Research projects for 2012

A Laser-Cooled Single Ion Optical Clock

Dr Patrick Baird and Dr Patrick Gill, NPL

The NPL has a number of projects involving laser-cooled ions and atoms. These are principally aimed at new frequency standards in the optical region, i.e., the development of new optical clocks. There are opportunities for Oxford doctoral students to conduct research in these areas in collaboration with NPL.

A laser-cooled, single ion optical clock

The ²S_1/2 – ²F_7/2 octupole transition at 467 nm in ¹⁷¹Yb+ is hugely forbidden by quantum mechanical selection rules, and as a result has extremely long lifetime against decay, and a correspondingly ultra-narrow natural linewidth (~ nHz). By electromagnetically trapping a single ¹⁷¹Yb⁺ ion and laser cooling it to about 1 mK under ultra-high-vacuum (UHV) conditions, it is possible to prepare an ion virtually at rest in an isolated environment. The narrow octupole transition can then be probed with a very stable, narrow-linewidth semiconductor laser to provide a highly accurate atomic clock. In addition, by simultaneously probing a second optical clock transition at 436 nm – this time an electric quadrupole transition: ²S_1/2 – ²D_3/2) using the same ¹⁷¹Yb⁺ ion, the frequency ratio between the two transitions can be accurately determined; this gives a very sensitive measure of any possible temporal variation in the fine structure constant, α. There is opportunity for an Oxford doctoral student to join an NPL research team developing these ideas. Experimental techniques used in this project include ion trapping, laser-cooling, laser stabilisation using optical cavities, single ion quantum detection and UHV systems.

Please click here for further details.

The project offers the possibility of funding through an EPSRC CASE award.
In the first instance, please contact: Dr Patrick Baird.

Cold Atoms in an Optical Lattice Potential

There is an opportunity for an Oxford student to undertake research on the NPL neutral lattice clock project. This research involves holding neutral strontium atoms in an optical lattice field in order to probe a clock transition; this offers potential for a future high-accuracy optical frequency standard and clock. Atomic clock systems bring together many aspects of experimental atomic physics research and in particular this project brings together magneto-optical trapping of neutral atoms together with cooling techniques, involving both broad- and narrow-line cooling transitions to cool clouds of neutral Sr atoms to microKelvin temperatures. The use of a “magic wavelength”, optical lattice trap, formed by counter-propagating laser beams, holds the atoms in space without perturbing the clock transition. The development and stabilisation of ultra-narrow linewidth lasers which are necessary for excitation of the clock transition will also be an essential part of the project.

Please click here for further details.

The project offers the possibility of funding through an EPSRC CASE award.
In the first instance, please contact: Dr Patrick Baird.

Novel laser plasma accelerators

The Lasers for Accelerators (L4A) group at the JAI (interdisciplinary but administratively based in particle physics) is a versatile team focused on several areas of cutting edge research in the boundary between accelerators and lasers. In particular, the group is developing an alternative method for laser plasma acceleration. Normally, the plasma oscillations are excited by a single extremely high intensity laser pulse, but it has been shown theoretically that large plasma oscillations can also be driven by a train of lower intensity pulses. This would enable the use of tabletop laser for plasma acceleration, rather than the use of national scale facilities. In this context, the L4A group is focusing on developing a suitable laser to drive plasma oscillations by a train of laser pulses in order to build a 1 GeV electron accelerator operating at 1 kHz. A PhD student project is available in this area within the JAI. The student would be working on developing new laser technologies in our laser lab in Oxford, including investigating methods of shaping a pulse train to efficiently excite a plasma oscillation. In conjunction with this work, the student would also be working on diagnostic methods for measuring the amplitude of plasma oscillation and using this to evaluate the best laser pulse trains for excitation. The project would therefore be primarily experimental in nature, and suit a student with an interest in laser or accelerator science. However, there is scope for the student to develop theoretical and simulation work, especially on the plasma and diagnostics side of the project.

The L4A group is also involved in all areas of interaction between accelerators and lasers and optics. This can cover areas ranging from the use of new laser technology for particle measurement to using lasers to actually accelerate the particles and everything in between, including the evaluation of optical fibres for data transport with radiation hard accelerator environments. A project the L4A group has been involved in for several years, in collaboration with RHUL and the Japanese High Energy Research Organisation (KEK), is the laserwire experiment to measure particle beam sizes of <1 micron. Laser is often the only method to measure the transverse size of electron bunches in a high intensity, well collimated electron beam, as foreseen for example by ILC, CLIC and new synchrotron radiation sources. For the laserwire project, the group has developed a special laser using photonic crystal rod fibre which delivers the required high energy laser pulses with exceptionally high mode quality. For more information see http://www.adams-institute.ac.uk/l4a

Application deadline: n/a
Funding source: STFC
Funding duration: 3.5 years

For more information please contact Dr Laura Corner, see contact information below

THIS PROJECT IS NOW FILLED.

Laser spectroscopy for combustion research

Our research is based on the interaction of laser light with molecules in gases. The intensity of the light induces both linear and non-linear responses leading to emission of light signals carrying information on the molecular species and its quantum state as well as thermodynamic properties of the gas. Our group has pioneered several types of laser-based diagnostics for measuring parameters in hostile environments such as flames and plasmas. Using non-linear light-molecule interactions we are now able to make measurements with unprecedented precision to understand the fundamental molecular processes occurring when fuels burn in flames or engines. This enables design of novel fuels and engines with reduced emissions and greater efficiency with the potential to significantly reduce global warming by the combustion of hydrocarbon fuels that will remain vital for energy and transportation for the foreseeable future. We work with colleagues in Engineering Science in Oxford and in Cambridge University, together with significant industrial support from major oil companies and automobile manufacturers. The studentship being offered will attract CASE support from one of our major industrial sponsors and will develop techniques of accurate and precise measurements in flames and engines using laser techniques developed in our group.

Application deadline: ASAP

Funding duration: 3 years

THESE PROJECTS ARE NOW FILLED

Several DPhil Studentships in Laboratory Astrophysics

We have multiple openings in our group for projects involving the use of large-scale laser system to simulate astrophysical environments in a laboratory. In particular, we are interested in:
- investigating the large scale magnetization of the universe driven by curved shocks;
- generating high Mach number collision-less shock waves;
- studying the magneto-hydrodynamics of cloud-wind interactions in the interstellar medium.
The experimental work will be conducted at a range of national and international laser facilities, up to the National Ignition Facility in Livermore, California - the largest laser system in the world. It is also envisioned that a significant fraction of the preparatory work will take place in the laser laboratory here in Oxford. In addition to laboratory work, students will have access to advanced - massively parallellelized - computer codes to numerically model the experiment and the astrophysical systems.

For more information please contact Professor Gregori, see contact information below

Laser-driven plasma accelerators

Electrons are pushed away from the front and back of an intense laser pulse as it propagates through a plasma, leading to the formation of a trailing longitudinal plasma wave. The longitudinal electric field in the plasma wave can be as high as 100 GV/m, more than three orders of magnitude larger than that found in conventional RF accelerators such as those used at CERN.
One factor which limits the energy to which particles can be accelerated is the distance over which the intensity of the driving laser can be maintained. Over the last few years our group has developed a technique for channelling laser pulses with peak intensities of up to 10¹⁸ Wcm^-2 over distances which are much longer than the limit set by diffraction. In collaboration with a group at Lawrence Berkeley National Laboratory (LBNL), we have used this technique to extend the length over which acceleration can be maintained by an order of magnitude and thereby generated electrons with energies of 1 GeV. This energy is of the order of that used in many synchrotrons around the world, but the plasma accelerator is only 33 mm long instead of 150 m! For further details, see Leemans et al .
Our work on laser-driven plasma accelerators is in three areas: (i) investigation of techniques for controlling the injection of electrons into the plasma wakefield; (ii) development of techniques for staging plasma accelerators, with a view to reaching beam energies beyond 1 GeV; (iii) development of applications of laser-driven plasma accelerators, particularly their application to the generation of x-rays. We pursue these goals by both experiment and numerical modelling. Some experiments are undertaken in our laboratories in Oxford, at facilities based at the Rutherford Appleton Laboratory (just outside Oxford), or with our collaborators in the USA and Europe.

Particle-in-cell simulation of a laser driven plasma wakefield (electron density): A laser pulse (yellow spot) propagates to the right and is trailed by a electron, but not ion-free, cavity (black). In this cavity a electron bunch (white dot) can be trapped and accelerated due to the ion's positive change.

Further reading

1. W. P. Leemans, S. M. Hooker et al., "GeV electron beams from a centimetre-scale accelerator," Nature Physics 2 696 (2006)
2. T.P. Rowlands-Rees, S. M. Hooker et al., "Laser-Driven Acceleration of Electrons in a Partially Ionized Plasma Channel," Phys. Rev. Lett. 100 105005 (2008)
3. M. Fuchs, F. Gruner, S. Karsch, S. M. Hooker et al., "Laser-driven soft-X-ray undulator source," Nature Physics 5 826 (2009)
4. T. Ibbotson, S. M. Hooker et al., "Laser-wakefield acceleration of electron beams in a low density plasma channel," Phys. Rev. ST Acc. Beams 13 031301 (2010)
5. S. I. Bajlekov, W. M. Fawley, C. B. Schroeder, R. Bartolini, and S. M. Hooker, "Simulation of free-electron lasers seeded with broadband radiation," Phys. Rev STAB 14 060711 (2011)

Quasi-Phase-Matched High-Harmonic Sources

Many processes in nature such as ultrafast chemical reactions, material dynamics, and electronic transitions, occur on femtosecond and attosecond timescales. In order to measure and understand such fast dynamics it is necessary to have a source which can provide temporal resolution on these timescales. One such source is high harmonic generation (HHG). HHG is an extreme nonlinear process which can occur when an intense laser pulse ionizes a gas, resulting in the emission of radiation with frequencies equal to the odd harmonics of the driving laser. These harmonics can extend to very high orders, as high as several hundred, corresponding to generated wavelengths in the soft x-ray region. This short-wavelength radiation has been shown to have high spatial and temporal coherence, allowing highly collimated beams to be generated with pulse durations as short as approximately 100 attoseconds. As such, HHG is now commonly used as a simple and reliable source of tunable short-wavelength radiation in a wide range of scientific disciplines.
However, the efficiency with which the harmonics are generated is very low: for generated photons with energies up to about 100eV the generation efficiency is of order 10^-7; for higher energy photons it is even lower. This limits the number of applications in which HHG can be used. One reason for the low generation efficiency is the difference between the phase velocities of the driving laser and the harmonic beam. This causes the intensity of each generated harmonic to oscillate with propagation distance with a characteristic period of 2L_c, where L_c is known as the coherence length. This problem can be overcome by suppressing harmonic generation in those regions where the generated harmonics are out of phase with the harmonic beam - a technique known as quasi-phase-matching (QPM). With N QPM zones the harmonic intensity is in principle increased by a factor of N².
QPM can be achieved in a number of ways such as using a train of counter-propagating pulses to periodically suppress HHG, or by promoting intensity-beating in a hollow-core waveguide to modulate harmonic generation.
We wish to take on a new student to optimize the QPM process at higher photon energies that have previously been possible, as well as to investigate the effect on QPM of changing the wavelength of the driving laser. The project will be largely, but not exclusively, experimental, and all experiments will be conducted in our ultrafast laser laboratory in Oxford. It is expected that part of this project will also involve identifying and conducting suitable proof-of-principle experiments to demonstrate the utility of QPM at high photon energies.

High-order harmonics generated in Argon: The enhancement of the signal at low wavelengths is the result of quasi-phase matching. In this case quasi-phase matching was achieved using a train of 4 ultrafast laser pulses counterpropagated in a hollow-core waveguide.

Further reading

1. T. Robinson, K. O'Keeffe, M. Landreman, S. M. Hooker, M. Zepf, and B. Dromey, "Simple technique for generating trains of ultrashort pulses," Optics Letters 32 2203-2205 (2007).
2. M. Zepf, B. Dromey, M. Landreman, P. Foster, and S. M. Hooker, "Bright Quasi-Phase-Matched Soft-X-Ray Harmonic Radiation from Argon Ions," Phys. Rev. Lett. 99 143901 (2007).
3. T. Robinson, K. O'Keeffe, M. Zepf, B. Dromey, and S.M. Hooker , "Generation and control of ultrafast pulse trains for quasi-phase-matching high-harmonic generation," JOSA B, 27 763-772 (2010).

THIS PROJECT IS NOW FILLED

The Fast, The Slow and The Fragile: Quantum correlations in room temperature solid state and ultracold atomic systems

Quantum correlations like entanglement have already been observed in various different physical systems. Two of the most extreme cases are solid state systems at room temperature where coherence is probed on a femto- to picosecond time scale ("The Fast"), and ultra-cold atomic samples which display quantum correlations with coherence times of seconds ("The Slow"). At high temperatures quantum correlations are destroyed quickly by thermal fluctuations, while at low temperatures they are destroyed by small stray perturbations ("The Fragile"). Amazingly, while the underlying physics is radically different in these two cases, both systems are theoretically described by similar models. In this project we will study such strongly correlated Hubbard and spin models. We will be particularly interested in setups where a strong laser drives them far out of equilibrium and entirely novel physics is revealed. For instance such driving can lead to local vibrational modes being excited which radically alter the properties of atoms/electrons in the system and dynamically affect global properties such as conductivity. The aim of this project is to develop theoretical models which are relevant to current experiments and analyse them analytically and numerically. Numerical study will form a large part of the project and will be based on existing time dependent DMRG techniques which we expect to develop further as part of this project. It is envisaged that this work is largely carried out in collaboration with experimental groups.

Application deadline: n/a
Funding source: DTA
Duration: 3.5 years

A Cryogenic Ion Trap for Quantum Computing;

We have recently received funding from the Oxford University John Fell Fund to build a cryogenic ion trap system to complement our existing highly successful ion trap quantum computing apparatuses (see www.physics.ox.ac.uk/users/iontrap ). Cryogenic cooling of the ion trap suppresses environmental noise from electric and magnetic fields, which allows the high-fidelity qubit maniuplations essential for quantum information processing. We are looking for an academically strong and highly-motivated student who is keen to pursue demanding experimental work.

The student will work with a post-doctoral researcher and spend the first12-18 months designing and building the apparatus. This is a challenging project during which the student will develop expertise in a wide range of state-of-the-art experimental techniques, including ultra-high vacuum, stabilized diode lasers, microwave and digital electronics, FPGA control systems, as well as cryogenics.

Application deadline: n/a
Funding source: TBC

Theoretical Quantum Optics of Ultracold Quantum Gases and Nanostructures

Both quantum optics and many-body physics of the lowest achievable temperatures are very active fields of modern research. However, the interaction between them is far from being complete. For example, in the most theoretical and experimental works on ultracold atoms, the role of light is reduced to a classical tool for preparing intriguing atomic states. In contrast, the main goal of this project is to develop a theory of the phenomena, where the quantum natures of both ultracold matter and light play equally important roles. The experiments on this ultimate quantum level of the light-matter interaction became possible just several years ago, which makes the interaction between the theory and experiment promising.

This project is focused on the development of models describing the interaction of quantum many-body systems (ultracold atoms or molecules) with quantized light. This interaction can be realised, for example, by trapping the quantum gas inside an optical cavity. In this case, the trapping potential cannot be described by a prescribed function as it is the case in most works. The potential is a quantum and dynamical variable, and the self-consistent solution for light and atomic dynamics is required. Moreover, the light can be used as a probe for the atomic state. As this is a fully quantum problem, the quantum nature of the measurement procedure has to be taken into account. In all those problems, the quantum light-matter entanglement plays a key role, allowing the application for quantum information processing, quantum simulations of condensed matter systems, and quantum metrology.

The ultimate goal of this project is to develop a theory of quantum control for strongly correlated many-body systems, which is currently unavailable. The theoretical methods to be used originate from atomic, optical and condensed matter physics. Moreover, the models developed for atoms will be applied for the solid state systems used in quantum nanophotonics (e.g. quasiparticles in semiconductor microcavities).

The candidates with interest and expertise in either Atomic, Molecular and Optical or Condensed Matter Physics are welcomed to apply. For more information, please, visit http://www2.physics.ox.ac.uk/contacts/people/mekhov

Application deadline: n/a
Project Reference:
Funding source: EPSRC*
Duration: 3 years

*Please note: Overseas fees are not included in this funding.

THESE PROJECTS ARE NOW FILLED

Creation and Diagnosis of Matter at TPa Pressures

This project aims to exploit our expertise in laser-induced dynamic compression and x-ray diffraction to make the first ever structural studies of solid matter above 1 TPa (10 megabars) using the JANUS, OMEGA, and National Ignition Facility (NIF) laser platforms in the US. At such pressures, the compression energy is sufficient to break all chemical bonds, providing a regime where new physics and chemistry are predicted to occur. The pressures we will finally achieve are comparable with those that exist at the centre of the giant planets. In a way, we are trying to 'recreate' Jupiter in the laboratory for a few billionths of a second, and, in that short time, to diagnose it via X-ray scattering. The experimental work will be supported by ab initio quantum calculations based on density functional theory.

Exterior of the NIF target chamber under construction: The square openings are for the quads of beamlines; the round openings will accommodate nearly 100 pieces of diagnostic equipment.

Picosecond X-ray Diffraction from Matter under Extreme Conditions

When a solid is suddenly subjected to Mbar pressures, such that its volume can change by many tens of percent, it starts to 'flow' like a liquid, even though it is still in a crystalline state. The fundamental physics of how this motion takes place is very poorly understood. Over the past few years we have been addressing this problem from an experimental and theoretical standpoint. From the experimental point of view, we have developed new techniques of X-ray diffraction that allow information about the crystal structure to be extracted on picosecond timescales. On the theoretical front, we use multi-million atom molecular dynamics simulations to track the motion of individual atoms under rapid compression. We have recently gained considerable insight into how simple single face centred cubic crystals deform and flow, and the logical next step is to look at the differences that occur in body centred cubic materials - and that study will be the main aim of this project. The project will involve using high power laser systems both in the UK and in the US (the high pressures are produced by laser ablation), and some interest in high performance computing could be an advantage, but is not essential.

The alpha-epsilon phase transition in laser-shocked iron: Simulated by multi-million atom molecular dynamics simulations

Application deadline: n/a
Project Reference: n/a
Funding source: AWE / EPSRC DTA
Eligibiliy for funding: Funding for Home and EU students only
Duration: 3.5 years