Theory Seminar
From MPQTheory

Date  Seminars 2017 (Go to Seminars 2016, Seminars 2015, Seminars 2014, and earlier therein)

09.01.2017  Bipartite charge fluctuations in Z_2 topological insulators and superconductors
Invited speaker: Loïc Herviou (Ecole Polytechnique, ENS) Bipartite charge fluctuations (BCF) have been introduced to provide an experimental indication of manybody entanglement.They are a very efficient and useful tool to characterize phase transitions in a large variety of chargeconserving models in one and two dimensions In this seminar, we study the BCF in generic one and twodimensional Z_2 (topological) models such as the Kitaev chain, spinorbit insulators, the graphene and the Haldane model, where the charge we observe is no longer conserved. In onedimension, we demonstrate that at phase transitions characterized by a linear dispersion, the BCF probe the change in a winding number that allows to pinpoint the transition and corresponds to the topological invariant for standard models. Additionally, we prove that a subdominant logarithmic contribution is still present at the exact critical point. Its quantized coefficient is universal and a characteristic of the critical model. In two dimensions, a similar structure appears. While the area term no longer reveal directly the phase transition, a subdominant logarithmic term is still present. Similarly to the entanglement entropy, it depends on the exact shape of the considered region, with contributions of the corner of the regions only. 
25.01.2017  Approaching nonAbelian Lattice Gauge Theories with Tensor Networks
Stefan Kühn In recent years the Tensor Network approach to lattice gauge theories has proven itself as promising alternative to the conventional Monte Carlo methods widely used. In contrast to Monte Carlo simulations, numerical methods based on Tensor networks do not suffer from the sign problem, thus allowing to address problems and parameter regimes which are inaccessible with Monte Carlo. However, even for for the simplest nonAbelian gauge models with dynamical fermions, the computational effort typically grows quickly. Hence, current Tensor Network simulations for nonAbelian models are rather limited. In this talk I will address the case of a SU(2) lattice gauge theory. I will show how, starting from a basis of color neutral states, the gauge field for systems on finite lattices with open boundary conditions can be integrated out, thus greatly reducing the degrees of freedom. While this formulation is completely general, it trivially allows to truncate the maximum colorelectric flux in the system, thus making it particularly suitable for a Tensor Network approach. As a proof of principle I will present numerical results for the low lying spectrum obtained with Matrix Product States for a family of truncated SU(2) models. 
01.02.2017  Decay of correlations in systems of fermions with longrange interactions at nonzero temperature
Senaida Hernandez We study correlations in fermionic systems with longrange interactions in thermal equilibrium. We prove a bound on the correlation decay between anticommuting operators based on longrange LiebRobinson type bounds. Our result shows that correlations between such operators in fermionic longrange systems of spatial dimension $D$ with at most twosite interactions decaying algebraically with the distance with an exponent $\alpha \geq 2\,D$, decay at least algebraically with an exponent arbitrarily close to $\alpha$. Our bound is asymptotically tight, which we demonstrate by numerically analyzing densitydensity correlations in a 1D quadratic (free, exactly solvable) model, the Kitaev chain with longrange interactions. Away from the quantum critical point correlations in this model are found to decay asymptotically as slowly as our bound permits. 
08.02.2017  An introduction to Variational MonteCarlo
Ivan Glasser Variational MonteCarlo methods are used to determine the energy of a wave function and to optimize its parameters in order to approximate the ground state of a manybody quantum system. In this talk I will give a general introduction to Variational MonteCarlo. Starting from the early days of MonteCarlo integration I will explain how these methods can be used to compute energies of wave functions. I will then give an overview of modern methods to optimize the energy of a wave function with many parameters. 
15.02.2017  Simulating nonEquilibrium systems with Matrix Product States
Julian Roos Understanding out of equilibrium remains a challenge for classical and quantum systems. There is no general nonequilibrium statistical mechanics framework to resort to, if one is interested in the statistical properties of observables in far from equilibrium situations. The theory of large deviations can fill this gap in some cases and Tensor Networks are one possibility to explore this problem from a numerical side. Matrix Product States can capture the properties of nonequilibrium stationary states of many classical and quantum models. On Wednesday I will discuss how people have used these techniques for the simplest problem of particle hopping on a 1D lattice. 
21.02.2017 at 14:00  Dissipation induced topological states: A recipe
Invited speaker: Moshe Goldstein (TelAviv University) It has recently been realized that drivendissipative dynamics, which usually tends to destroy subtle quantum interference and correlation effects, could actually be used as a resource. By proper engineering of the reservoirs and their couplings, one may drive a system towards a desired quantumcorrelated steady state, even in the absence of internal Hamiltonian dynamics. An intriguing class of quantum phases is characterized by topology, including the quantum Hall effect and topological insulators and superconductors. Which of these noninteracting topological states can be achieved as the result of purely dissipative Lindbladtype dynamics? Recent studies have only provided partial answers to this question. In this talk I will present a general recipe for the creation, classification, and detection of states of the integer quantum Hall and 2D topological insulator type as the outcomes of coupling a system to reservoirs, and show how the recipe can be realized with ultracold atoms and other quantum simulators. The mixed states so created can be made arbitrarily close to pure states. I will discuss ways to extend this construction to other topological phases, including nonGaussian ones, such as fractional quantum Hall state. 
01.03.2017 at 14:00  Characterizing manybody states at finite temperature via a Klein twist
Invited speaker: HongHao Tu (LudwigMaximiliansUniversität  München ) In this talk, I will describe an ongoing work on how universal data for distinguishing different phases may be extracting from thermal states of quantum manybody systems. This approach relies on a Klein bottle partition function (defined by twisting the usual partition function in imaginarytime axis) and is relevant for situations where nonchiral conformal field theories govern the bulk or edge physics (e.g. 1d critical states and 2d timereversal invariant topological insulators). Benchmark results will be provided for several 1d critical models. 
02.03.2017 at 11:30  Majorana quasiparticles from angular momentum conservation
Invited speaker: Fernardo Iemini (International Centre for Theoretical Physics  Trieste, Italy) We show how angular momentum conservation can stabilise a quasitopological phase of matter supporting Majorana qausiparticles as edge modes. Differently from typical scenarios, where such quasiparticles require the presence of superconductivity, we investigate orbital SU (2) × SU (2) Hubbard models in the presence of spinorbit coupling. The latter reduces the global spin symmetry to an angular momentum parity symmetry, which provides an extremely robust protection mechanism that does not rely on any coupling to additional models. The emergence of Majorana edge modes is elucidated using field theory techniques, and corroborated with numerical simulations. Our results pave the way toward the observation of Majorana edge modes with Alkalineearthlike fermions in optical lattices, where the basic ingredients for our recipe  spinorbit coupling and strong interorbital interactions  have been observed over the last two years. 
08.03.2017  Almost Conserved Local Operators in MBL systems
Nicola Pancotti Long time dynamics of nonintegrable systems holds the key to fundamental questions (thermalization). Analytical tools can only apply to particular cases (integrable models, perturbative regimes). Numerical simulations, limited in time, have found evidence of different time scales. A new numerical technique for constructing slowly evolving local operators was introduced by Kim et al. in Phys. Rev. E 92, 012128 (2015). Those operators have a small commutator with the Hamiltonian and they might give rise to long time scales. In this work, we apply this technique to the many body localization problem. We show that this method can not only signal the difference between the ergodic and localized phases, but it is also sensitive to the presence of the Griffith region between both. 
15.03.2017  Efficient representation of fully manybody localized systems using tensor networks
Invited speaker: Thorsten Wahl (Oxford University) Manybody localization (MBL) is currently an intensely studied topic and characterized by the fact that certain strongly disordered systems fail to thermalize. For sufficiently strong disorder in one dimension, all eigenstates of MBL systems fulfill the area law of entanglement. This makes tensor network states ideally suited to represent such fully manybody localized systems. Building on the ansatz proposed in Phys. Rev. B 94, 041116(R) (2016), I will present a tensor network that is able to capture the full set of eigenstates of such MBL systems efficiently: For a given system size, local observables can be approximated with an error that decreases as an inverse polynomial of the computational cost, which is an exponential improvement over the previous ansatz. If the system size is increased, the computational cost needs to grow only linearly with the system size in order to keep the accuracy fixed. The technique turns out to be highly accurate deep in the localized regime and maintains a surprising degree of accuracy in predicting certain local quantities even in the vicinity of the dynamical phase transition. Finally, the power of the technique is demonstrated on systems of 72 sites, where clear signatures of the phase transition can be seen. 
22.03.2017  A generalisation of the injecitvity condition for PEPS
Andras Molnar Projected Entangled Pair States (PEPS) is an ansatz believed to be suitable for analytical and numerical investigation of ground states of manybody Hamiltonians. To design a PEPS that admits certain (local) symmetries one has to understand when two different PEPS tensors give rise to the same state. This question in the full generality is however undecidable, it is therefore important to find relevant classes of tensors for which it can be answered. One such class is injcetive PEPS. Two injective PEPS describe the same state if and only if their tensors are related with a gauge transformation on the virtual space. Here we provide a generalisation of this class. This generalisation includes states that fail to be injective for purely geometrical reasons (so called corner problem). We show under which condition can two such states be equal. We also show that symmetries give rise to invertible Matrix Product Operators (MPO) on the boundary degrees of freedom. These MPOs can be used to assign an element of the third cohomology of the symmetry group to the state the same way as in the classification of the Symmetry Protected Topological (SPT) phases. 
23.03.2017 at 14:00  DMRG with Subspace Expansion on SymmetryProtected Tensor Networks
Invited speaker: Claudius Hubig (LudwigMaximiliansUniversität  München) The Density Matrix Renormalisation Group when applied to matrix product states is the method of choice for groundstate search on onedimensional systems and still highly competitive even in unfavourable circumstances, such as critical systems and higher dimensions. In this talk, I will discuss two separate methods which can be used to improve the computational efficiency of DMRG and related methods on matrixproduct states and beyond. The first component is the implementation of both abelian and nonabelian symmetries in an entirely general way suitable also for higherrank tensors as encountered in e.g. tree tensor network states. The second ingredient, the subspace expansion, allows for a fully singlesite DMRG algorithm with favourable linear scaling in the local dimension of the tensor network. Even for common problems, this results in a considerable speedup over the traditional twosite DMRG method or the density matrix perturbation approach for groundstate search at reduced algorithmic complexity. Additionally, the subspace expansion can potentially be used in a large set of other algorithms, such as the TDVP or the variational application of a matrixproduct operator onto a matrixproduct state. 
27.03.2017 at 11:30  Decoding Protocols for Classical Communication on Quantum Channels
Invited speaker: Matteo Rosati (Scuola Normale Superiore, Pisa, Italy ) I discuss the transmission of classical information via quantum carriers with focus on the decoding stage. While the optimal transmission rate and encoding have been well studied in the past providing viable solutions for freespace or opticalfiber communication, a practical decoder is still difficult to design. This is due to the requirement of performing joint measurements over several transmission modes and their difficult implementation. I approach the problem from several points of view presenting ideas for decoding algorithms and practical devices, especially for communication with coherent states of the electromagnetic field. 
29.03.2017  HighFidelity Hot Gates for Generic SpinResonator Systems
Invited speaker: Martin Schütz (Harvard University) We propose and analyze a highfidelity hot gate for generic spinresonator systems which allows for coherent spinspin coupling, in the presence of a thermally populated resonator mode. Our scheme is nonperturbative, applies to a broad class of physical systems, including for example spins coupled to circuitQED and surface acoustic wave resonators as well as nanomechanical oscillators, and can be implemented readily with stateoftheart experimental setups. We provide and numerically verify simple expressions for the fidelity of creating maximally entangled states under realistic conditions. 
30.03.2017 at 14:00  Synthetic gauge fields: from topology to the Unruh effect
Invited speaker: Alessio Celi (ICFO) The theoretical and experimental progress in quantum simulation with ultracold atoms of the last decay have pushed the realm of simulable models well deep into condensed matter and has started touching highenergy and gravitational physics. In this talk, I will focus on the possibilities opened by synthetic gauge fields. In the first part, after a brief review of "synthetic dimension" idea, I will discuss in which sense and to what extend a narrow Hofstadter model, as for instance obtained with synthetic lattices, displays the topological properties of a large 2D system pierced by the same flux. In particular, I will show that such narrow systems allow for a surprising accurate measurement of the Chern number through an appropriate Laughlin pump experiment. In the second part of the talk I will move to an apparently unrelated subject, the simulation of Dirac fermions in artificial curved spacetime. In fact, I will show that such simulation can be achieved through laser assisted tunneling and synthetic gauge fields. As an application, I will present our recent proposal for observing the Unruh effect with ultracold atoms. 
10.04.2017 at 15:30  Quantum steerability as a nesting problem
Invited speaker: Chau Nguyen (Max Planck Institute for the Physics of Complex Systems) We are to discuss quantum steerability, which was recently discovered as the third type of quantum nonlocality besides quantum nonseparability and Bell nonlocality. A bipartite quantum state is said to be steerable (from Alice's side) if the corresponding EinsteinPodolskyRosen steering experiment can be convincingly verified (from Alice's side). We show that the problem of determining the steerability of a bipartite quantum state can be stated as a nesting problem of convex objects in a linear space. Nesting criteria are then proposed. As the first application, we prove the conjecture on the steerability of Tstates, confirming them as the first class of twoqubit states of lower symmetry than the Werner states of which the steerability can still be fully characterised. As the second application, we discuss our recent progress in understanding the longstanding question on the steerability of the Werner states beyond projective measurements. 
11.04.2017 at 14:00  Computational power of symmetryprotected topological phases
Invited speaker: David Stephen (University of British Columbia) In manybody physics, many essential properties of a quantum state are determined by the phase of matter in which it resides. Recent years have witnessed tremendous progress in the discovery and classification of quantum phases, and it is thus pertinent to ask: what can a phase of matter be used for? A standout example in quantum information processing is the use of topological phases for errorresilient quantum computation. The exchange statistics of anyonic excitations present in these phases determine the possible logical gates and also label the topological phase itself. In this talk, I will make a similar connection for the symmetryprotected topological (SPT) phases in one dimension. I will show that the computational power of quantum states, defined via their use as resources for measurementbased quantum computation (MBQC), is uniform within certain SPT phases. This uniform computational power is determined using the same algebraic structure that classifies the SPT phases, namely group cohomology. These results give insight into the structure of MBQC resource states, and highlight how the classification of quantum phases can contribute to our understanding of the power of quantum computation. 
19.04.2017  A Projector Quantum Monte Carlo Method for nonlinear wavefunctions
Invited speaker: Lauretta Schwarz (University of Cambridge) The projected imaginary time evolution of Full Configuration Interaction Quantum Monte Carlo (FCIQMC) can be reformulated in terms of a Lagrangian minimization which naturally admits polynomial complex wavefunction parameterizatons, thereby circumventing the exponential scaling of the FCIQMC approach. While previously these nonlinear wavefunctions have traditionally been used in the area of Variational Monte Carlo, we consider recent developments for the identification of deeplearning neural networks to optimize this Lagrangian, which can be written as a modification of the propagator for the wavefunction dynamics. We demonstrate the capability of this approach with a Correlator Product State wavefunction, a form of Tensor Network State, and use it to find solutions to the stronglycorrelated Hubbard model, as well as abinitio systems, including a linear hydrogen chain and a fully periodic, abinitio graphene sheet. The number of variables which can be simultaneously optimized greatly exceeds alternative formulations of Variational Monte Carlo, allowing for systematic improvability of the wavefunction flexibility towards exactness, whilst combining traditional Variational and Projector Quantum Monte Carlo approaches. 
26.04.2017  Effective theory for correlations of states obtained from conformal field theory
Benedikt Herwerth We study states of one and twodimensional spin systems that are constructed as correlators within the conformal field theory of a massless, free boson. In 1d, these states are good descriptions of ground states of XXZ spin chains and in 2d, they are similar to lattice Laughlin states. We show that the zz correlations in these states are determined by an action that is a modification of the original freeboson theory. The truncation of this action to quadratic order provides a solvable, effective theory for the correlations in our states. The mass term in this effective theory explains the behavior of the correlations, which decay polynomially in 1d and at the edge of a 2d system and exponentially in the bulk of a 2d system. We test the validity of our approximation by comparing it to Monte Carlo computations. 
03.05.2017  Projected Entangled Pair States wIth Virtual Symmetries
Henrik Dreyer Topologically ordered quantum states are the first kind of longrange entangled states that we are beginning to understand. A complete understanding, however, has only been achieved at finetuned points (stringnets, quantum doubles, …). Meaningful (i.e., still topological) generalisations of these special points are hard to find. On the other hand, tensor network states have been used to find exact, efficient representations of these finetuned points in several cases. The idea of this work is to find a suitable generalisation of quantum double states in the tensor network language. To this end, I will introduce Ginjective tensor network states for finite G and then study an example of a SU(2)invariant class of tensors. In this case, topological order breaks down. I will sketch how we arrive at this conclusion and hint at the mechanism behind the transition. 
10.05.2017  PEPS for chiral topological systems
Anna Hackenbroich Projected entangled pair states (PEPS) in two spatial dimensions provide an efficient representation for the ground states of many nonchiral systems with instrinsic topological order such as quantum double and string net models. In this framework, a virtual symmetry of the local tensor leads to a nonvanishing topological entanglement entropy and ground state degeneracy on nontrivial surfaces. On the other hand it is much less clear whether PEPS can also describe chiral topological systems. In the case of noninteracting fermions it has been shown that such descriptions do not exist since any gaussian fermionic PEPS with topological features has algebraically decaying correlation functions. After providing some background on chiral topological phases and their possible PEPS representations I will focus on a PEPS describing a critical chiral topological spin liquid for spin 1/2 particles on a square lattice. In particular I will describe the construction of this state in terms of a local tensor with SU(2) symmetry, analyse its virtual symmetries and discuss the properties of the chiral mode appearing in its entanglement spectrum. 
17.05.2017  Quantum power methods for ground state problems
Yimin Ge Quantum computers are expected to have a deep impact in the simulation of large quantum systems. Of particular interest is the ability to study their ground states, which are classically often intractable. In this talk, I will present a new quantum algorithm for this task inspired by the classical power iteration method. The algorithm prepares a good approximation of the ground state using techniques which were recently developed in the context of QLSP solvers for implementing operators that have suitable Fourier or Chebyshev series representations. Unlike algorithms based on adiabatic evolution or naive phase estimation, this algorithm provides a certification of success. The runtime is quadratically better than the naive phase estimation method and polynomial in the classical iteration length, and moreover has little memory requirements. This makes it an attractive candidate for potential applications of small quantum computers. Joint work with J. Tura and J.I. Cirac. 