DEPARTMENT OF PHYSICS AND ASTRONOMY

Excited-state positronium formation from helium, argon and xenon

D.J. Murtagh, D.A. Cooke, G. Laricchia,

Physical Review letters, 102, 133202 (2009)

Paper selected as 'Editors' suggestion'

Positronium is the quasi-stable bound-state of a positron and an electron. It is readily formed in encounters of positrons with matter and its own interactions with atoms and molecules, including its fragmentation and even its combination with another positronium atom, are amenable to experimental investigations. Interest in positronium encompasses the quest for the understanding of fundamental matter/antimatter interactions, tests of collision physics theories and of bound-state QED calculations, the analysis of energetic events occurring in the galactic centre (where it is estimated 93% of all annihilations occur through the decay of Ps) and problems of medical relevance.

Now a team at UCL have measured for the first time the probability for its formation in an excited (2P) state in collisions of positrons with helium, argon and xenon atoms. They measured coincidences between the remnant-ion and the Lyman-alpha photon from positronium and found maximum values to increase from approximately 0.06 ± 0.01 in helium, to 0.12 ± 0.04 in argon and 0.26 ± 0.09 in xenon. These results might have repercussions also in the development of beams of positronium and evaluation of its survival probability in dense media.

Detection of the transit of the planetary companion to HD 80606

Stephen Fossey, Ingo Waldmann and David Kipping

Paper accepted for publication in Monthly Notices of the RAS Letters.
A preprint is available at www.arxiv.org 

These findings have also been featured in the 'The Times' newspaper at http://www.timesonline.co.uk/tol/news/uk/science/article6135796.ece and on the BBC website- see http://news.bbc.co.uk/1/hi/sci/tech/8020594.stm

We have made one of the first detections of the transit of the extrasolar planet HD 80606b. The planet is unusual in that it has a highly eccentric orbit, which in itself may hold clues as to its origin and orbital evolution. The key observations were made using UCL's observatory (the University of London Observatory) in Mill Hill, NW London, with the direct participation of UCL undergraduates.

The observation of a transit of HD 80606b - its passage in front of the star as seen from Earth - enables a precise determination of its radius, which turns out to be close to that of Jupiter; from its now-resolved mass of 3.9 Jupiter masses, the density of HD 80606b is found to be the amongst the highest of the known transiting planets.

We can also resolve precisely the inclination of the orbit, which turns out to be in a near-grazing configuration (see figure), and the orbital eccentricity is tightly constrained by our data.

Dr. Greg Laughlin, who co-ordinated a global observing campaign on HD 80606b via his web site www.oklo.org, noted on his site, "It's certainly been a long time since an observational astronomical discovery of this magnitude has been made from within the London City Limits."

HD 80606b has the longest period (111 days) for a transiting planet yet discovered, and its orbit is also the most eccentric - it orbits its parent star more like a comet, moving in a couple of months from just inside the 'habitable zone' of its parent star, before plunging close to the star to experience an 800-fold increase in irradiation. This unusual orbit may be due to the influence of other planets in the system, or due to the parent star being part of a binary star system.

The observations at ULO were carried out by UCL undergraduates in a collaboration set up by Dr. Steve Fossey. A 10-hour series of observations was conducted on the night of Feb 13/14, when the transit occurred, by Ingo Waldmann, Maria Duffy, Stephen Fawcett, Yilmaz Gul and Cherry Ng, all undergraduate students taking degrees in the Dept of Physics and Astronomy at UCL, supervised by Steve Fossey.

Ingo Waldmann, a 4th-year Natural Sciences MSci student at UCL, obtained and reduced the observations as part of his final-year project. The final author is Dave Kipping, a postgraduate student at UCL, who developed an eccentric-orbit modelling code which enabled a precise analyis of the transit light curve.

The detection was made using two of ULO's smaller telescopes which are well set up for this kind of photometry - a 35-cm Celestron and a 25-cm Meade telescope. Careful observation, and rigorous calibration and data-reduction procedures, enable us to contribute important results in observational astrophysics, even from London. Our results have provided some of the most precise measurements of the transit yet obtained by astronomers.

Independent detections of the transit of HD 80606b have also been reported  by Moutou et al. and Garcia-Melendo & McCullough (see www.exoplanet.eu for details).

ULO website: www.ulo.ucl.ac.uk

Links to other articles relating to this research

http://www.ulo.ucl.ac.uk/news/index.html#hd80606b-news-reports

For further details on this discovery please see UCL News http://www.ucl.ac.uk/news/news-articles/0903/09030301

Fig 1:
The geometry of the transit (courtesy of Greg Laughlin (UCSC) www.oklo.org

FIG 2:
Light curves of exoplanet HD80606b captured at ULO. On the time axis, HJD 2454876.5 corresponds to midnight on Feb 13/14 2009.  (The Meade light curve has been shifted vertically by -0.023 units for clarity.) The brightening of the star by about 1% as the planet completes its transit is clearly detected.

The Computational Power of Correlations

Janet Anders and Dan Browne

Physical Review Letters 102, 050502 (2009)

In this Letter, Anders and Browne study the intrinsic computational power of the correlated outputs of quantum measurements, a phenomenon previously exploited in measurement-based quantum computation (MBQC). Exploiting computational complexity theory, they provide a classification of the computational power of research states for measurement-based quantum computation. This allows the identification of a classical computational analogue of MBQC. Remarkably, this leads, for the first time, to a unified formulation of the Bell inequalities and the Greenberger Horne-Zeilinger effect, two of the most well- studied examples of quantum Physics' incompatibility with a local realistic model of nature. This research opens up the possibility of a more systematic study of the quantum correlations of many-particle systems and gives new insights into the mechanisms through which measurement-based quantum computation obtains its power.

Nuclear & Particle Physics by Brian Martin

Nuclear & Particle Physics is an introduction to the subject and provides a readable and up-to-date overview of both the theoretical and experimental aspects of nuclear and particle physics, with an emphasis on the phenomenological interpretation of experimental phenomena.

Completely revised, this edition incorporates several new and greatly expanded sections, including additional material on:

• Penning trap measurements of nuclear masses

• nuclear beta decay

• time reversal invariance and tests via measurements of electric dipole moments

• nuclear weapons

• experimental searches for the Higgs boson

• neutrino physics, including oscillation experiments and experiments on neutrinoless double beta decay

• CP violation in the decays of B mesons and consequences for the standard model

• particle astrophysics

There is also a new Appendix on gauge invariance and the Higgs mechanism, tables of data for nuclear and particle physics and additional problems.

This is an invaluable text for all undergraduate physics and astronomy students taking courses in nuclear and particle physics.

Measurement-based quantum computation

H. J. Briegel, D. E. Browne, W. Dür, R. Raussendorf & M. Van den

Nest
Nature Physics 5, 19- 26 (2009).

Quantum computation offers a promising new kind of information processing, where the non-classical features of quantum mechanics are harnessed and exploited. A number of models of quantum computation exist. These models have been shown to be formally equivalent, but their underlying elementary concepts and the requirements for their practical realization can differ significantly. A particularly exciting paradigm is that of measurement-based quantum computation, where the processing of quantum information takes place by rounds of simple measurements on qubits prepared in a highly entangled state. In this progress article we review recent developments in measurement-based quantum computation with a view to both fundamental and practical issues, in particular the power of quantum computation, the protection against noise (fault tolerance) and steps towards experimental realization.

Finally, we highlight a number of connections between this field and other branches of physics and mathematics.