April 30, 2018, 11:30 a.m.
Exact description of the boundary theory of the Kitaev Toric Code with open boundary conditions
May 1, 2018, 11:30 a.m.
Journal club: Variational quantum eigensolvers
Journal club of quantum computation and information theory In the first meeting we will discuss the following paper: https://arxiv.org/pdf/1509.04279.pdf Discussion: Xavier Bonet
May 3, 2018, 2 p.m.
Loop analysis of linear resistance in high-Tc materials and Dirac materials
Bad metals including the high T_c superconductors display an exotic resistance that is linear in both temperature and magnetic field. This hallmark of strong correlations is poorly understood. We show that Fourier transforming the magnetoconductance with respect to magnetic field obtains a curve describing the area distribution of loops traced by fermions within the sample. Analysis of this area distribution reveals that linear resistance is caused by scattering and quantum interference, but with more large loops than occur in ordinary 2-D and 3-D materials where scattering destroys quantum coherence and limits loop size. This limit is absent in linear resistance materials, resulting in larger loops limited only by thermal decoherence. Linear resistance signals that quantum coherence is maintained in the presence of scattering. The linear relations discussed here provide a new lens for interpreting the the high-Tc phase diagram, and we describe some of the resulting insights.
May 4, 2018, 2:30 p.m.
Oort 276 (Casimir room)
Dynamics of quantum measurement and the measurement problem
Quantum mechanics allows ideal measurements to be treated by postulates, which has led to various postulate-based interpretations of varying merit. In a laboratory a measurement is performed with a physical apparatus. Much can be learned from solving the Curie-Weiss model for quantum measurement, where the z-component of a spin 1/2 is measured with a Curie-Weiss magnet. In this exactly solvable model three dynamical mechanisms have been identified: 1) truncation of the density matrix (disappearance of off-diagonal "Schrodinger cat" terms) 2) registration, where the macroscopic pointer benefits from a first order phase transition 3) subensemble relaxation inside the magnet after decoupling it from the spin The which-basis question and the ready-state of the apparatus find obvious meanings. While 1) and 2) are concordant with the postulates, 3) is related to the quantum measurement problem: how can one describe individual events in the ensemble approach? While this is trivial classically (a coin lies face up or down), it is believed to be unsolvable within quantum mechanics. Our approach allows to formulate minimal postulates to connect to the reality in laboratories. Most "quantum probabilities" should be left without interpretation. The Born rule is connected to indications of the macroscopic magnet, while the state of the microscopic spin is inferred from it. The frequency interpretation emerges.
May 4, 2018, 3:20 p.m.
Oort 276 (Casimir room)
Thermal transport in a charge two-channel Kondo setup
Gerwin van Dalum
The original Kondo model describes the behaviour of a non-magnetic metal containing a single magnetic impurity. Motivated by the experimental realization of a remarkable quantum dot device [Iftikhar et al., Nature 526, 233 (2015)], we consider the more complicated charge two-channel Kondo (2CK) model. In particular, we discuss the thermal transport properties of this 2CK model due to a temperature gradient between the leads. We predict a violation of the Wiedemann-Franz law in the Fermi liquid regime, and propose that the heat conductance may provide a way to experimentally verify the Majorana character of the dot region at the critical non-Fermi liquid point.
May 4, 2018, 4:30 p.m.
Oort 276 (Casimir room)
Jette van der Broeke
May 4, 2018, 5:10 p.m.
Oort 276 (Casimir room)
Topological Mechanics of Critically Coordinated Lattices
Frames consisting of nodes connected pairwise by rigid rods or central-force springs, possibly with preferred relative angles controlled by bending forces, are useful models for systems as diverse as architectural structures, crystalline and amorphous solids, sphere packings and granular matter, networks of semi-flexible polymers, proteins, and origami. Particularly interesting today is the increasing number of 3D-printed micron-scale metamaterials. This talk will present an overview of the elastic and vibrational properties of versions of these frames, called Maxwell lattices, whose constraints match the translational degrees of freedom of their nodes. They include the square, kagome, and pyrocholore and lattices and their modifications with nearest-neighbor central-force springs as well as jammed packings of soft spheres. Like the Su-Schrieffer-Heeger model of Polyacetylene, topological insulators, and Weyl semi-metals, these lattices have a topological characterization that in their case determines the number and nature of their zero-energy edge modes, the nature of their long-wavelength elasticity, and whether or not they have isolated topologically protected Weyl-like zero modes in the bulk. If time permits, the talk will present a mechanical model whose vibrational spectrum reproduces the electronic spectrum of graphene with different hopping for each of the three bond directions.
May 8, 2018, 11:30 a.m.
Journal club: Introduction to quantum error correction codes
Introduction to quantum error correction codes Discussion by: Marcello Caio Paper: https://arxiv.org/pdf/0904.2557.pdf Comments: Marcello will give a general introduction of QEC codes. From the paper he will discuss the "Introduction", "Stabilizer codes" and "Fault-tolerant gates".
May 8, 2018, 2 p.m.
First lecture on Elasticity
Lecture I: Fundamentals A. Two views of elasticity: Lagrangian and phase of mass-density waves of crystals B. Non-linear stress and Strain C. Linearized Bulk phonon spectrum D. Surface Rayleigh waves
May 9, 2018, 4:30 p.m.
How isotropic is the Universe?
A fundamental assumption of the standard model of cosmology is that the large-scale Universe is isotropic. Because of its centrality, it is essential to test this assumption. Breaking isotropy leads to Bianchi cosmologies, a set of solutions to the Einstein field equations of which only the subset describing rotating universes was previously tested against data. In this talk, I present a general test of isotropy considering, for the first time, all the degrees of freedom of anisotropic expansion. We analyse cosmic microwave background data from Planck, carrying out the first joint analysis of temperature and polarization data for this purpose. We also show that improved constraints on anisotropy may be obtained by extending the likelihood to high ell. For the vector mode (associated with rotating universes), we obtain a limit on the anisotropic expansion that is an order of magnitude tighter than previous Planck results using the CMB temperature only. We recover upper limits for all the other modes, with the weakest one arising from the regular tensor modes. We disfavour anisotropic expansion of the Universe with odds of 121,000:1 against.
May 15, 2018, 11:30 a.m.
Journal club: TBD
Discussion by: Tom O'Brien Paper: TBD Comments:
May 15, 2018, 2 p.m.
Second lecture on Elasticity
Lecture II: Sample materials A. Nematic Elastomers: Ward identities and spontaneous broken symmetry B. Smectic-A liquid crystal and graphene: fluctuation breakdown of elastic response C. Filamentous networks
May 16, 2018, 7:30 p.m.
Metamaterials and Topological Mechanics
Metamaterials are engineered to have properties not found in nature, such as a negative optical index of refraction, one-way light or vibration waves, or exotic elastic behavior. Made-to-order structures at length scales as short as a micron can now be fabricated with advanced materials processing like 3D printing. Topology is a unifying mathematical concept related to properties that do not change under continuous changes in parameters. Topological materials, like topological insulators, have bulk excitation spectra characterized by topological invariants, associated with the opening of bandgaps, that determine physical properties like the Hall conductivity or the nature of edge excitations. This talk will discuss mechanical metamaterials with topologically protected zero-energy edge states in an idealized limit. These states exist at every wavenumber on a surface so that any surface shape distortion costs no energy. Changing the topological class of the material causes one or more zero modes per wavenumber to move from one side of a sample to the opposite creating rigid and supersoft edges.
May 17, 2018, 2 p.m.
Eigenstate Thermalization -- from interacting spins to quantum field theory, and beyond.
In this talk I will discuss recent developments and applications of Eigenstate Thermalization Hypothesis, the underlying microscopic mechanism explaining thermalization of isolated quantum ergodic systems. I will start with a brief review of the subject in case of many-body quantum systems and then formulate the hypothesis for the conformal field theories. In the second part of the talk I will discuss the connection between Eigenstate Thermalization and Random Matrix Theory and will argue that thermalization of quantum systems involves a new timescale, which is parametrically longer than Thouless time. The talk is based on arXiv:1804.08626, arXiv:1710.10458, arXiv:1610.00302.
May 18, 2018, 11:30 a.m.
Scale-Invariance in the cuprate superconductor
Arkady Shekhter (Florida)
Metallic behavior in the "strange metal" state and the very nature of its charge carriers are the most pressing open questions in the physics of high-temperature superconductivity of cuprates. Zero-temperature collapse of a line of phase transitions as well as evidence for enhanced electronic correlations approaching critical doping, p~0.19, suggest a quantum critical point, i.e. colapse of intrinsic energy scale inside superconducting dome. Our recent high-magnetic-filed resistance and Hall measurements in LSCO cuprates provide dynamic evidence for vanishing energy scale in cuprates near critical doping (x=0.20). At very high magnetic fields (up to 93T) the resistivity is linear in magnetic field. Measurements at multiple temperatures reveal a temperature-field competition characteristic of scale-invariant transport behavior. Such behavior of magneto-transport in the strange metal state of cuprates is incompatible with well established picture of quasiparticle transport in conventional metals.
May 22, 2018, 3 p.m.
Doppelganger dark energy: modified gravity with non-Universal couplings after GW170817
The recent observation of GW170817 told us that Gravitational Waves (GWs) propagate at the speed of light in the nearby universe (or very very close to it). For dark energy models, this means that theories with higher derivatives terms - which modify the propagation speed of GWs - are at a first glance excluded unless same (background dependent) cancellations between the free functions of the theory are imposed. Nevertheless, the propagation speed of GWs could have been different than that of light at high redshift where constraints are still loose. In this talk, I will present a scalar-tensor model with a non trivial higher derivative structure in which the current value of the propagation speed is a dynamical limit. This can be achieved imposing that dark matter and dark energy interact and that their densities are proportional to each other.
May 24, 2018, 1:15 p.m.
Splitting of electrons and violation of the Luttinger sum rule
Eoin Quinn (Amsterdam)
We present a framework for organising the correlations of interacting electrons, which allows us to describe a regime of strongly correlated behaviour. We highlight two ways to characterise the electronic degree of freedom, either by the canonical fermion
May 29, 2018, 11:30 a.m.
Casimir Room (273)
Comparison of error mitigation strategies in a hydrogen molecule quantum simulation
In this project, we have used a novel quantum-classical variational algorithm to perform a quantum simulation of the hydrogen molecule under realistic error rates of a quantum device. We have shown that under such noise parameters it is not possible to reach accurate results of the hydrogen molecule dissociation curve. In order to reduce the errors of the experiment, we have developed an error mitigation protocol based on physical constrains of the problem that we have called "Parity verification measurement". Moreover, we have used two previously developed error suppression protocols and investigate their performance in our variational algorithm for the hydrogen molecule. Also, we have investigated the combination of all three methods to estimate their error under current error rates. Finally, we have shown that it is possible to calculate the hydrogen molecule dissociation with accuracy below a the chemical accuracy threshold. This proof-of-principle result shows that error mitigation methods will be useful to perform accurate calculation with small quantum computers.
- May 29, 2018, 11:30 a.m. Xavier Bonet-Monroig (Instituut-Lorentz) Comparison of error mitigation strategies in a hydrogen molecule quantum simulation
May 29, 2018, 2 p.m.
Third lecture on Elasticity
Lecture III: Elasticity of Topological Mechanical Networks A. Introduction to topological mechanics B. Guest-Hutchinson modes and their consequences. C. Surface excitations D. Real materials and the breaking of topological constraints
- May 29, 2018, 2 p.m. Tom Lubensky (Lorentz Professor) Third lecture on Elasticity
May 31, 2018, 2 p.m.
- May 31, 2018, 2 p.m. ST Luca Delacrétaz (Stanford) TBA
June 13, 2018, 3 p.m.
Anjan Ananda Sen
- June 13, 2018, 3 p.m. CS Anjan Ananda Sen (Center For Theoretical Physics, Jamia Millia Islamia Central University) TBD
June 13, 2018, 7:30 p.m.
The reasonable and unreasonable effectiveness of hydrodynamics in exotic quantum matter
Hydrodynamics has a long and glorious history, describing phenomena ranging from flows of water, patterns of weather, to star and galaxy formations. During the last decade, it has also played important and often surprising roles in characterizing various types of exotic quantum matter. In particular, it helped formulating a new class of quantum matter: quantum liquids without quasiparticles. I will briefly review these stories and explain physical reasons behind these phenomenal new successes of hydrodynamics. I will then discuss some recent theoretical developments in reformulating hydrodynamics based on action principle. The reformulation enables a new recent application: characterizing quantum many-body chaos, where the role of hydrodynamics has been rather unexpected and its success remains mysterious.
- June 13, 2018, 7:30 p.m. CE Hong Liu (MIT) The reasonable and unreasonable effectiveness of hydrodynamics in exotic quantum matter
June 15, 2018, 11:30 a.m.
Theories of spin-nematic excitonic insulators in graphite under high magnetic field and quantum multi-criticality in disordered Weyl semimetal
In the first part of the talk, I will present our phenomenological theory for metal-insulator transitions in graphite under high magnetic field. In the second part of the talk, I will discuss our recent theory on multicriticality in disordered Weyl semimetal.
- June 15, 2018, 11:30 a.m. LS Ryuichi Shindou (Beijing) Theories of spin-nematic excitonic insulators in graphite under high magnetic field and quantum multi-criticality in disordered Weyl semimetal
LS = Lorentz Seminar, Casimir room (276), Oort building
CE = Colloquium Ehrenfestii, De Sitter lecture room, Oort building
SBM = Soft & Biological Matter Seminar, Casimir room (276), Oort building
CS = Cosmology Seminar, Casimir room (276), Oort building
ST = String Theory Seminar, Casimir room (276), Oort building