Scientific Series

The parity invariance of spinfoam gravity is an open question. Naively, parity breaking should reside in the sign of the Immirzi parameter. I show that the new Lorentzian vertex formula is in fact independent of this sign, suggesting that the dynamics is parity-invariant. The situation with boundary states and operators is more complicated. I discuss parity-related pieces of the transition amplitude and graviton propagator in the large-spin 4-simplex limit. Numerical results indicate patterns similar to those in the Euclidean case. In particular, parity-related components of the graviton propagator differ by a phase. I discuss possible resolutions of this issue.

Ideas about information are pervasive, yet the fundamental nature and structure of information - if indeed it has one! - remains elusive. Work done from many different perspectives, including those of physics, biology, logic, computer science, statistics, and game and decision theory, has yielded insights into various aspects of information. Could there be a comprehensive, unified theory? We shall chart a particular path, focusing on the idea of *information flow*, and show how common mathematical structures arise in the description of information flow in computer science, logic and quantum information.

Correlations in quantum states are sometimes inaccessible if only restricted types of quantum measurements can be performed, an effect known as quantum data hiding. For example highly entangled states shared by two parties might appear uncorrelated if the parties can only measure locally their shares of the state and communicate classically with each other. In this talk I will first discuss how a better understanding of the peculiar type of correlations found in quantum data hiding states is useful in addressing two challenges of quantum information theory: the design of efficient algorithms for determining if a quantum state is entangled, and the establishment of area laws in gapped local Hamiltonians. Second, I will present new efficient ways of generating data hiding, e.g. employing random local quantum circuits, and will briefly discuss the relevance of this approach to the problem of proving quick equilibration of quantum systems unitarily interacting with a large environment.

We use the mathematical language of sheaf theory to give a unified treatment of non-locality and contextuality, which generalizes the familiar probability tables used in non-locality theory to cover Kochen-Specker configurations and more. We show that contextuality, and non-locality as a special case, correspond exactly to *obstructions to the existence of global sections*. We describe a linear algebraic approach to computing these obstructions, which allows a systematic treatment of arguments for non-locality and contextuality. A general correspondence is shown between the existence of local hidden-variable realizations using negative probabilities, and no-signalling. Maximal non-locality is generalized to maximal contextuality, and characterized in purely qualitative terms, as the non-existence of global sections in the support. Some ongoing work with Shane Mansfield and Rui Soares Barbosa is described, which identifies *cohomological obstructions* to the existence of global sections, opening the possibility of applying the powerful methods of cohomology to non-locality and contextuality.

Cosmic strings are a generic prediction of Grand Unified Theories that can leave a sufficient imprint in the Cosmic Microwave Background anisotropies to open an observational window into an otherwise unreachable high energy domain. Being formed as topological defects of a Higgs field, they are also naturally coupled to various other fields, that can lead to superconducting-like currents, hence radically changing their structure and properties. After having summarized the standard string network behaviour and its cosmological effects, I will concentrate on the current-carrying properties and show how those can modify drastically the overall picture. In particular, I will exhibit the many current case, including the special non abelian situation that requires more care to be fully understood.

The fluid/gravity correspondence relates hydrodynamic flows in D spacetime dimensions to black hole dynamics in D+1 dimensions. It is an extension of black hole thermodynamics, where charges are upgraded into local currents, and the Bekenstein-Hawking entropy - into a local entropy current. I will propose a definition for a generalized local current of Wald entropy relevant to higher-curvature gravity. As an example, I will consider 5d gravity with a U(1) gauge field and a gravitational Chern-Simons term. This corresponds to a charged fluid in 4d spacetime, whose charge current has a gravitational anomaly. I will discuss the effects of this anomaly on the generalized entropy curent and the flow equations.

We provide a microscopic understanding of the nucleation of topological quantum liquids that arise due to interactions between non-Abelian anyons. With the pairwise anyon interactions typically showing RKKY-type oscillations in sign, but decaying exponentially with distance, we show that the character of the nucleated phase is fully determined by anyon interactions beyond nearest neighbor exchange. We investigate this issue in the context of Kitaev's honeycomb lattice model. In the presence of vortex lattices, depending on microscopic parameters such as the vortex lattice spacing, we observe the nucleation of several distinct Abelian topological phases, that differ in their band structure and Chern number description. By employing an effective model of Majorana fermions, we show that these phases can be fully predicted from the vortex-vortex interactions. Corresponding microscopic results should hold for vortices forming an Abrikosov lattice in a p-wave superconductor or quasiholes forming a Wigner crystal in non-Abelian quantum Hall states.

It is usually assumed that the quantum wave-particle duality can have no counterpart in classical physics. We were driven into revisiting this question when we found that a droplet bouncing on a vibrated bath could couple to the surface wave it excites. It thus becomes a self-propelled "walker", a symbiotic object formed by the droplet and its associated wave. Through several experiments, we addressed one central question. How can a continuous and spatially extended wave have a common dynamics with a localized and discrete droplet? Surprisingly, quantum-like behaviors emerge; both a form of uncertainty and a form of quantization are observed. This is interesting because the probabilistic aspects of quantum mechanics are often said to be intrinsic and to have no possible relation with underlying unresolved dynamical phenomena. In our experiment we find probabilistic behaviors and they do have a relation with chaotic individual trajectories. These quantum like properties are related in our system to the non-locality of a walker that we called its "wave mediated path memory". The relation of this experiment with the pilot wave model proposed for quantum mechanics by de Broglie will be discussed.

The Enriched Xenon Observatory (EXO) collaboration has observed the two-neutrino double beta decay of 136Xe with EXO-200, a prototype to the full EXO detector in development. This second order process, predicted by the Standard Model, has been observed for several nuclei but not for 136Xe. The observed decay rate provides new input to matrix element calculations and to the search for the more interesting neutrino-less double-beta decay, the most sensitive probe for the existence of Majorana particles and the measurement of the neutrino mass scale. The motivation to search for neutrino-less double-beta decay will be discussed. An overview of experimental efforts, and the status of calculations of nuclear matrix elements will be given. The EXO-200 detector and underground site at WIPP, New Mexico, will be presented, and the observation of the two-neutrino decay discussed. The presentation will then focus on the development of EXO Full, a multi-tonne detector with the ability to identify the daugther ion as a powerful background reduction tool, with SNOLAB a possible site.

Several mechanisms can lead to production of particles during inflation. I discuss how this phenomenon can induce a contribution to the primordial spectrum of gravitational waves with unusual properties: the tensors produced this way can violate parity; can have a large three-point function; can have a relatively large tensor-to-scalar ratio even if inflation occurs at low energies; finally, their spectrum can display a feature that can be directly detected by second-generation gravitational interferometers such as advanced LIGO.

Recently, we developed a user friendly scheme based on the quantum kinetic equation for studying thermal transport phenomena in the presence of interactions and disorder . This scheme is suitable for both a systematic perturbative calculation as well as a general analysis. We believe that this method presents an adequate alternative to the Kubo formula, which for thermal transport is rather cumbersome. We have applied this approach in the study of the Nernst signal in superconducting films above the critical temperature. We showed that the strong Nernst signal observed in amorphous superconducting films, far above Tc, is caused by the fluctuations of the superconducting order parameter. We demonstrated a striking agreement between our theoretical calculations and the experimental data at various temperatures and magnetic fields. My talk will include a general description of the quantum kinetic approach, but mainly I will concentrate on the Nernst effect in superconducting films. I will use this example to discuss some subtle issues in the theoretical study of thermal phenomena that we encountered while calculating the Nernst coefficient. In particular, I will explain how the Nernst theorem (the third law of thermodynamics) imposes a strict constraint on the magnitude of the Nernst effect.

Bizon and Rostworowski have recently suggested that anti-de Sitter spacetime might be nonlinearly unstable to transfering energy to smaller and smaller scales and eventually forming a small black hole. We consider pure gravity with a negative cosmological constant and find strong support for this idea. While one can start with a single linearized mode and add higher order corrections to construct a nonlinear geon, this is not possible starting with a linear combination of two or more modes. One is forced to add higher frequency modes with growing amplitude. The implications of this turbulent instability for the dual field theory are discussed.