Theory Seminar
From MPQTheory

Date  Seminars 2014 (Go to Seminars 2013, Seminars 2012, Seminars 2011, and earlier therein)

15.01.2014  High Energy Physics in the Light of Quantum Simulators
Eliška Greplová The goal of this seminar is to provide an overview of some of the latest work on the simulation of high energy physics using ultracold atoms. The basics of continuous and lattice field theory together with the gauge principle will be reviewed, and the engineering of the corresponding Hamiltonians using optical lattices will be discussed. The presentation will be mainly based on the following references: [1] Phys. Rev. A 88, 023617 (2013)

22.01.2014  Beyond Adiabatic Elimination
Vanessa Paulisch Adiabatic elimination is a commonly used method for approximating the dynamics of multilevel systems. This procedure is, however, somewhat ambiguous and it is not clear how to improve on it systematically. Parts of my bachelor thesis, which deal with these problems, will be presented in this seminar. In addition, illustrative examples, applications and an alternative to the adiabatic elimination procedure for Raman transitions [1] will be discussed. The most important results of my thesis can be found on arXiv:1209.6568. [1] R. Han et al. (2013) Raman transitions without adiabatic elimination: A simple and accurate treatment. J. Mod. Opt. 60, 255

29.01.2014  An elementary quantum network of single atoms in optical cavities
Guest speaker: Carolin Hahn (MPQ, Quantum Dynamics Group) Quantum networks are at the heart of quantum communication and distributed quantum computing. Single atoms trapped in optical resonators are ideally suited as universal quantum network nodes capable of sending, receiving, storing, and releasing photonic quantum information. The reversible exchange of quantum information between such singleatom cavity nodes is achieved by the coherent exchange of single photons. In my talk I will discuss the first experimental realization of an elementary quantum network consisting of two atomcavity nodes that are located in remote, independent laboratories an MPQ in Garching [1]. We demonstrate the faithful transfer of arbitrary quantum states and the creation of entanglement between the two atoms. We characterize the fidelity and lifetime of the maximally entangled Bell states and manipulate the nonlocal state via unitary operations applied locally at one of the nodes. This cavitybased approach to quantum networking offers a clear perspective for scalability. [1] S. Ritter et al., “An elementary quantum network of single atoms in optical cavities”, Nature 484, 195200 (2012)

05.02.2014  The numerical renormalization group
Stefan Kühn I will give a very basic introduction to the ideas of the renormalization group. After some background has been established, I would like to present the Kadanoff block spin method for the Ising model as a simple example for the application of the renormalization group. Finally I will discuss the numerical renormalization group approach to the singleimpurity Anderson model.

12.02.2014  Fiberbased FabryPerot microcavities: A powerful tool for experiments with quantum and nanosystems
Guest speaker: David Hunger (MPQ, Laser Spectroscopy Group) State of the art optical microcavities store light within volumes of wavelength scale for millions of optical cycles. Such spatiotemporal confinement of light can dramatically enhance lightmatter interactions. To provide this capability on an accessible platform, we have developed microscopic FabryPerot cavities based on lasermachined optical fibers. The design achieves small mode volumes and large quality factors, combined with full tunability and open access to the cavity field. I will present our current efforts to build efficient single photon sources and to perform ultrasensitive microscopy with such cavities.

19.02.2014  Open quantum manybody systems: when interactions meet dissipations
Guest speaker: Zi Cai (LMU Munich) Understanding quantum systems embedded into environment is of particular theoretical and practical importance. The situation becomes particularly interesting and complex when the system itself is already an interacting manybody system, which provides novel perspectives to quantum manybody physics. In this talk, I will present two examples to show how the conspiracy of dissipation and interaction can significantly change the behaviors of the quantum systems and give rise to novel phenomena, including the algebraic decoherence behavior and a dissipationinduced localization. Our numerical methods can be applied to various open quantum manybody systems ranging from the Rydberg atoms and trapped ions to quantum computational systems based on solidstate devices (e.g. rfSQUIDs, Solidstate qubits in diamonds nanostructures and quantum wells).

26.02.2014  Quantum information processing with closed timelike curves
Ivan Glasser There is now a signiﬁcant body of results on quantum interactions with closed timelike curves (CTCs) in the quantum information literature. In this talk I will present and compare two theories that make an attempt at treating time travel quantummechanically : Deutsch's theory of CTCs and the theory of CTCs through postselected teleportation. Using a CTC we will kill our grandfathers and point out how the two theories have a different way of solving the causality paradox. Then we will show how CTCs enable us to distinguish nonorthogonal quantum states and to clone an arbitrary quantum state. Finally we will discuss the ambiguities that appear in nonlinear extensions of quantum mechanics when dealing with proper and improper mixtures and show how these ambiguities may affect the interpretation of our previous results.

12.03.2014  Continuous Matrix Product States
Benedikt Herwerth Continuous matrix product states, introduced by F. Verstraete and J. I. Cirac in 2010, are an interesting generalization of the successful matrix product ansatz to continuous theories. As such, they provide a new, variational approach to study quantum field theories. In this seminar, we will discuss the definition of continuous matrix product states, their relation to discrete matrix product states, applications and we will consider possible future directions. The talk will mainly focus on the studies

19.03.2014  Area law and Tensor Networks
Andras Molnar Area laws emerging from studies of the black hole entropy are a surprising feature of Nature. It tells us that information contained in a region is located on the boundary. This philosophy motivated the study of area laws for spin systems. I will overview these area laws and discuss their relation to the approximations by MPS and PEPS.

02.04.2014  Precision physics at low energy: The proton radius puzzle
Guest speaker: Randolf Pohl (MPQ, Laser Spectroscopy Group) The recent discovery of the Higgs particle at CERN is a tremendous success of human ingenuity. Disappointingly, however, no sign of physics beyond the Standard Model has been seen so far. Precision measurements at low energy can give a complementary access to BSM physics. Here, two experiments involving muons, have displayed a large discrepancy to the Standard Model: The muon (g2) measurement and the measurement of the proton rms charge radius from laser spectroscopy of muonic hydrogen. After a brief introduction to some precision tests of the Standard Model I will focus on the "proton radius puzzle".

09.04.2014  Inflation
Speaker: Lucas Clemente I will give a very basic introduction to the theory of cosmic inflation, why it solves the horizon & flatness problems and how quantum fluctuations during inflation explain the presentday structure of the universe. I will also talk about recent observations of the microwave background (WMAP, Planck, BICEP) and discuss how they confirm inflation. Please find my slides (once finished) at https://clemente.io/inflation

23.04.2014  Properties of topological codes
Speaker: Xiaotong Ni The field of quantum error correction has borrowed many ideas that come from the study of topological order. One notable example is the toric code. In this talk I'm going to talk about some properties of these topological codes, with a focus on the so called "cleaning lemma" and the usage of holographic principle.

30.04.2014  Simulation of dynamic Abelian and nonAbelian gauge theories with ultracold atoms
Speaker: Erez Zohar In this talk I will present the ongoing MPQTAU collaboration, in which we use several methods of ultracold atoms trapped in optical lattices as quantum simulators for 1+1 and 2+1 dimensional dynamic gauge theories. The simulating methods are different and use various implementations  either BECs or single atoms, and allow the simulation of the gauge field and coupled matter dynamics and the observation of confinement, as well as measuring Wilson Loops, in Abelian (U(1)  compact QED and Z_N) or nonAbelian (SU(N)) lattice gauge theories. [1] Zohar, E. and Reznik, B.  Phys. Rev. Lett. 107, 275301 (2011)

07.05.2014  Relativistic attosecond physics
Guest speaker: Laszlo Veisz (MPQ, Attosecond Physics Group) Laserplasma interactions beyond 10^18 W/cm^2, the so called relativistic intensity threshold, under certain circumstances generate pulses with the shortest durations to date, lying in the attosecond (10^18 s) time domain. Utilizing an unparalleled laser source experiments are conducted to produce (a) intense isolated attosecond XUV and Xray pulses via high harmonic generation in gases or dense plasmas as well as (b) relativistic femtosecond and attosecond electron bunches from laserplasma interaction. These secondary beams open up various fields of applications, among others nonlinear XUV science or electron diffraction in the attosecond domain.

14.05.2014  Boundary theories for chiral Projected Entangled Pair States
Speaker: Thorsten Wahl I will investigate the topological character of the chiral Gaussian fermionic states introduced in [Phys. Rev. Lett. 111, 236805 (2013); arXiv:1308.0316]. These are topological Projected Entangled Pair States (PEPS), which are special in that they possess chiral edge modes, i.e., have a nonvanishing Chern number. I will show that this property arises from a symmetry on the virtual modes of the PEPS, which also leads to a universal correction to the area law for the zero Rényi entropy and a (double) degeneracy of the local parent Hamiltonian on the torus. Based on that symmetry, one can build string operators from which one can obtain the other degenerate ground state.

21.05.2014  Quantum simulation of twodimensional U(1) critical systems: A Higgs particle and the possible help of string theory for the optical conductivity
Guest speaker: Lode Pollet (LMU Munich) Quantum simulators are special purpose devices designed to provide physical insight in a specific quantum problem that is hard to study in the laboratory and impossible on a computer. However, before they can be used they require calibration. For cold atomic systems, quantum Monte Carlo simulations have played a key role there. They established a few years ago that the thermodynamic properties of the experimental system are in onetoone agreement with the simulations of the corresponding model. The synergy between the two approaches has dramatically progressed since then, to each other’s benefice: In the main part of this talk, I will focus on the dynamical properties of a U(1) critical system in (2+1) dimensions focusing on the existence of the amplitude mode or Higgs particle, and on the optical conductivity, which we compare against predictions from the AdS/CFT correspondence. Finally, I will discuss some open problems for this approach to quantum simulation.

28.05.2014  Quantum Error Correction for Metrology
Guest speaker: Eric Kessler (Harvard University, USA) Due to the particular experimental relevance, the effects of environmental noise in quantum metrology has been studied intensely in recent years. Fundamental bounds for parameter estimation of noisy channels have been derived, and experimental techniques have been developed to counter the compromising effects noise, such as dynamical decoupling. In this talk, I will introduce an alternative, errorcorrectionbased approach to improve quantum sensing protocols in the presence of noise. Guided by several simple examples in the context of nanoscale sensing using nitrogenvacancy centers in diamond, I derive a set of conditions under which it is possible to correct for errors arising from the coupling to the environment without perturbing the signal of the quantum measurement. The scheme is complementary to current dynamical decoupling techniques, and can improve sensing under realistic experimental conditions by several orders of magnitude. The perspective of Heisenberglimited sensitivity in the presence of noise will be briefly discussed.

04.06.2014  Introduction to Superconducting Qubits
Vanessa Paulisch Superconducting qubits are solid state electrical circuits with applications in quantum information processing. They are made of superconducting inductors, capacitors and Josephson junctions, where the latter forms the central element due to it's nonlinear behaviour. In contrast to microscopic quantum systems, they tend to be well coupled to other circuits/”qubits”. I will explain the simplest and most basic “qubit”, the CooperPairBox, in detail and compare it to other implementations of superconducting qubits. Furthermore, I will discuss their coupling, readout and coherence times, supplemented with recent experimental achievements.

18.06.2014  Gauge color codes
Guest Speaker: Hector Bombín (Perimeter Institute, Canada) I will describe a new class of topological quantum error correcting codes with surprising features. The constructions is based on color codes: it preserves their unusual transversality properties but removes important drawbacks. In 3D, the new codes allow the effectively transversal implementation of a universal set of gates by gauge fixing, while errordectecting measurements involve only 4 or 6 qubits. Furthermore, they do not require multiple rounds of error detection to achieve faulttolerance.

25.06.2014  The computational power of blackbox normalizer circuits
Juan BermejoVega Normalizer circuits are families of quantum circuits that generalize Clifford circuits (qubit operations widely used in QIT) to high dimensional spaces associated to groups. They can be efficiently simulated with classical computers although they can comprise quantum Fourier transforms, logic gates that are essential in Shor’s factoring algorithm. In this talk we will present new families of normalizer circuits are associated to infinite groups and blackbox groups. We will show that blackbox normalizer circuits can achieve exponential quantum speedups and implement several celebrated quantum algorithms, including Shor’s: this yields a precise formal connection between Clifford/normalizer circuits and famous quantum algorithms that break publickey cryptosystems. Joint work with Cedric YenYu Lin and Maarten Van den Nest.

09.07.2014  Efficient Estimation of Quantum States and Gates: A Survey and New Results
Guest Speaker: Steve Flammia (University of Sydney, Australia) I will survey recent approaches for efficient estimation of quantum states and channels. This includes a host of novel methods based on powerful mathematical techniques, including compressed sensing, convex optimization, matrix product states, and more. In every case, these methods give provable asymptotic speedups over the prior state of the art, and enhance performance numerically as well. The focus will be on joint work with David Gross, YiKai Liu, Joel Wallman, and Jens Eisert.

24.07.2014  The Asymptotic Cooling of HeatBath Algorithmic Cooling
Guest Speaker: Sadegh Raeisi (IQC Waterloo, Canada) The purity of quantum states is a key requirement for many quantum applications. Improving the purity is limited by fundamental laws of thermodynamics. Here we are probing the fundamental limits for a natural approach to this problem, namely heatbath algorithmic cooling(HBAC). The existence of the cooling limit for HBAC techniques was proved by Schulman et al. in, the limit however remained unknown for the past decade. Here for the first time we find this limit. In the context of quantum thermodynamics, this corresponds to the maximum extractable work from the quantum system.

06.08.2014  Quantum spin liquids at the vicinity of Mott transition
Guest Speaker: Yi Zhou (Zhejiang University, China) We study quantum spin liquid states (QSLs) at the vicinity of metalinsulator transition. Assuming that the low energy excitations in the QSLs are described chargeful quasiparticles with the same Fermi surface structure as in the corresponding metallic phase, instead of charge neutral spinons, we propose a phenomenological Landaulike low energy theory for the QSLs and show that the usual U(1) QSLs with spinon Fermi surface is a representative member of this class of spin liquids. Based on our effective low energy theory, an alternative picture to the BrinkmanRice picture of Mott metalinsulator transition is proposed. The charge, spin and thermal responses of QSLs are discussed as well as collective modes under such a phenomenology, which are consistent with existing experiments and can be verified by future experiments.

16.09.2014  Composite Fermion Theory of Fractional Quantum Hall Effect
Yinghai Wu In this talk, we will first review the composite fermion theory of strongly interacting electrons in magnetic field in two dimensions. This theory successfully explains the incompressible fractional quantum Hall states, the compressible Fermiliquidlike states, and the Wigner crystal states in the lowest Landau level that have been observed in various systems hosting twodimensional electron gases. We then apply the composite fermion theory to study bosonic systems. From exact diagonalization studies of twocomponent bosons in synthetic magnetic field, we find evidence for several incompressible quantum Hall states as well as a compressible Fermiliquidlike state. A quantitatively accurate microscopic understanding of these states is provided by the composite fermion theory. We also briefly discuss some other recent works on fractional quantum Hall states of electrons.

17.09.2014  Quantum simulation of the Schwinger Model: Is it feasible?
Stefan Kühn During the last years there has been a variety of proposals to quantum simulate gauge theories. These simulators might offer an alternative route to address problems like out of equilibrium dynamics or certain regions of the phase diagram which are currently intractable to Monte Carlo simulations. Despite these promising prospects there are also limitations, as for example infinite dimensional Hilbert spaces for the gauge degrees of freedom typically have to be represented by finite dimensional systems and noise in a real experiment might break the gauge invariance. Hence, it is a crucial question how those limitations might affect the performance. In this week's seminar I am going to address these questions for two particular realizations of the Schwinger model. I will present numerical evidence suggesting that even with a small finite dimension for the gauge degrees of freedom quite accurate results are possible and that the adiabatic ground state preparation is to some extent possible in the presence of gauge invariance breaking noise.

24.09.2014  Measuring the Chern number of Hofstadter bands with bosonic atoms
Guest Speaker: Monika Aidelsburger (LMU Munich) The simulation of eletrons moving in periodic potentials exposed to large magnetic fields with ultracold atoms in optical lattices has motivated several successful experimental works about the realization of uniform artificial magnetic fields. One main challenge in this context is the implementation of experimental probes revealing the nontrivial topology of energy bands. Here I report about direct measurements of the transverse Hall deflection of ultracold bosonic atoms in artificially generated Hofstadter bands. In combination with the measured occupation of the different Hofstadter bands we were able to obtain an experimental value for the Chern number of the lowest band with good precision. This result constitutes the first Chernnumber measurement in a nonelectronic system. The artificial magnetic field was generated using a new alloptical technique, which enables flux rectification in a staggered optical superlattice based on laserassisted tunneling.

08.10.2014  Frequencyresolved photon correlations: discovering sources of quantum light
Guest Speaker: Carlos Sánchez (Universidad Autónoma de Madrid, Spain) Photon counting statistics is one of the main fields in quantum optics. A considerable step forward in this direction has been the recent development of an efficient theory to compute time and frequencyresolved Norder correlation functions. By spanning these correlations over all the possible frequencies of detection, one can reveal a plethora of features hidden in the standard correlation functions. Here I show that some regions of these maps of correlations correspond to quantum emission that cannot be accounted by classical physics, and how these correlations can be exploited to engineer a source that emits all of its light in bundles of Nphotons.

15.10.2014  Magnetometry with nitrogenvacancy defects in diamond
Martin Schütz Advancements in magnetic detection and imaging have contributed immensely to a wide range of scientific areas from fundamental physics and chemistry to practical applications such as data storage industry and medical science. Due to its atomic size and exceptionally long spincoherence times, the isolated electronic spin of the nitrogenvacancy (NV) centre in diamond offers unique possibilities to be employed as a nanoscale quantum sensor for detection and imaging of ultraweak magnetic fields. I will try to give a pedagogical introduction to the the basic physical principles and discuss first applications of NV magnetometers in the context of nanomagnetism, mesoscopic physics and life sciences.

22.10.2014  Quantum criticality and infinite dimensional matrix product states
Ivan Glasser Quantum phase transitions occur in systems at zero temperature, but they also have an influence on a larger region in the finite temperature phase diagram, where quantum criticality is realized. In this talk I will give an introduction to quantum phase transitions and quantum criticality, focusing on the example of the exactly solvable Ising model in a transverse field. I will then present a way of using infinite dimensional matrix product states to construct one dimensional spin models displaying quantum criticality in their low temperature phase diagram. These models have analytical purifications and their properties can be investigated using MonteCarlo simulations. The results for the phase diagram of such a model will be presented and I will comment on the possibility to realize this model with a simple Hamiltonian.

29.10.2014  Tensor network methods for lattice gauge theories
Guest speaker: Luca Tagliacozzo (ICFO, Spain) Recently tensor network (TN) methods have been extended so to better address lattice gauge theories (LGT). TN can be used either as an analytical tool to better understand LGT or as a variational ansatz for their numerical simulations. I will present results in both directions for 2D LGT with continuous symmetry groups.

05.11.2014  Excited States in Spin Chains from Conformal Field Theory
Benedikt Herwerth Exactly solvable models play an important role in our understanding of physical phenomena. In the past years, methods from conformal field theory (CFT) have been applied to construct wave functions as infinite matrix product states and corresponding parent Hamiltonians. We discuss a model that is obtained from the SU(2) WessZuminoWitten CFT at level one and that is equivalent to the HaldaneShastry model. We obtain wave functions for the excited states of the spin system from correlation functions in the CFT. The construction is based on the current algebra of the CFT and therefore reflects its structure at the level of the spin system.

12.11.2014  Matrix product states in nonequilibrium dynamics
András Molnár We review a simple exactly solvable model in nonequilibrium dynamics, the Asymmetric Simple Exclusion Process. We show the phyiscs of the model through mean field approximation, then present the exact solution via matrix product states. The talk is mostly based on the review by Blythe and Evans: Nonequilibrium steady states of Matrix Oroduct Form: a solver's guide.

19.11.2014  Detecting nonlocality in manybody quantum states
Guest speaker: Jordi Tura (ICFO, Spain) One of the most important steps in the understanding of quantum manybody systems is due to the intensive studies of their entanglement properties. Much less, however, is known about the role of quantum nonlocality in these systems. This is because standard manybody observables involve correlations among few particles, while there is no multipartite Bell inequality for this scenario. Here we provide the first example of nonlocality detection in manybody systems using only twobody correlations. To this aim, we construct families of multipartite Bell inequalities that involve only second order correlations of local observables. We then provide examples of systems, relevant for nuclear and atomic physics, whose ground states violate our Bell inequalities for any number of constituents. Finally, we identify inequalities that can be tested by measuring collective spin components, opening the way to the experimental detection of manybody nonlocality, for instance with atomic ensembles. The talk will be based on the following studies

26.11.2014  Microscopic Origin of the 0.7Anomaly in Quantum Point Contacts
Guest speaker: Jan von Delft (LMU) The conductance of a quantum point contact exhibits an unexpected shoulder at $simeq 0.7 (2 e^2/h)$, known as the "0.7anomaly", whose origin is still subject to debate. Proposed scenarios for explaining it have evoked spontaneous spin polarization, ferromagnetic spin coupling, the formation of a quasibound state leading to the Kondo effect, Wigner crystallisation, various treatments of inelastic scattering, and a smeared van Hove peak in the local density of states. In my talk, I will argue that the 0.7anomaly arises from "slow spin fluctuations" in the quantum point contact. The microscopic origin of these slow (ferromagnetic) spin fluctuations is the presence of a smeared van Hove peak in the local density of states at the bottom of the lowest onedimensional subband of the point contact. This peak in the local density of states, which reflects the fact that electrons are being slowed down while they cross the 1D barrier constituting the QPC, amplifies interaction effects and enhances the magnetic spin susceptibility and inelastic scattering rate. I will present theoretical calculations and experimental results that show good qualitative agreement for the dependence of the conductance on gate voltage and magnetic field, including the behavior of the effective lowenergy scale that governs the strength of the magnetic response. Finally, I will argue that "slow spin fluctuations" can be viewed as the common ground shared by several of the seemingly contradictory scenarios for explaining the 0.7anomaly that are currently on the market. In particular, slow spin fluctuations arise also in the scenarios evoking a quasibound state, ferromagnetic spin coupling and Wigner crystallization. Common ground can also be found with the spin polarization scenario if one is willing to reinterpret "spontaneous spin polarization" to mean a slowly fluctuating ferromagnetic spin configuration that looks static on short time scales, but averages to zero over longer times.

03.12.2014  Outofequilibrium dynamics and thermalization of string order
Guest speaker: Leonardo Mazza (Scuola Normale Superiore, Pisa, Italy) Motivated by recent experiments on ultracold gases, I will discuss the outofequilibrium dynamics of a system characterized by string order, e.g. the onedimensional Mott insulator or the spin1 Haldane phase. When a closed quantum system evolves outofequilibrium, it is important to assess whether local observables attain stationary expectation values and whether the state becomes locally indistinguishable from a thermal state, according to the general thermalization framework. However, string observables are nonlocal observables and do not easily fit into the standard thermalization theory: it is thus interesting to investigate their equilibration dynamics. Using algorithms based on matrixproduct states, I will show the first instances of thermalization via such multisite observables for a spin1 chain in the Haldane phase after a sudden global parameter quench of the Hamiltonian. Thermalization occurs only for scales up to a horizon growing at a well defined speed, due to the finite maximal velocity at which string correlations can propagate. Combining anlytical and numerical arguments, it is possible to demonstrate the existence of a LiebRobinson bound for the spreading of string correlations in the system, which complements the known (and experimentallyobserved) bound for twopoint correlations. Differently from local order, string order can be abruptly destroyed by a global quantum quench: this situation is fully characterized. A qualitatively similar behavior is found for the string order of the Mott insulating phase in the BoseHubbard chain. This paves the way towards an experimental testing of our results in present coldatom setups. References: L. Mazza, D. Rossini, M. Endres and R. Fazio, Outofequilibrium dynamics and thermalization of string order, Phys. Rev. B 90, 020301(R) (2014).

17.12.2014  The dynamics of a Quantum phase transition: Emergence of coherence in melting and expanding Mott insulators
Guest speaker: Ulrich Schneider (LMU) Optical lattices have emerged in the last ten years as a very versatile platform to study quantum manybody physics in a clean and wellcontrolled environment and can therefore act as a quantum simulator for condensedmatter systems. Due to the much slower timescales compared to electrons in a solid, atoms in optical lattices are especially suited to study the outofequilibrium dynamics of interacting manybody systems, which presents one of the most challenging problems in manybody theory. I will present a detailed study of the dynamics of the phase transition from Mott insulator to superfluid, in which we particularly investigated how fast phase coherence between lattice sites can develop and spread. Furthermore, I will present the first experimental study of a transient quasicondensation into finite momentum states that we realized using an expanding 1D Mott insulator of hardcore bosons. 