Presentations are more important than papers: my first youtube video

July 27, 2015

During the past 5 years or so I have come to believe that presentations are actually more important than scientific papers. As a consequence, I have recently spent quite a lot of energy learning how to give better presentations. This is a truly fascinating and rewarding topic. While I find it is difficult, if not downright impossible, to master good public speaking, I’ve very much enjoyed trying to improve how I give my presentations.

Today I’d like to annouce the appearance of my first youtube video:

This is a recording of a talk I recently gave to graduate students here in Hannover. The objective of the talk was to share and channel advice I’ve received in the past years on how to give a good presentation. While I don’t claim to be especially good at giving good talks myself (the excruciating experience of watching myself on video for essentially the first time only serves to underline this!) I have learnt a great deal from other excellent speakers, and I hope that I can at least share a couple of the tips and tricks I’ve learnt.

Depending on the reception to this video it might signal a change in the way I will go about communicating our research. I’ve recently noticed that I am spending an increasing amount of time watching videos of talks at conferences, video tutorials, and miscellaneous other videos (cat videos, unboxings, etc. 🙂 ). It truly is a supremely powerful medium of communication, combining both visual and auditory modes of delivery, and, given that you can pause and skip, I have found it to often be superior to attending talks.

I am still passionate about open science, and open notebook science, and I am always contemplating better and more efficient ways to implement at least some core principles of openness. This is why this blog and my twitter account have become so neglected as of late: I’ve just found that github provides an amazingly useful, superior, and simple tool to achieve this. If you want to know what I’m doing on any given day then you can check out my activity there. (Basically all of my notes are now stored openly there.)

However, github is not the right tool for communicating and sharing ideas. Here I think video is superior, and youtube a natural platform. We’ll see.

I do hope you enjoy watching my video; any comments, suggestions, and criticisms are (actually) welcome!


An introduction to the continuous limit construction I

May 19, 2014

In this post I’d like to begin to explore what is meant by the continuum limit of a quantum lattice system. This post is meant to serve as the first in a series of intuitive overviews of the ideas involved in the open science project “continuous-limits-of-quantum-lattice-systems” hosted on github.

The continuous limit is a power tool in the condensed-matter theorist’s toolkit: by identifying the appropriate effective field theory modelling the low-energy large-scale physics of a complex quantum system one can bring the fully developed apparatus of (perturbative) field theory and the renormalisation group to bear on a problem, often delivering results unavailable via any other means.

Now I’m pretty sure I’m not alone in feeling confused by much of the available physical literature on this topic. Over the past decade I’ve tried to understand the process whereby a field theory is produced to describe a given quantum lattice system. However, up until recently, this has always seemed like a kind of mysterious black magic to me. I know it has to do with symmetries etc. etc.. But this didn’t really help me! I had so many questions. E.g., how exactly does the state of the effective field theory relate to that of the original lattice system? And, for that matter, how do you know what quantities are “fieldlike” and which don’t admit a field-theoretic representation? That is, what has most puzzled me is the quantitative side of things: ideally what I would like is some kind of map which associates, one to one, lattice quantities with field quantities in an operationally transparent way.

Thus I was very excited when I discovered that there is indeed such a map and, further, is naturally associated with the quantum de Finetti theorem. Here I’d like to explain the idea behind this construction using the quantum information theoretic language of exchangeable states.

Read the rest of this entry »


Returning to open science: continuous limits of quantum lattice systems

April 28, 2014

As I mentioned in my previous post, I have been working for some five years on trying to understand quantum field theory from a quantum-information perspective. This has finally come to a fruition of sorts: I’m pretty sure I have an operationally motivated way to build nontrivial nongaussian quantum field states using a variety of tensor network states.

The input to the procedure is any family of tensor network states (or, indeed, any family of states) whose correlation functions diverge in a controllable way as a function of a scale parameter. The procedure then produces a continuum limit with the corresponding quantum field data modelling the quantum fluctuations around the limit.

There are two main ideas behind the procedure: (1) it begins by extending the mean-field formalism of Hepp and Lieb (developed later by Verbeure and coworkers) to identify the emergent continuous large-scale degrees of freedom describing the classical bulk fluctuations (remarkably the continuous degrees of freedom are not prescribed beforehand) — this is a kind of generalised law of large numbers result; and then (2) by exploiting a generalised quantum central limit-theorem type argument the quantum fluctuations around the bulk are then identified and the emergent quantum field operators are subsequently identified. The applicability of this procedure is contingent on the family of input states satisfying certain criteria, which essentially boils down to the ability to tune the correlation length in a controlled way.

A nontrivial result is that several tensor network states naturally satisfy the criteria required by the continuum limit procedure: in particular, for the continuum limit of the matrix product state and projected entangled-pair state classes we recover their recently introduced continuous counterparts and for tree tensor network classes arising from Kadanoff block spin renormalisation and the multi-scale renormalisation ansatz class we obtain continuum descriptions generalising the recently introduced continuous MERA.

For me the most exciting discovery in all of this is that there are simply an enormous number of non-gaussian states which can serve as fixed points of Wilson’s RG and give rise to very reasonable renormalisable QFTs.

An open science experiment

I gave up on open science a while ago (see this post for details). However, I’ve always wanted to give it another try.

The open-source software (OSS) movement is often held up as a model for how open science should work and it occurred to me recently we could exploit a powerful tool used in OSS to facilitate scientific collaborations, namely, github. Thus today I’d like to announce a new github-based open-science project based on the aforementioned continuum limit construction: I’ve created a github repository for this project and uploaded the latex source of a paper I’ve been working on for some time describing this construction. It is my hope that this initial incomplete draft could serve as the basis for a collaborative project on understanding how to implement Wilsonian renormalisation for tensor network states. Read the rest of this entry »