Wednesday, April 29, 2015

Probing non-equilibrium dynamics in a quantum many-body system

This week at UQ there was a fascinating Quantum Science Seminar by Jorg Schmiedmayer, describing some beautiful ultra cold atom experiments.  Much of the talk is nicely discussed in a book chapter Does an isolated quantum system relax? [Answer is yes].
Some of the most recent results are in a Science paper, Experimental observation of a generalised Gibbs ensemble that appeared this month.

The experiments involve a one-dimensional Bose gas that can be described as a Luttinger liquid. This means that many properties, even non-equilibrium ones, can be calculated analytically and compared to experiment. This is a theorists dream!

Here are a few things that stood out.

Relaxation from a non-equilibrium state to the thermal equilibrium state occurs on several different time scales. First there is rapid relaxation to a "quasi-steady state" described by a Generalised Gibbs ensemble [This idea goes back to Jaynes] that involves an effective temperature [that one can even theoretically calculate in terms of the bose gas interaction strength].

There is a characteristic length scale associated with the relaxation.

One can even directly measure higher order correlation functions (4th, 6th and 10th order!), as seen below. Furthermore, in a Luttinger liquid [a non-interacting boson field theory] these should factorise in terms of the 2nd order correlation function [reflecting Wick's theorem]. One can even test this experimentally.

One can also simulate the sine Gordon theory and vary the coupling constant and so move through the associated phase transition.

Tuesday, April 28, 2015

Sabbaticals are undervalued

Take one if you can.

A great privilege and opportunity of having a tenured faculty position is the possibility at most institutions of taking a sabbatical. Roughly one year away for every six years of service or one semester away every seventh semester.

I am concerned that increasingly many faculty don't take sabbaticals and that some institutions or departments (at least in Australia) discourage them. I think they are particularly important for Australians because of our geographic isolation. [For some strange reason at UQ it is called a Special Studies Program].

There are many obstacles to taking sabbaticals: finance, family commitments, keeping a group running, inertia, .... For some US faculty they also have to come up with some of the salary....

I have actually only ever taken one sabbatical (Oxford in 2004). However, for the past 12 years I have had few teaching and admin. responsibilities and have had freedom to travel. I am actually in the process of applying for one for the second half of 2016. As part of the process I had to recycle the report I wrote at the end of the last one about my activities and how I and the university benefited. I recalled that it was a productive time that lead to new ideas and collaborations. But, only reading the report did I recall just how great the benefits were. Much of what happened in the following five years or so would not have been possible without things that happened on the sabbatical.

So take one! Encourage your colleagues to as well! I think they are especially important for people weighed down with heavy teaching and/or admin responsibilities.

Are sabbaticals becoming less common? Why? Have you benefitted from one or more?

Saturday, April 25, 2015

Don't confuse necessary and sufficient conditions

In Carl Caves recent UQ Quantum Science Seminar "Quantum metrology meets Quantum Information Science" as an aside he also made an important side point.

People often erroneously assume that the converse of a statement is true (i.e.  A implies B means that B implies A). This came up because a few referees had said that some of the results he presented in the seminar were "obvious". Roughly speaking, this concerns the issue of trying to determine what input quantum state to an interferometer will produce a "physical" output state. He found that the input state had to be "physical" (by some well-defined technical criteria). Showing this is non-trivial. However, it is obvious a physical input state is sufficient to produce a physical output state. But, that does not mean it is necessary. Showing this turned out to be quite non-trivial.

I can immediately think of two other cases where scientists made similar errors of conflating necessary and sufficient conditions.

The first case is the existence of quasi-crystals. It was well known that a periodic array of atoms is sufficient to produce sharp diffraction peaks. However, many people erroneously assumed that this was also necessary. I emphasise this point when I teach undergraduates about quasi-crystals.

The second concerns Angle-dependent MagnetoResistance Oscillations in quasi-two-dimensional metals. Not long after their experimental discovery AMRO was explained in terms of a coherent interlayer transport and a three-dimensional Fermi surface. It was subsequently more or less assumed that observing AMRO was evidence for a three-dimensional Fermi surface.  However, in 1998, Perez Moses and I showed that a 3D Fermi surface was not necessary for the existence of AMRO.

Friday, April 24, 2015

Is quantum entanglement really needed?

On tuesday at UQ Carl Caves gave a Quantum Science Seminar "Quantum metrology meets Quantum Information Science".

One side point he made was that just because quantum entanglement is glamorous and beloved by luxury journals does not mean that you actually always need it to optimise any and every task. A specific example is in this paper which states:
The Heisenberg limit is thus achieved without any entanglement between the arms of the interferometer. In fact, Jiang, Lang, and Caves [4] showed that the state ψinoptis the only nonclassical product state, i.e., not a coherent state, that produces no modal entanglement after a beam splitter. These results indicate that, as in Ref. [18], modal entanglement is not a crucial resource for quantum-enhanced interferometry.

Wednesday, April 22, 2015

A basic but important research skill, 6: skepticism



In particular, students should not just believe something because

- it is an exciting idea.

Basic but important questions to ask are:

What is the evidence? How reliable is the evidence?
Is there an alternative explanation, particularly a simpler one?



Tuesday, April 21, 2015

Calibrating a ruler for hydrogen bond lengths

I have just finished a paper with Bijyalaxmi Athokpam and Sai  Ramesh,
Isotopic fractionation in proteins as a measure of hydrogen bond length

If a deuterated molecule containing strong intramolecular hydrogen bonds is placed in a hydrogenated solvent it may preferentially exchange deuterium for hydrogen. This preference is due to the difference between the vibrational zero-point energy for hydrogen and deuterium.  It is found that the associated fractionation factor $\Phi$  is correlated with the strength of the intramolecular hydrogen bonds. This correlation has been used to determine the length of the H-bonds (donor-acceptor separation) in a diverse range of enzymes and has been argued to support the existence of short low-barrier H-bonds.

Starting with a potential energy surface based on a simple diabatic state model for H-bonds we calculate $\Phi$ as a function of the proton donor-acceptor distance $R$.  For numerical results, we use a parameterization of the model for symmetric O-H.... O bonds.  We consider the relative contributions of the O-H stretch vibration, O-H bend vibrations (both in plane and out of plane), tunnelling splitting effects at finite temperature, and the secondary geometric isotope effect. We
compare our total $\Phi$ as a function of $R$ with NMR experimental results for enzymes, and in particular with an empirical parametrisation $\Phi(R)$, used previously to determine bond lengths.

I welcome any comments or suggestions.

Monday, April 20, 2015

Declining universities

Universities (and their problems) are certainly increasingly in the news. Here are a few things I read recently and recommend. Unfortunately, a lot of it is discouraging.

The Economist ran a cover story The World is Going to University (but is it worth it?)  and special report. It particularly documents the "massification" that is going on and the associated problems. Did you know China has hired almost 100,000 new faculty in the past few decades!

UQ economist John Quiggin wrote an excellent piece Rank delusions in the (USA) Chronicle of Higher Education.
[If it is behind a pay-wall you can read it on his blog].
Basically annual university rankings are a pointless exercise that just tell us what we already know. They are actually basically the same as 100 years ago! In contrast, the top 50 companies on the Dow Jones index are completely different.

The eminent British literary critic Terry Eagleton has a biting piece The Slow Death of the University (also in the Chronicle). In the midst of the humorous and engaging rant about the state of British universities, a major and profound point is

[The British government should] also seek to restore the honorable lineage of the university as one of the few arenas in modern society (another is the arts) in which prevailing ideologies can be submitted to some rigorous scrutiny. What if the value of the humanities lies not in the way they conform to such dominant notions, but in the fact that they don’t?
Aside: I really like Eagleton as a public intellectual. On my personal blog about theology, I have written several posts about him.

Ireland seems to be abandoning any basic research. Everything must have clear commercial benefits.

Tonight the Australian TV show Four Corners ABC [publicly funded channel] is running a story, Degrees of Deception about universities lowering academic standards in order to enrol and graduate mediocre international students who pay hefty tuitions. There is a related article in the Australian today (behind a paywall unless you Google "Education's double bind").

Thursday, April 16, 2015

Teaching enhances research enhances teaching



Wednesday, April 15, 2015

You really should turn off your email occasionally

I should too.

I don't have a phone.
But, why spoil a good holiday?
This was incredibly efficient.

Most things can wait.



Monday, April 13, 2015

Are American universities Crystal Cathedrals?

The documentary The Ivory Tower is worth watching.


The hype and limited potential of MOOC’s is discussed.


This dramatic change has been forced by financial mismanagement.



Tuesday, April 7, 2015

Superfluid helium on prime time TV


Saturday, April 4, 2015

Effective tutorials, II.






I welcome other ideas.

Wednesday, April 1, 2015

What does it mean to "observe" a Fermi surface?


People make silly unjustified claims to get published.

At first I was excited when I saw the Nature Communications paper
Quasiparticles and Fermi liquid behaviour in an organic metal 


The abstract of the paper states


It is rather noisy!


What do you think?






I am gobsmacked because in 1996 a book was published

