Colloquia are Wednesdays at 4:00 p.m. and will be presented online via zoom. All meetings are password protected. If you are not a student/faculty member, pre-register with Veronica Lingo to attend a lecture.

August 19 — "Conformational change in matter and mind"

August 26 — "A Surprisingly Promising Approach to a Fundamental Theory of Physics"

September 2 — "Driven Quantum Materials"

  • Presenter: Andrea Cavalleri, Max Planck Hamburg
  • Host: Dan Dessau
  • Time: 9:00 A.M. MDT
  • URL: https://cuboulder.zoom.us/j/93814388408
  • Abstract: I will discuss how coherent electromagnetic radiation at infrared and TeraHertz frequencies can be used to drive collective excitations in complex solids. Important examples involve the nonlinear control of the crystal lattice, to induce atomic-structural deformations often not possible in equilibrium, and have been used by our group to induce interesting collective states at high temperatures, including magnetic order, ferroelectricity and non-equilibrium superconductivity. 

September 9 — "Topological photonics"

  • Presenter: Mohammed Hafezi, University of Maryland and JQI
  • Host: Ana Maria Rey 
  • Time: 9:00 A.M. MDT
  • URL: https://cuboulder.zoom.us/j/93785716475
  • Abstract: There are many intriguing physical phenomena that are associated with topological features -- global properties that are not discernible locally. The best-known examples are quantum Hall effects in electronic systems, where insensitivity to local properties manifests itself as conductance through edge states which are insensitive to defects and disorder. In the talk, we first discuss how similar physics can be explored with photons; specifically, how various topological model can be simulated in various photonics systems, from ring resonators to photonic crystals. We then discuss how the integration of strong optical nonlinearity can lead to intriguing quantum phenomena, such as a topological source of quantum light and chiral quantum optics. These results may enable the development of classical and quantum optical devices with built-in protection for next-generation optoelectronic and quantum technologies. In the end, we discuss an emerging field at the interface of quantum optics and correlated electron systems, with the goal of creating and manipulating many-body states of light-matter hybrids with new functionalities, such as high-Tc superconductors.  

September 16 — "Undecidability in Quantum Physics: Hilbert’s Second and Sixth Problems Meet"

  • Presenter: David Perez-Garcia, Universidad Complutense de Madrid
  • Host: Rahul Nandkishore
  • Time: 9:00 A.M. MDT
  • URL: https://cuboulder.zoom.us/j/95984357829
  • Abstract: The pioneering work of Goedel and Turing in the 30s showed that there exist problems in mathematics and computer science that cannot be solved. They are called undecidable. Since then, several problems in physics have been shown to be undecidable too. In this talk I will show that many interesting properties of a quantum many body system are indeed undecidable. This negative result has, however, a positive side. It predicts the existence of a new effect that we name "size-driven quantum phase transition”. I will present this effect, its characteristic features, as well as our recent ideas to try to observe it.

September 23 — "Fracton order: From quantum hard drive to foliated manifold"

  • Presenter: Xie Chen, Caltech
  • Host: Rahul Nandkishore
  • Time: 9:00 A.M. MDT
  • URL: https://cuboulder.zoom.us/j/93512943770
  • Abstract: One major open problem in quantum information theory is how to build a quantum hard drive, i.e. a quantum mechanical system that can store quantum information reliably for a very long time without active error correction. No completely satisfying solution to this problem has been found, but in the search for possibilities a whole new class of quantum many-body models have been discovered with exotic properties that have never been seen before. This new class of models, dubbed the “fracton” models, host point excitations that cannot move freely, have robust ground state degeneracy that increases with system size, and have unusually slow thermalization dynamics even in the absence of any disorder. In this talk, I will introduce some of the most important fracton models and present a systematic framework for characterizing their universal properties. By realizing that the model properties depend on the foliation structure of the underlying manifold, we are able to reveal the hidden equivalence relation between many seemingly different models and identify the direction of search for new nontrivial features.

September 30 — "Drawing the line between kinematics and dynamics in special relativity and in quantum mechanics"*

  • Presenter: Michel Janssen, University of Minnesota
  • Host: Allan Franklin
  • Time: 9:00 A.M. MDT
  • URL: https://cuboulder.zoom.us/j/99462004738
  • Abstract: The mathematical equivalence of matrix and wave mechanics papers over an important difference in what Heisenberg and Schrödinger took to be their key insights. For Schrödinger it was that atomic physics calls for an underlying wave theory just as geometric optics had in the 19th century. For Heisenberg it was that atomic physics calls for a new general framework for doing physics just as electrodynamics had in the early-20th century. Following Heisenberg rather than Schrödinger, I present a few case studies in special relativity and quantum mechanics in which problems that seem to call for dynamical solutions were solved instead by an appeal to the new kinematical frameworks introduced by these new theories. I use these case studies to bring out some parallels between the standard take on special relativity and a more controversial information-theoretic take on quantum mechanics and then use the former to argue for the latter.    
    *Based on joint work with Michael Janas and Mike Cuffaro

October 7

October 14

October 21

October 28

November 4

November 11

November 18

November 25 — Fall Break, No Colloquium

December 2

December 9

For more information about colloquia this semester, contact: Rahul Nandkishore.