Scientific Series

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.

This talk focuses on an application of a WKB technique that is a generalization of the Born-Oppenheimer approximation to the Schwinger model of angular momentum. This work makes it possible to express the asymptotic limits of higher 3nj symbols in terms of the asymptotic limits of lower 3nj symbols, when only a subset of quantum numbers are taken to be large. After an introduction of the the technique, we show that the 15j symbol has different asymptotic limits in terms of a combination of several Ponzano Regge actions when different subsets of quantum numbers are taken large.

In this talk, I will present a first principle construction of a holographic dual for gauged matrix models. The dual theory is a closed string field theory coupled with an emergent two-form gauge field defined in one higher dimensional space. The bulk space with an extra dimension emerges as a well defined classical background only when the two-form gauge field is in the deconfinement phase. Based on this, it is shown that critical phases that admit holographic descriptions form a novel universality class with a non-trivial quantum order.

In this talk, I will discuss constraints on dark matter (DM) from collider physics, neutron stars and DM halo shapes. Monojet plus missing energy searches at hardron colliders limit DM interactions with quarks and gluons, which provide a complementary probes of DM to direct detection. Stars can capture ambient DM particles. The captured scalar DM particles may form a Bose-Einstein condensate, leading to a black hole at the center of the host neutron star that eventually causes its destruction. The observation of old neutron stars exclude a wide range of the DM-neutron scattering cross-section for the scalar asymmetric DM. In various well-motivated models, DM has self-interactions which may leave imprints on galactic dynamics. I will discuss an upper bound on DM self-interaction cross section derived from elliptical DM halo shapes which is about two orders of magnitude stronger than the result from the Bullet Cluster.

The time variation of physical constants has been much discussed in the literature, motivated by claims of fine structure constant variations together with several theoretical ideas. Although it is well understood (by most, but not all!) cosmologists that one must consider only dimensionless constants, most discussions of the strength of gravity involve "G", which is of course dimensional. I discuss some applications of variations of "G" on cosmological observables, stressing the need to stay dimensionless. "Constants" might also vary in space. An idea which is perhaps less crazy is that cosmological parameters might vary across the observable Universe. I show how this leads to dipole modulation, which corresponds to a correlation between neighbouring multipoles in maps of the cosmic microwave sky. Searches for such signals could lead to constraints on the variation of the cosmological model on the largest accessible scales.

Sound waves with long-distance propagation are both a consequence of hydrodynamics, and a danger to hydrodynamics' very existence, as they violate the assumption of local equilibration. In the talk, I will discuss what the thermally excited sound and shear waves do to viscosity. In 2+1 dimensions, the shear viscosity and the diffusion constant cease being independent transport coefficients. In 3+1 dimensions, the fluctuations render the second-order hydrodynamics invalid.

Constant mean curvature (uniform K) hypersurfaces extend to future null infinity in asymptotically flat spacetimes. With conformal compactification, the entire hypersurface can be covered by a finite spatial grid, eliminating any need an "outgoing wave" boundary condition or for extrapolation to find gravitational wave amplitudes. I will discuss the asymptotic behavior near future null infinity, how this can be simplified by suitable gauge conditions, and how this determines the physical Bondi energy and momentum of the system. Numerical results for how Bowen-York parameters in the conformally flat initial value problem are related to the physical energy and momentum in systems with single and binary black holes will be presented.

We construct a class of entangled supersymmetric states which is used as a non-local resource in the CHSH game. This class of super entangled states is more non-local then maximally entangled states if the supersymmetric degrees of freedom are accessible to measurement. Consequently, we show that the winning probability for the CHSH game is greater than cos2(pi/8) corresponding to an expected value greater than Tsirelson's bound.

Low-temperature phases of strongly-interacting quantum many-body systems can exhibit a range of exotic quantum phenomena, from superconductivity to fractionalized particles. One exciting prospect is that the ground or low-temperature thermal state of an engineered quantum system can function as a quantum computer. The output of the computation can be viewed as a response, or 'susceptibility', to an applied input (say in the form of a magnetic field). For this idea to be sensible, the usefulness of a ground or low-temperature thermal state for quantum computation cannot be critically dependent on the details of the system's Hamiltonian; if so, engineering such systems would be difficult or even impossible. A much more powerful result would be the existence of a robust ordered phase which is characterised by its ability to perform quantum computation. I'll discuss some recent results on the existence of such a quantum computational phase of matter. I'll outline some positive results on a phase of a toy model that allows for quantum computation, including a recent result that provides sufficient conditions for fault-tolerance. I'll also introduce a more realistic model of antiferromagnetic spins, and demonstrate the existence of a quantum computational phase in a two-dimensional system. Together, these results reveal that the characterisation of quantum computational matter has a rich and complex structure, with connections to renormalisation and recently-proposed concepts of 'symmetry-protected topological order'.

Classical constraints come in various forms: first and second class, irreducible and reducible, regular and irregular, all of which will be illustrated. They can lead to severe complications when classical constraints are quantized. An additional complication involves whether one should quantize first and reduce second or vice versa, which may conflict with the axiom that canonical quantization requires Cartesian coordinates. Most constraint quantization procedures (e.g., Dirac, BRST, Faddeev) run into difficulties with some of these issues and may lead to erroneous results. The Projection Operator Method involves no gauge fixing, no auxiliary variables of any kind, and can treat simultaneously any and all kinds of constraints. It also admits a phase space path integral formulation with similar features.

I will discuss three ways in which (the string landscape and) eternal inflation is fun: (1) because it motivates revisiting some beautiful, classic calculations; (2) because its global description requires asking novel questions with possible broad ramifications; and (3) because it leads to experimental predictions.