Physics Colloquia

Fall 2017 (usually Fridays 2:00 PM in SE 319)

Date Speaker Title
Sept 22 Yidun Wan Experimentally Probing Topological Order and Its Breakdown via Modular Matrices
Oct 6 Yongge Ma Black Hole Entropy in Loop Quantum Gravity
Oct 13 Constantine Deliyannis What Can the Light Element Tracers (Li, Be, B) Tell Us About Stellar Interiors (and Cosmology)?


Colloquium Abstracts

Experimentally Probing Topological Order and Its Breakdown via Modular Matrices

Yidun Wan  (Fudan University)

The modern conception of phases of matter has undergone tremendous developments since the first observation of topologically ordered states in fractional quantum Hall systems in the 1980s. In this paper, we explore the question: How much detail of the physics of topological orders can in principle be observed using state of the art technologies? We find that using surprisingly little data, namely the toric code Hamiltonian in the presence of generic disorders and detuning from its exactly solvable point, the modular matrices -- characterizing anyonic statistics that are some of the most fundamental finger prints of topological orders -- can be reconstructed with very good accuracy solely by experimental means. This is a first experimental realization of these fundamental signatures of a topological order, a test of their robustness against perturbations, and a proof of principle -- that current technologies have attained the precision to identify phases of matter and, as such, probe an extended region of phase space around the soluble point before its breakdown. Given the special role of anyonic statistics in quantum computation, our work promises myriad applications both in probing and realistically harnessing these exotic phases of matter.


Black Hole Entropy in Loop Quantum Gravity

Yongge Ma (Beijing Normal University)

The statistical origin of black hole entropy is the great mystery which has to be explored by any quantum gravity theory. The quasi-local notion of isolated horizon was proposed in order to formulate the horizon of a black hole by phase space functions which can be quantized. The isolated horizon can be thought as an inner boundary of a four-dimensional asymptotically flat spacetime region. Because of the isolated horizon condition, it turns out that, with different gauge choices, the boundary degrees of freedom can be described either by a SU(2) Chern-Simons theory with punctures or by a SO(1,1) BF theory with sources. In both cases the entropy of the isolated horizon can be calculated in the framework of loop quantum gravity. The leading order contributions of the microscopical degrees of freedom are both proportional to the area of the horizon, while the sub-leading quantum corrections are different for different approaches. The BF theory description of the boundary degrees of freedom is applicable in arbitrary dimensions of spacetime.


What Can the Light Element Tracers (Li, Be, B) Tell Us About Stellar Interiors (and Cosmology)?

Constantine Deliyannis (Indiana University)

Helioseismology strongly supports the Standard Solar Model, yet the current surface solar lithium (Li) abundance is about a factor of 50 lower than that predicted by this model, betraying a secret past life of our Sun. In fact, nearly all stars exhibit lower surface Li abundances than predicted by standard theory. I will discuss how these Li abundances, together with surface stellar Be and B abundances inform us about physical processes occurring inside stars that are not included in standard theory, such as rotationally-induced mixing, diffusion, and mass loss. I will point out implications for testing Big Bang Theory when trying to determine the primordial Li abundance from Li observations in the oldest stars.