AtMol Research Seminar Archive
Index Current Seminars

Michealmas 2011

Date Speaker Institution
Controlled quantum dynamics of ultra-cold atoms
5th October (CLC 407) Dr Christoph Weiss Durham University

The talk will start with transferring ideas from condensed matter physics like "photon assisted tunnelling" to ultra-cold atoms in periodically shaken optical lattices. For these, the "photons" are time-dependent potential modulations in the kHz-regime. Periodically shaken optical lattices allow to study many-particle quantum dynamics far from the mean-field regime both theoretically and experimentally. The talk will also include our proposal to generate mesoscopic quantum superpositions via scattering of bright quantum matter-wave solitons.

C-Field Methods for Molecules and Atoms, with Applications to Bragg Scattering
12th October (Ph30) Prof. Crispin Gardiner University of Otago

We present a model of a coupled bosonic atom-molecule system, using the recently developed c-field methods as the basis in our formalism. We derive expressions for the s-wave scattering length and binding energy within this formalism, and by relating these to the corresponding experimental parameters, we can accurately determine the phenomenological parameters in our system. Using this model, we formulate the basic theoretical methods for Bose-Einstein condensation of atoms close to a Feshbach resonance, in which the tunable scattering length of the atoms is described using a system of coupled atom and molecule fiuelds. These include the Thomas-Fermi description of the condensate profile, the c-field equations, and the Bogoliubov-de Gennes equations, and the Bogoliubov excitation spectrum for a homogeneous condensed system. We apply this formalism to the special case of Bragg scattering from a uniform condensate, and find that for moderate and large scattering lengths, there is a dramatic difference in the shift of the peak of the Bragg spectra, compared to that based ona structureless atom model. The result is compatible with the experimental results of Papp et al. [PRL 101, 135301 (2008)]. Finally we wimplement a full detailed simulation of the Bragg scattering from a condensate cloud including condensate inhomogeneity and three-body loss, and modelling correctly the essential experimental details. Results from these simulations are in very good quantitative agreement with the experimental results, confirming the importance of the resonance bound state in the dynamics of the condensate for fast experiments like Bragg scattering.

Journal Club Talk
19th October Mr Michael Köppinger Durham University

Latest measurements of the neutrino velocity produced at CERN and detected at the OPERA neutrino experiment result in an early arrival of the muon neutrinos with respect to the expected time for travelling with the speed of light. This talk gives an overview of the experiment and the results are presented.

Ultrafast entangling gates between nuclear spins using optically excited triplet states
26th October Dr John Morton University of Oxford

Entanglement is a necessary ingredient in most emerging quantum technologies, including quantum repeaters, quantum information processing (QIP) and the strongest forms of quantum cryptography. Nuclear spins, such as those in liquid state nuclear magnetic resonance have been powerful in the development of quantum control methods, however, due to the weak nuclear spin polarisation, these demonstrations contained no entanglement and ultimately constitute classical simulations of quantum algorithms. Furthermore, nuclear spins exhibit very weak interactions (often in the range 10 - 100 Hz)leading to slow gate operation times.

We have shown how a coupled electron spin can be used to hyperpolarise 31P nuclear spins in silicon, to obtain an initial state of sufficient purity to create an entangled state between the two spins [1]. The state was verified using density matrix tomography based on geometric phase gates, and had a fidelity of 98% compared with the ideal state at this field and temperature (3.4 T, 2.9 K). A coupled electron spin can also dramatically reduce time taken to perform single-qubit gates to nuclear spin, using Aharanov-Anadan geometric phase gates [2].

However, in both cases, the long nuclear spin coherence time (which can exceed seconds) is compromised by the permanent presence of a coupled electron spin [3]. Here we report how a transient electron spin, arising from an optically excited triplet state, can be used to hyperpolarise, couple and measure two nearby nuclear spins. The presence of the triplet state alone enhances the nuclear-nuclear coupling by two orders of magnitude over that of the ground state. We further extend this enhancement to five orders of magnitude by exploiting the spinor nature of the electron, reducing the nuclear spin entangling gate time to hundreds of nanoseconds. We find that different nuclear spin Bell states show very different decoherence rates, giving insights into decoherence induced by the temperature presence of the triplet state. The entangling gate we demonstrate can be widely applied to systems comprising an electron spin coupled to multiple nuclear spins, such as NV centres, while our successful use of a transient electron spin motivates the design of new molecules where nuclear spins are coupled to photo-excited triplet states.

[1] S. Simmons et al. Entanglement in a Solid State Spin Ensemble, Nature 470 69 (2011) [2] JJL Morton et al., Bang-bang control of fullerene qubits using ultra-fast phase gates, Nature Physics 2 40 (2006) [3] JJL Morton et al. Solid state quantum memory using the 31P nuclear spin, Nature 455 1085 (2008)

Ion-neutral reactive collisions in the cold regime
2nd November Dr Stefan Willitsch Universität Basel

Laser- and sympathetically-cooled ions in traps have recently emerged as attractive systems for a wide range of applications. In the talk, we will highlight two recent developments in the field. First, we will discuss recent experiments for the initialization of sympathetically-cooled molecular ions in a well-defined internal (rotational-vibrational) quantum state which pave the way for future applications in quantum technology, precision spectroscopy and cold chemistry [1,2,3]. Second, we will present recent results on reactive collisions between laser-cooled ions and ultracold atoms in combined ion-atom traps which illustrate general features of cold ion-neutral chemical reactions and highlight the important role of radiative chemical processes in the cold regime [4]. We will conclude the presentation with an outlook on future directions.

[1] X. Tong et al., Phys. Rev. Lett. 105 (2010), 143001
[2] X. Tong et al., Phys. Rev. A 83 (2011), 023415
[3] J. Mur-Petit et al., arXiv:1106.3320 [quant-ph]
[4] Felix H.J. Hall et al, arXiv:1108.3739 [physics.atom-ph]
Complex flows of complex fluids
9th November Dr Suzanne Fielding Durham University

Quantum degenerate strontium gases and repulsive polarons in a strongly interacting Fermi mixture
16th November Dr. Florian Schreck Universität Innsbruck

I will present two ultracold atom experiments. In the first part of my talk, I will describe our journey to quantum degenerate gases of the alkaline-earth element strontium. In the second part, I will discuss the preparation of a strongly interacting 6Li-40K Fermi mixture and our study of repulsive polarons in that mixture.

Domain walls in Nano wires
23rd November (1:10 pm, buffet lunch for everyone at 12:30) Dr Dan Allwood University of Sheffield

Soft magnetic materials have a magnetisation configuration that depends strongly on the size and shape of a magnetic element. The extended geometry of patterned magnetic nanowires creates a magnetic easy axis along the wires’ length. Opposite magnetic regions (domains) are then separated by ‘domain walls’ of finite size. The position and motion of domain walls can be controlled using the element geometry, materials properties, externally applied magnetic fields and internal electric currents.

Controlling the position of domain walls for data storage applications has been the subject of much international research over the past decade. However, domain walls in these nanowires are also nanoscale sources of magnetic field, acting effectively as magnetic monopoles. In this talk, we will see some of the effects of this and how it can be applied to control a secondary system.

In particular, the magnetic field from a domain wall can be used to interact with ultra-cold paramagnetic atoms prepared in a low-field seeking quantum state. Recent experiments with colleagues at Durham University have demonstrated this interaction with a reconfigurable atom mirror, made from an array of serpentine magnetic nanowires. Future experiments may see zero-dimensional atom traps created above mobile domain walls. By using the atoms as ‘qubits’, the geometry of the nanowires could then allow entanglement of trapped atoms via excited states. The magnetic nanowire architecture is highly scalable and has the potential to support a viable quantum computation system.

Journal Club Talk
30th November Ms Kirsteen Butler Durham University

This talk gives an overview of preparations for the LISA (Laser Interferometer Space Antenna) experiment. The array will consist of three independent spacecraft flying in formation, with each craft carrying a test mass inside. These masses will then be used to detect gravitational waves from changes in the distances between them. Some results for testing the stability of the lasers involved are presented.

Rochester Lecture
7th December (4:30pm, W103) Prof. Jeremy Baumberg University of Cambridge

Generations of students have been taught that light can be focussed down only as tightly as a spot of order of its wavelength. This several hundred nanometer scale limit would imply that nano-scale active elements cannot interact strongly with light. However this turns out to be untrue, and we are able to demonstrate the localisation of light tighter than 1nm. This has two principal benefits: spectroscopy can be used to probe such nano-scale architectures containing individual objects,and light can be concentrated to high intensity in ultra-small volumes.

Both these have utility in a variety of new applications, which open up other areas of science.