Below is a list of DPhil projects for 2022 within the Atomic and Laser Physics sub-department; potential graduate students are encouraged to contact supervisors if they have any questions with regard to these projects.

Please note, some of the projects below will be presented at our Open Day on 26th November 2021. Please email to register for a place. 

Quantum computing with trapped ions

Dr Chris Ballance

Trapped-ion devices have demonstrated, on a small number of qubits, all the building-blocks required to build a quantum computer with precision better than any competing technology. The aim of this project is to develop and utilise a world-class intermediate-scale quantum computer that, by virtue of high-fidelity any-qubit-to-any-qubit entangling gates along with low error rates, will operate at a performance level currently unachievable in any other architecture.

This is a challenging project which will push the limits of laser technology, quantum/classical control techniques, and quantum algorithm design.

The project will involve both experimental and theoretical work, including:
- building an apparatus that uses a newly developed type of trapped-ion qubit
- obtaining precision coherent control over individual atomic ions
- developing and applying new theoretical tools to understand and optimise many-qubit couplings

For further details

Fast, high-fidelity entanglement via optical phase control

Dr Chris Ballance

Trapped-ion devices have demonstrated, on a small number of qubits, all the building-blocks required to build a quantum computer with precision better than any competing technology. However the speed of these devices, limited by the entangling gates, has not increased commensurately. The aim of this project is to change this by exploiting optical phase control to significantly speed up trapped-ion entangling gates whilst also removing several currently limiting fundamental sources of error.

In preliminary work, we have recently demonstrated the first high-speed entangling logic gates for trapped-ion qubits [Schafer et al., Nature 555, 75 (2018)]. We achieved a fidelity of 99.8% for a 1.6µs gate time, close to the highest reported two-qubit gate fidelities of 99.9%, but more than an order of magnitude faster. Over the course of this project we will extend this proof-of-concept technique to demonstrate the first high-speed control of multi-qubit registers.

The project will involve both experimental and theoretical work, including:

  • developing and numerically modeling phase-controlled fast entangling gate dynamics
  • building a new apparatus optimised for high-speed multi-qubit entangling gates
  • sophisticated classical control techniques to precisely control the optical interaction phase of multi-qubit register

For further details

Breaking symmetry with light: ultra-fast ferroelectricity and magnetism from non-linear phononics

Professor Andrea Cavalleri and Professor Paolo Radaelli

A collaboration between Prof. Paolo G. Radaelli and Prof. Andrea Cavalleri, who holds a joint appointment between the Clarendon Laboratory and the Max Planck Institute for the Structure and Dynamics of Matter, (Hamburg).

The use of light to control the structural, electronic and magnetic properties of solids is emerging as one of the most exciting areas of condensed matter physics. One promising field of research, known as femto-magnetism, has developed from the early demonstration that magnetic ‘bits’ in certain materials can be ‘written’ at ultra-fast speeds with light in the visible or IR range [1]. More radically, it has been shown that fundamental materials properties such as superconductivity can be ‘switched on’ transiently under intense illumination [2]. Recently, the possibilities of manipulating materials by light have been greatly expanded by the demonstration of mode-selective optical control, whereby pumping a single infrared-active phonon mode results in a structural/electronic distortion along the coordinates of a second, anharmonically coupled Raman mode – a mechanism that was termed ‘nonlinear phononics’. Crucially, the Raman distortion is partially rectified, meaning that it oscillates around a different equilibrium position than in the absence of illumination. Very recently, it was realised that, under appropriate conditions, the rectified Raman distortion can transiently break the structural and/or magnetic symmetry of the crystal. Such symmetry breaking persist for a time corresponding to the carrier envelope of the pump, which can be less than a picosecond, and can give rise to the ultra-fast emergence of ferroic properties such as ferromagnetism and ferroelectricity. Through symmetry analysis and first-principle calculations, we have identified several promising ‘photo-ferroic’ materials that should display these effects, with potential applications in ultra-low-power information storage, ultra-fast electronics and many more.

This DPhil project will give the successful candidate the opportunity to pioneer this new field of research. Initial experiments on the candidate ‘photo-ferroic’ materials that we have already identified will be performed at the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg, Germany. As a mode-selective pump, we will employ coherent laser radiation in the THz or far-IR range with sub-ps carrier envelopes, while the transient emergence of the ferroic properties will be probed with second-harmonic generation (SHG) and/or Faraday rotation of near-infra-red light. Later on in the project, changes in the crystal and magnetic structures of the materials will be probed with X-rays at free electron laser sources such as the European XFEL in Hamburg. Meanwhile, the candidate will develop search strategies for new classes of ‘photo-ferroic’ materials, based on symmetry and density functional theory calculations. He/she will develop the materials specifications in collaborations with crystal growers in Oxford and elsewhere, and will be involved hands on in all aspects of the design and realisation of the experiments and the data analysis.

The experimental part of this project will be predominantly based in Hamburg, so it is essential for the candidate to be willing and able to be based in Germany for extended periods during the DPhil.

[1] Femtomagnetism: Magnetism in step with light. Uwe Bovensiepen, Nature Physics 5, 461 - 463 (2009)
[2] See for example M. Mitrano,et al., ‘Possible light-induced superconductivity in K3C60 at high temperature’, Nature, 530, 461–464 (2016)
[3] Nonlinear phononics as an ultrafast route to lattice control, M. Först et. al., Nature Physics, 7, 854–856 (2011)

Experiments with ultracold atoms and Bose-Einstein condensates

Professor Chris Foot

Professor Foot’s research group investigates the fascinating properties on ultracold quantum gases at temperatures of a few tens of nanokelvin. Samples are produced using laser light to cool atoms from room temperature and then confining them in a magnetic trap where evaporation leads to further cooling. These processes increase the phase-space density by many orders of magnitude to reach quantum degeneracy (Bose-Einstein condensate for atoms with integer spin). The research group in Oxford has pioneered new methods of trapping ultracold atoms using a combination of radiofrequency and static magnetic fields that is a very powerful tool for probing 2D quantum systems.

Research on `Investigating non-equilibrium physics and universality using two-dimensional quantum gases’ is funded by a new EPSRC grant [1]. Coherent splitting and matter-wave interference techniques enable comprehensive read out the quantum information from these systems to study fundamental questions such as how an isolated quantum system evolves towards equilibrium (or quasi-equilibrium states). We are working with theoretical colleagues to make detailed comparison with quantum statistical mechanics. Understanding the fundamental quantum physics of many-body system has important implications for quantum devices and technology based on them.

Future research directions include experiments on weak quantum measurements (sometimes called quantum non-demolition) on atoms in double-well potentials (bosonic Josephson junctions), squeezed states of the atoms and extensions to quantum gases that are a mixture of multiple species such a rubidium and strontium atoms.


Optical lattice clocks for fundamental physics and redefinition of the second

Professor Chris Foot

This project has been filled for 2021/22.

This is an experimental project, working with some of the UK’s best atomic clocks at the National Physical Laboratory. The clocks are based on strontium atoms, which are laser-cooled to a temperature of 1 µK, and then trapped in an optical-lattice dipole trap. The trapped atoms are probed using an ultra-stable clock laser, which is tuned in frequency to address a narrow optical transition at 429 THz. By measuring whether the clock laser excites the atoms or not, we can steer the laser to “tick” at a rate matching the narrow atomic transition frequency. Following this procedure, the clock laser can measure the passage of time to 18 digits of precision – enough to resolve the gravitational redshift from a change in height of just a few cm on the surface of the earth.

Already optical lattice clocks reach fractional frequency uncertainties and instabilities more than 100 times lower than the best caesium primary frequency standards. As a result, optical lattice clocks are a likely candidate for a future redefinition of the SI second. However, before such a redefinition, it must be shown that these systems can be engineered to run reliably, and that frequencies derived from such clocks are reproducible. We achieve this through real-time comparison of NPL’s clocks with others across Europe via optical fibre links, and across the globe via satellites including the soon-to-be-operational Atomic Clock Ensemble in Space (ACES). These ultra-precise clock-clock comparisons also provide valuable insight into many open problems in fundamental physics, underpinning the hunt for dark matter, tests for violations of relativity, and constraints on possible variations in fundamental constants. To extend the reach of clocks towards new applications, new techniques must be developed to overcome the current limitations on clock performance. This effort will be the focus of the current PhD placement, for instance by exploring how quantum entanglement can be leveraged to supress frequency instability arising from quantum projection noise by engineering ‘spin-squeezed’ states. This project has been filled for 2021/22.

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

Professor Gianluca Gregori

We are looking for DPhil experimental/theoretical/computational positions to study and simulate in the laboratory extreme astrophysical conditions. The research work 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 on theoretical topics involving strongly coupled and partially degenerate plasmas - which are particularly relevant for describing white dwarf structure. We are also interested in applying machine learning techniques such as Graph Neural Networks and symbolic regression in order to extract the emergent properties of such systems, including viscosity and thermal conductivity.

2. The understanding of the generation and amplification of magnetic fields in the Universe. We are particularly interested in 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. In addition, we are looking at performing experiments at CERN (using the HiRadMat facility) to generate ultra-bright electron-positron beams and study their propagation into a preformed plasma. Such experiments aim at replicating some of the coherent processes occurring in Gamma Ray Bursts and address key questions that cannot be answered by gamma-ray telescopes alone.

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.

Measuring transport properties of dense plasmas using X-ray photon correlation spectroscopy.

Prof Gianluca Gregori, Prof Sam Vinko and Prof Justin Wark

Short summary:

We intend to use X-ray photon correlation spectroscopy (XPCS) in novel ways to extract effective transport coefficients as a function of scales in dynamic HED materials. Recent developments in XPCS have demonstrated it as a powerful diagnostic at x-ray free-electron laser facilities such as LCLS and XFEL, enabling the tracking of atomic-scale structure and dynamics

with unprecedented Spatio-temporal resolution. Viscosity and other transport properties are essential for the fundamental understanding and prediction of the dynamical behavior of extreme materials. Photon correlation spectroscopy provides this information by characterizing electron

density fluctuations across a broad range of length scales and timescales, while X-ray scattering allows probing such motions in dense matter. The combination of these techniques is known as X-ray photon correlation spectroscopy (XPCS). In XPCS, large-intensity fluctuations known as ‘speckles’ arise from the scattering pattern. A single snapshot in time of the speckles yields information about spatial variations while for dynamic targets,

information is extracted from the time auto-correlation of the speckle patterns.

The role of the student would be to design the implementation of XPCS at XFEL on laser-driven samples, as well as perform the experiments and the data analysis. The latter will also be complemented by comparing our experimental data with the predictions of numerical simulations - ab initio Density Functional Theory and Bohm Molecular Dynamics for microscopic transport and multi-physics hydrodynamic modelling for macroscopic transport properties.

This is joint project between STFC and Trinity College. The studentship is fully funded for UK fees students. As the project funding is linked to Trinity College, the successful candidate would be allocated to Trinity if awarded the scholarship. 

Plasma accelerators

Professor Simon Hooker

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. Laser-driven plasma accelerators could therefore drive novel, very compact sources of particles and ultrafast radiation.

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. For this reason applications to work in this area should be made to the sub-departments of Atomic & Laser physics AND to Particle Physics.

Our work in this area is undertaken in our new high-power laser lab in Oxford, and at national laser facilities in the UK and elsewhere. We have recently been awarded a £2M, 4-year grant from EPSRC to support our research programme. Further information on our research can be found on the laser-plasma accelerator group website

We are offering two DPhil projects to start in October 2022, as outlined below.

1. Controlled injection and acceleration in all-optical plasma channels
Conventional electron-beam-driven light sources (i.e. synchrotrons and free-electron lasers) use electron bunches with energies of a few GeV. An Oxford-Berkeley collaboration were the first to generate electron beams with comparable energy from a laser-plasma accelerator. Reaching this energy requires that the driving laser pulse, which has an intensity of around 10^18 W / cm^2, is guided over several centimetres — well beyond the distance over which diffraction occurs.

The Oxford group has recently developed a new type of plasma channel generated by auxiliary laser pulses. Since they are free-standing, these channels are immune to laser damage, and hence they are very promising stages for future multi-GeV plasma accelerators operating at kilohertz pulse repetition rates.

In order to drive the most interesting (and challenging) applications — such as driving very compact free-electron lasers, or future particle colliders — the electron bunches generated by the plasma accelerator must be of very high quality. In other words, they must have low energy spread, be of small transverse size, and have a low transverse momentum. Further in order to reach very high bunch energies it may be necessary to couple two or more plasma accelerator stages together.

In this project we will bring these ideas together to investigate methods for controlling the injection of high-quality electron bunches into laser wakefields driven in all-optical plasma channels and transporting them between plasma accelerator stages. The project will involve experimental work in Oxford and at international high-power laser facilities, and numerical simulations using particle-in-cell codes and other numerical tools.

2. Multi-pulse laser wakefield accelerators
Most work to date on laser-driven plasma accelerators has been done with single driving pulses, which must have an energy of order 1 J and a duration shorter than the plasma period (around 100 fs). These demanding parameters can be generated by Ti:sapphire laser systems. However, Ti:sapphire lasers have very low efficiencies (< 0.1%) and (at these pulse energies) are limited to pulse repetition rates below 10 Hz.

Many potential applications of laser-plasma accelerators — such as light sources and future particle colliders — require operation at much higher pulse repetition rates (at least in the kilohertz range) and much higher ‘wall-plug’ efficiencies. New types of laser have recently become available which are very efficient, and which can provide joule-level pulses at pulse repetition rates in the kHz range. However, these pulses are too long (a few picoseconds) to drive a plasma wave directly.

We have recently proposed a new concept for spectral- and temporal-modulation of picosecond-duration pulses to convert a picosecond laser pulse into a train of short pulses spaced by the plasma frequency. This approach, combined with the all-optical plasma channels described above, appears to be a promising route to achieving multi-GeV, kHz repetition rate plasma accelerators.

In this project we will seek to demonstrate these ideas for the first time, by demonstrating the generation of pulse trains, their application to the excitation of laser wakefields, and acceleration of electrons in those wakefields. The project will involve experimental work in Oxford and at international high-power laser facilities, and numerical simulations using particle-in-cell codes and other numerical tools.

Demonstration of a cavity-based photonic entangler

Dr Axel Kuhn

Entanglement, in which two quantum systems can exhibit correlations that are greater than the limit allowed by classical physics, is one of the most intriguing predictions of quantum mechanics. Entanglement between remote atoms or ions is a key resource for quantum computing, and plays a central role in NQIT, the Oxford-led quantum technology hub.

We propose two schemes for entangling remote atoms: one probabilistic and one deterministic. In the probabilistic scheme, two distant atoms each emit a photon which are combined at the two input ports of a 50:50 beamsplitter. If the photons are detected at different output ports, then the atoms are projected into an entangled state. In place of a simple beam splitter, we also anticipate using more complex photonic networks [A. Holleczek, PRL 117, 023602 (2016)] in combination with active optical photon switching and routing. In the deterministic scheme, an atom emits a single photon which is reabsorbed by a second atom by running the emission process in reverse [J. Dilley, PRA 85, 023834 (2012)]. In doing so, the state of the first atom is entangled with that of the second. In both schemes, a high-finesse optical cavity is used to enhance the light-atom interactions.

Currently, we have two optical cavity experiments with random atom loading. The first phase of the project will be to build an optical dipole trap to permanently hold single atoms in the cavities. The feasibility of this approach has recently been demonstrated [D. Stuart, arXiv:1708.06672], and suitable fibre-tip and FIB-milled cavity mirrors are at present under development. The second phase will be to generate and quantify the entanglement between the two remote atoms using full Bell-state tomography.

Person specification

This is a highly challenging experimental project which will push the limits of laser and optical technology. It would suit a student with experience in atomic and laser physics and a keen interest in exploring quantum phenomena experimentally.

Work environment

The research team does encompass two postdocs and four graduate students which operate three laboratories dedicated to cavity-qed and atom-photon coupling in cavities at the Physics department of the University of Oxford. The work space is well equipped, comprising four vacuum chambers for studying atom-photon coupling in cavities, a large number of ECDL and fibre lasers for atom manipulation, a frequency comb for synchronously stabilising all laser and cavity frequencies, and a large battery of single-photon counters. The project builds on the current work by other graduate students in our group, atom-cavity coupling and strong cavity coupling.

The new student will directly contribute towards achieving cavity-mediated remote entanglement. All necessary apparatus exists within NQIT, including high-finesse cavities, vacuum chambers, and all lasers for trapping and driving the photon production process. Close support on a day-to-day basis will be provided by at least one Oxford PDRA for the duration of the project.

Superresolution imaging via linear optics in the far-field regime

Professor Alexander Lvovsky

Rayleigh's criterion defines the minimum resolvable distance between two incoherent point sources as the diffraction-limited spot size. Enhancing the resolution beyond this limit has been a crucial outstanding problem for many years. A number of solutions have been realized; however, all of them so far relied either on near-field or nonlinear-optical probing, which makes them invasive, expensive and not universally applicable. It would therefore be desirable to find an imaging technique that is both linear-optical and operational in the far-field regime. A recent theoretical breakthrough demonstrated that “Rayleigh’s curse” can be resolved by coherent detection the image in certain transverse electromagnetic modes, rather than implementing the traditional imaging procedure, which consists in measuring the incoherent intensity distribution over the image plane. To date, there exist proof-of-principle experimental results demonstrating the plausibility of this approach. The objective of the project is to test this approach in a variety of settings that are relevant for practical application, evaluate its advantages and limitations. If successful, it will result in a revolutionary imaging technology with a potential to change the faces of all fields of science and technology that involve optical imaging, including astronomy, biology, medicine and nanotechnology, as well as optomechanical industry.

The group website can be found here: quantum and optical technology group

Optical neural networks

Professor Alexander Lvovsky

This position is funded by a Marie Curie Innovative Training Network.

Machine learning has made enormous progress during recent years, entering almost all spheres of technology, economy and our everyday life. Machines perform comparably to, or even surpass humans in playing board and computer games, driving cars, recognizing images, reading and comprehension. It is probably fair to say that a modern machine will perform better than a human in any environment it has complete knowledge of. These developments however impose growing demand on our computing capabilities, including both the size of neural networks and the processing rate. This is particularly concerning in view of the decline of Moore’s law.

The project is to implement artificial neural networks using optics rather than electronics. The training of neural network consists of linear operations (matrix multiplication) combined with nonlinear activation functions applied to individual units. Both these operations can be implemented optically using lenses, spatial light modulators and nonlinear optical techniques such as saturable absorption. However, one crucial element of the training procedure - so-called backpropagation - has so far remained elusive. Our group has developed an idea to overcome this obstacle and implement pure optical backpropagation in a neural network, thereby enabling the training that is practically electronics-free. We confirmed the viability of this approach by simulation. Our next goal – and the goal of this doctoral research project – is to set up an experiment and test the method in a practical setting.

The group website can be found here: quantum and optical technology group

Chip-based quantum computing with trapped-ion qubits

Professor David Lucas

Trapped ions constitute near-perfect qubits with unrivalled quantum logic performance. Microfabricated “chip” traps are a promising avenue for scaling up to the large numbers of qubits required for future quantum computers. We have previously demonstrated the highest precision elementary qubit operations, using chip traps (see papers here and here ), where the qubit control was by performed by microwave electronic techniques. We plan to combine these techniques with integrated optical elements (as recently demonstrated elsewhere, e.g. here and here) to provide a fully integrated and scalable platform for quantum information processing.

We are looking for a highly motivated first-class student to join these world-leading projects. For further information, please see our web page.

Ion trap-integrated optical cavities for fast networked quantum computation

Professor David Lucas

The Oxford team has recently demonstrated the generation of networked quantum entanglement with the best combination of speed and fidelity in the world see this article in Physics World(link is external). Entangling qubits via photons in this way opens the way to scalable quantum computing via a network of small processors. However, at present the speed of entanglement generation is limited by the fact that most photons emitted by the trapped-ion qubits are not captured. A solution to this problem is to use optical cavities, which can in principle increase the photon collection efficiency near unity, and permit MHz entanglement rates between the network nodes. The goal of this project will be to fabricate and test an integrated ion trap / cavity system, using cutting-edge microfabrication technology. Trapped-ion quantum computing forms the major hardware research area of the UK Quantum Computing and Simulation Hub.

We are looking for a highly motivated first-class student to join these world-leading projects. For further information, please see our web page.

Maximising plasma turbulence in the hot spot of inertial fusion targets

Professor Peter Norreys

The student will investigate, using relativistic fluid theory and Vlasov-Maxwell simulations, the local heating of a dense plasma by two crossing electron beams generated during multi-PW laser-plasma interactions with a pre-compressed, inertial fusion target. Heating occurs as an instability of the electron beams that drives Langmuir waves, which couple non-linearly into damped ion-acoustic waves and into the background electrons. Initial simulations show a factor-of-2.8 increase in electron kinetic energy with a coupling efficiency of 18%. By considering the collisionless energy deposition of these electron beams, we are able to demonstrate, via sophisticated radiation-hydrodynamic simulations, that this results in significantly increased energy yield from low convergence ratio implosions of deuterium-tritium filled “wetted foam" capsules, as recently demonstrated on the National Ignition Facility. This approach promises to augment the heating of the central hot spot in these targets, and is attractive as a complementary approach that of fast ignition inertial fusion.

The student will:

  • Simulate (Vlasov or possibly particle-in-cell) parameter scan of the energy cascade. The question is how dependent are we upon the electron energy, thermal spread, divergence, beam-to-background density ratio.
  • Simulate the energy cascade process in an inhomogeneous plasma.
  • Simulate energy cascade using finite beams.
  • Help design experiments verifying the energy cascade process.

The student will also use machine learning to study the optimisation of the energy deposition process.

Quantum electrodynamics (QED)

Prof Peter Norreys 

“Quantum electrodynamics (QED), described by Feynman as "the jewel of Physics", is an exquisite description which brings together Einstein's special theory of relativity with the non-intuitive world of quantum mechanics. It is one of the most successful models of modern physics, laying the foundations of our understanding of the interactions of matter and light. Yet, despite continuing attempts, one of its key predictions remains unproven by experiment. Classical physics states that light cannot interact with itself the absence of matter - shine two beams of light at each other and they pass through, unaffected. However, QED predicts that photons, particles of light, should be able to scatter off each other even in complete vacuum. If this can be proven experimentally, and the photon scattering patterns analysed, it would provide not only confirmation of our understanding of the Standard Model of Physics, but enable us to explore some theories of "Beyond the Standard Model" physics which are needed to completely explain physical properties of our Universe.

Our four year project brings together a team of academic experts from UK and international universities with experienced staff from the Central Laser Facility, the UK's national experimental laser site, to firstly build and test new laser optics and detection equipment required for these experiments and then use this instrumentation to perform the ultimate experiments using the world's highest power lasers at the "Super-intense Ultrafast Laser Facility" in China.  Our team also combines outstanding theorists in Oxford Physics who will create mathematical models for the photon interactions, with leading scientists who have contributed to some of the greatest physical discoveries in recent times, such as the detection of the Higgs boson at the Large Hadron Collider. Together they will enable the optimisation of our experimental procedures, provide predictions on signatures which would be generated by phenomena beyond the standard model of physics and constrain our experimental results. Please apply to join our project for your DPhil and join us in the unique physics investigation.”

Laser-plasma interaction physics for inertial fusion and extreme field science

Professor Peter Norreys

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.

My team is working on:
• 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

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, including applications of machine learning - I can offer projects in all of these areas More specifically:

Project 1. I have applied for funding of a studentship via the EUROFusion Enabling Research grant “Foams as a Pathway to Energy from Inertial Fusion (FoPIFE)”. The student will help develop our understanding of wetted foam implosions using high power lasers as well as design and implement high energy laser experiments on at the Central Laser Facility and Ecole Polytechnique devoted to understanding the behaviour of laser-irradiated foam targets. We will know the outcome in December 2020 of the grant application.

Project 2. I have been awarded an STFC grant to implement a new optical diagnostic on the AWAKE run II experiment at the CERN laboratory, in conjunction with the John Adams Institute. Our aim is the visualise plasma wakefields as they evolve in the 10 metre long plasma column. This will be the first time that the structure of a beam-driven wakefield accelerator will be measured in the laboratory and promises exciting discoveries of the real structure of wakefields generated by the Super Proton Synchrotron beam (operating at 400 GeV). The student will help with the design and implementation of the oblique angle frequency domain holographic set-up, visualise the outcome and compare the data with state of the art computer simulations.

Project 3. I am about to submit a large collaborative grant application to UKRI-EPSRC on photon-photon scattering using intense laser pulses. The idea is to ‘polarise the vacuum’ using three intense laser pulses. The virtual electron-positron vacuum is then polarised by the electric field and behaves like a dielectric medium, with the generation of a fourth beam that has a distinct wavelength and direction. This will be the first tests of photon scattering in the low photon energy, strong field limit. It might provide tests of physics beyond the standard model. We expect to know the outcome of this application in June 2020.

Single impurity in a dipolar Bose-Einstein condensate

Dr Robert Smith

A single impurity interacting with a quantum bath is a simple (to state) yet rich many-body paradigm that is relevant across a wide sweep of fields from condensed matter physics to quantum information theory to particle physics. The aim of this project is to experimentally explore this physics using the highly controllable platform of a ultracold bath of Erbium atoms in which potassium impurities can be imbedded. The special feature of Erbium atoms is their large magnetic dipole moments which result in long-range and anisotropic dipole-dipole interactions in addition to the short-range contact interactions more normally seen in cold atom systems. This opens new avenues in a range of topics from polaron physics to information flow in open quantum systems.

Exploring Planetary-Core Matter with X-ray Free-Electron Lasers (XFELs)

Sam Vinko and Justin Wark

X-Ray free-electron-lasers (FELs) produce beams of x-rays less than 100-fsec in duration at sufficient brightness that x-ray diffraction patterns of crystals can easily be obtained in a single shot.  High power optical lasers, synchronized to the FEL, via laser ablation of thin surface layers, can for a few nanoseconds generate solids at pressures similar to those found inside giant planets.  The combination of the two thus allows the exploration of the phase diagram of types of matter in a completely new regime, and which does not exist on earth.  The Oxford team is one of the leading international groups in this area.

At present we have shown how complicated phase transitions can take place on these short timescales.   We know the density of the materials we generate from the diffraction pattern, and also have methods to determine the multi-million atmosphere pressures that we reach using sophisticated interferometric techniques.  However, the missing piece of information is a good measure of temperature – we know the material remains solid (we see the crystal structure), and we know it has been heated as it has been compressed, but exactly by how much remains elusive.  The amount will depend on how the lattice deforms during the nanosecond compression waves.

In this project the student will explore and evaluate three ideas for temperature measurement, as well as further our understanding of the underlying heating and deformation mechanisms.  The three methods involve inelastic scattering from phonons, the Debye-Waller effect, and direct measurement of the electron temperatures with resonant inelastic scattering.  A combination of both classical molecular dynamics and quantum density functional theory will inform experiments that we will pursue at the European X-Ray Laser (XFEL) in Hamburg, and thus the project will be a combination of both theory and experiment. 

We expect to offer a project in this area that will be fully-funded for UK students.  Under the auspices of Oxford Centre for High Energy Density Science(OxCHEDS) and via a partnership with our industrial sponsors at AWE, the project will attract a substantial uplift in the normal student stipend. Further details are available upon request.

For a flavour of the sort of work we do, some of our recent papers in this area are:

1) Molecular dynamics simulations of inelastic x-ray scattering from shocked copper

O Karnbach, PG Heighway, D McGonegle, RE Rudd, G Gregori, JS Wark

Journal of Applied Physics 130 (12), 125901

2) Molecular dynamics simulations of inelastic x-ray scattering from shocked copper

O Karnbach, PG Heighway, D McGonegle, RE Rudd, G Gregori, JS Wark

Journal of Applied Physics 130 (12), 125901

3) Femtosecond X-ray diffraction studies of the reversal of the microstructural effects of plastic deformation during shock release of tantalum

M Sliwa, D McGonegle, C Wehrenberg, CA Bolme, PG Heighway, ...

Physical review letters 120 (26), 265502

Exploring Quantum Plasmas with X-ray Free-Electron Lasers (XFELs)

Dr Sam Vinko

The advent of high-brightness 4th generation free-electron laser (FEL) light sources has revolutionised our ability to study extreme states of matter with unprecedented precision and control. The addition of new high-repetition rate, high energy laser drivers to FEL beamlines, such as the Dipole laser at the high-energy-density (HED) endstation of the European XFEL, will allow for a host of new compression experiments in well-controlled high-energy-density conditions to be investigated. In particular, the capability to tune the inter-atomic spacing between atoms in plasmas and compressed solids to the point where inner-shell electrons start overlapping, interacting and hybridizing is of great interest as it constitutes a new quantum frontier in dense plasmas. This novel regime is one where quantum effects and correlation may be sustained up to very high temperatures, and can now be accessed for the first time in the laboratory.

DPhil projects exploring this quantum plasma regime are available, with a focus on theoretical, computational, or experimental research.

Our experimental efforts are undertaken at large-scale FEL facilities, such as LCLS in California and the European XFEL in Hamburg, where we deploy a range of spectroscopy and x-ray diffraction techniques to understand how high-energy-density systems can be generated, and how they behave in extreme conditions of temperature and pressure. For a flavour of the sort of work we do, some of our recent papers in this area are:

1) Humphries et al., Mapping the Electronic Structure of Warm Dense Nickel via Resonant Inelastic X-ray Scattering(link is external)
2) Vinko et al.,Time-Resolved XUV Opacity Measurements of Warm Dense Aluminum(link is external)
3) Van den Berg et al., Clocking Femtosecond Collisional Dynamics via Resonant X-Ray Spectroscopy(link is external)

Our experimental work closely ties into computational modelling (density functional theory, collisional-radiative atomic kinetics), and the application of advanced statistical tools and machine learning to help interpret complex experimental measurements in large-dimensional spaces:

4) Kasim et al.,Building high accuracy emulators for scientific simulations with deep neural architecture search(link is external) Also see reports on this work in Nature Physics(link is external) and Science(link is external)
5) Hollebon et al.,Ab initio simulations and measurements of the free-free opacity in aluminum(link is external)
6) Kasim et al., Inverse problem instabilities in large-scale modelling of matter in extreme conditions(link is external)

Searching for the Universal Functional with Machine Learning and Differentiable Density Functional Theory

Kohn–Sham density functional theory (KS-DFT) is among the most popular approaches to modelling the quantum electronic structure of molecules and extended systems, and is widely used across physics, chemistry and material science. It is an exact formulation of the full quantum manybody problem, but in practice many approximations are needed to treat the very challenging electron exchange and correlations (XC) effects. Central to the theory is the existence of a universal functional – one independent of potentials external to the manybody electron system. This functional incorporates all the XC effects and can thus model complex physical systems exactly, with the added benefit of a low computational cost typical reserved for less accurate mean field approximations. Alas, this universal functional, also know as the exact XC functional, remains unknown.

Recent developments in our research group have shown how the XC functional can be efficiently modelled by a deep neural network embedded within a fully-differentiable KS DFT framework (details). Remarkably, this approach allows us to use experimental observables directly from nature to infer the properties of the XC functional in a way that transcends specific materials, elements, and properties.

This project will make use of these groundbreaking early developments to further shape our understanding of the XC functional and its impact on the prediction of complex quantum properties for advanced materials discovery. Work will include adding further constraints to train and validate the deep-learned XC neural network, extending code capabilities to larger systems, modelling dynamical properties, and designing materials based on required physical or chemical properties. 

Candidates should be familiar with and interested in writing and modifying software, coding in python (including PyTorch), and using automatic differentiation. A solid background in atomic and/or condensed matter physics will be beneficial.