Research projects for 2015

Professor Christopher Foot

Experiments on a network of cold-atom quantum systems

The quantum systems that we use are Bose-Einstein condensates in which two states of the atoms are linked by a transition that is weakly driven with microwave radiation to form a superfluid Josephson juntion (analogous to superconducting JJs). We shall interconnect these systems by measuring the population differences across each JJ, non-destructively, implementing feedback control by fast independent adjustments of each JJ. This 'semi-classical' network of quantum systems includes quantum fluctuations and so it can simulate the full quantum dynamics for a certain subset of Hamiltonians, including the Ising model; finding the minimum of the energy function is important since many computationally demanding mathematical optimisation problems may be formulated as ground-state search algorithms of an Ising model. Our network of quantum systems is distinct from quantum networks, i.e., a quantum network of quantum systems where the interconnections are quantum, as well as the nodes. With this system we plan to:i) investigate annealing protocols (that determine ground states) using precise quantum optical measurements and with extreme isolation from environmental noise, ii) produce spin squeezing in samples of ultracold atoms by continuous measurements and show how such quantum properties can enhance nonlinear high-precision interferometry, and iii) study systems analogous to topological models (with Prof. Ruostekoski, Southampton, Maths Dept.), leading to experimentally testable predictions.

Email
c [dot] foot1 [at] physics [dot] ox [dot] ac [dot] uk

Professor Gianluca Gregori

High Energy Density Laboratory Astrophysics - Scaling the Cosmos to the Laboratory

We are looking for DPhil experimental/theoretical positions to study and simulate in the laboratory extreme astrophysical conditions. The research is focussed on the following themes:

1. Investigation of the equation of state of ultra dense matter as the one occurring in the core of giant planets (such as Jupiter and many exoplanets). The experimental work involves using high power laser facilities to compress the matter to densities above solid and then applying x-ray techniques to probe its microscopic state. Interested students can also focus their work onto theoretical topics involving strongly coupled and partially degenerate plasmas - which are particularly relevant for describing white dwarf structure.

2. The understanding of the generation and amplification of magnetic fields in the Universe. We are particularly interested at the role of turbulence (and dynamo) in producing the present day values of magnetic fields in cluster of galaxies. Experiments on large laser facilities are planned in order to simulate in the laboratory intra-cluster turbulence and measure the resultant magnetic field generation and amplification by dynamo.

3. Quantum gravity with high power lasers. The idea is to use high intensity lasers to drive electrons to very high accelerations and then observe effects connected to the Unhruh-Hawking radiation. The ideal candidate is expected to work in defining the required experimental parameters and the proposal for a future experiment.
Our group has access to several laser facilities (including the National Ignition Facility, the largest laser system in the world). Students will also have access to a laser laboratory on campus (currently hosting the largest laser system in the department), where initial experiments can be fielded. Further details can be found by browsing our research web-page http://www.physics.ox.ac.uk/users/gregorig/

Prospective candidates are encouraged to contact Prof Gregori for further information.

Email
g [dot] gregori1 [at] physics [dot] ox [dot] ac [dot] uk

Professor Simon Hooker

1. Plasma accelerators

In a laser wakefield accelerator an intense laser pulse propagating through a plasma excites a trailing plasma wave via the action of the ponderomotive force, which acts to expel electrons from the region of the laser pulse. The longitudinal electric field in this plasma wakefield 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. Particles injected into the correct phase of the plasma wave can therefore be accelerated to energies of order 1 GeV in only a few tens of millimetres.

Theoretical and experimental work on plasma accelerators in Oxford is undertaken by a collaboration of research groups in the sub-departments of Particle Physics and Atomic & Laser Physics.

Please note that applications to work in this area should be made to the sub-department of Particle Physics, as explained at http://www2.physics.ox.ac.uk/research/laser-plasma-accelerators/recruitment this page also gives further information on DPhil projects which are available in this area.

2. Ultrafast lensless imaging with OPA-driven high-harmonic generation

An intense laser pulse interacting with the atoms or molecules of a gas can drive a highly nonlinear polarization which in turn generates coherent radiation at harmonics of the laser. This high-harmonic generation (HHG) process can generate ultrafast (in the attosecond range) pulses of radiation in the important soft x-ray spectral region, with potential applications in ultrafast imaging and in time-resolved science.

The efficiency of HHG is low unless steps are taken to phase-match or quasi-phase-match (QPM) the generation process. The wavelength of the driving laser is also important in determining the brightness and range of photon energies which can be generated. Our group has developed several techniques for QPM and demonstrated that these methods can increase the energy in the harmonic beam by more than an order of magnitude.

In this project we will use femtosecond optical parametric amplifiers (OPAs) to determine the optimum driving laser wavelength for generating x-rays of a given photon energy. We will also investigate a new QPM technique based on modulation of the polarization state of the driving laser in a birefringent waveguide.
The bright x-ray beams generated by these methods will be used for coherent diffraction imaging. This method images objects without conventional optics (which are not available at these wavelengths); it does so by recording the intensity diffraction pattern of the object and using sophisticated algorithms to overcome the "phase-retrieval problem" to deduce the object. Using a wavelength-tunable x-ray source we will seek to image specific elements in a sample, and as a test of these ideas we will the size and shape of particles formed in precipitation-strengthened Al alloy.

This work will be supported by a 4-year research programme funded by EPSRC.

Further information available from:
https://www2.physics.ox.ac.uk/research/plasma-accelerators-and-ultrafast...

Email
s [dot] hooker1 [at] physics [dot] ox [dot] ac [dot] uk

Dr Steve Kolthammer and Professor Ian Walmsley

Quantum photonics

Experimental quantum optics plays a key role in our growing understanding of the essential features of quantum physics, from foundational principles to consequences in information processing. To date, however, most experiments have been restricted to simple quantum systems composed of just one or two photons. A major aim for the field is developing methods to realise increasingly complex states from which remarkable properties are predicted to emerge. The Quantum Photonics team in the Ultrafast Quantum Optics Group works at the forefront of this challenge, using novel optical devices that enable the study of new large-scale quantum systems. We are developing photonic chips using the latest fabrication technologies that enable integrated quantum light sources and programmable multiport interferometers, optical fibre networks for time-domain manipulation of quantum light, and superconducting detectors that efficiently count single photons. We use these tools to study applications in quantum computation and metrology, as well as fundamental issues including randomness, the quantum-classical boundary, and macroscopic quantum systems.
We invite applications from outstanding, motivated students interested in joining our Quantum Photonics team to undertake a DPhil research project. You will gain expertise in experimental quantum optics, including knowledge of optics, lasers, integrated photonics, and quantum information. Your work will be carried out as part of a large team of experimental and theoretical quantum physicists, both in Oxford and collaborating institutions worldwide.

An overview of our research group is found at https://www2.physics.ox.ac.uk/research/ultrafast-quantum-optics-and-opti.... For further information please contact Dr Steve Kolthammer or Professor Ian Walmsley.

Email
s [dot] kolthammer [at] physics [dot] ox [dot] ac [dot] uk
I [dot] Walmsley [at] physics [dot] ox [dot] ac [dot] uk

Dr David Lucas and Professor Andrew Steane

Quantum Computing with Trapped Ions, Lasers and Microwaves

Quantum computers offer the prospect of dramatic increases in information processing power, but this potential will only be realized if the qubits which hold the quantum information can be manipulated sufficiently precisely, and if the system can be scaled up to larger numbers of qubits. At Oxford, we have this year reported the best qubit in the world, consisting of a single ion held in a microfabricated "chip trap". The qubit has a coherence time of nearly one minute, its quantum state can be prepared and read out with a fidelity of over 99.9%, and we can perform single-qubit quantum logic gates with an average fidelity measured to be 99.9999%. The key to achieving these results, which now define the state of the art, was the use of microwave techniques: the ion trap itself is a novel design, being the first to incorporate on-board microwave circuitry, and is built using a technology which is in principle scalable to much larger numbers of qubits. In related experiments we have recently demonstrated the highest-fidelity (99.9%) two-qubit quantum logic gate in the world, implemented using laser manipulation of the same type of qubit (hyperfine ground states in calcium-43 ions). Together, these results represent the first demonstration, in any physical system, of all fundamental qubit operations with sufficient fidelity for fault-tolerant quantum computing. We plan to scale our systems up to larger numbers of qubits by interfacing trapped-ion qubits using photonic qubits. We are looking for one or two highly motivated first-class students to join these world-leading projects.

Email
d [dot] lucas1 [at] physics [dot] ox [dot] ac [dot] uk
a [dot] steane1 [at] physics [dot] ox [dot] ac [dot] uk

Professor Dieter Jaksch

1) Simulation of two-dimensional strongly-correlated quantum systems using high-performance tensor network theory algorithms

Tensor network theory (TNT) provides efficient and highly accurate methods for simulating many-body quantum systems, which cannot be represented exactly for all but the smallest systems due to the exponential growth of the number of parameters required with system size. The many-body wave function, and the operators that act on them, are represented as a network of tensors (multi-dimensional arrays of numbers) which is manipulated by performing a series of tensor manipulations such as reshaping, contracting and factorising. This computational/numerical DPhil project focusses on developing new algorithms for treating two-dimensional systems that utilise TNT, and will be added to the existing TNT software library. This high-performance library has been developed in our group for the last two years, and already has many users. The project will provide algorithms that are the first of their kind freely available as part of a software library, and will be used not only by members of our group, but research groups throughout the UK. The code is being developed in C, with OpenMP and MPI also being used to implement a hierarchical parallelisation scheme. The DPhil student will collaborate with members of our group to design routines that can be used by them to solve physics problems. The student will also have the chance to work with scientific software engineering experts who provide us with advice on producing high-quality sustainable software and on optimising our codes for running on large–scale supercomputers, such as the national supercomputing cluster ARCHER.

2) Optically steering, manipulating and cooling strongly correlated electron systems

In the past decade there have been pioneering experiments which have shown how laser light can manipulate, measure and selectively cool not only atoms or ions, but also single modes in macroscopic opto-mechanical devices. A major long-term aim in our group, in collaboration with Prof. Andrea Cavalleri Oxford/Hamburg experimental group, is to apply these techniques to strongly correlated electron systems such as Mott insulators and cuprate superconductors. Broadly this work plans to identify and realize "hidden" phases of materials, that are metastable out-of-equilibrium states which only exist while the system is driven. A key example of this would be to devise techniques to engineer the coupling of a stacked cuprate material to a surrounding cavity so that emissions into the cavity mode result in the cooling of superconducting phase fluctuations. These phase fluctuations are thought to be responsible for the transition to a non-superconducting state, thus even moderate cooling of this specific degree of freedom, as opposed to the entire material, may provide a novel route to stabilizing superconductivity above its critical temperature.

This theory DPhil project will work towards this grand challenge by investigating in detail the interaction between various strongly correlated electron systems and light both in and out of a cavity. Regimes of moderate and strong coupling to desired degrees of freedom, such as superconducting order parameter for stacks of Josephson junctions, will be determined. Various approaches will be pursued, such as using the cavity to perform continuous weak measurements to steer the state of the system, or strongly driving structural modes of the material to dynamically modulate its electronic properties. Combinations of phenomenological, mean-field, and numerical techniques will be applied to characterise the response of the system. Insight from these studies should lay the foundations for gauging the physical parameter space in which techniques for phase cooling are possible, and to what extent.

3) Quantum probes of quantum systems, impurities in cold atomic gases

Researchers in many areas have recently separately considered extracting information about a quantum system by bringing it temporarily and coherently into contact with another smaller quantum system, a probe, which is then measured. This has several advantages over traditional methods for measuring properties of a quantum system: It has the potential to be non-destructive; the potential to exploit entanglement and superposition of a perhaps spatially-extended probe in order to extract information directly about complicated correlation functions; and can involve strong interactions and thus occur on smaller time-scales than, say, linear response, and measure non-equilibrium properties. This theoretical DPhil project focuses primarily on impurity atom probes of cold atomic gases, how they could be realised, what information could be extracted or new regimes probed, and the role such probes could play in current or near-future experiments (our theory group is in contact with the experimental groups of Chris Foot, Oxford, and Stefan Kuhr, Strathclyde). Initial avenues of exploration would include using a highly-trapped impurity atom (localised to a few nm rather than the μm resolution of light) to probe a cold atomic gas on length-scales at which mean-field descriptions break down and the corpuscular nature of the gas appears, or using multiple atomic probes to identify whether number conservation occurs in the Bose-Einstein condensation of an atomic gas. This collaborative project will build upon the numerical and analytical expertise of the group in describing the evolution of impurities in cold atom systems, giving the student a firm background in these methods specifically and the vast and exciting area of cold atom physics in general.

Email
d [dot] jaksch1 [at] physics [dot] ox [dot] ac [dot] uk

Professor Peter Norreys

Laser-Plasma Interaction Physics for Inertial Fusion and Extreme Field Science

Intense lasers have extraordinary properties. They can deliver enormous energy densities to target, creating states of matter in the laboratory that are otherwise only found in exotic astrophysical phenomena, such as the interiors of stars and planets, the atmospheres of white dwarfs and neutron stars, and in supernovae. The behaviour of matter under these extreme conditions of density and temperature is a fascinating area of study, not only for understanding of fundamental processes that are, in most cases, highly non-linear and often turbulent but also for their potential applications for other areas of the natural sciences. These include the development of:

• Inertial confinement fusion - applications include energy generation and the transition to a carbon-free economy and the development of the brightest possible thermal source for neutron scattering science
• New high power optical and X-ray lasers using non-linear optical properties in plasma
• Novel particle accelerators via laser and beam-driven wakefields - including the AWAKE project at CERN, as a potential route for a TeV e-e+ collider
• Unique ultra-bright X-ray sources
• New physics at the intensity frontier

The understanding of these physical processes requires a combination of observation, experiment and high performance computing models on the latest supercomputers. My team is versatile, combining experiment, theory and computational modelling. I can offer projects in all of these areas – please contact me to discuss your interests and let’s take it from there.

Application deadline: n/a
Funding source: EPSRC DTA - tbc
Duration: 3 years

Email
p [dot] norreys1 [at] physics [dot] ox [dot] ac [dot] uk

Dr Josh Nunn and Professor Ian Walmsley

1) D.Phil. in quantum memory and photonic applications

We invite applications from first-class students to join our group in Oxford for a D.Phil project on quantum memory, to start in 2014 (start date negotiable), supervised jointly by Joshua Nunn and Ian Walmsley.
The Ultrafast Group in Oxford has pioneered a GHz bandwidth optical memory in cesium vapour based on Raman scattering, with a world-leading time-bandwidth product exceeding 3000. Memories of this kind are a key component of future quantum technologies. The next steps in our research are to demonstrate single-photon operation, which requires engineering the density of scattering states to suppress noise and boost the memory efficiency. We are also working on an integrated memory in hollow-core optical fibre. In this project, the student will combine these ideas and develop a broadband quantum-capable integrated memory in the first two years of work. This will then be applied to the synchronisation of photon sources, or other photonic primitives, in the final year. The project is focussed on enabling the next generation of quantum photonic processors by synchronisation. The student will gain experimental skills in quantum optics, fibre optic and waveguide design, optical cavities, non-linear optics and atomic physics, as well as theoretical modelling of coherent light-matter interactions and quantum networks. There is also considerable opportunity for collaboration with other teams in the group working on integrated photonic circuits and advanced photodetection, and with other research groups both nationally (Southampton; Imperial) and internationally (Paderborn, Germany; NIST, USA), which are jointly funded.

For more details please contact Josh Nunn or Ian Walmsley:

Email
j [dot] nunn1 [at] physics [dot] ox [dot] ac [dot] uk
I [dot] Walmsley1 [at] physics [dot] ox [dot] ac [dot] uk

2) D.Phil. in diamond photonics with NV ensembles

We invite applications from first-class students to join our group in Oxford for a D.Phil project on diamond photonics, to start in 2014 (start date negotiable), supervised jointly by Joshua Nunn and Ian Walmsley.
Quantum information processing using light offers radical new technologies such as super-fast quantum computers and super-secure quantum communication. But a key stumbling block is the ability to engineer strong light-matter interactions. These are required both to mediate photonic logic gates, and to enable the efficient storage and switching of photonic wavepackets to form a scalable architecture. Our group has developed one of the world's leading quantum memories based on Raman scattering in an atomic vapour. We are now investigating ways to integrate this memory inside a solid medium. In this project, the student will design an implementation of the memory based on the Raman interaction in an ensemble of nitrogen-vacancy (NV) defect centres in diamond, and build an experiment to demonstrate this solid state Raman memory. Looking further ahead, the same Raman interaction could be used to generate heralded, near-deterministic non-linearities at the single photon level based on Stokes scattering followed by measurement-induced quantum back-action. These research tasks are open-ended but the demonstration of a broadband light-matter interface in the solid state will be a transformative step forward in quantum photonics that will have a broad impact on the community. The student will have the opportunity to participate in active collaborations with Steven Prawer in Melbourne (fabrication) and Gerard Milburn in Brisbane (theory).

For more details please contact Josh Nunn or Ian Walmsley:

Email
j [dot] nunn1 [at] physics [dot] ox [dot] ac [dot] uk
I [dot] Walmsley1 [at] physics [dot] ox [dot] ac [dot] uk

Dr Josh Nunn

1) Quantum memories and light-matter interactions with atomic and atom-like ensembles

The Oxford Memories Group have developed a unique platform for the quantum-coherent storage of light pulses. This is a key enabling technology for scalable photonic information processing: wide-area quantum communications, quantum enhanced sensing, and all-optical quantum computing. We are developing integrated memories based on cesium vapour in micro-cavities and hollow-core fibres, along with combined storage / non-linear switches in Rydberg atoms, and diamond (both NV and SiV colour centres).

We invite applications from excellent undergraduate students with an interest in quantum information science and the experimental skills to bring these nascent technologies to maturity. The flexibility to apply design principles across several platforms will be an advantage.

This work will be closely affiliated with the UK Quantum Technologies Network, a national government programme focussed on bringing quantum technologies to market.

Contact: Joshua Nunn

Email
j [dot] nunn1 [at] physics [dot] ox [dot] ac [dot] uk

2) Quantum transport

This project is focussed on the exploration of noise-assisted energy transport in quantum networks, and related phenomena. We are currently constructing an optical simulator based on a network of coupled optical cavities. Initial experiments will leverage the formal equivalence between single-particle quantum transport in e.g. bio-molecules, and classical interference in optical systems, to study the efficiency of diffusive and ballistic transport in the presence of controlled noise, disorder and network topology. But the project is open-ended. Extensions with non-classical (anti-bunched) input fields and non-linear (Kerr) elements may be explored, which will provide a platform to study transport physics beyond the single-particle regime. Networks involving active optical switching or atomic memories may also be explored. Finally, ultrafast non-linear optical scattering experiments may also be employed to directly study de-localised transport in molecular systems.

We invite applications from excellent undergraduate students with an interest in quantum information and/or quantum biology, and an experimental background in either quantum or non-linear optics.

This work will be closely affiliated with the Oxford Martin Programme on Bio-Inspired Quantum Technologies ( http://www.oxfordmartin.ox.ac.uk/research/programmes/quantum).

Contact: Joshua Nunn

Email
j [dot] nunn1 [at] physics [dot] ox [dot] ac [dot] uk

Dr Sam Vinko

X-ray Interactions with Matter on X-ray Free Electron Light Sources

Over the past few years there has been a revolution in X-ray science: the advent of the world’s first hard X-ray free electron laser (FEL), the Linac Coherent Light Source (LCLS) in California, in one step in 2009 increased the spectral brightness of X-ray sources over that of any synchrotron by a factor of a billion. Spatially coherent, monochromatic, femtosecond X-ray pulses can now be routinely produced over a wide spectral range, accessing the spatial and temporal scales of atomic processes simultaneously for the first time. When focused to micron-sized spots, the unique characteristics of X-ray and XUV FELs provide a range of new opportunities to generate and study matter in extreme conditions with unprecedented accuracy and control, at temperatures of several millions of degrees Celsius and at atomic densities of order 1023 per cm3 and above. Such conditions are interesting as they are found towards the interior of stars and in other astrophysical objects, but are also directly relevant to inertial confinement fusion research. Focused FEL beams also show great promise to be used as a type of “giant microscope”: to look at chemistry and biological processes in real time, and to image in 3D tiny nano-crystals, protein molecules, and viruses. These objects are either too small or their processes too fast to be investigated via current techniques, but could be accessed using coherent X-ray diffraction imaging on FELs. For this novel technique to be truly valuable, however, the full information required must be collected before the ultra-high intensity of the FEL turns the sample into a hot-dense plasma, and obliterates it. The way in which this process takes place is however still relatively poorly understood. For these reasons, our research focuses on understanding the fundamental X-ray-matter interaction process that leads to the creation of hot dense plasmas, and on studying their structure, dynamics and evolution in time.

One or two research projects will be made available to start in October 2015, for students interested in experimental and/or theoretical work on X-ray matter interactions and dense plasmas. On the experimental side, the work will be conducted as part of an international collaboration on key FEL facilities: the LCLS FEL at Stanford in California, the FERMI FEL in Trieste, Italy, and the FLASH FEL in Hamburg, Germany. On the theoretical and computational side, the work will combine a range of techniques such as collisional-radiative simulations and first-principle density functional theory studies of strongly-coupled systems, all at the cutting edge of our current modelling capabilities of these extreme conditions.

For further information please contact Dr Sam Vinko:

Email
s [dot] vinko [at] physics [dot] ox [dot] ac [dot] uk

Professor Ian Walmsley

Attoscience and Strong Field Physics

Recently, nonlinear frequency conversion has been extended to provide access to coherent extreme ultraviolet (XUV) and soft x-ray radiation. This has enabled experiments routinely performed at synchrotron and free-electron laser facilities to be moved into a compact laboratory environment, and the underlying physics has given birth to attoscience. This emerging field concerns the observation of electron and atomic dynamics at the microscopic level, where the timescale is measured in attoseconds (10-18 s). Such fundamental measurements will have a wide reaching impact throughout the sciences, and potential applications include computation, energy harvesting and healthcare. The Attoscience and Strong Field Physics section of the Ultrafast Group are engaged in all aspects of attoscience research from the production of energetic few-cycle laser pulses, to the generation, all-optical characterization and use of XUV radiation.
Applications are invited from outstanding students interested in joining the Attoscience and Strong Field Physics team to pursue a DPhil research project. You will work with an advanced laser system on a daily basis and gain a deep understanding of ultrafast optics, XUV technology and state-of-the-art imaging techniques. In addition to a strong experimental focus, you will also have the opportunity to develop strong computational skills.
The main research activities are summarised at https://www2.physics.ox.ac.uk/research/ultrafast-quantum-optics-and-opti.... For further information please contact Dr. Patrick Anderson or Professor Ian Walmsley.

Email
P [dot] Anderson1 [at] physics [dot] ox [dot] ac [dot] uk
I [dot] Walmsley [at] physics [dot] ox [dot] ac [dot] uk

Professor Justin Wark

Studying High Pressure Matter with X-Ray Lasers

AVAILABLE FOR 2014 ENTRY!

An opportunity exists for a DPhil project in the Physics Department at the University of Oxford to use the brightest X-ray source on the planet to study the response of matter to pressures normally only found towards the centre of planets. The student will work under the supervision of Professor Justin Wark in the sub-department of Atomic and Laser Physics. The project, funded by Lawrence Livermore National Laboratory in California, will entail both an experimental and theoretical study of matter under extreme conditions. On the experimental side, the student will use intense optical lasers to subject matter to many millions of atmospheres for timescales in the sub-nanosecond regime. In conjunction, the most intense x-ray laser on earth - the Linac Coherent Light Source at SLAC, will be used to obtain few-femtosecond single-shot diffraction images as the matter deforms or forms new phases. The experiments will be complemented by both classical molecular dynamics simulations, and by ab initio density functional theory calculations. The work builds on previous successful research recently published at the highest level (Science, 342, 220 [2013]). Full funding exists (fees and stipend) for successful UK or EU candidates for a period of 3.5 years. Interested candidates should have a upper-second or first class degree (or equivalent) in Physics, be interested in both experimental and theoretical physics, and be willing to spend some short periods in the US for experiments and collaborations. Enquiries can be made to Professor Justin Wark.

Email
j [dot] wark1 [at] physics [dot] ox [dot] ac [dot] uk