# Theory Seminar

 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, 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 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 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 non-interacting fermion systems. The third part of the talk will be non-quantum: 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 one-dimensional harmonically trapped ideal system with an impurity Guest speaker: Artem Volosniev (University of Aarhus, Denmark) We study impure mesoscopic one-dimensional ensembles with majority particles either non-interacting fermions or bosons. Assuming a short-range 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 Chern-Weil 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 system-bath interactions. 25.03.2015 Protected gates for topological quantum field theories Guest speaker: Robert König (TUM, Garching, Germany) We give restrictions on locality-preserving unitary automorphisms U, which are protected gates, for 2-dimensional topologically ordered systems. For generic anyon models, we show that such unitaries only generate a finite group, and hence do not provide universality. For non-abelian models, we find that such automorphisms are very limited: for example, there is no non-trivial 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) The strong interactions of elementary particles are described theoretically in the framework of Quantum Chromodynamics (QCD). The most promising way to solve QCD is given by numerical simulations using Markov Chain Monte Carlo (MCMC) Methods, in which the space-time continuum is replaced by a lattice. We shall demonstrate that since the invention of this approach by K. Wilson the conceptual, algorithmic and supercomputer developments have progressed so much that today realistic simulations of lattice-QCD become possible, bringing us close to a, at least, numerical solution of QCD. As some examples, we will show results for the hadron spectrum, resonances and the structure of nucleon. We will also discuss open problems, in particular those where MCMC methods are not applicable and where tensor network techniques have a strong potential to make substantial progress. 01.04.2015 Can quantum devices evolve? Xiaotong Ni There is a long (but somewhat sparse) history of people using ideas from artificial intelligence to design classical devices, and in the recent years this approach has gained some attention in the quantum community. In this talk I will give a review on some selected works, and discuss the possibility of using these ideas to help design quantum memories. 08.04.2015 Sensing with nitrogen-vacancy centers in diamond Guest speaker: Nan Zhao (Beijing Computational Science Research Center, China) Nitrogen-vacancy (NV) center is one of the most promising systems for quantum information processing. NV centers are also excellent sensors for detecting weak signals. In this talk, I will first discuss the microscopic decoherence mechanisms of NV center electron spins. Based on the decoherence mechanisms, I will talk about recent progress on single-molecule nuclear magnetic resonance using NV centers and other applications of NV centers in sensing weak signals. 15.04.2015 Non equilibrium dynamics and entanglement in the transverse field Ising chain Guest speaker: Leda Bucciantini (University of Pisa, Italy) I will discuss the long time dynamics of some relevant observables in the quantum spin Ising chain that evolves unitarily from an initial out of equilibrium configuration. The aim is that of understanding the behaviour of out of equilibrium many body quantum systems, observed in recent ultra-cold atom experiments, in terms of few laws coming from quantum field theory or quantum mechanics, in a similar way as classical thermodynamics is based on statistical mechanics and ensemble formulation. 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 non-invasive measurability, using Bell’s construction of stochastic pilot-wave 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 pilot-wave theory again helps to clarify the situation. 22.04.2015 Many body gates: from small chains to networks Nicola Pancotti During the last years a great effort has been made in order to characterize, implement and optimize entangling gates between two distant particles. Here we study the evolution of a quantum complex system and we search whether there exists an optimal time t* in which a perfect entangling gate G is implemented. The aim of this thesis is dual. On one side we propose a brand new numeric method acquired from the machine learning community which gives a good speed up compared to the previous ones. On the other side we look for new, unknown solutions Jopt that parametrize the Hamiltonian so that exp(−iH(Jopt )t) = G ⊗ S; S is an operator which leaves the rest of the system in an unknown (don’t care) state. We start from a very simple 3-spins chain, for which we already know the solution. We move then forward to a N-spins chain and finally we enlarge the line to obtain a network. We aim to find perfect topologies and optimal two body interactions capable of implementing fast and high fidelity gates such as entangling, Toffoli, CCZ, etc. 29.04.2015 Deterministic generation of many-body entanglement and photonic states assisted by dissipation Vanessa Paulisch Recent efforts to couple cold atoms to nanophotonic waveguides are expected to yield novel ways to engineer and control light-matter interactions at the quantum level. By tailoring the waveguide properties, one can reach a regime of purely collective dissipation. This allows for the preparation of many-body entangled states which are protected from collective dissipation. These states can me mapped to superradiant states, which in turn generate photonic states. We show how to deterministically and reversibly generate arbitrary superpositions of Fock states. 06.05.2015 String breaking in a non-abelian gauge theory – Studying real-time dynamics with Matrix Product States Stefan Kühn In recent years there has been quite some effort to apply Matrix Product States and more general Tensor Networks to lattice gauge theories. Contrary to the standard Euclidean time Monte Carlo approach, which is widely used to study lattice gauge theories numerically in the nonperturbative regime, Tensor Networks do not suffer from the sign problem and allow simulating time evolution. They might therefore offer an alternative route to address certain regions of the phase diagram and to study real-time dynamics of problems that are currently intractable with Monte Carlo methods. In this week's seminar I am going to talk about string breaking in an 1+1 dimensional SU(2) lattice gauge theory. After discussing the theoretical background and how the model can be addressed with Matrix Product States, I will present some numerical results on real-time dynamics of string breaking. 18.05.2015 Holographic quantum error-correcting codes: Toy models for the bulk/boundary correspondence Guest speaker: Fernando Pastawski (Caltech, USA) In this talk I will introduce a family of exactly solvable toy models of a holographic correspondence based on a novel construction of quantum error-correcting codes with a tensor network structure. The building block for these models are a special type of tensor with maximal entanglement along any bipartition, which gives rise to an exact isometry from bulk operators to boundary operators. The entire tensor network is a quantum error-correcting code, where the bulk and boundary degrees of freedom may be identified as logical and physical degrees of freedom respectively. These models capture key features of entanglement in the holographic correspondence; in particular, the Ryu-Takayanagi formula and the negativity of tripartite information are obeyed exactly in many cases. I will describe how bulk operators may be represented on the boundary regions mimicking the Rindler-wedge reconstruction. 27.05.2015 An introduction to Schramm-Loewner Evolutions Ivan Glasser Schramm-Loewner Evolutions are families of random planar curves. They describe interfaces of some models of statistical physics, such as percolation and the Ising model, in the continuum limit. In this talk I will give an introduction to the theory of Schramm-Loewner Evolutions. Lattice models that give rise to Schramm-Loewner Evolutions in the continuum will be introduced and their main properties, such as conformal invariance in the scaling limit, will be discussed. I will then show how Schramm-Loewner Evolutions arise from these properties in the continuum. Finally I will give a short overview of the current knowledge about Schramm-Loewner Evolutions and I will briefly mention the relationship between Schramm-Loewner Evolutions and Conformal Field Theory. 02.06.2015 Transport signatures of long-range nuclear-spin coherence in a quantum-dot spin valve Guest speaker: Stefano Chesi (Beijing Computational Science Research Center, China) Several types of quantum-dot spin valves were recently realized. For such systems, we have analyzed the efficient transfer of angular momentum into the nuclear bath and the detection of nuclear-spin coherence through transport signatures. Flip-flop processes between electron and nuclear spins in the quantum dot are allowed by the hyperfine interaction. Long-range nuclear-spin coherence can induce a strong enhancement of such spin-flip transition rates, by an amount proportional to the number of nuclear spins. Under a finite voltage bias, the enhancement is revealed by an intense current burst analogous to superradiant light emission. Instead, fast local dephasing for the nuclear spins leads to an incoherent evolution analogous to spontaneous emission. Through a combination of simple rate equations and a more general master equation we have characterized these two regimes and the crossover between them. We also discuss our ongoing work about related schemes, but with unpolarized contacts. We generally assume uniform hyperfine couplings, which yield the strongest coherent enhancement. We propose realistic strategies, based on isotopic modulation and wavefunction engineering in core-shell nanowires, to realize this analytically solvable "box-model" of hyperfine couplings. 03.06.2015 Entanglement generation in 3-mode optomechanical systems Guest speaker: Yingdan Wang (Chinese Academy of Sciences, Beijing, China) A 3-mode optomechanical system (e.g., with two electromagnetic fields coupled to a single mode of a mechanical resonator) is a key element of hybrid quantum systems. We find that large steady-state entanglement can be achieved in a 3-mode optomechanical system by effectively laser cooling a delocalized Bogoliubov mode. This approach allows one to surpass the bound on the maximum stationary intracavity entanglement possible with a coherent 2-mode interaction. In particular, we find that optimizing the relative ratio of optomechanical couplings, rather than simply increasing their magnitudes, is essential for achieving strong entanglement. We also provide analytic insight into the generation of stationary itinerant photon entanglement in such system. We identify the parameter regime of maximal entanglement, and show that strong entanglement is possible even for weak many-photon optomechanical couplings. We also show that strong tripartite entanglement can be generated between the photonic and phononic output fields. 10.06.2015 Long-range entanglement generation between spatially separated electronic spins Guest speaker: Mónica Benito Gonzáles (ICMM, Madrid, Spain) We propose to use Quantum Hall edge channels in order to generate long-range entanglement between spatially separated spin qubits. Since the entanglement is actively stabilized by purely dissipative dynamics, our scheme is inherently robust against noise and imperfections. 16.06.2015 Measuring the universal scaling of many-body entanglement Guest speaker: Philipp Hauke (IQOQI Innsbruck, Austria) Quantum correlations are strongly enhanced near quantum critical points, which opens attractive prospects for applications such as quantum-enhanced metrology. A major obstacle, however, is the difficulty of observing many-body entanglement. We show that the quantum Fisher information, a witness for genuinely multipartite entanglement, is equivalent to routinely measured response functions such as the dynamical susceptibility. This equivalence is independent from microscopic details. Moreover, we derive the universal scaling of the quantum Fisher information, and illustrate it at three examples, the Mott-insulator–superfluid transition of hard-core bosons, and the Ising model in a transverse field with nearest-neighbor as well as infinitely-ranged interactions. Our work makes multipartite entanglement accessible to experiments studying quantum phase diagrams at nonzero temperatures, most importantly also in the context of solid-state samples. 16.09.2015 Tba Thorsten Wahl Tba