Scientific Series

We construct the q-deformed spinfoam vertex amplitude using Chern-Simons theory on the boundary 3-sphere of the 4-simplex. The rigorous definition involves the construction of Vassiliev-Kontsevich invariant for trivalent knot graph. Under the semiclassical asymptotics, the q-deformed spinfoam amplitude reproduce Regge gravity with cosmological constant at nondegenerate critical configurations.

In general relativity, the fields on a black hole horizon are obtained from those in the bulk by pullback and restriction. In quantum gravity, it would be natural to obtain them in the same manner. This is not fully realized in the quantum theory of isolated horizons in loop quantum gravity, in which a Chern-Simons phase space on the horizon is quantized separately from the bulk. I will outline an approach in which the quantum horizon degrees of freedom are simply components of the quantized bulk degrees of freedom. A condition is imposed on the quantum states to encode the existence of a horizon. I will present evidence that solutions to this condition have properties on the horizon that are remarkably similar to those of Chern-Simons theory. Instrumental in formulating the horizon condition are novel flux operators that use the Duflo isomorphism and seem to represent some type of quantum deformed SU(2). I will review their definition and summarize what I know about their properties.

It's usually assumed that youtube is just for kittens, babies, and music videos. However, youtube is also the highest-traffic site on the internet and it turns out it's actually a darn good place to teach people about physics! We'll start with the story and analysis of how my video series Minutephysics grew from a fun project to a youtube channel with 120,000 subscribers and 9 million views in just a few months, then discuss how media & technology (especially videos) can facilitate good (and bad) communication, and finally talk about how you can harness the power of the internet in your own physics outreach. And of course we'll watch a few cool videos along the way. As a primer, feel free to check out www.youtube.com/minutephysics

The experimental violation of Bell inequalities using spacelike separated measurements precludes the explanation of quantum correlations through causal influences propagating at subluminal speed. Yet, it is always possible, in principle, to explain such experimental violations through models based on hidden influences propagating at a finite speed v>c, provided v is large enough. Here, we show that for any finite speed v>c, such models predict correlations that can be exploited for faster-than-light communication. This superluminal communication does not require access to any hidden physical quantities, but only the manipulation of measurement devices at the level of our present-day description of quantum experiments. Hence, assuming the impossibility of using quantum non-locality for superluminal communication, we exclude any possible explanation of quantum correlations in term of finite-speed influences.

We consider the effect of an in-plane current on the magnetization dynamics of a quasi-two-dimensional spin-orbit coupled nanoscale itinerant ferromagnet. By solving the appropriate kinetic equation for an itinerant electron ferromagnet, we show that Rashba spin-orbit interaction provides transport currents with a switching action, as observed in a recent experiment (I. M. Miron et al., Nature 476, 189 (2011)). The dependence of the effective switching field on the magnitude and direction of an external magnetic field in our theory agrees well with experiment.

The partition function on the three-sphere of many supersymmetric Chern-Simons-matter theories reduces, by localization, to a matrix model. In this talk I will describe a new method to study these models in the M-theory limit, but at all orders in the 1/N expansion. The method is based on reformulating the matrix model as the partition function of a Fermi gas. This new approach leads to a completely elementary derivation of the N^{3/2} behavior for ABJM theory and other quiver Chern-Simons-matter theories. In addition, the full series of 1/N corrections to the original matrix integral can be simply determined by a next-to-leading calculation in the semiclassical expansion of the quantum gas.

We investigate the theoretical implications of scale without conformal invariance in quantum field theory. We argue that the RG flows of such theories correspond to recurrent behaviors, i.e. limit cycles or ergodicity. We discuss the implications for the a-theorem and show how dilatation generators do generate dilatations. Finally, we discuss possible well-behaved non-conformal scale-invariant examples.

The advent of large spectroscopic surveys of galaxies in the early 1980s has shown us that galaxies assemble in large scale structures. Recently, cosmic voids have received more attention through the availability wide and deep galaxy surveys. Voids have a simple phase space structure and thus are easier to model than cluster of galaxies. I will present two important applications of the precise analysis of voids in the context of constraining the equation of state of dark energy. First I will discuss how they could be used to have a much better determination of the expansion factor than using traditional methods, like Baryonic Acoustic Oscillations. Second, I will show that voids is maybe the only large-scale structure for which the dynamics can be finely modelled, notably through the use of the Monge-Ampere-Kantorovitch orbit reconstruction method. For the two above cases, I will present how we can mathematically define cosmic voids, the methods that have been developed to find them and some results based on N-body simulations for constraining the Dark Energy equation of state.

Many-body entanglement, the special quantum correlation that exists among a large number of quantum particles, underlies interesting topics in both condensed matter and quantum information theory. On the one hand, many-body entanglement is essential for the existence of topological order in condensed matter systems and understanding many-body entanglement provides a promising approach to understand in general what topological orders exist. On the other hand, many-body entanglement is responsible for the power of quantum computation and finding it in experimentally stable systems is the key to building large scale quantum computers. In this talk, I am going to discuss how our understanding of possible many-body entanglement patterns in real physical systems contributes to the development on both topics. In particular, I am going to show that based on simple many-body entanglement patterns, we are able to (1) completely classify topological orders in one-dimensional gapped systems, (2) systematically construct new topological phases in two and higher dimensional systems, and also (3) find an experimentally more stable scheme for measurement-based quantum computation. The perspective from many-body entanglement not only leads to new results in both condensed matter and quantum information theory, but also establishes tight connection between the two fields and gives rise to exciting new ideas.

One of the major obstacles in quantum information processing is to prevent a quantum bit from decoherence. One powerful approach to protect quantum coherence is dynamical decoupling. I will present some recent progress of diamond-based quantum information processing using dynamical decoupling. The other promising approach is to use topological quantum systems, which are intrinsically insensitive to local perturbations. I will discuss some ideas to create and probe topological quantum systems. Furthermore, I will propose a hybrid platform between topological and conventional quantum systems, which can combine the advantages from both.

I will discuss magnetic properties of superconductors, first in a model independent way and then by using holographic models. This approach has the advantage of highlighting the generic features of superconducting materials and, at the same time, the predictions of specific models. I will start with the Meissner effect and the vortices. Given the importance of the magnetic field dynamics in these phenomena, I will describe how to introduce a dynamical gauge field in holography. Then I will show that the holographic superconductor, like all known high-temperature superconductors, is Type II. Finally, I will discuss the case of hollow cylindrical superconductors threaded by an axial magnetic field. The physics of these systems is periodic with respect to the magnetic flux, with a period that depends on weather non-local quantum effects are suppressed or not. In the holographic model these effects are not suppressed when the radius of the cylinder is below the critical value at which the Hawking-Page phase transition takes place.

I will describe our recent work on a new topological phase of matter: topological Weyl semimetal. This phase arises in three-dimensional (3D) materials, which are close to a critical point between an ordinary and a topological insulator. Breaking time-reversal symmetry in such materials, for example by doping with sufficient amount of magnetic impurities, leads to the formation of a Weyl semimetal phase, with two (or more) 3D Dirac nodes, separated in momentum space. Such a topological Weyl semimetal possesses chiral edge states and a finite Hall conductivity, proportional to the momentum-space separation of the Dirac nodes, in the absence of any external magnetic field. Weyl semimetal demonstrates a qualitatively different type of topological protection: the protection is provided not by the bulk band gap, as in topological insulators, but by the separation of gapless 3D Dirac nodes in momentum space. I will describe a simple way to engineer such materials using superlattice heterostructures, made of thin films of topological insulators. References: arXiv:1110.1089; Phys. Rev. Lett. 107, 127205 (2011); Phys. Rev. B 83, 245428 (2011)."