Talks

Does quantum mechanics really tell us that particles, molecules, and maybe even cats, can be in two places at once? Does it force us to deny a reality that is independent of our observation? How can scientists disagree about what quantum mechanics means and yet still agree that it is right? Joseph Emerson, co-writer of the award-winning documentary “The Quantum Tamers”, will address these questions and then describe, drawing on preview clips from the documentary, how the weirdness of the quantum world is now being harnessed for a ‘quantum information revolution’ that includes quantum teleportation, super-secure quantum communication, and the exponential power of quantum computation.

In recent years the characterization of many-body ground states via the entanglement of their wave-function has attracted a lot of attention. One useful measure of entanglement is provided by the entanglement entropy S. The interest in this quantity has been sparked, in part, by the result that at one dimensional quantum critical points (QCPs) S scales logarithmically with the subsystem size with a universal coefficient related to the central charge of the conformal field theory describing the QCP. On the other hand, in spatial dimension d > 1 the leading contribution to the entanglement entropy scales as the area of the boundary of the subsystem. The coefficient of this "area law" is non-universal. However, in the neighbourhood of a QCP, S is believed to possess subleading universal corrections. In this talk, I will present the first field-theoretic study of entanglement entropy in dimension d > 1 at a stable interacting QCP - the quantum O(N) model. Our results confirm the presence of universal corrections to the entanglement entropy and exhibit a number of surprises such as different epsilon -> 0 limits of the Wilson-Fisher and Gaussian fixed points, violation of large N counting and subtle dependence on replica index.

Self-assembly refers to any thermodynamic process in which a bunch of particles (molecules, biomolecules, polymers, colloids) come together in solution to form an ordered structure. In living things it is a widely used and robust manufacturing tool: DNA, RNA and proteins spontaneously form three dimensional structures, and supramolecular structures emerge from protein aggregates with staggering degrees of ordering and specificity. By contrast, most synthetic systems in soft condensed matter do not assemble robustly. In this talk I will discuss experiments on simple systems that allow us to probe the physics and thermodynamics of self-assembly. We use systems consisting of small numbers (N ⇐ 12) of confined spherical colloidal particles to understand what physical parameters (interactions) determine how a system will assemble. We find that the probability of self-assembling a particular configuration can be understood in terms of the geometry of sphere packings. The geometrical model gives some insights into how phase transitions emerge as N approaches the bulk limit. At the same time, it yields some general insights into the design principles for robust self-assembly.

While many expect that the best place for a theorist is in an environment where they are surrounded by fellow theorists (e.g. Perimeter Institute), there are significant advantages for the theorist and scientific progress to spend time in a data-oriented environment. Starting in September, I will be a NASA Postdoctoral Program Fellow at Goddard Space Flight Center in Greenbelt, Maryland. In this talk, I will describe the way in which a theorist can contribute to an experimental environment in the field of cosmology. More specifically, I will discuss modified gravity and setting observational priorities for the Joint Dark Energy Mission.

Primordial non-Gaussianity has been traditionaly constrained using three-point function of the cosmic microwave background. Two years ago, however, Dalal et al have shown that non-Gaussianity of the local type induces a scale dependent bias for biased tracers of the underlying dark matter structure. This allows constraining of the primordial non-Gaussianity from measurements of large-scale structure provided by redshift surveys. I will discuss the technique, its theoretical aspects, it surprising resilience towards systematics and current results from the real data. I will also show some preliminary new results: extension to the two field inflationary models and the analogue of the Dalal effect in the Lyman alpha forest.