Theory Seminar
From MPQTheory

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 wellknown LeggettGarg inequalitites. In this talk I will discuss a condition called nosignaling 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 manybody states
Yimin Ge It is commonly believed that area laws for entanglement entropies imply that a quantum manybody 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 manybody 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 postselection 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, Barcelona, 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 spin1/2 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  Nonlocal 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 nonlocal 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 BermejoVega 20 years after Shor discovered his factoring algorithm, the field of quantum computation is still missing a theory that explains how quantum speedups 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 manybody 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 oncecalled "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 nonexperts. 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 chargedensitywave 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 nontechnical 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  Classical Simulation with Matchgates and Holographic Algorithms
Henrik Dreyer Classical Simulation of Quantum Computations is concerned with investigating the boundary between what is computable on a classical device and a quantum computer. Several avenues have been pursued towards this end (including also Clifford circuits and Tensor Networks), the ultimate question being the origin of the quantum speedup. In this talk, a basic introduction to Matchgate theory will be given and then used to simulate a certain class of quantum circuits. Subsequently, we explore the equivalence of these circuits to noninteracting fermion systems. The third part of the talk will be nonquantum: Matchgates can be used to find classical counting algorithms in which an exponential number of terms cancel. Due to their similarity with quantum interference, these are called “Holographic Algorithms”. Holographic Algorithms have received notable coverage within the Computer Science community since it has been speculated that they are of importance for the P vs. NP problem.

17.03.2015  A onedimensional harmonically trapped ideal system with an impurity
Guest speaker: Artem Volosniev (University of Aarhus, Denmark) We study impure mesoscopic onedimensional ensembles with majority particles either noninteracting fermions or bosons. Assuming a shortrange interparticle potential I will first discuss an eigenspectrum for a fermionic host medium; there a very strong repulsive interaction realizes a spin chain, the coupling coefficients for which can be established if the masses of impurity and majority are the same. Next I will address some ground state properties of an impurity in a bosonic bath and I also sketch what happens if the interaction strength is suddenly changed.

18.03.2015  Chern classes and some applications to condensed matter physics
Anna Hackenbroich Chern numbers are an important concept in both condensed matter and high energy physics. In this talk, I will give a basic introduction to real Chern classes of complex vector bundles from the perspective of ChernWeil theory. I will explain the importance of Chern classes for physical applications and, by considering the Haldane model, show how they are used in the topological classification of Bloch band structures.

24.03.2015  Work extraction from quantum systems
Guest speaker: Martí Perarnau (ICFO, Barcelona, Spain) Traditional thermodynamics deals with macroscopic systems in thermal equilibrium, where one only has control over a few macroscopic variables (volume, temperature, etc). This minimal level of control is in contrast with the great progress over the last decades in the coherent control of single quantum systems. Based on the notion of passive states, in this talk I will describe an extension of the standard notions of thermodynamics (in particular the problem of work extraction) for highly controllable small quantum systems. Concretely, I will discuss how the maximal work extraction principle is modified in the presence of entangling operations, correlations, finite baths, and strong systembath interactions.

25.03.2015  Protected gates for topological quantum field theories
Guest speaker: Robert König (TUM, Garching, Germany) We give restrictions on localitypreserving unitary automorphisms U, which are protected gates, for 2dimensional topologically ordered systems. For generic anyon models, we show that such unitaries only generate a finite group, and hence do not provide universality. For nonabelian models, we find that such automorphisms are very limited: for example, there is no nontrivial gate for Fibonacci anyons. More generally, systems with computationally universal braiding have no such gates. For Ising anyons, protected gates are elements of the Pauli group.These results are derived by relating such automorphisms to symmetries of the underlying anyon model: protected gates realize automorphisms of the Verlinde algebra. We additionally use the compatibility with basis changes to characterize the logical action This is joint work with M. Beverland, O. Buerschaper, F. Pastawski, J. Preskill and S. Sijher.

30.03.2015  Status and open Problems in Lattice Gauge Theory Computations
Guest speaker: Karl Jansen (DESY, Zeuthen, Gemany) Tba

01.04.2015  Tba
Xiaotong Ni Tba

08.04.2015  Tba
Guest speaker: Nan Zhao (Beijing Computational Science Research Center, China) Tba

16.04.2015  Leggett–Garg Inequalities, Pilot Waves and Contextuality
Guest speaker: Guido Bacciagaluppi (University of Aberdeen, UK) In this talk we first analyse Leggett and Garg’s argument to the effect that macroscopic realism contradicts quantum mechanics. After making explicit all the assumptions in Leggett and Garg’s reasoning, we argue against the plausibility of their auxiliary assumption of noninvasive measurability, using Bell’s construction of stochastic pilotwave theories as a counterexample. Violations of the Leggett–Garg inequality thus do not provide a good argument against macrorealism per se. We then apply Dzhafarov and Kujala’s analysis of contextuality in the presence of signaling to the case of the Leggett–Garg inequalities, with rather surprising results. An analogy with pilotwave theory again helps to clarify the situation. 