## Friday, April 22, 2016

### The Pittsburgh Quantum Institute: PQI2016 - Quantum Challenges

For the last 2.5 days I've been at the PQI2016:  Quantum Challenges symposium.  It's been a very fun meeting, bringing together talks spanning physical chemistry, 2d materials, semiconductor and oxide structures, magnetic systems, plasmonics, cold atoms, and quantum information.  Since the talks are all going to end up streamable online from the PQI website, I'll highlight just a couple of things that I learned rather than trying to summarize everything.

• If you can make a material such that the dielectric permittivity $\epsilon \equiv \kappa \epsilon_{0}$ is zero over some frequency range, you end up with a very odd situation.   The phase velocity of EM waves at that frequency would go to infinity, and the in-medium wavelength at that frequency would therefore become infinite.  Everything in that medium (at that frequency) would be in the near-field of everything else.  See here for a paper about what this means for transmission of EM waves through such a region, and here for a review.
• Screening of charge and therefore carrier-carrier electrostatic interactions in 2d materials like transition metal dichalcogenides varies in a complicated way with distance.  At short range,  screening is pretty effective (logarithmic with distance, basically the result you'd get if you worried about the interaction potential from an infinitely long charged rod), and at longer distances the field lines leak out into empty space, so the potential falls like $1/\epsilon_{0}r$.  This has a big effect on the binding of electrons and holes into excitons in these materials.
• There are a bunch of people working on unconventional transistor designs, including devices based on band-to-band tunneling between band-offset 2d materials.
• In a discussion about growth and shapes of magnetic domains in a particular system, I learned about the Wulff construction, and this great paper by Conyers Herring on why crystal take the shapes that they do.
• After a public talk by Michel Devoret, I think I finally have some sense of the fundamental differences between the Yale group's approach to quantum computing and the John Martinis/Google group's approach.  This deserves a longer post later.
• Oxide interfaces continue to show interesting and surprising properties - again, I hope to say more later.
• On a more science-outreach note, I learned about an app called Periscope (basically part of twitter) that allows people to do video broadcasting from their phones.  Hat tip to Julia Majors (aka Feynwoman) who pointed this out to me and that it's becoming a platform for a lot of science education work.
I'll update this post later with links to the talks when those become available.