Research projects for October 2018

Dr Patrick Baird

DPhil Research Opportunity on Ultra-High-Q Microresonators for Optical Frequency Comb Generation

Ultra-High-Q microresonators are extremely powerful devices that allow confining enormous amounts of light into tiny micron-scale mode volumes. This enables the observation of nonlinear optical effects at low threshold powers, making microresonators ideal components for future integrated photonic circuits and for the next generation of optical computing.

A key application that utilizes nonlinear optics in microresonators is the generation of optical frequency combs. Optical frequency combs have been first developed 15 years ago and have become ubiquitous tools for spectroscopy, precision metrology and optical clocks. In 2005 optical frequency combs have been awarded with the Nobel Prize in Physics. Microresonators have recently shown to be promising candidates for shrinking optical frequency comb generators into chip-based devices for out-of-the-lab use [1]. However, the underlying physics of optical comb generation in microresonators is far from being fully understood and fundamentally different from conventional mode-locked laser comb generators [2].

There is opportunity for an Oxford D.Phil student to join a recently established research team at the National Physical Laboratory (NPL) to work on microresonator-based frequency comb generation. At present there are several research projects available both on the applied side of frequency comb generation in microresonators as well as on the fundamental research side with the goal of understanding the physics of the underlying comb generation process.

NPL provides an ideal environment for research on optical frequency combs with an existing infrastructure of optical atomic clocks and conventional frequency combs that can be used for future experiments.

[1] Del'Haye et.al, Optical frequency comb generation from a monolithic microresonator
Nature 450, 1214-1217 (2007)
[2] Kippenberg, Holzwarth, Diddams, Microresonator-Based Optical Frequency Combs
Science 332, 555-559, (2011)

For further information please contact either Dr Patrick Baird or Dr Pascal Del'Haye. There is funding for a least one studentship on this project.

Email
p.baird1@physics.ox.ac.uk
pascal.delhaye@npl.co.uk

Professor Christopher Foot

1. Experiments with ultracold atoms and Bose-Einstein condensates

Professor Foot’s research group investigate the fascinating properties on ultracold quantum gases at temperatures of a few tens of nanokelvin. The 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 enables the investigation of the properties of 2D quantum systems. We are setting up experiments to work with multiple atoms species, e.g. two isotopes of Rb, or Rb and K atoms, in which atoms of one species act as ‘quantum probes’ of the surrounding gas. This will enable us to read out the quantum information from these systems and investigate new aspects of quantum thermodynamics and statistical physics. This work on understanding the fundamental quantum physics of many-body system has important implications for quantum devices and technology based on them. We also plan to carry out experiments on the transport properties in 1D quantum systems. Experiments with cold atoms allow single particles to be imaged and their properties measured thus providing deep insights that are complementary to work on quantum matter in Condensed Matter.

Email
c.foot1@physics.ox.ac.uk

2. Development of cold-atom sources for quantum technologies

This project is suitable for those with an interest in developing new apparatus that will be applied to practical devices. We have developed a compact source of cold rubidium atoms based on a novel (patented) design of magneto-optical trap and we are collaborating with a laser company based in Scotland (M Squared Lasers) to commercialise this work. In this project we will develop a compact source of laser-cooled strontium atoms working with that company and other interested parties. Strontium requires a considerably different approach to that used for rubidium and developing the experimental methods into a robust and reliable system is the primary task. We anticipate that there will be considerable demand for sources of cold strontium since they are necessary for clocks that use atoms in optical lattices (strontium clocks are currently the world-leading technology), matter-wave interferometers (for which one of the strontium isotopes is very suitable), as well as more fundamental research work on ultracold molecules and quantum gases. In summary, strontium has considerable advantages for quantum technology but it is not (yet) used so widely as rubidium because it requires more complicated apparatus. The technical issues will be addressed in this project and the cold-atom source will be demonstrated by building a matter-wave interferometer. Collaborations with other research teams to demonstrate other applications are likely. It is expected that this project will be fully supported by a studentship for 4 years. Further information is available from Professor Foot.

Email
c.foot1@physics.ox.ac.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.gregori1@physics.ox.ac.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

The project involves the generation of high-order harmonics of ultrafast visible laser pulses, and the application of this radiation to coherent diffraction imaging (CDI). High-harmonic generation (HHG) is a fascinating process in which an intense, femtosecond-duration laser pulse strongly drives the valance electrons of an atom to induce a highly nonlinear polarization; these oscillating dipoles radiate the odd harmonics of the driving field to generate a coherent beam of harmonics extending from the visible into the soft X-ray region.

CDI is a new method for "lensless imaging,” which is invaluable in spectral regions (such as the X-ray region) in which conventional optics are not available. In essence, CDI works by recording the diffraction pattern of an object, and inverting this using fast phase-retrieval algorithms to deduce the original object.

We are interested in: (i) developing more efficient HHG sources, with improved properties, by using optical parametric amplifiers (OPAs) to optimize the wavelength of the driving laser; (ii) using the HHG beams for element-specific X-ray imaging of microscopic objects.

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

Further details are available at: https://www2.physics.ox.ac.uk/research/plasma-accelerators-and-ultrafast...

Further details from Prof Simon Hooker.

Email
s.hooker1@physics.ox.ac.uk

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 thin films that efficiently count single photons. We use these tools to study applications in quantum computation and metrology, as well as fundamental issues including the quantum-classical boundary and macroscopic quantum systems.

Potential research projects include:

* The quantum-classical transition in large optical networks: Optical fields provide a promising route to create quantum correlations across a network consisting of many nodes that span large distances. The preservation, manipulation, and measurement of multipartite quantum features, including entanglement, in these systems is an important challenge. We plan to develop practical theoretical tools and experimental methods to study the operation of large-scale quantum networks and how they differ from their classical counterparts.

* Quantum-enhanced biological sensing and imaging: Precisely designed quantum optical fields have potential to probe light-sensitive biological samples with unmatched precision. We will study the design of novel quantum-powered microscopes based on stimulated emission and phase-contrast imaging. The end goal is a laboratory demonstration of performance that exceeds the capability of a device using only classical optics.

* Direct quantum simulation and analogue computation: photonic coprocessors have potential to add purpose-built hardware to quantum computers. The fundamental properties of light enable unique construction of processors based on the quantum interference of non-interacting bosonic fields. We will study analogue computation based on the principles of boson sampling, and investigate the design of few-mode photonic simulators that can be used to study the physics of complex molecular and condensed matter 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 Professor Ian Walmsley.

Email
I.Walmsley@physics.ox.ac.uk

Professor 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 recently developed the highest-performing 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 qubit technology, of all fundamental qubit operations with sufficient fidelity for fault-tolerant quantum computing. As an application, we used our quantum logic gate to make the first test of Bell's Inequality with two different species of trapped atoms. 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. For further information, please see our web page at www.physics.ox.ac.uk/users/iontrap or email

[table=Email]
d.lucas1@physics.ox.ac.uk
table]

Professor Dieter Jaksch

1) Designing Out-of-Equilibrium Many-Body Quantum Systems

Many key existing and emerging technologies, ranging from switches in current computing and data networks to quantum devices for high-precision sensing, derive their functionality from out-of-equilibrium physics. As electronic devices become smaller, better understanding and control of this physics at the quantum level will become crucial to developing future technologies, and to address major challenges, such as developing energy-efficient switching and communications links. How to exploit the advantages of increasingly complex devices in the quantum regime and in the presence of noise and decoherence is intrinsically an issue of out-of-equilibrium many-body quantum physics. It is therefore crucial to put methods in place now that will underpin the design of out-of-equilibrium quantum systems, and lead to novel quantum devices.

This theoretical DPhil project will be attached to the EPSRC programme grant DesOEQ which aims to explore, understand, and design out-of-equilibrium quantum dynamics that are relevant for future technologies, using quantum simulators with atomic gases in optical potentials. These simulators offer a unique level of controllability on a microscopic level, where interactions and trapping potentials are quantitatively understood from first principles. Their size and time scales make it possible to directly image and manipulate the system on the level of a single atom, as well as to track and control dynamics in real time. The DPhil project will specifically consider steady-states of driven, dissipative ultracold atomic systems with the aim of reaching otherwise inaccessible quantum phases [1,2]. For instance, the project will study the dynamics near transitions between different phases of the driven Hubbard model investigated in [1] and how the physics is affected when coupling the system to a dissipative bath.

[1] J. R. Coulthard, S. R. Clark, S. Al-Assam, A. Cavalleri, and D. Jaksch, Phys. Rev. B 96, 085104 (2017)
[2] F. Görg, M. Messer, K. Sandholzer, G. Jotzu, R. Desbuquois, T. Esslinger, arXiv:1708.06751 (2017)

2) Long-ranged interactions in ultracold molecules

Many new applications in quantum science and technology have emerged from the full quantum control of ultracold atoms. Molecules offer even greater possibilities owing to their rich internal structure, controllable long-range dipole-dipole interactions and stronger couplings to electric and microwave fields. However, quantum control of molecules is fundamentally different from controlling atoms and needs to be developed in order to exploit their potential. The EPSRC programme grant QSUM develops this control through a coherent research programme of interconnected experimental and theoretical projects.

This theoretical DPhil project is attached to the QSUM grant and will investigate the physics of arrays of ultracold polar molecules interacting via long-range dipole-dipole potentials. These systems promise to be a fundamentally new tool for understanding the quantum physics of strongly correlated many-body systems. For example, such systems will allow us to understand the exotic quantum phases of dipolar quantum gases and to simulate lattices of interacting spins. They will reveal fundamental aspects of quantum magnetism, and allow us to explore new regimes of superfluidity, including stable topological phases relevant to quantum information processing. To address these questions this DPhil project will develop theoretical methods and algorithms based on tensor network theory to describe long-ranged molecular interactions in lattice systems. These will then be used to study the quantum phases of molecular arrays and their potential as a quantum technologies platform.

Email
d.jaksch1@physics.ox.ac.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. 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 – please contact me to discuss your interests and let’s take it from there.

Email
p.norreys1@physics.ox.ac.uk

Professor Ian Walmsley

Cavity opto‐mechanics: from quantum foundations to quantum technologies

Cavity quantum opto-mechanics is one of the newest and fastest growing areas of quantum science internationally and was recently noted by the Nature Publishing Group as a milestone in physics [1]. Central to opto-mechanics is radiation pressure (the force exerted by simply the reflection of light), which is used to control the motion of micro‐ or nano‐fabricated mechanical resonators. Using this interaction in combination with the techniques of quantum optics provides a route to experimentally generate quantum states of motion at a truly macroscopic scale. This rich avenue offers significant potential to contribute to both fundamental and applied science, allowing quantum-enhanced weak force sensing with unprecedented precision, and even offering one of the most promising routes to observe quantum gravitational phenomena with a table-top experiment [2,3]. Specific DPhil projects can be tailored to the student’s interests and can focus in one of two directions: (i) to experimentally develop techniques to generate and probe non-classical states of motion [4,5] to empirically explore fundamental physics, such as potential quantum gravity phenomena [2,3], or (ii) to utilise quantum states of motion to develop ultra-sensitive force sensors that can play a key role in the next generation of quantum technologies.

Successful DPhil applicants will be part of a vibrant international team of researchers working in close collaboration with several research groups in Oxford, London, and overseas, including Australia, Germany, Japan, and the United States. You will gain an expertise in experimental quantum optics, lasers, cryogenics, as well as simulation and theoretical modelling in a number of different areas of physics. You will also have the opportunity to spend some of your DPhil abroad to work with our collaborators. Recent opto-mechanics graduates have went on to pursue exciting careers in Google, NASA, and IBM, as well as becoming internationally leading academics.

[1] http://www.nature.com/milestones/milephotons/full/milephotons23.html
[2] R. Penrose, General Relativity and Gravitation 28, 581 (1996).
[3] I. Pikovski, M. R. Vanner, M. Aspelmeyer, M. S. Kim, and C. Brukner, Nature Physics 8, 393 (2012).
[4] K. C. Lee, etal., Science 334, 1253 (2011).
[5] M. R. Vanner, M. Aspelmeyer, M. S. Kim, Physical Review Letters 110, 010504 (2013).

For further information on this DPhil opportunity please see our group webpage

https://groups.physics.ox.ac.uk/QMLab/

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.

Research projects are available to 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.vinko@physics.ox.ac.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
I.Walmsley@physics.ox.ac.uk

Professor Justin Wark

Studying High Pressure Matter with X-Ray Lasers

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.wark1@physics.ox.ac.uk