 21.01.2015 Area laws and efficient descriptions of quantum many-body states Yimin Ge It is commonly believed that area laws for entanglement entropies imply that a quantum many-body state can be faithfully represented by efficient tensor network states - a conjecture frequently stated in the context of numerical simulations and analytical considerations. In this talk, I will show that this is in general not the case, except in one dimension. It turns out that the set of quantum many-body states which satisfy an area law for all Renyi entropies contains a subspace of exponential dimension. This implies that there exist area law states which do not have an efficient description in a very general sense, including tensor network states such as polynomial PEPS or MERA. Not even a quantum computer with post-selection can efficiently prepare all quantum states fulfilling an area law, and moreover not all area law states can be eigenstates of local Hamiltonians. I will also discuss variations of these results with translational and rotational invariance as well as decaying correlations. [Based on work with Jens Eisert, arXiv:1411.2995] 04.02.2015 Local temperature in interacting spin systems Guest speaker: Senaida Hernández (ICFO, Spain) In standard thermodynamics, temperature is a local quantity: a subsystem of a large thermal system is in a thermal state at the same temperature as the whole system. For strongly interacting systems, however, the locality of temperature breaks down. We explore the possibility of associating an effective thermal state to subsystems of infinite chains of interacting spin-1/2 particles. We study the effect of correlations and criticality in the definition of this effective thermal state and discuss the possible implications for the classical simulation of thermal quantum systems. 10.02.2015 Non-local Adiabatic Response of a Localized System to Local Manipulations Guest speaker: Shivaji L. Sondhi (Princeton University, USA) We examine the response of a system localized by disorder to a time dependent local perturbation which varies smoothly with a characteristic timescale $\tau$. We find that such a perturbation induces a non-local response, involving a rearrangement of conserved quantities over a length scale $\sim \ln \tau$. This effect lies beyond linear response, is absent in undisordered insulators and highlights the remarkable subtlety of localized phases. The effect is common to both single particle and many body localized phases. Our results have implications for numerous fields, including topological quantum computation in quantum Hall systems, quantum control in disordered environments, and time dependent localized systems. For example, they indicate that attempts to braid quasiparticles in quantum Hall systems or Majorana nanowires will surely fail if the manipulations are performed asymptotically slowly, and thus using such platforms for topological quantum computation will require considerable engineering. They also establish that disorder localized insulators suffer from a statistical orthogonality catastrophe. 11.02.2015 Twenty Years After Shor Juan Bermejo-Vega 20 years after Shor discovered his factoring algorithm, the field of quantum computation is still missing a theory that explains how quantum speed-ups emerge. This slows down progress in finding new quantum algorithms. In this talk, we apply an extension of the generalized stabilizer formalism (an extension of a successful quantum information paradigm for describing quantum many-body states) to study the structure of quantum algorithms for abelian and nonabelian hidden subgroup problems. We use our tools to explain the success of Shor's algorithm and the former kind, and to discuss the failure of a research program that aimed at solving the latter and finding the once-called "Holy Grail of quantum computation": an efficient quantum algorithm for the Graph Isomorphism problem. 12.02.2015 Introduction to the AdS/CFT correspondence, its generalizations and applications Guest speaker: Johanna Erdmenger (MPP, Munich, Germany) I will give an introduction to the AdS/CFT correspondence and its generalization for non-experts. AdS/CFT is a map between strongly coupled quantum field theories and classical theories of gravitation which originates from string theory. Moreover, I will introduce applications of this approach to quantum liquids and to strongly correlated systems. I will also comment on the relation between AdS/CFT and tensor networks. 04.03.2015 Emergence of Chiral Order in SU(2) lattice gauge theory in 1D Guest speaker: Pietro Silvi (University of Ulm, Germany) We study the ground state properties of the simplest quantum link model undergoing a SU(2) lattice gauge invariance, in one spatial dimension. We observe the existence of a charge-density-wave phase, where matter nucleates into localized hadrons forming a crystalline structure. We sketch the phase diagram and analyse the transition to other equilibrium phases. 05.03.2015 Information Lost and Information Regained - An overview of the black hole information paradox Guest speaker: Abhiram Kidambi (MPP, Munich, Germany) Black holes are one of the most mysterious concepts in physics and carry with them a large number of unsolved problems and counterintuitive results, none of which are more troubling than the information paradox.The black hole information paradox, proposed by Stephen Hawking in 1976, revealed unitarity violation in black hole evaporation. Hawking showed that when a pure quantum state enters a black hole, it should be released in the form of thermal radiation during black hole evaporation thereby violating unitarity. In the recent years, there has been an increase in interest from the quantum gravity community with the aim of understanding and resolving this paradox. Attempts to save unitarity have resulted in even more counterintuitive paradoxes which boils down to the incompatibility between quantum mechanics, the equivalence principle and effective field theory principles. In this talk, I will give a non-technical and unbiased overview of the black hole information paradox (both original and current) and current status of resolution schemes. Disclaimer: Most of the community is divided on the resolution schemes. The quantum origin of these resolution schemes are largely unclear. It is fair to say that there is no unanimously favoured resolution yet. 11.03.2015 Tba Henrik Dreyer Tba 18.03.2015 Tba Anna Hackenbroich Tba 25.03.2015 Tba Guest speaker: Robert König (TUM, Garching, Germany) Tba 01.04.2015 Tba Xiaotong Ni Tba 08.04.2015 Tba Guest speaker: Nan Zhao (Beijing Computational Science Research Center, China) Tba