Scientific Series

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A polynomial-time algorithm for the ground state of 1D gapped local Hamiltonians

Thomas Vidick Weizmann Institute of Science

May 14, 2014

PIRSA:14050039
What can we learn from modifing gravity ?

Claudia de Rham Imperial College London
May 14, 2014

PIRSA:14050003
Astrophysics
Unitarity and crossing symmetry in the S-matirx of large N Chern-Simons theory with fundamental matter

Shiraz Minwalla Tata Institute of Fundamental Research (TIFR)
May 13, 2014

PIRSA:14050006
High Energy Physics
Gapless spin liquids in frustrated Heisenberg models

Federico Becca SISSA International School for Advanced Studies
May 13, 2014

PIRSA:14050074
Condensed Matter
New strong interactions and the t t-bar asymmetry

Joachim Brod University of Cincinnati
May 13, 2014

PIRSA:14050092
High Energy Physics
Random models of inflation

Andrew Liddle University of Lisbon
May 13, 2014

PIRSA:14050007
Astrophysics
The quest for self-correcting quantum memory

Olivier Landon-Cardinal California Institute of Technology
May 12, 2014

PIRSA:14050028
Quantum Physics
Curvature in Noncommutative Geometry

masoud khalkhali Western University
May 09, 2014

PIRSA:14050064
Mathematical Physics
Bulk-Edge Correspondence in 2+1-Dimensional Abelian Topological Phases

Jennifer Cano University of California System
May 09, 2014

PIRSA:14050011
Condensed Matter
On the role of the extra kinetic term coupling ƛ in classical Hořava-Lifshitz gravity

Luis Pires Radboud Universiteit Nijmegen
May 08, 2014

PIRSA:14050061
General Relativity & Quantum Cosmology
Black holes without Lorentz symmetry

Thomas Sotiriou University of Nottingham
May 08, 2014

PIRSA:14050002
General Relativity & Quantum Cosmology
Scattering of emerging excitations in Matrix Product States

Jutho Haegeman Ghent University
May 06, 2014

PIRSA:14050010
Condensed Matter

A polynomial-time algorithm for the ground state of 1D gapped local Hamiltonians

Thomas Vidick Weizmann Institute of Science

May 14, 2014

PIRSA:14050039

Computing ground states of local Hamiltonians is a fundamental problem in condensed matter physics. We give the first randomized polynomial-time algorithm for finding ground states of gapped one-dimensional Hamiltonians: it outputs an (inverse-polynomial) approximation, expressed as a matrix product state (MPS) of polynomial bond dimension. The algorithm combines many ingredients, including recently discovered structural features of gapped 1D systems, convex programming, insights from classical algorithms for 1D satisfiability, and new techniques for manipulating and bounding the complexity of MPS. Our result provides one of the first major classes of Hamiltonians for which computing ground states is provably tractable despite the exponential nature of the objects involved. /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-style-parent:""; font-size:11.0pt; font-family:"Calibri","sans-serif";}
What can we learn from modifing gravity ?

Claudia de Rham Imperial College London
May 14, 2014

PIRSA:14050003
Astrophysics
I will review models of modified gravity in the infrared and show how extra degrees of freedom present in these theories get screened via the Vainshtein mechanism. That mechanism comes hand in hand with its own share of peculiarities: classical superluminalities, strong coupling and perturbative non-analyticity of the S-matrix to name a few. From a traditional effective field viewpoint such effects are disastrous but I will present the first hints in understanding these theories beyond the traditional perspective and their implications not only for gravity but also for our understanding of a certain class of field theories.
Unitarity and crossing symmetry in the S-matirx of large N Chern-Simons theory with fundamental matter

Shiraz Minwalla Tata Institute of Fundamental Research (TIFR)
May 13, 2014

PIRSA:14050006
High Energy Physics
We present explicit computations and conjectures for 2 → 2 scattering matrices in large N U(N) Chern-Simons theories coupled to fundamental bosonic or fermionic matter to all orders in the ’t Hooft coupling expansion. The bosonic and fermionic S-matrices map to each other under the recently conjectured Bose-Fermi duality after a level-rank transposition. The S-matrices presented in this paper may be regarded as relativistic generalization of Aharonov-Bohm scattering. They have unusual structural features: they include a non analytic piece localized on forward scattering, and obey modified crossing symmetry rules. We conjecture that these unusual features are properties of S-matrices in all Chern-Simons matter theories. The S-matrix in one of the exchange channels in our paper has an anyonic character; the parameter map of the conjectured Bose-Fermi duality may be derived by equating the anyonic phase in the bosonic and fermionic theories.
Gapless spin liquids in frustrated Heisenberg models

Federico Becca SISSA International School for Advanced Studies
May 13, 2014

PIRSA:14050074
Condensed Matter
We present our recent numerical calculations for the Heisenberg model on the square and
Kagome lattices, showing that gapless spin liquids may be stabilized in highly-frustrated
regimes. In particular, we start from Gutzwiller-projected fermionic states that may
describe magnetically disordered phases,[1] and apply few Lanczos steps in order to improve
their accuracy. Thanks to the variance extrapolation technique,[2] accurate estimations of
the energies are possible, for both the ground state and few low-energy excitations.
Our approach suggests that magnetically disordered phases can be described by Abrikosov
fermions coupled to gauge fields.

For the Kagome lattice, we find that a gapless U(1) spin liquid with Dirac cones
is competitive with previously proposed gapped spin liquids when only the nearest-neighbor
antiferromagnetic interaction is present.[3,4] The inclusion of a next-nearest-neighbor term
lead to a Z_2 gapped spin liquid,[5] in agreement with density-matrix renormalization group
calculations.[6] In the Heisenberg model on the square lattice with both nearest- and
next-nearest-neighbor interactions, a Z_2 spin liquid with gapless spinon excitations is
stabilized in the frustrated regime.[7] This results are (partially) in agreement with recent
density-matrix renormalization group on large cylinders.[8]

[1] X.-G. Wen, Phys. Rev. B {\bf 44}, 2664 (1991); Phys. Rev. B {\bf 65}, 165113 (2002).
[2] S. Sorella, Phys. Rev. B {\bf 64}, 024512 (2001).
[3] Y. Iqbal, F. Becca, S. Sorella, and D. Poilblanc, Phys. Rev. B 87, 060405(R) (2013).
[4] Y. Iqbal, D. Poilblanc, and F. Becca, Phys. Rev. B 89, 020407(R) (2014).
[5] W.-J. Hu, Y. Iqbal, F. Becca, D. Poilblanc, and D. Sheng, unpublished.
[6] H.-C. Jiang, Z. Wang, and L. Balents, Nat. Phys. 8, 902 (2012);
S. Yan, D. Huse, and S. White, Science 332, 1173 (2011).
[7] W.-J. Hu, F. Becca, A. Parola, and S. Sorella, Phys. Rev. B 88, 060402(R) (2013).
[8] S.-S. Gong, W.Z., D.N. Sheng, O.I. Motrunich, and M.P.A. Fisher, arXiv:1311.5962 (2013).
New strong interactions and the t t-bar asymmetry

Joachim Brod University of Cincinnati
May 13, 2014

PIRSA:14050092
High Energy Physics
The CDF and D0 experiments at Tevatron measure a top-quark forward-backward
asymmetry significantly larger than the standard-model prediction.
We construct a model that involves new strong interactions at the electroweak scale
and can explain the measured asymmetry. Our model possesses a flavor symmetry
which allows to evade flavor and collider constraints, while it still permits flavor-violating
couplings of order 1 which are needed to generate the asymmetry via light t-channel vectors.
Random models of inflation

Andrew Liddle University of Lisbon
May 13, 2014

PIRSA:14050007
Astrophysics
Rather than writing down specific functional forms, one can generate inflation models via stochastic processes in order to explore generic properties of inflation models. I describe our explorations of the phenomenology of randomly-generated multi-field inflation models, both for canonical fields and for a braneworld-motivated scenario. Implications of some recent observational results, including BICEP2, will be discussed.
The quest for self-correcting quantum memory

Olivier Landon-Cardinal California Institute of Technology
May 12, 2014

PIRSA:14050028
Quantum Physics
A self-correcting quantum memory is a physical system whose quantum state can be preserved over a long period of time without the need for any external intervention. The most promising candidates are topological quantum systems which would protect information encoded in their degenerate groundspace while interacting with a thermal environment. Many models have been suggested but several approaches have been shown to fail due to no-go results of increasingly general scope. In a nutshell, 2D topological models and many 3D topological models have point-like excitations which propagate freely and change the groundstate at any non-zero temperature. A recent suggestion is to introduce effective long-range interactions between those point-like excitations. In this presentation, I will first explain the desiderata for self-correction, review the recent advances and no-go results, and describe the current endeavours to define a self-correcting system in 2D and 3D. Time permitting, I will briefly present our recent work on the thermal instability of models which aim to introduce effective long-range interactions between point-like excitations (joint work with Beni Yoshida, John Preskill and David Poulin).
Curvature in Noncommutative Geometry

masoud khalkhali Western University
May 09, 2014

PIRSA:14050064
Mathematical Physics
After the seminal work of Connes and Tretkoff on the Gauss-Bonnet theorem for the noncommutative 2-torus and its extension by Fathizadeh and myself, there have been significant developments in understanding the local differential geometry of these noncommutative spaces equipped with curved metrics. In this talk, I will review a series of joint works with Farzad Fathizadeh in which we compute the scalar curvature for curved noncommutative tori and prove the analogue of Weyl's law and Connes' trace theorem. Our final formula for the curvature matches precisely with the one computed independently by A. Connes and H. Moscovici. I will then report on our recent work on the computation of scalar curvature for noncommutative 4-tori (which involves intricacies due to violation of the Kähler condition). We show that metrics with constant curvature are extrema of the analogue of the Einstein–Hilbert action. A purely noncommutative feature in these works is the appearance of the modular automorphism from Tomita–Takesaki theory of KMS states in the final formulas for the curvature.
Bulk-Edge Correspondence in 2+1-Dimensional Abelian Topological Phases

Jennifer Cano University of California System
May 09, 2014

PIRSA:14050011
Condensed Matter
The same bulk two-dimensional topological phase can have multiple distinct, fully-chiral edge phases. We show that this can occur in the integer quantum Hall states at fillings 8 and 12 with experimentally-testable consequences. We also show examples for Abelian fractional quantum Hall states, the simplest examples being at filling fractions 8/7, 12/11, 8/15, 16/5. For all examples, we propose experiments that can distinguish distinct edge phases. Our results are summarized by the observation that edge phases correspond to lattices while bulk phases correspond to genera of lattices. Since there are typically multiple lattices in a genus, there are usually many stable fully chiral edge phases corresponding to the same bulk. We show that fermionic systems can have edge phases with only bosonic low-energy excitations and discuss a fermionic generalization of the relation between bulk topological spins and the central charge. The latter follows from our demonstration that every fermionic topological phase can be represented as a bosonic topological phase, together with some number of filled Landau levels. Our analysis also leads to a simple demonstration that all Abelian topological phases can be represented by a Chern-Simons theory parameterized by a K-matrix.
On the role of the extra kinetic term coupling ƛ in classical Hořava-Lifshitz gravity

Luis Pires Radboud Universiteit Nijmegen
May 08, 2014

PIRSA:14050061
General Relativity & Quantum Cosmology
We study the classical constraint algebra of Hořava-Lifshitz gravity, where due to the breaking of 4d diffeomorphism symmetry, there is a new dimensionless coupling absent in GR and whose role is not yet clear. Starting from two apparently contradictory results, we show how the role of the extra coupling differs between the projectable and non-projectable versions of the theory. In particular, we see how in the latter, it gives rise to a non-trivial constraint algebra, akin to the conditions seen in the CMC gauge of GR.
Black holes without Lorentz symmetry

Thomas Sotiriou University of Nottingham
May 08, 2014

PIRSA:14050002
General Relativity & Quantum Cosmology
Our current definition of what a black hole is relies heavily on the assumption that there exists a finite maximum speed of propagation for any signal. Indeed, one is tempted to think that the notion of a black hole has no place in a world with infinitely fast signal propagation. I will use concrete examples from Lorentz-violating gravity theories to demonstrate that this naive expectation is not necessarily true.
Scattering of emerging excitations in Matrix Product States

Jutho Haegeman Ghent University
May 06, 2014

PIRSA:14050010
Condensed Matter
We review the formalism of matrix product states and one of its recent generalisations which allows to variationally determine the dispersion relation of elementary excitations in generic one-dimensional quantum spin chains. These elementary excitations dominate the low energy effective behaviour of the system. We discuss recent work where we show how we can also describe the effective interaction between these excitations – as mediated by the strongly correlated ground state – and how we can extract the corresponding S matrix. With these two ingredients, we can already build a highly non-trivial low-energy description of any microscopic Hamiltonian by assuming that higher order scattering processes are negligible. This allows to extract accurate information about the behaviour of the system under perturbations or at finite temperature, as we illustrate using the spin 1 Heisenberg model.

Title	Speaker(s)	Date	Talk Type	Info link
A polynomial-time algorithm for the ground state of 1D gapped local Hamiltonians	Thomas Vidick	2014-05-14	Scientific Series	View details
What can we learn from modifing gravity ?	Claudia de Rham	2014-05-14	Scientific Series	View details
Unitarity and crossing symmetry in the S-matirx of large N Chern-Simons theory with fundamental matter	Shiraz Minwalla	2014-05-13	Scientific Series	View details
Gapless spin liquids in frustrated Heisenberg models	Federico Becca	2014-05-13	Scientific Series	View details
New strong interactions and the t t-bar asymmetry	Joachim Brod	2014-05-13	Scientific Series	View details
Random models of inflation	Andrew Liddle	2014-05-13	Scientific Series	View details
The quest for self-correcting quantum memory	Olivier Landon-Cardinal	2014-05-12	Scientific Series	View details
Curvature in Noncommutative Geometry	masoud khalkhali	2014-05-09	Scientific Series	View details
Bulk-Edge Correspondence in 2+1-Dimensional Abelian Topological Phases	Jennifer Cano	2014-05-09	Scientific Series	View details
On the role of the extra kinetic term coupling ƛ in classical Hořava-Lifshitz gravity	Luis Pires	2014-05-08	Scientific Series	View details
Black holes without Lorentz symmetry	Thomas Sotiriou	2014-05-08	Scientific Series	View details
Scattering of emerging excitations in Matrix Product States	Jutho Haegeman	2014-05-06	Scientific Series	View details

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A polynomial-time algorithm for the ground state of 1D gapped local Hamiltonians

What can we learn from modifing gravity ?

Unitarity and crossing symmetry in the S-matirx of large N Chern-Simons theory with fundamental matter

Gapless spin liquids in frustrated Heisenberg models

New strong interactions and the t t-bar asymmetry

Random models of inflation

The quest for self-correcting quantum memory

Curvature in Noncommutative Geometry

Bulk-Edge Correspondence in 2+1-Dimensional Abelian Topological Phases

On the role of the extra kinetic term coupling ƛ in classical Hořava-Lifshitz gravity

Black holes without Lorentz symmetry

Scattering of emerging excitations in Matrix Product States

A polynomial-time algorithm for the ground state of 1D gapped local Hamiltonians

What can we learn from modifing gravity ?

Unitarity and crossing symmetry in the S-matirx of large N Chern-Simons theory with fundamental matter

Gapless spin liquids in frustrated Heisenberg models

New strong interactions and the t t-bar asymmetry

Random models of inflation

The quest for self-correcting quantum memory

Curvature in Noncommutative Geometry

Bulk-Edge Correspondence in 2+1-Dimensional Abelian Topological Phases

On the role of the extra kinetic term coupling ƛ in classical Hořava-Lifshitz gravity

Black holes without Lorentz symmetry

Scattering of emerging excitations in Matrix Product States