PHY670 Seminar in Theoretical Physics 

Fall 2015: organized by Tzu-Chieh Wei, Federico Bonetti, and Sam McDermott

12/10 Thur 2:30pm Jonathan Heckman (UNC at Chapel Hill)
Title: The Big List of Little Strings

12/7 Mon 1:00pm   Alexander Turbiner (Simons Center)
Title: 3-body quantum Coulomb problem: where we are

Abstrract: Current status of 3-body Coulomb problem (negative hydrogen ion H-,helium atom, lithium ion, H2+ ion etc) will be described.Major emphasis will be given to a question of stability vs Coulomb charge and to analytic structure of the ground state energy. The existence of two critical charges with associated square-root and essential singularities, respectively, is predicted. The 2nd excited, weakly-bound state of H- is predicted as a result.

12/3 Thur 2:30pm Po-Yao Chang (Rutgers)
Title: Surface and vortex states in topological nodal superconductors

Besides topological band insulators, which have a full bulk gap, there are also gapless phases of matter that belong to the broad class of topological materials, such as topological semimetals and nodal superconductors.We study examples of gapless topological phases, focusing in particular on nodal superconductors, such as nodal noncentrosymmetric superconductors (NCSs). We compute the surface density of states of nodal NCSs and interpret experimental measurements of surface states. In addition, we investigate Majorana vortex-bound states in both nodal and fully gapped NCSs using numerical and analytical methods. We show that different topological properties of the bulk Bogoliubov-quasiparticle wave functions reflect themselves in different types of zero-energy vortex-bound states. In particular, in the case of NCSs with tetragonal point-group symmetry, we find that the stability of these Majorana zero modes is guaranteed by a combination of reflection, time-reversal, and particle-hole symmetries. Finally, by using a dimensional reduction procedure from higher-dimensional topological insulators and superconductors, we derive a classification of topologically stable Fermi surfaces in semimetals and nodal lines in superconductors. 

12/2 Wed 2:30pm   Simeon Hellerman (Tokyo U., IPMU)
Title: Quantum Information Theory of the Gravitational Anomaly

Abstract: Recent papers have appeared describing certain peculiarities of measures of quantum-mechanical entanglement in local quantum field theories with nonvanishing gravitational anomaly.  We show that these peculiarities follow from the fact that the Hilbert space of a gravitationally anomalous QFT simply does not ever tensor factorize at all into Hilbert spaces in complementary regions, and thus the defining assumption on which the concept of quantum entanglement is based, always fails in gravitationally anomalous theories.  We show the breakdown of all known definitions of entanglement when the gravitationally anomaly is present, and give a general proof that no such definition exists, based on a series of tight logical connections between tensor factorizations, boundary conditions, and the gravitational anomaly.

11/23 Mon 2:00pm   Lorenzo Di Pietro (Weizmann)
Title: “Universal Properties of Cylinder Partition Functions”

11/18 Wed 2:30pm   Joint HET/RIKEN/YITP Seminar 
David Shih (Rutgers)
Title: "Natural SUSY Today"

11/11 Thur 2:30pm Matt Low (IAS) 
Title: Composite Spin-One Resonances at the LHC

11/4 Thur 2:30pm Ramis Movassagh (IBM) 
Title: A counterexample to the area law for quantum matter
Entanglement is a quantum correlation which does not appear classically, and it serves as a resource for quantum technologies such as quantum computing. The area law says that the amount of entanglement between a subsystem and the rest of the system is proportional to the area of the boundary of the subsystem and not its volume. A system that obeys an area law can be simulated more efficiently than an arbitrary quantum system, and an area lawprovides useful information about the low-energy physics of the system. It was widely believed that the area law could not be violated by more than a logarithmic factor (e.g. based on critical systems and ideas from conformal field theory) in the system’s size. We introduce a class of exactly solvable one-dimensional models which we can prove have exponentially more entanglement than previously expected, and violate the area law by a square root factor.  We also prove that the gap closes as n^{-c}, where c \ge 2, which rules out conformal field theories as the continuum limit of these models. It is our hope that the mathematical techniques introduce herein will be of use for solving other problems.
(Joint work with Peter Shor).

Phys. Rev. Lett. 109, 207202

11/3 Wed 2:30pm  Matteo Baggioli (Barcelona IFAE)
Title:  "Solid applications of massive gravity"

11/2 Mon 2:00pm  Dhritiman Nandan (Humboldt University)
Title: Hidden structures in form factors of N+4 sYM

10/29 Thur 2:30pm Prateek Agrawal (Harvad) 
Title: The Cosmological Constant Problem in Scalar Gravity

10/22 Thur 2:30pm Juven Wang (Harvard/IAS)
Title: Geometric phase, string braiding statistics and spacetime surgery

Topological invariances are useful tools to classify and characterize distinct states of condensed matter.  Berry phase is one of such invariances, which can be generalized to the non-Abelian geometric matrix when energy eigenstates degenerate.  For topological orders, specifically the many-body quantum states with bulk energy gap and with topology-dependent ground state degeneracy, the geometric matrix encodes the braiding statistics of energetic excitations. Apart from Berry phase, we also use the wave function overlap to derive the geometric matrix. We study string and particle statistics of 2+1 and 3+1 spacetime dimensions. We utilize aspects of both quantum topology and spacetime topology to derive new formulas analogous to Verlinde’s via the spacetime surgery. This provides new insights for higher dimensional topological states of matter.

If there is extra time left, I will give a brief survey on the similar notion of geometric matrix for symmetry-protected topological states (SPTs), the SPT invariants and its field theory representation.

10/21 Wed 2:30pm Olaf Hohm (MIT)
Title: Introduction into Exceptional Field Theory

10/14 Wed 2:30pm Simone Giombi (Princeton U)
Title: Sphere Free Energy and the epsilon Expansion

Abstract: I will discuss the dimensional continuation of the sphere free energy in conformal field theory. This provides a smooth interpolation between a-anomaly coefficients in even dimensions and F-values in odd dimensions, and suggests a generalization of the existing a-theorems and F-theorems to continuous dimensions. As concrete applications, I will explain how to use the Wilson-Fisher epsilon expansion to find the value of F for the 3d Ising CFT and related O(N) symmetric models, and for the conformal phase of the 3d QED with N_f massless fermions. 

10/12 Mon 2:30pm  Roman Orus (U Mainz)---special seminar 
Title: Entanglement, tensor networks, and topological quantum order

Abstract: Topological order in a 2d quantum matter can be determined by the topological contribution to the entanglement Renyi entropies. However, when close to a quantum phase transition, its calculation becomes cumbersome. In this talk I will show how topological phase transitions in 2d systems can be much better assessed by multipartite entanglement, as measured by the topological geometric entanglement of blocks. Specifically, I will present an efficient tensor network algorithm based on Projected Entangled Pair States (PEPS) to compute this quantity for a torus partitioned into cylinders, and then use this method to find sharp evidence of topological phase transitions in 2d systems with a string-tension perturbation. When compared to tensor network methods for Renyi entropies, this approach produces almost perfect accuracies close to criticality and, on top, is orders of magnitude faster. Moreover, I will show how the method also allows the identification of Minimally Entangled States (MES), thus providing a very efficient and accurate way of extracting the full topological information of a 2d quantum lattice model from the multipartite entanglement structure of its ground states. If time allows I will also present briefly other ongoing projects at our group involving the use of tensor networks to study large-spin Kagome quantum antiferromagnets, 1d symmetry-protected topological order, continuous unitary transformations, and (1+1)d lattice gauge theories.

10/8 Thur 2:30pm Travis Maxfield (U Chicago)
Title: A Landscape of Field Theories

Abstract: Studying a quantum field theory involves a choice of space-time manifold and a choice of background for any global symmetries of the theory. We argue that many more choices are possible when specifying the background. In the context of branes in string theory, the additional data corresponds to a choice of supergravity tensor fluxes. We propose the existence of a landscape of field theory backgrounds, characterized by the space-time metric, global symmetry background and a choice of tensor fluxes. As evidence for this landscape, we study the supersymmetric six- dimensional (2, 0) theory compactified to two dimensions. Different choices of metric and flux give rise to distinct two-dimensional theories, which can preserve differing amounts of supersymmetry.

10/1 Thur 2:30pm  Yiming Zong (YITP)
Title: Gravothermal evolution of galactic dark matter halos with velocity-dependent self-interactions

Abstract : 

9/30 Wed 2:30pm David Cosower (IAS & Saclay)
Title: Cross-Order Relations from Maximal Unitarity


9/24 Thur 2:30pm  Alexandros Anastasiou (Imperial College)
Title: Gravity as the square of Yang-Mills

Abstract : A recurring theme in the attempts of understanding the quantum theory of gravity is the idea of “Gravity as the square of Yang-Mills”. This involves the tensoring of the multiplet content of two super-Yang-Mills theories to obtain the multiplet content of a supergravity theory. A complete understanding of this correspondence requires studying how both global and local symmetries originate from the corresponding Yang-Mills factors. In the first part of the talk I will show how we can construct the global (U-duality) supergravity symmetries out of the corresponding global Yang-Mills ones (R-symmetries) in all spacetime dimensions between 3 and 10. In the second part I will demonstrate how the local symmetries of the gravity theory (in linearised approximation) can also be derived from local gauge invariance of the Yang-Mills factors. This will be done through a specific example in 4 spacetime dimensions.

9/23 Wed 2:30pm Jelle Hartong (U Brussels)
Title: Holographic Reconstruction of 3D Flat Space-Time from Null Infinity

Abstract: Over the recent years a lot of progress has been made in extending holographic dualities (at the gravity level) to space-times that are not asymptotically AdS. Space-times such as Lifshitz, warped AdS and Schroedinger geometries (relevant for holographic applications to condensed matter systems and to Kerr/CFT) have to a greater or lesser extent been put on a firm holographic foundation in the regime where the bulk dynamics is described by the Einstein-Hilbert action plus matter. An important role in these developments has been played by non-relativistic symmetry groups and novel types of boundary geometries such as Newton-Cartan geometries. In this talk it will be shown that also asymptotically flat space-times can be studied holographically using similar methods. I will show that the boundary geometry at future null infinity is described by a so-called Carrollian geometry and that one can define a boundary energy-momentum tensor whose associated conserved currents span the infinite dimensional BMS_3 algebra.

9/17 Thur 2:30pm Harikrishnan Ramani (YITP)
Title: The WW story

Abstract:Run 1 of the LHC has reported routine excesses(around 20%) in WW measurements both at 7 and 8 TeV.Accounting for higher order QCD effects through pT resummation can explain some of the discrepancy. The CMS experiment obtained good theory-experiment  agreement partly due to accounting for higher order corrections calculated by our group. A closely related approach; jet veto resummation has also had success explaining this excess. In this seminar I will talk about the WW experiment set up, transverse momentum resummation and reweighting methods. I shall also compare the two resummation procedures(transverse momentum and jet veto resummation).

9/10 Thur 2:30pm Zhibai Zhang (CityTech CUNY)
Title: Black Rings in Supergravity

Abstract: Black rings are five-dimensional black holes with non-spherical event horizons. Their existence indicates that the black hole uniqueness theorems of four dimensions do not apply in five dimensions, and that the classification of black holes in higher dimensions is more rich. In this talk, I will give a brief review of the general properties of black rings and their construction, present new black ring solutions in supergravity and discuss their thermodynamics. I will also discuss some possible future directions.

9/3 Thur 3:30pm (note special time) Tzu-Chieh Wei (YITP)
Title: Hamiltonian quantum computer in one dimension

Abstract: There are several approaches for quantum computation (QC), such as the standard circuit model, adiabatic QC, measured-based QC, etc. In addition, quantum computation can be achieved by preparing an appropriate initial product state of qudits and then letting it evolve under a fixed Hamiltonian. The readout is made by measurement on individual qudits at some later time. Feynman was the first to propose such a Hamiltonian quantum computer, but his Hamiltonian involves 4-body interaction among qubits that are not geometrically local. Can a Hamiltonian quantum computer be constructed even in a one-dimensional geometry with  short-ranged (i.e. nearest few neighbors) interaction? It turns out that it is possible with a nearest-neighbor (i.e., 2-local) interaction, but the individual spins need to have at least 8 levels (i.e., the local Hilbert space has dimension d=8).  Here we consider the compromise between the locality k and the local dimension d in one spatial dimension for universal Hamiltonian quantum computers. As the locality k increases, it is expected that the minimum required d can decrease. In particular, for 3-local interactions (k=3) how much lower than 8 can the local dimension d? We provide a construction that uses d=5, i.e. spin-2. The resulting Hamiltonian is, however, not translationally invariant. When one imposes translation invariance, the required d will need be higher. We also provide another construction with d=8 translationally invariant Hamiltonian that is universal (in the sense of quantum computation). One implication of our results is that simulating generic 1D chains of spin-2 particles belongs to the complexity class BQP and is fact BQP-complete.

* Only basic quantum mechanics is used in this talk and no background on quantum computation is required

8/26 Wed 2:30pm Michael Spillane (YITP) 
Title: Thermal Corrections to Renyi Entropies on S^{d-1}
Abstract: Interest in entanglement entropy and its generalization R\'{e}nyi entropy is growing due to its applications in a variety of fields in physics, from quantum computation and phase transitions to black hole physics and holography.  In this talk I will discuss the effects of a small finite temperature on both measures of entanglement.  The primary focus will be on calculating corrections for a free fermion on an n-sphere.  Via conformal maps and a Boltzmann expansion of the density matrix, the corrections can be related to a specific two point function on a conical space. This work expands on previous work in Refs. arXiv:1407.1358 and arXiv:1411.6505.