David B. Cassidy: trapping positronium

David Cassidy

We talk to Professor David B. Cassidy from the Atomic, Molecular, Optical and Positron Physics group at University College London about his work with positronium – an exotic atom made of matter and antimatter – and more.

Q: Which research projects are you and your group currently working on?

My group works on positronium physics.  Positronium (which has the chemical symbol Ps) is a hydrogen-like atom composed of a positron bound to an electron.  Being made of a particle and an antiparticle, this is not a stable arrangement; the positron and electron tend to annihilate each other.  The ground state atoms live either for 142 ns, or in a different spin configuration for only 0.125 ns, before this happens.  Our work has an emphasis on the production of excited states, partly because these are less prone to annihilation.  Using a positron trap we generate a pulsed positronium gas which we probe with lasers in order to create and study excited Ps states.  This includes highly excited Rydberg states, which can live for quite a long time.  Ultimately we want to be able to manipulate these states, and eventually capture them into traps so we can perform spectroscopic measurements (among other things).

Q: What motivated you to pursue this field of research?

I have always been involved in positron physics since doing my PhD at UCL in the late 1990s.  After various different postdoctoral positions in the US I started working with Allen Mills who had bought a positron trap from a company called First Point Scientific (now out of business unfortunately).  The trap was great because, after a few years getting it to work, it allowed us to do new experiments on high density positronium where we could look at Ps-Ps scattering and Ps molecule formation.  It also made it possible to do experiments where we could probe Ps with lasers.  This had not been done very much at the time, and the new trap technology made it a lot easier.  We found that there were lots of fairly easy experiments we could do (the low hanging fruit was everywhere).  These were nevertheless interesting experiments and they laid the groundwork for many other things.  When I moved back to the UK in 2013 it was obvious that we hadn’t even scratched the surface of what could be done.

Thin meso-structured film imaged by SEM

In recent work by Professor Cassidy, positronium interaction with thin meso-structured films is studied. These images show such a film in cross-section (a) and from above (b). From Søren L Andersen et al 2015 J. Phys. B: At. Mol. Opt. Phys. 48 204003

Q: Where do you think the field is heading?

It is always difficult, and perhaps even foolhardy, to try and predict where an entire field is heading.  There are many promising areas to be explored, and lots of good people working hard.  We are very much focussed on producing long lived Rydberg states that we can manipulate using well developed methods (thankfully developed by others, so we don’t have to know too much).  The goal is to use these methods to control Ps atoms so that we can make better use of them in other experiments.  If we can produce slow focussed beams we can do scattering measurements, high-resolution spectroscopy, or gravity measurements, to name just a few possibilities.  In general the overall direction I want to go in is to make positronium more a part of the atomic physics realm, as opposed to some fringe area that most people aren’t even aware of.

Q: What do you consider to be the hot topics in AMO physics at the moment?

I don’t care what the “hot topics” are.  You might as well ask me what bandwagon I want to jump on.  I think people should do research because they think it is important and interesting, not because the guy down the corridor is doing it, or because funding agents have decided that it’s good for business.

Q: What current problem facing humanity would you like science to provide a solution to?

Well, it’s maybe not such a big problem for humanity, but I would really like to know where all the antimatter went, what dark matter is, whether or not dark energy even exists and if all these things are somehow connected.  By studying systems containing antimatter it might just be possible to say something about such matters. I eagerly await the next breakthrough to address the rest.

Q: What advice would give to young scientists?

Quite often PhD students and post docs seem to believe that anything other than a future in academia is selling out.  However, the job market is really difficult at the moment, and there are many intellectually (and financially) rewarding career paths outside of the academic world, so my advice would be to keep an open mind about these options.  For those who are sure they want to remain in a University environment I would say that it is very advantageous to be able and willing to travel anywhere in the world.  Not only does this provide the maximum number of options, but it is the best way to broaden your scientific horizons.  Plus, living in different places around the world can be a lot of fun, even if it is just for a few years!


On behalf of JPhysB I would like to thank Professor Cassidy for answering our questions and for recently publishing “Positronium emission and cooling in reflection and transmission from thin meso-structured silica films” in our journal.  We have just begun publishing our Special Issue on Antihydrogen and positronium where you can read Professor Cassidy’s paper and more positronium research.

CC-BY logoThis work is licensed under a Creative Commons Attribution 3.0 Unported License

Image 1: copyright David Cassidy; used with permission.

Image 2: The meso-structured silica thin film imaged by SEM (a) in cross section and (b) from a top view, from Søren L Andersen et al 2015 J. Phys. B: At. Mol. Opt. Phys. 48 204003

Categories: Journal of Physics B: Atomic, Molecular and Optical Physics

Tags: , , , , , , , ,

2 replies


  1. Introducing Letters from the Journal of Physics | JPhys+
  2. Positrons and antiatoms – JPhys+
%d bloggers like this: