AtMol Research Seminar Archive
Index Current Seminars

Unless otherwise advertised, all talks are in room Ph30 at 1:00pm on Wednesdays.

Michaelmas Term 2013

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 non-zero 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 non-equilibrium problem.

Following a brief review of the history of problems of phase transitions in coupled matter-light 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 self-organization 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)
[2] J. Keeling, M. J. Bhaseen and B. D. Simons, Phys. Rev. Lett 105 043001 (2010); 
M. J. Bhaseen, J. Mayoh, B. D. Simons and J. Keeling, Phys. Rev. A 85, 013817 (2012).
[3]J. Keeling, M. J. Bhaseen and B. D. Simons arXiv:1309.2464

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 speed-up of computation.

Recently boson sampling has been implemented experimentally using integrated photonics and fibre-based linear optics networks [2-4]. 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 speed-up.

[1] S. Aaronson and A. Arkhipov, Proceedings of the 43rd Annual ACM Symposium on Theory of Computing, 333-342 (2011).
[2] M. A. Broome et al., Science 339, 794-798 (2013).
[3] J. B. Spring et al., Science 339, 798-801 (2013).
[4] A. Crespi et al., Nature Photonics 7, 545-549 (2013).
[5] C. Gogolin et al., arxiv:1306.3995v2 (2013).

Exchange potentials in Hartree-Fock 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 single-electron 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 single-particle 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 semi-metallic. Typically, the lack of screening is thought to be cause of this anomaly. Semi-conductors 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 semi-conductors and also give too small or zero band gaps for some transition-metal 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.)
In fact, we exploit the benefit of treating exchange accurately, but with a local single-particle potential, rather than a non-local one.

Towards precision measurement with anti-hydrogen
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. Anti-hydrogen is the perfect candidate for testing CPT symmetry as its matter counterpart, hydrogen, is extremely well-studied.

Following the magnetic confinement of cold anti-hydrogen in 20120 [1], the first spectroscopic measurement of the internal states of an anti-atom has been performed [2]. This journal club talk will explore the techniques for producing cold anti-hydrogen 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)
[2] C. Amole et al. Nature 463, 439 (2012)
[3] C. Amole et al. Nature Communications 4, 1785 (2013)

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.

Bose-Einstein Condensates of Photons
Wed 13 Nov 2013 Rob Nyman Imperial College London

Photons are bosons, but they don't normally make Bose-Einstein condensates because their number is not conserved in thermal equilibrium. However, light pumped into a fluorescent dye can exchange energy with the dye-solvent 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, high-reflectivity 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 Bose-Einstein condensate (BEC) will form, as first achieved in 2010[1].

The equation of motion which describes a photon BEC is very similar to the Gross-Pitaevskii 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 angle-resolved 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 dye-microcavity 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)
[2] R.A. Nyman and M.H. Szymanska, arXiv:1308.3588 (2013)

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:
(i) Collapse and revival and cat state preparation in spin systems.
(ii) "Cooling" a superconducting device into a cat state.

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 Bose-gas in the presence of a classical, far red-detuned, 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 spatially-structured 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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