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First measurements of the differential positronium-formation cross-sections

Positrons are the antimatter version of electrons and so their fate in a matter world is ultimately to annihilate. However, prior to this, a positron may combine with an electron to form a matter-antimatter hybrid called positronium. This is akin to a hydrogen atom with the proton replaced by a positron. Fundamental to our understanding of the physical universe, positron and positronium are these days also acknowledged as being fantastically useful in practical applications such as probing material properties and medical diagnostics. However, there is still much that we do not know for sure about the details of the interactions of these particles with ordinary matter. For example if, in a collision with an atom or molecule, a positron captures an electron, in which directions is the positronium likely to travel and with what probability? More...

Published: Jun 17, 2015 12:35:19 PM

CO2 Satellite

New calculations to improve carbon dioxide monitoring from space

How light of different colours is absorbed by carbon dioxide (CO2) can now be accurately predicted using new calculations developed by a UCL-led team of scientists. This will help climate scientists studying Earth’s greenhouse gas emissions to better interpret data collected from satellites and ground stations measuring CO2. More...

Published: Jun 15, 2015 10:29:10 AM

Watt Steam Engine

On quantum scales, there are many second laws of thermodynamics

New research from UCL has uncovered additional second laws of thermodynamics which complement the ordinary second law of thermodynamics, one of the most fundamental laws of nature. These new second laws are generally not noticeable except on very small scales, at which point, they become increasingly important. More...

Published: Feb 10, 2015 11:55:53 AM

Current Research

Our research involves the cooling, manipulation and trapping of atoms, molecules, and more complex particles using optical fields. Current work is studying the focusing of molecules and atoms, and the transport of these species within optical lattices.  This includes the creation of stationary cold molecules by deceleration in optical lattices, and the study of the dynamics of molecular motion within these structures using coherent Rayleigh scattering. We are currently working on cooling these molecules to microkelvin temperatures for experiments in the quantum regime. Other work is exploring mechanisms for cooling larger nanoscale to microscale particles, trapped by light, to temperatures where the quantum nature of there motion becomes apparent. For further information please contact Peter Barker.


Cavity optomechanics with optically trapped particles

The field of optomechanics, in particular the cooling of small mechanical oscillators, is currently one of the most exciting and rapidly growing areas of physics. An important question is whether one can not only reach, but also operate these devices in the quantum regime. In addition to the fundamental interest, there are promising possibilities for highly sensitive measurements of weak forces at the quantum limit (in other words with a displacement limited only by the width of the ground state of the mechanical oscillator). It will also have applications for generating entanglement between the light field and the mechanical modes. We are currently exploring cavity cooling of nanoscale polarisable particles. Such large particles (in the 100 nm size range) interact strongly with a cavity field, allowing both trapping and cooling by the same field. Since in these cooling schemes no internal resonance is involved, no detailed knowledge of the internal structure of the particle is required. Significantly, unlike usual cavity optomechanics schemes, where the object is physically connected to the environment, the nanoparticle to be cooled is held in a conservative potential so is well isolated from the environment. This confers unique advantages, over conventional optomechanical schemes, in relation to the ultimate objective of cooling to the ground state. A research blog on this topic can be found here.

Trapped nanosphere 1

We collaborate so far with M. Shneider at Princeton University, T. Monteiro at UCL and J. Ruostekoski at the University of Southampton.

Creating ultra-cold molecules by sympathetic cooling with rare gas atoms

A research blog for this project can be found at http://pfbarker.wordpress.com/. Laser cooling has been of primary importance in the exploration of ultra-cold interactions between trapped atoms, for quantum atom optics, and for precision metrology through high-resolution spectroscopy. It has also provided a well-controlled testing ground for condensed matter physics, and more recently quantum information. Attention has now turned to molecules because of the quite different interactions that can occur between ultra-cold molecules. They are seen as ideal candidates in the search for CPT violation, for exploring ultra-cold chemistry, and for the creation of novel quantum fluids using dipolar molecules. Unfortunately, laser cooling, which has been so successful for many atomic species cannot be applied to molecules and thus new methods are required to reach this new regime in molecular and ultra-cold physics.


Our work addresses this problem by utilising the very general techniques of sympathetic and evaporative cooling to create ultra-cold molecules from cold stationary molecules that have been created by optical Stark deceleration. To undertake this work we will bring ultra-cold laser cooled rare gas atoms such as argon, krypton and xenon into thermal contact with stationary cold molecules initially at temperatures in the 10 mK to 1 K range. We will use elastic collisions between the cold rare atoms and the molecules co-trapped within an optical field to thermalise the mixture, bringing it to a common temperature below the 1 mK bottleneck that currently exists. Although in principle sympathetic and evaporative cooling are applicable to many molecular species, this project will explore the cooling of molecular hydrogen and benzene by collisions with ultra cold rare gas atoms, producing molecules in the 100 microkelivn temperature range and below. An important part of this programme will be the study, via experiment and theory, the atom-molecule collisions that are vitally important for sympathetic and evaporative cooling, as well as for the potential ultra-cold molecular chemistry that can be studied once the molecules are cooled into the microkelvin temperature regime. This research will utilise our optical Stark decelerator and will build a magneto-optical trap for producing ultra-cold rare gas atoms. We are building a deep optical trap that will be capable of holding atomic or molecular species for the long periods (seconds) required for thermalisation.


We collaborate with Prof J. Tennyson and Dr P. Barletta in the THAMOS group of AMOPP at UCL.

Optical manipulation of molecules


Recently, coherent manipulation of neutral molecules using the large conservative potentials produced by pulsed optical fields has demonstrated deflection, rotation, alignment and focusing of molecules. At the same time, the creation of cold molecules using both optical and other means has become an important new area of atomic and molecular and optical physics. We are currently studying the manipulation and trapping of molecules within strong optical fields. Recently we have begun to explore the effects of laser induced molecular alignment on the dipole force. In particular, we are interested in tailoring the force by controlling the alignment of the molecule with respect to the polarization of the field. Recent results confirm this and we are now studying whether we can use this process for state selection of molecules.

The figure illustrates how we measure velocity changes induced in neutral molecules by the dipole force from a single focused laser beam (shown in red) within a time-of-flight spectrometer. The velocity is measured by ionising the neutral molecules using a resonant probe beam (shown in blue) and the time-of-flight can be related to their velocity.

Coherent Rayleigh scattering

Coherent Rayleigh scattering (CRS) is a new and practical non-linear flow and combustion diagnostic probe. The CRS technique measures the microscopic and bulk properties of a gas by analysis of Bragg scattering light from laser induced perturbations in the gas. This research program is aim further developing the CRS technique for measurement of temperature and density in the gas phase with high spatial (100 micron) and temporal (10 ns) resolution. Initial work is focusing on characterising CRS in well controlled environments where gas composition, collisions and strong fields can be carefully studied.  The second major component of this research applies the CRS technique to prototypical high speed flow and combustion environments.  We aim to demonstrate a system capable of single shot measurements of transient events, over a wide range of temperatures, pressures and species concentrations.

Further details of this work can be found in the following references:

Tailoring the optical dipole force for molecules by field-induced alignment, S. M. Purcell and P. F. Barker, Phys. Rev. Lett. 103, 153001 (2009)

Sympathetic cooling by collisions with ultracold rare gas atoms, and recent progress in optical Stark deceleration, P.F. Barker, S. M. Purcell, P. Douglas, P. Barletta, N. Coppendale, C. Maher-McWilliams, J. Tennyson, Faraday Discussions 42, 175 (2009)

Self-organisation and cooling of a large ensemble of particles in optical cavities, Y.K. Zhao, W.P. Lu, P.F. Barker, G. J. Dong, Faraday Discussions 42, 311 (2009)

Towards sympathetic cooling of large molecules: cold collisions between benzene and rare gas atoms, P. Barletta, J. Tennyson, P.F. Barker, New Journal of Physics 11, 055029 (2009)

Creating ultracold molecules by collisions with ultracold rare-gas atoms in an optical trap, Barletta P, Tennyson J, Barker P.F., Phys. Rev. A 78, 052707 (2008)

Spectra of molecular gases trapped in deep optical lattices, P.F. Barker, S.M. Purcell, M.N. Shneider, Phys. Rev A. 77, 063409 (2008)

Molecular transport in pulsed optical lattices, M.N. Shneider, P.F. Barker, S.F. Gimelshein, App. Phys. A 89, 337 (2007)

Spectral Narrowing in Coherent Rayleigh Scattering, H. T. Bookey, M. N. Shneider, and P. F. Barker, Phys. Rev. Lett. 99, 133001 (2007)

Cooling molecules in optical cavities, W. Lu, Y. Zhao, P.F. Barker, Phys. Rev. A 76, 013417 (2007)

Separation of binary gas mixtures in a capillary with an optical lattice, M.N. Shneider, S.F. Gimelshein, P.F. Barker, Laser Physics Letters 4, 519 (2007)