Log · Research

Conference Log: Bad Honnef

I’m just back from a 4-day conference in Bad Honnef, just south of Bonn. It’s a beautiful place! My last workshop log went down fairly well, at least within the research community, so I figured I’d do another as a summary of what was discussed at the conference. Despite the length of this post, there’s still no space to go into detail, so I’ve included lots of links to the relevant people/papers for anyone interested.

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The official conference photo, taken by Jutta Lang.

Day One

The day kicked off with a 90-minute talk by Eugene Demler on coherent dynamics in quantum matter. Eugene told us about a way of experimentally measuring certain properties known as correlation functions that gave me a few interesting ideas relating to some previous work I did, as well as some examples of systems that exhibit interesting time dependences.

He was followed by Thomas Gasenzer who talked about nonthermal fixed points, a way of understanding the physics that dominates the behaviour of materials up until long times when the true thermal fixed points takes over. Alessandro Silva was up next, talking about dynamical phase transitions – these are when the behaviour of a material sharply changes over time, and deserve a blog post of their own one day. I think the model that Silva presented is one that my own techniques can also be used to study, but not one we’ve looked at yet – maybe something to think about for the future.

After lunch, we had my clear favourite talk of the meeting as Anushya Chandran talked about how to model the coupling between a many-body localised system to a thermal system and whether MBL can survive. In her early model, it can’t – but will this result survive extending the model to more realistic situations? [Edit 04/06/17: This work is now available in pre-print form here.] Also: shoutout to Anushya’s postdoc Phil Crowley – Phil and I had a lot of detailed and really useful chats about some techniques I’m currently working on.

Ehud Altman came next. He talked not about the strong disorder RG from which I mostly know of him, but more about the dynamics of thermalisation and the paradox of how classical hydrodynamics can emerge at long times from a quantum model in which entanglement (a very quantum thing) seems to dominate the behaviour.

Next, each of the non-speaking participants gave a 1-minute flash presentation of our posters – there were a lot of us so this took well over an hour. This is a situation where having your name at the very end of the alphabet doesn’t do you any favours – the audience were all a bit asleep by the time I took the stage…

Day Two

The day kicked off with my surprise second favourite talk of the meeting as Anatoli Polkovnikov gave a very lucid introduction to periodically driven quantum systems – think taking a quantum material and shaking it, then asking what this does to its properties. I’ve never worked on these systems (yet!) but I’d really like to. Anatoli mostly focused on the high-frequency expansion with a few thoughts towards MBL and thermalisation.

This is the point where already I began to stop taking detailed notes on every talk as some started to veer a little far from my own field. We had talks from Takashi Oka, Tomaž Prosen and Michael Sentef which were all a bit out of my comfort zone, finishing with a talk from Marcus Heyl at the end of the day with more on dynamical quantum phase transitions and ways to probe them.

The day ended with possibly the longest and certainly the warmest (!) poster session I’ve ever attended. My work was largely equilibrium physics, so at a non-equilibrium conference it didn’t get a huge amount of attention – to be honest, after a few hours of the sweltering heat, I retreated back to my air-conditioned room for a while before dinner and did a little work. Conferences are fun, but sometimes they can be a bit exhausting and it’s good to find a place to hide and recharge for a little while.

Day Three

The morning after a conference dinner that included free drinks, the slightly bleary-eyed audience rocked up to the first lecture of the day to hear Sebastien Diehl talk about driven open quantum systems. I took a lot of notes on this talk – it’s a research area that’s still very new to me and I still have a lot to learn.

There was then another burst of specialist talks that I endeavoured to follow as best I could, to varying degrees of success. Diehl was followed by Aditi Mitra, talking about quenches in fermionic systems, then Emanuele Dalla Torre talking about driven spin chains – he made some interesting points I hadn’t really considered about how driven systems (new to me) differ from equilibrium systems (my regular fare).

After lunch came Philipp Werner with an enlightening discussion of nonequilibrium dynamical mean field theory, a technique I’d heard of but never used. This was followed by Emanuel Gull, who talked more on the nuts and bolts of quantum Monte Carlo techniques used to numerically compute time-evolution. I’ve never been a huge fan of QMC, but the more I learn about it, the more I appreciate how powerful it is.

At the end of the day came Dieter Jaksch, talking about how driving a quantum system can stabilise order, and how (counter-intuitively) driving a system can reduce fluctuations in a similar way as decreasing the temperature, something that could come in very useful for engineering practical quantum technologies.

Day Four

The day started with Stefan Kehrein, the man who literally wrote the book on flow equation techniques, talking about flow equation techniques (ofc) for calculating quantum entanglement entropy. This is something I’ve played with a little, but ran up against some fundamental problems. Kehrein is currently developing a way that avoids the issues I encountered, but it’s unclear whether it can be applied to the really interesting situations yet – hopefully though!

Corinna Kollath came next, discussing interesting topological effects that can be cooked up in ultracold atomic systems using artificial gauge fields, the connection between topology and dissipation and a brief talk about aging dynamics that was quite useful to me, as I’d been attempting a similar analysis of my own data during one of the previous talks.

The penultimate talk was given by my boss, Marco Schiró, discussing drive and dissipation in quantum light-matter systems, taking in some work done in collaboration with the man who taught me quantum field theory, as well as some really cool newer investigations being done with his PhD student Orazio Scarlatella, who’s doing some fascinating work with a non-equilibrium version of the Bose-Hubbard model. The very final talk was given by Achim Rosch about systems with approximate conservation laws, how and when those laws apply and how to calculate things with them – quite important in the context of a lot of modern condensed matter research.

And with that, the meeting was over! Honorary mentions to Anne-Maria Visuri, Kush Saha, Francesco Peronaci, Arya Dhar, Mateo Puviani, Matias Mikkelsen, Philipp Weiss, Kathrin Luksch, Anastasia Dietrich, Steven Mathey and Marc Alexander for the pleasure of their company and conversation at various points during the week, and thank you to conference organisers Martin Eckstein and Michael Knap for putting together the meeting. Apologies to all those I didn’t manage to say goodbye to – I had to leave pretty sharply at the end to catch the first of my 5 trains back to Paris!

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