Interacting Quantum Systems Driven Out of Equilibrium - day 2

Continuing into day 2 of our workshop:

• Bryce Gadway of the University of Illinois spoke about using cold atoms in an optical lattice to simulate topological and disordered systems.  His group has implemented an optical lattice constructed not by interference of retroreflected lasers, but by interference between a laser and counterpropagating beams frequency shifted by precise, controlled amounts.  As a non-atomic physics person I'm a bit fuzzy on the details, but the point is that this allows his group to put in place precise control of the on-site potential of each lattice site and to dial in designer phase shifts associated with tunneling between adjacent sites, on demand.  This means it is possible to study transport problems (like Bloch oscillations) as well as introducing designer, time-varying, site-specific disorder if desired.  
• I spoke about my group's work on heating and dissipation in atomic- and molecular-scale junctions driven out of equilibrium (and into a steady state) by electronic bias.  I framed the discussion in terms of how hard it is to obtain truly local information about vibrational and electronic distributions in such driven systems.  On the vibration side, if you're interested, I suggest looking here, here, and here, with a recent related result here.  On the electronic front, I talked about published (here and here) and some unpublished data looking at electronic shot noise at high biases in atomic-scale metal junctions.  
• Eugene Demler from Harvard (my grad school classmate) gave a nice talk that addressed nonequilibrium aspects of both cold atoms and electrons.   For example, he and collaborators have developed some theoretical machinery for looking at a cold atom version of the orthogonality catastrophe - what happens if you suddenly "turn on" interactions between a cold Fermi gas and a single impurity, and watch the dynamics.  These same theoretical techniques can be applied to solid state systems as well.  (This is just a subset of what was presented.)
• Ryo Shimano from Tokyo University gave a very pretty talk about optical manipulation and driving of the Higgs mode inside superconductors.  You can hit a superconductor with THz radiation as a pump, and then probe at some delay with additional THz radiation.  If the pump is at the right frequency (energy half the superconducting gap, in the s-wave case), you can excite collective sloshing of the condensate (see here and scroll down to the first example).  As you might imagine, things get more rich and complicated with more exotic superconductors (multiband or unconventional).
• Emil Yuzbashyan from Rutgers presented a look at the fundamental issues involved in non-thermal steady states of ensembles of quantum particles at long times after a quench (a sudden change in some parameter).  As I wrote in the first-day discussion, the interesting question here is when does the system evolve seemingly coherently (i.e., the particles slosh around in recurring patterns, just as a Newton's cradle ticks back and forth), and when does the system instead tend toward a long-time state that looks like a randomized, thermalized condition?   To see how this relates to classical mechanics, see these articles (here and here) that I need to find time to read.  
• Lastly, my colleague Matt Foster from Rice spoke about quenched BCS superfluids, topology, spectral probes, and gapless (topological) superconductivity under intense THz pumping.  This was a neat pedagogical talk about this work.  It touches some of the same issues as the Shimano talk above.  One aspect that I found interesting to consider:  You can have a system where a quench drives some collective oscillations, and those collective oscillations act as a Floquet perturbation, changing the effective band structure and giving rise to nonlinearities that continue the oscillations.  Wild stuff - here are the slides.  

We then had a lunch that segued smoothly into a relaxed poster session for students and postdocs.  Overall, it was a great workshop and a good chance to get people from diverse areas of CM and AMO physics together over common interests in nonequilibrium response of quantum systems.  Thanks to everyone who was able to come, to our staff organizer who made everything actually come together, and of course to our sponsors (NSF DMR, ICAM-I2CAM, the Gordon and Betty Moore Foundation, Lakeshore, Advantest, and Coherent)!