Durham Atomic and Molecular Physics: Part of the JQC Durham–Newcastle 
AtMol Research Seminar Archive

Date  Speaker  Institution 

Superradiance of cold atoms in optical cavities  
Wed 9 Oct 2013 in Ph8  Dr Jonathan Keeling  St Andrews University 
Questions about the collective behaviour of large numbers of atoms (or artificial atoms) interacting with radiation have a long history, dating back at least to the work of Dicke in 1954. A particularly notable result is that above a critical coupling strength, the ground state of the Dicke model is predicted to become one with a nonzero photon number. However, complications arise from when the diamagnetic coupling (the A^2 term) between light and matter is included, suggesting that the Dicke phase transition may be an unobservable artifact. Nonetheless, in 2010, this transition was observed [1] in a system of cold atoms in an optical cavity, where a generalised Dicke model arises as the effective description of this nonequilibrium problem. Following a brief review of the history of problems of phase transitions in coupled matterlight systems, I will discuss examples of collective superradiant behaviour in systems of cold atoms, considering both Bosonic [1,2] and Fermionic [3] atoms. In contrast to the bosonic case, Pauli blocking leads to lattice commensuration effects that influence selforganization in the cavity light field. This includes a sequence of discontinuous transitions with increasing atomic density and tricritical superradiance. [1] K. Baumann, C. Guerlin, F. Brennecke and T. Esslinger, Nature 464,
1301 (2010)


Boson sampling: Is quantum really faster?  
Wed 16 Oct 2013  Hannes Busche  Durham University 
Boson sampling, i.e. sampling the output distribution of a set of indistinguishable photons (or bosons) undergoing a linear unitary transformation, is a task considered to be "hard" to solve using classical computers [1]. It has therefore been proposed as an alternative to an universal quantum computer to demonstrate a quantum speedup of computation. Recently boson sampling has been implemented experimentally using integrated photonics and fibrebased linear optics networks [24]. This journal club talk will introduce the concept and experiments and summarise a resulting debate [5] whether boson sampling is really suitable to demonstrate quantum speedup. [1] S. Aaronson and A. Arkhipov, Proceedings of the 43rd Annual ACM Symposium on Theory of Computing, 333342 (2011).


Exchange potentials in HartreeFock and density functional theory  
Wed 23 Oct 2013  Dr Nikitas Gidopoulos  Durham University 
In the theory of electronic structure, the Hartree Fock method is the earliest and most intuitive approximation, that treats electrons as independent particles, while at the same time respecting fully the exchange symmetry. It was proposed around eighty years ago and its study typically forms the elementary introduction to the theory of electronic structure in any textbook. It follows we believe we understand the theory and its limitations rather well. I shall present and discuss the results of a series of calculations. Conceptually, these calculations are easy to understand: we calculate the HF ground state Slater determinant (Phi) for a system and then we calculate its ground state singleelectron density. Then, (with density functional theory in mind) we use a simple algorithm to "invert" the HF density in order to obtain the effective Hamiltonian with a local singleparticle potential, whose ground state (different from Phi) has the same density as Phi. Finally, we study the band structure of this local potential for the various systems of interest. We have applied this method to a number of representative systems where either the HF approximation or the common, density functional theory (DFT) approximations (or both) fail to give a qualitatively correct band structure. For example, HF predicts every system to be insulating, including simple metals, that are predicted to be semimetallic. Typically, the lack of screening is thought to be cause of this anomaly. Semiconductors are also predicted to have a large gap. On the other hand, common approximations in DFT are accurate for metals but predict too small band gaps for semiconductors and also give too small or zero band gaps for some transitionmetal oxides that are antiferromagnetic insulators (and considered as strongly correlated systems that cannot be described by such approximations). We will see that our method gives a reasonably accurate description for all these systems, despite the fact that the underlying theoretical model is the HF approximation and no correlation effects are included in the calculations. (Defining correlation as whatever is added beyond a HF calculation.)


Towards precision measurement with antihydrogen  
Wed 30 Oct 2013  Stefan Kemp  Durham University 
One of the great unsolved problems in modern day physics is the imbalance between matter and antimatter in the universe. The violation of CP symmetry by the weak force cannot account for this imbalance alone. However, if CPT symmetry were violated, the mismatch would be closer to being explained implying physics outside of the standard model. Antihydrogen is the perfect candidate for testing CPT symmetry as its matter counterpart, hydrogen, is extremely wellstudied. Following the magnetic confinement of cold antihydrogen in 20120 [1], the first spectroscopic measurement of the internal states of an antiatom has been performed [2]. This journal club talk will explore the techniques for producing cold antihydrogen and summarise the recent work in both probing the hyperfine structure and measuring the gravitational mass [3] of such a system. [1] G. B. Andresen et al. Nature 468, 673 (2010)


Manipulating terahertz light  
Wed 6 Nov 2013  Andrew Gallant  Durham University 
The field of plasmonics has generated considerable interest in recent years. This talk focuses on the applications of plasmonics in the terahertz region. THz surface plasmon based devices can be constructed using the standard techniques of semiconductor processing technology, as the characteristic length scales are commensurate with the wavelength (i.e. a fraction of a millimetre). As well as providing a background introduction to terahertz systems, this talk will cover surface plasmon based sensors (including biological sensors); techniques for guiding, concentrating and manipulating THz radiation; and new methods for THz imaging and microscopy. 

BoseEinstein Condensates of Photons  
Wed 13 Nov 2013  Rob Nyman  Imperial College London 
Photons are bosons, but they don't normally make BoseEinstein condensates because their number is not conserved in thermal equilibrium. However, light pumped into a fluorescent dye can exchange energy with the dyesolvent mixture, coming to thermal equilibrium without destroying the photons. The electronic structure of the dye sets a minimum energy, preventing the photon from being destroyed altogether. Placing this system between two curved, highreflectivity mirrors, the light is confined for about a nanosecond, which is much faster than the picosecond it takes for thermalisation. Therefore thermal equilibrium can be achieved at room temperature with a photon number determined by the pumping intensity. At a fixed temperature, and sufficient density, a BoseEinstein condensate (BEC) will form, as first achieved in 2010[1]. The equation of motion which describes a photon BEC is very similar to the GrossPitaevskii equation which is familiar from atomic BEC. However, the effects of continual pumping and decay via cavity mirrors also appear, making the equation complex. The parameters of this equation, especially the interactions, are not yet known. I will discuss an experimental method for inferring the strength of interactions in photon BEC, by observing these excitations using angleresolved spectroscopy of the light that leaks through the mirrors[2]. Even very weak interactions should be detectable this way. I have recently started constructing an experimental apparatus to produce and study BEC of photons in this dyemicrocavity system. The project's aims include measuring the interaction strength, characterising the coherence properties of the condensate, and fabricating mirror shapes which would allow observation of 1D gases of photons as well as photon BECs with mesoscopic numbers. I will discuss the recent progress of the experiment as well as the theoretical description of photon BEC. [1] J. Klaers et al, Nature vol 468, p545 (2010)


Optical atomic clocks for more than just telling the time  
Wed 20 Nov 2013  Rachel Godun  NPL 
Abstract TBA 

Quantum metrology and creating the quantum resources for it  
Wed 27 Nov 2013  Tim Spiller  Leeds University 
Quantum information technologies (QIT) offer advantages over
their conventional IT counterparts, due to exploitation of fundamental
features of quantum physics such as entanglement. Quantum
communication technologies are now available, whereas large scale
quantum computers are still a long way off. Measurement and sensing
technologies, based on modest and practical quantum resources, could
be the next QIT to emerge commercially. In this talk I'll give a
simple review of the ideas that underpin quantum metrology. A key
issue for metrology is the preparation of suitable quantum resources.
I'll highlight two examples of such resource preparation:


Instabilities and "phonons" of optical lattices in hollow optical fibres  
Wed 4 Dec 2013  Mike Gunn  Birmingham University 
Instabilities are predicted for a sufficiently long hollow photonic optical fibre containing a one dimensional Bosegas in the presence of a classical, far reddetuned, confined weak electromagnetic mode. We examine both a single beam with Bose gas (a type of Brillouin instability) and the case of a standing wave, or optical lattice. The instabilities of these driven systems have pronounced spatial structure of combined modulational instabilities in the electromagnetic and Bose density fields. Near the critical wave vectors for the instability the coupled modes of the BEC and light can be interpreted as "phonons" of the optical lattice. We conjecture these spatiallystructured instabilities for the optical lattice, which we predict at weak fields, develop into the source of spatially homogeneous heating predicted for strong fields. 

Extraordinary Seminar  Zeeman and Microwave molecular decelerators for making cold molecular beams  
Thurs 5 Dec 2013 in Ph30 at 3:00pm  Taka Momose  University of British Columbia, Canada 
Abstract TBA 

Seminar Title TBA  
Wed 11 Dec 2013  Speaker TBA  University 
Abstract TBA 
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