AtMol Research Seminar Archive
Index Current Seminars

Easter Term 2007

Date Speaker Home
Bragg Scattering of a Cooper Paired Fermi Gas
April 18th Dr. Katharine Challis Atomic and Molecular Physics

Bragg scattering was first developed as a technique for studying the structure of crystalline solids, but it has since been used extensively to manipulate and probe ultra-cold Bose gases. Recent theoretical proposals suggest that Bragg scattering experiments could provide a signature of superfluidity and pairing in ultra-cold Fermi gases. However, Bragg scattering of a Cooper paired Fermi gas has not yet been observed.

We have considered Bragg scattering of an ultra-cold Cooper paired Fermi gas. Our calculations are based on a mean-field approach and predict a pair-correlated Bragg scattering mechanism which is a direct consequence of Cooper pairing. The correlated scattering has a distinct signature in momentum space, and may be useful as an experimental probe of pair correlations in a Fermi gas.

Venue: Ph 327

A Cloak of Invisibility: Harry Potter Does Electromagnetism
April 25th Prof. Sir John Pendry FRS Imperial College

Refractive materials gives us some limited control of light: we can fashion lenses, and construct waveguides, but complete control of light is beyond simple refracting materials. Ideally we might wish to channel and direct light as we please just as we might divert the flow of a fluid. Manipulation of Maxwell's equation shows that we can achieve just that provided we have access to some highly unusual material properties. Metamaterials open the door to this new design paradigm for optics and provide the properties required to give complete control of light. One potential application would be to steer light around a hidden region, returning it to its original path on the far side. Not only would observers be unaware of the contents of the hidden region, they would not even be aware that something was hidden. The object would have no shadow.

The 2007 Rochester Lecture starts at 4:30pm in the Appleby Lecture Theatre.

MSci vivas
April 30th to May 4th
Calculation of Bound and quasi-bound states of van der Waals forces and prediction of Feshbach resonances in Ultracold moloecular collisions
May 2nd Mr. Musie Beyene Jeremy Hutson Chemistry Group

Long range forces play a critical role in the dynamics of molecules in the cold and ultracold temperature regimes. For example it has been demonstrated that bound and quasi-bound states of the van der Waals interaction induce resonances in collisions at these temperatures. A significant challenge in the the field of ultracold molecular matter is to understand and manipulate the dynamic at these temperatures.

In the presentation I will describe the method we use to calculate the energies of bound and quasi-bound states of the van der Waals interaction in the presence of a field and how this information is used to predict the position of Feshbach resonances which can be used to tune scattering properties. I will discuss collision properties across a resonance in a multichannel scattering problem and present recent calculations and observations on the He-NH system.

Investigating (Bio)molecular Structure and Dynamics Using Ultrafast Spectroscopy
May 9th Dr Neil T. Hunt University of Strathclyde, Glasgow

Hydrogen bonding in liquids plays an important role in the structure and chemistry of biomolecules. From stabilising biopolymer molecules into structures such as the α-helix or β-sheet to the reversible bond formation that allows the action of enzymes, the hydrogen bond (H-bond) is central to all biological processes. Ultrafast spectroscopic methods have been used to study the dynamics of two biologically relevant systems; the peptide linkage model compound N-methylacetamide (NMA) and poly-L-Lysine (PLL) on both sides of the α-helix to random coil transition.

In the case of NMA the rotation/diffusion timescale shows unusual behaviour as a function of temperature. This is caused by a transition from normal to fractional Stokes-Einstein-Debye dynamics due to the formation of hydrogen bonded chain structures. The resulting heterogeneity leads to decoupling of the rotational timescale from the bulk viscosity.

The dynamics of PLL show marked differences between the α-helix and random coil states which are caused by differences in the nature of solvent-peptide H-bonding. These changes are thought to be influential in processes such as protein folding and structural rearrangements in enzyme binding pockets.

The technique and applications to biomolecules of ultrafast 2D-IR spectroscopy will be introduced and future directions discussed.

References:
Hunt NT; Kattner, L; Shanks, RP; Wynne K, J. Am. Chem. Soc. 129, 3168 - 3172 (2007);
Hunt, NT; Wynne, K, Chem. Phys. Lett. 431, 155-159 (2006).

Venue: Ph8 ;   Joint seminar with Condensed Matter.

Using quantum reflection from semiconductor surfaces to control electrons and ultra-cold atoms
May 16th Prof Mark Fromhold University of Nottingham

I will show how the reflection of quantum-mechanical waves from semiconductor surfaces can provide sensitive control of electrons and ultra-cold atoms.

Firstly, I will focus on electrons in "superlattices", comprising alternating layers of different semiconductor materials. Multiple reflections of electron waves from the layer interfaces can create a unique type of chaotic electron motion. The abrupt onset of chaos produces a sharp increase in the measured current flow by creating unbound electron orbits, which propagate through intricate web patterns in phase space and imprint themselves on the quantum-mechanical wavefunctions.

Next, I will consider how room-temperature semiconductor surfaces can be used to manipulate atoms cooled to nK temperatures. At such low temperatures, quantum-mechanical reflection can shield the atoms from the disruptive influence of the surface. By considering recent experiments performed at MIT on Bose-Einstein condensates, I will show that inter-atomic interactions and the aspect ratio of the condensate both play a crucial role in the reflection process. I will also consider how surfaces that are etched on nanometre and micrometre scales can be used to increase the reflection probability and control the shape of the reflected atom cloud.

Venue: Ph8 ;   Joint seminar with Condensed Matter.

HERSCHEL & ALMA : OBSERVING THE COOL UNIVERSE
May 23rd Ms Meltem Akyilmaz Atomic and Molecular Physics

Herschel Space Observatory is the fourth cornerstone mission of European Space Agency and is a part of the Horizon 2000 long-term plan. ALMA (Atacama Large Millimetre Array) is an international telescope project that North America, Europe, Japan and Chile contribute by providing funding and instruments. Herschel and ALMA will be observing the 'Cool Universe', detecting cold, faint and distant objects. In this talk, a review of key scientific drives for these projects will be presented. The importance of observations in far infrared and sub-millimetre wavelengths will be addressed. Observational challenges will be mentioned and the characteristics of the instruments will be given. There will be a period during which both Herschel and ALMA will be in operation. The synergies between these two projects will also be mentioned in details.

Cavity QED with arrays of microfabricated cavities
May 30th Dr Jonathan Goldwin Imperial College London

The study of single neutral atoms in high-finesse optical cavities is leading to important advances in the field of quantum information processing. In the strong-coupling regime a single photon is enough to saturate an atomic transition, allowing coherent control over the internal state of the atom. In the context of quantum computing, one outstanding technical challenge is to make these systems scalable.

We are performing experiments using arrays of microfabricated cavities. Each plano-concave resonator consists of an etched spherical mirror and a coated single-mode optical fibre, allowing open access to the intra-cavity field mode. The small mode volume leads to a large single-photon Rabi frequency, and light is conveniently coupled into and out of the cavities via the fibres themselves. Here I will present measurements characterising the optical properties of the cavities, as well as our recent detection of atoms dropped from magneto-optically trapped clouds. Finally I will discuss our plans for future experiments involving quantum degenerate gases and small arrays of cavities.

Strong atom-light interactions in Rydberg systems
June 6th Prof. Charles S Adams Atomic and Molecular Physics

Highly excited Rydberg atoms are strongly interacting even over mesoscopic distance scales. For example, the interaction energy between two n=80 rubidium Rydberg atoms separated by 5 microns is more than 4 orders of magnitude larger than their decoherence rate due to spontaneous emission. By coupling a ground state atom to a Rydberg state one can map the strong interatomic interactions into photon interactions. We report experimental results on coherent coupling to highly excited Rydberg states using electromagnetically induced transparency (EIT) [1] and discuss the potential of Rydberg EIT for cross phase modulation of single photons.

[1] AK Mohapatra TR Jackson CS Adams, Phys Rev Lett 98, 113003 (2007).

Dynamics and control of Rydberg wavepackets
June 13th Dr. Jan R.R. Verlet Chemistry Department

A superposition of high-lying Rydberg states is excited using a picosecond pulse, forming a non-stationary electronic wavepacket. The subsequent dynamics are monitored using an interference technique and pulsed-field ionisation. We have looked at two systems - the Xe atom and the NO diatomic molecule. In Xe, we can optically access two dominant Rydberg series of differing angular momentum and observe a beat between the two series in the wavepacket dynamics. Using a pair of well-defined and intuitive optical pulses, we demonstrate that either series can be removed from the initial wavepacket, leaving only a single angular momentum component. We have extended our coherent control scheme to more complex systems such as NO, where one also accesses two Rydberg series, but in this case they not only have differing angular momentum, but also converge to differing rotational states of the ion. Finally, we show that we can use this approach to attain coherent control over the branching ratio of Rydberg state autoionisation versus predissociation.

Quantum Resonances in a Quantum Kicked Rotor
June 20th Mr. Paul Halkyard Atomic and Molecular Physics

Quantum chaos describes the field of work that examines the behaviour of the quantum counterparts of classically choatic systems and encompasses many peculiar effects. One such effect is the appearance of quantum resonances. For a classical ensemble subjected to quantum kicked rotor dynamics, these resonances manifest themselves as a quadratic increase in the kinetic energy for narrow momentum widths and a linear increase for broad momentum widths. Transient behaviour will be discussed with some preliminary results given for higher order resonances.