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Young Researchers Conference 2008

Conference Date: 
Lundi, Décembre 8, 2008 (All day) to Vendredi, Décembre 12, 2008 (All day)

 

This conference  provides an opportunity for  young physicists to experience the dynamic environment and research opportunities offered to Postdoctoral Fellows pursuing their research careers at Perimeter Institute.  Invited participants enjoy a multidisciplinary conference, which highlights new developments in theoretical physics contributed by their peers.  The main areas of the conference are quantum information, quantum foundations, string theory, quantum gravity, cosmology and elementary particle theory.  The speakers are postdoctoral level theorists from around the world and the event is also attended by Perimeter Institute resident scientists.

 

This event is open to invited attendees only. 

 

 

Nabamita Banerjee, Harish-Chandra Research Institute

Giulio Chiribella, Universita degli Studi di Pavia, Italy

Roger Colbeck, ETH Zurich

Jonathan Engle, Albert Einstein Institute, Germany 

Adrienne Erickcek, Caltech

Enrique Fernandez-Borja, University of Valencia

Anna Gustavsson, Imperial College London

Yevgeny Kats, Harvard

Michael Kiermaier, MIT

Louis LeBlond, Texas A&M

Netanel Lindner, Technion - Israel Institute of Technology

Alberto Montina, University of Firenze

Arvind Murugan, Princeton

Aseem Paranjape, TIFR Mumbai

Gil Paz, IAS

Roberto Pereira, Marseille

Fabian Schmidt, University of Chicago

Michael Seevinck, Utrecht University 

Sarah Shandera, Columbia University

Yanwen Shang, University of Toronto 

Lorenzo Sindoni, SISSA, Italy

Yuji Tachikawa, Institute for Advanced Study

Ethan Thompson, University of Washington

Gonzalo Torroba, Rutgers University

Jon Yard, Los Alamos National Laboratories

 

Nabamita Banerjee, Harish-Chandra Research Institute

Asymptotic Expansion of the N=4 Dyon Degeneracy

We study various aspects of power suppressed as well as exponentially suppressed corrections in the asymptotic expansion of the degeneracy of quarter BPS dyons in N=4 supersymmetric string theories. In particular we explicitly calculate the power suppressed corrections up to second order and the first exponentially suppressed corrections. We also propose a macroscopic origin of the exponentially suppressed corrections using the quantum entropy function formalism. This suggests a universal pattern of exponentially suppressed corrections to all four dimensional extremal black hole entropies in string theory.


Giulio Chiribella, Universita degli Studi di Pavia, Italy

Admissible transformations of quantum networks and their applications in quantum information processing

Quantum operations are known to be the most general state transformations that can be applied to parts of compound systems compatibly with the probabilistic structure of quantum mechanics.  What about the most general transformations of quantum operations?  It turns out that any such general transformation can be realized by a quantum network with an open slot in which the input operation can be inserted, thus programming the resulting circuit. Moreover, one can recursively iterate this construction, generating an infinite hierarchy of admissible transformations and proving their realization within the circuit model of quantum mechanics.  These results provide the basis of a new method to optimize quantum networks for information processing tasks, including e.g. gate estimation, discrimination, programming, and cloning.   As examples of application, I will present here the optimal quantum networks for estimation of group transformations, for the alignment of reference frames with multiple communication rounds, and for universal cloning of unitary transformations. 


Roger Colbeck, ETH Zurich

The impossibility of partially local hidden variable models for quantum theory and the relation to cryptography

More than 40 years ago, Bell ruled out completely local hidden variable models as an explanation for quantum correlations.  However, a new type of hidden variable model has recently been brought to light by the work of Leggett.  Such a model has both local and non-local parts.  Roughly speaking, having a local part means that the measurement outcomes can be guessed with better than 50% success.  In this talk, I will explain that there exist quantum correlations for which any hidden variable model must have a trivial local part. I will then discuss how an extension of the original theorem implies that these correlations can be used to enhance the quality of a private random string.


Jonathan Engle, Albert Einstein Institute, Germany 

LQG spinfoam models: basic ideas and motivations

We give an overview of what we have called the "LQG spinfoam models," that provide a spinfoam dynamics for LQG, for arbitrary values of the Barbero-Immirzi parameter in both Lorentzian and Euclidean signatures.  The key motivation behind these models was to modify the Barrett-Crane model, by handling more carefully certain constraints, called simplicity constraints, which become second class in the quantum theory. As a result, the kinematics of the models exactly match those of LQG.  The goal of this talk is to provide a broad picture of the significance of these models and their basic ideas.


Adrienne Erickcek, Caltech

Inflationary Origins of the Cosmic Power Asymmetry

WMAP measurements of CMB temperature anisotropies reveal a power asymmetry: the average amplitude of temperature fluctuations in one hemisphere is larger than the average amplitude in the opposite hemisphere at the 99% confidence level.  This power asymmetry may be generated during inflation by a large-amplitude superhorizon perturbation that causes the mean energy density to vary across the observable Universe.  Such a superhorizon perturbation would also induce large-scale temperature anisotropies in the CMB; measurements of the CMB quadrupole and octupole (but not the dipole!) therefore constrain the perturbation's amplitude and wavelength.  I will show how a superhorizon perturbation in a multi-field inflationary theory, the curvaton model, can produce the observed power asymmetry without generating unacceptable temperature fluctuations in the CMB.  I will also discuss how this mechanism for generating the power asymmetry will be tested by forthcoming CMB experiments.


Enrique Fernandez-Borja, University of Valencia

Black hole entropy in Loop Quantum Gravity

I will review the present status of the black hole entropy computation in Loop Quantum Gravity within the isolated horizon framework. Starting from the recently discovered discretization effect, I will give an overview of the subsequent developments that have been obtained motivated by it. Through this further analysis of the problem I will present some new related results and the promising new open windows that they give rise to.


Anna Gustavsson, Imperial College London

Constrained Quantum Dynamics

I will discuss a new framework that has been introduced to investigate the dynamics of constrained quantum systems, and the procedure for its implementation. The approach makes use of the fact that the space of pure states in quantum theory has both a Riemannian structure, the unitary-invariant Fubini-Study metric, and a symplectic structure. As a consequence, the idea of constrained quantum motion can be realised by requiring that the dynamics should lie on a suitably specified submanifold of the state space. I will show that for certain classes of systems the framework can be viewed as an extension of Dirac's theory for constrained systems applied to quantum mechanics. I will illustrate some of the results through examples and I will also discuss how the framework can be extended to include mixed quantum states.


Yevgeny Kats, Harvard

A Toy Model of Unparticle Physics

I will discuss a simple two-dimensional theory, whose unparticle sector is a modification of the Schwinger model, that gives new insights into the qualitative features of unparticle physics. I will analyze the transition between the short-distance perturbative physics and large-distance unparticle behavior. Then I will show how to compute processes that involve unparticle self-interactions, for which nontrivial higher n-point functions of the conformal theory are essential.


Michael Kiermaier, MIT

On-shell methods in Quantum Field Theory

The efficient computation of scattering amplitudes in quantum field theory has many important applications, ranging from the computation of QCD backgrounds at the LHC to the study of the perturbative finiteness of N=8 supergravity. "On-shell methods" are a crucial ingredient in the computation of gauge theory and gravity amplitudes because they are far more efficient than traditional Feynman diagram techniques.

I give an introduction to the basic concepts used in this field. I explain one particularly elegant method, the MHV vertex expansion, and outline how we recently proved the validity of this expansion in N=4 Super Yang-Mills Theory.


Louis LeBlond, Texas A & M

Tachyon Mediated Non-Gaussianity I will discuss the various sources of non-Gaussianity (NG)  in a class of multi-field models of inflation. I will show that there is both an intrinsic and a local contribution to the NG although they both have the same shape. It is also possible in this class of models that the dominant part to the 3-pt function comes from loop diagrams. These models are of the hybrid type and while they occur naturally in string theory, the conditions for the NG to be important are not generic.


Netanel Lindner, Technion-Israel Institute of Technology

A photonic cluster state machine gun

We present a method which can be used to convert certain single photon sources, such as quantum dots, into devices capable of emitting large strings of photonic cluster states in a controlled and pulsed “on demand” manner. Such sources greatly alleviate the resources required to achieve linear optical quantum computation. Standard spin errors, such as dephasing, are shown to affect only 1 or 2 of the emitted photons at a time. This allows for the use of standard fault tolerance techniques, and shows that the machine gun can be fired for arbitrarily long times. Using realistic parameters for current semiconductor quantum dot sources, we conclude high entangled-photon emission rates are achievable, with Pauli-error rates less than 0.2%.


Alberto Montina, University of Firenze

Does the wave-function concern information or reality

In the standard interpretation of quantum mechanics, the wave-function provides a complete statistical information of systems. It does not represent a real field, but is similar to the concept of probability distribution of classical mechanics. In the framework of this interpretation, no attempt of representing an underlying reality by means of well-defined quantities is made.

Conversely, an ontological theory of quantum mechanics provides a classical description by means of well-defined quantities, whose values are partially hidden and the state of our knowledge is represented by probability distributions. I will show that in any ontological Markovian theory the wave-function must be promoted to the rank of a real field, i.e. it must correspond to elements of reality.

This implies that the number of ontological variables must grow exponentially with the physical size. Some consequences of this theorem are finally sketched.


Arvind Murugan, Princeton

2+1dimensional gauge gravity duality

I discuss our recent investigations into 2+1 dim Chern-Simons theories with gravity duals that have reduced supersymmetry. Many new phenomena such as fractional statistics arise in 2+1 dim field theory that make this duality interesting and subtle. I focus on our work involving an example of such a duality with minimal supersymmetry and propose a field theoretic dual for a long known vacuum of gauged supergravity on AdS_4.

I also argue that 2+1 dim duality might present a favorable landscape for constructing non-supersymmetric conformal fixed points at large but finite N.


Aseem Paranjape, TIFR Mumbai

Backreaction from Averaging in Cosmology

There is an ongoing debate in the literature concerning the effects of averaging out inhomogeneities ("backreaction'') in cosmology. In particular, it has been suggested that the backreaction can play a significant role at late times, and that the standard perturbed FLRW framework is no longer a good approximation during structure formation, when the density contrast becomes nonlinear. After a brief introduction to the problem, I will show using Zalaletdinov's covariantaveraging scheme that as long as the metric of the universe can be described by the perturbed FLRW form, the corrections due to averaging remain negligibly small. Further, using a fully relativistic and reasonably generic model of pressureless spherical collapse, I will show that as long as matter  velocities remain small (which is true in this model even at late times), the perturbed FLRW form of the metric can be explicitly recovered.  Together with the observation that real peculiar velocities are in fact nonrelativistic, these results imply that the backreaction remains small  even during nonlinear structure formation.  


Gil Paz, Institute for Advanced Study 

Supersymmetric U(1)' Models

Extension of the minimal supersymmetric standard model (MSSM) that include a U(1)' gauge symmetry are motivated by top-down constructions and offer an elegant solution to the MSSM mu problem. In this talk I will describe some of the opportunities that such models offer, such as a new mechanism for mediation of supersymmetry breaking, as well as some of the challenges in constructing viable supersymmetric U(1)' models.


Roberto Pereira, Marseille

Loop quantum gravity vertex for spin foam gravity

New spin foam models for gravity have been recently proposed to deal with the shortcomings of the Barrett-Crane model. In particular, they draw a closer connection between the Loop Quantum Gravity and the Spin Foam approaches to non perturbative quantum gravity. In this talk, I will present the construction for the case of Lorentzian signature and finite Immirzi parameter. An area operator can be defined and its spectrum agrees with the one defined in LQG. Finally, the amplitude is shown to be finite after a suitable regularization.


Fabian Schmidt, University of Chicago

Structure Formation in Modified Gravity

Instead of adding another dark component to the energy budget of the Universe in trying to explain the accelerated expansion, one can ask whether the cause is in fact the laws of gravity itself on the largest scales. In this talk, I will consider a sub-class of so-called f(R) gravity theories which closely follow the LambdaCDM expansion history, while at the same time evading tight Solar System constraints on gravity. I will present new results from cosmological N-body simulations which consistently solve for the modified gravitational force. In particular, I will discuss the effects of modified gravity on structure formation, dark matter halo properties, and cosmological observables.


Michael Seevinck, Utrecht University

Deep Hidden Variables

Despite over 40 years of research on Bell-type inequalities and the question of non-locality, new technical results that have general foundational relevance can still be obtained. In this talk will present a number of new results that deal with the question of how to discern local, quantum and no-signaling correlations.

  1. I will present a non-trivial no-signaling inequality that discerns no-signaling correlations from general correlations - the first to our knowledge. This inequality has a striking similarity with the CHSH inequality, yet it is crucially different.
  2. I will next discuss interesting relationships that can be inferred between some well-known conditions at different hidden- variable levels (such as the conditions of outcome and parameter independence). The upshot of the analysis will be that which conditions are to be obeyed by different kinds of correlations and which are not, depends on the level of consideration. A conclusive picture therefore depends on which hidden-variable level is considered to be fundamental.
  3. I will further comment on interesting relationships that exist between inferences on the surface and subsurface level. Here the surface level deals with experimentally accessible probabilities (e.g., via relative frequencies) and the sub-surface level deals with probabilities that are conditioned on a hidden-variable (or the quantum state). The most interesting such a relationship is thefollowing: any deterministic hidden-variable theory that obeys no- signaling and gives non-local correlations must show randomness at the surface, i.e., the surface probabilities cannot be deterministic. This is the case in Bohmian mechanics but this result shows it to be generic.

Throughout the talk I will show how these three topics are related, and comment on the foundational impact of the results obtained.


Sarah Shandera, Columbia University

Statistics in single field inflation

Non-Gaussianity is a powerful observable that may reveal important properties of the fundamental physics of inflation, with qualitative and quantitative features of higher order correlation functions distinguishing between models. Here I will discuss the structure of correlation functions in the most general single field inflation model and explain why this information is important for making use of observations from the CMB and large scale structure.


Yanwen Shang, University of Toronto

IR modification of gravity and the forbidden mass range of spin-2 particles in De Sitter space

One of the most challenging problems in theoretical physics today is the so called cosmological constant problem. While current observations are consistent with the prediction of GR with an unexplainable tiny cosmological constant, it remains possible that it's the deviation of the law of gravity at large distance from Einstein's theory that resolves the puzzle. In this talk, I will briefly review some of the theoretical attempts we made along this line, in particular, the so called "classically constrained gravity" and its implications in quantum cosmology. I will also present some most recent study on massive spin-2 particles in De Sitter space, and describe a model, initially motivated by DGP theory, which allows one to explore the Higuchi forbidden mass range of the graviton on the De Sitter background.


Lorenzo Sindoni, SISSA, Italy

Emergent gravity: lessons from two models

While analogue models for gravity have so far provided some insights on the kinematical aspects of general relativity, the emergence of gravitational dynamics is still unclear. In this talk I will present twomodels which aim at filling this gap. In the first one a BEC model is considered, to uncover the gravitational dynamics hidden in these systems. In particular, the emergence of a modified Newtonian dynamics and of the cosmological constant will be discussed. A second model will be used to show how Nordstrom theory of gravity emerges from a system defined in Minkowski spacetime, showing how background independent theories of gravity could emerge from background dependent models.


Yuji Tachikawa, Institute for Advanced Study

Story of a & c

In two dimensional CFTs the Zamolodchikov's c-theorem is fundamental in that it shows that the number of degrees of freedom decreases along the renormalization group flow.  I will give a short history of and discuss recent developments in the quest to find its four-dimensional analogue using the central charges 

a & c.


Ethan Thompson, University of Washington

The holographic description of non-relativistic conformal field theories

The AdS/CFT correspondence has recently been extended to field theories satisfying the non-relativistic generalization of conformal symmetry, the Schroedinger symmetry.  These holographic descriptions offer the potential to do calculations in the strong coupling regime of experimentally-realized condensed matter systems, such as fermions at unitarity.  In this talk, we will outline the holographic formulation of such NRCFTs at zero temperature.  We will then discuss the embedding of the appropriate geometry into IIB supergravity, and the finite temperature generalization that results.  We will conclude with a brief discussion of the holographic description of non-relativistic conformal hydrodynamics and current research projects.


Gonzalo Torroba, Rutgers University

Four dimensional dynamics of string compactifications

We report on recent progress in understanding string compactifications to four dimensions, preserving minimal supersymmetry. We develop a general formalism to construct the kinetic terms of the low energy degrees of freedom. At strong warping, new light Kaluza-Klein modes appear, which change the effective action for the complex and Kahler moduli. We explain how to determine these new fields starting from 10d, and find their couplings to the zero mode sector.


Jon Yard, Los Alamos National Laboratories

Surprises in the theory of quantum channel capacity

A quantum channel models a physical process which adds noise to a quantum system by interacting with the environment.  Protecting quantum systems from such noise can be viewed as an extension of the classical communication problem introduced by Shannon sixty years ago.  A fundamental quantity of interest is the quantum capacity of a given channel.  It measures the amount of quantum information that can be transmitted with vanishing error, in the limit of many independent transmissions over that channel.   In this talk, I will show that certain pairs of channels, each with a capacity of zero, can have a strictly positive capacity when used together.  This unveils a rich structure in the theory of quantum communication that is absent from Shannon's classical theory.   This is joint work with Graeme Smith (IBM) which was published in the Sept. 26 issue of Science.