Theory and Computation Group

Theory and Computation Research Activity

The Theory and Computation group is the focus of theoretical and computational modelling activities in the ATI, bringing together a large variety of advanced computational tools used on dedicated high-performance computing platforms. Please find out more about our research areas.

PhD projects

Collective dynamics and coherence in hybrid cavity-spins ensembles

Supervisor

Eran Ginossar

Research Group

Theory and Advanced Computation

Type

Theoretical

Techniques used

  • Theoretical techniques of quantum  mechanics for systems out of equilibrium

Student will require

  • Good analytical skills

Objectives

  • Study the non-equilibrium dynamics and phase diagram of the system
  • Develop proposals for experimental implementation of efficient hybrid architectures with existing quantum technologies

Project description

Solid state materials such as diamond with point defects are realizing ensembles of weakly interacting electronic spins. These are leading candidates for storing quantum information in hybrid superconducting circuits due to their relatively weak decoherence and relaxation processes [1,2]. The interaction between the spins and the cavity gives rise to a collective dynamics which is challenging to model and understand especially the different steady-state phases that exist in this system when it is driven by external fields. 

[1] Schuster DI, Sears AP, Ginossar E, DiCarlo L, Frunzio L, Morton JJ, Wu H, Briggs GA, Buckley BB, Awschalom DD, 'High-cooperativity coupling of electron-spin ensembles to superconducting cavities.'. Phys. Rev. Lett. 105, 140501 (2010)

[2] Alex Kamenev, 'Field Theory of Non-Equilibrium Systems', Cambridge University Press, New York, 2011.

Multi-physics Modelling of High-Power Microwave Transistors

Supervisor

Peter Aaen

Type

Experimental, Theoretical

Student will require

  • Applicants are expected to have a first-class or 2:1 degree or equivalent in Electrical Engineering with a solid background in Microwave Engineering an additional advantage.

Funding source

Funded PhD Project (UK Students Only)

Project description

The Advanced Technology Institute at the University of Surrey hosts researchers using fundamental and applied research for the design, manipulation and fabrication of optical and electronic devices/systems. Our researchers, from a diverse range of scientific backgrounds and backed by state-of-the-art research equipment and facilities, are currently designing materials, devices, and systems to address the grand challenges faced by society

We are currently seeking to fill a PhD position in the area of multi-physics modelling of microwave power transistors. These transistors have important roles in mobile phone infrastructure, satellite communications, terrestrial broadcast, avionic and weather radar systems, and microwave backhaul, to name but just a few applications. Market demands for increased power and efficiency show no sign of abating and more circuit functions will be incorporated into a design. For the successful realization of high-frequency and high-power devices based on GaN, LDMOS, and GaAs technologies, we need to consider a comprehensive view of models, simulation methods, design techniques, all while incorporating the inevitable coupling that occurs between device physics, electromagnetics and the thermal environment.

The student will contribute to the development of a multi-physics simulation platform to simulate power transistors and power-amplifiers. The student should expect to be involved in detailed physical and numerical simulation using software packages like Agilent’s Advanced Design System, ANSYS’s HFSS, Sonnet’s em, as well as undertaking some practical measurements and programming in MATLAB. The student should be aware that this project might involve working closely with an Industrial partner (depending on the availability of project sponsors). 

Please send your CV and covering letter to: Dr. Peter H. Aaen, Reader, Advanced Technology Institute, via email to p.aaen@surrey.ac.uk, University of Surrey, Guildford, Surrey GU2 7XH, including transcripts of marks for your undergraduate and/or MSc with your application.

Informal enquiries may be directed to Dr Peter H. Aaen (p.aaen@surrey.ac.uk).

Optimal coherent control for quantum information processing

Supervisor

Eran Ginossar

Research Group

Theory and Advanced Computation

Type

Theoretical

Collaborations

University of Maryland

Techniques used

  • Analytical derivations in quantum mechanics of open systems
  • Numerical simulations for integration of stochastic differential equations, partial differential equations and optimization algorithms
  • Usage of high-end computing resources such as HPC linux clusters and GPU programming

Student will require

  •  Good knowledge of solving problems in quantum mechanics
  •  Some experience with numerical packages and programming skills

Objectives

  • Study theoretically the question of the speed of control involving large state spaces
  • Use techniques of quantum coherent control theory to find novel protocols for control of complex qubit cavity systems in circuit quantum electrodynamics architectures
  • Develop new algorithmic approaches for performing large scale numerical simulations of open quantum systems

Project description

Quantum information processing is implemented by precisely controlling the dynamics of the quantum state of a system that realises a register of qubits. These include operations such as reading and writing the state of the qubit, unitary gates and resets, which together are the basic building blocks of a quantum computer [1,2]. One of the central challenges with current quantum computing architectures is to find optimal design parameters and pulses that would control the dynamics of a register of qubits with high fidelity and speed [3,4].

The goal of this project is to answer what is the optimal control that can be achieved with respect to these building block tasks given the realistic constraints of existing implementations of qubits. This research will involve understanding the limits to controllability on the Hamiltonian system and finding the optimal control pulses with exact numerical simulations applied to the full open system. This project is an opportunity to both advance our understanding of control of open quantum systems as well as assist the ongoing experimental efforts in quantum information processing.

[1] M. D. Reed, L. DiCarlo, B. R. Johnson, L. Sun, D. I. Schuster, L. Frunzio, R. J. Schoelkopf, 'High-Fidelity Readout in Circuit Quantum Electrodynamics Using the Jaynes-Cummings Nonlinearity', Phys. Rev. Lett. 105, 173601 (2010)

[2] Johnson BR, Reed MD, Houck AA, Schuster DI, Bishop LS, Ginossar E, Gambetta JM, DiCarlo L, Frunzio L, Girvin SM, Schoelkopf RJ.  Quantum non-demolition detection of single microwave photons in a circuit, Nature Physics 6, pp. 663-667 (2010).

[3] Ginossar E, Bishop LS, Schuster DI, Girvin SM. (2010) Protocol for high-fidelity readout in the photon-blockade regime of circuit QED, Phys. Rev. A 82, 022335 (2010)

[4] Herschel A. Rabitz, Michael M. Hsieh, Carey M. Rosenthal, Quantum Optimally Controlled Transition Landscapes, Science 303, 1998 (2004)

Physics and applications of hyperuniform disordered photonic materials

Supervisor

Marian Florescu

Research Group

Theory and Advanced Computation

Type

Theoretical

Collaborations

Princeton University, New York University, San Francisco State University.

Techniques used

Computer simulations and/or analytical calculations.

Student will require

Potential students should have a strong interest in modeling the physics of micro and nanostructured photonic structures and devices.

Objectives

  • To elucidate the fundamental correlation between geometrical and optical characteristics of hyperuniform disordered materials
  • To investigate novel light trapping mechanisms and advanced optical functionalities in hyperuniform disordered photonic materials with applications to nano-scale optical components including novel types of optical cavities and arbitrary-bending angle waveguides, nonlinear wave mixing and resonantly enhanced optical transmission.

Project description

The field of photonics has advanced tremendously recently through the development of micro and nanostructured photonic materials. An important class of such materials is represented by photonic crystals (PC), structures that may present frequency ranges over which the light propagation is prohibited for all directions and polarizations. Due to their unique ability to mold the flow of light and to control the light-matter interaction, PC led to a broad new frontier both in basic science and technology. 

Until recently, the only materials known to have sizeable complete photonic band gaps (PBG) were photonic crystals, periodic structures, and it was generally assumed that long-range periodic order was instrumental in the PBG formation. It has been though discovered that this assumption is false, and a new class of materials with large complete band gaps, namely hyperuniform disordered microstructures, has been introduced [1]. This class of materials characterized by suppressed density fluctuations (hyperuniformity) includes isotropic, translationally-disordered structures and quasicrystals with crystallographically-forbidden rotational symmetries [2]. Due to their distinctive optical and structural properties, non-crystallographic PBG materials are expected to facilitate unprecedented capabilities for controlling light [3] that could result in potential breakthroughs in optical communications, energy harvesting and conversion systems, and non-linear optics. The applications of non-crystallographic PBG materials include optical insulators and waveguides without the constraints on shape or direction, improved absorption efficiency of solar and energy conversion devices or flexible platforms for integrated optical micro-circuitry.

The project focuses primarily on the photonic properties of hyperuniform dielectric structures, but it will lay the foundation for the investigation of the electronic and phononic properties of other types of hyperuniform disordered solids, including amorphous silicon, graphene and biophotonic materials.

[1] M. Florescu, S. Torquato, and P. Steinhardt, “Designer disordered materials with large complete photonic band gaps”, Proceedings of the National Academy of Sciences 106, 20658 (2009).
[2] M. Florescu, S. Torquato, and P. Steinhardt, ”Complete band gaps in two-dimensional photonic quasicrystals”,  Physical Review B 80, 155112 (2009).
[3] W. Man, M. Florescu, K. Matsuyama, P. Yadak, S. Torquato, P. Steinhardt and P. Chaikin, "Experimental observation of photonic bandgaps in hyperuniform disordered material," Conference on Lasers and Electro-Optics (CLEO) and the Quantum Electronics and Laser Science Conference (QELS) (Optical Society of America, Washington, DC, 2010).

Single-photon sources in photonic band gap materials

Supervisor

Marian Florescu

Research Group

Theory and Advanced Computation

Type

Theoretical

Collaborations

Hearne Institute for Theoretical Physics, Louisiana State University

Techniques used

Computer simulations and/or analytical calculations.

Student will require

Potential students should have a strong interest in modeling the physics of micro and nanostructured photonic structures and devices.

Objectives

  • To investigate the confinement  of the electromagnetic radiation in cavities and waveguides realized in microstructured photonic materials, necessary to describe a series of experimental studies and to design new applications.
  • To develop a fundamental understanding of how to tailor the photonic characteristics in periodic and aperiodic microstructured photonic reservoirs to achieve strong light-matter interaction and controllable single-photon emission.
  • To explore mechanisms that enable facilitate enhanced, unidirectional generation of single photons with a high repetition rate.

Project description

In recent years, quantum optical information processing has attracted much attention, mostly for its applications to secure communication protocols and the possibility of solving efficiently computational tasks impossible to solve on a classical computer. It was demonstrated that efficient quantum computing can be implemented using only single-photon sources, passive linear optical elements, and detectors. The optical approaches to quantum computation benefit from the lack of decoherence of photons and the relative ease of photon manipulation. Single-photon sources [1] are thus essential ingredients in any optical implementation of quantum computing. A single-photon source emits in a deterministic way one photon at a time, with each photon being indistinguishable from the others [2]. Identifying the optimal system able to generate “on demand”, unidirectional pulses of single photons with a high-repetition rate (in a so-called "photon gun" device) [3], [4] has proven to be a complex task and is a subject of great interest to the scientific community. Present-day research considers photon emission from single atoms or molecules, quantum dot structures, or chemical compounds.

This project will investigate the possibility of achieving single-photon emission in suitably designed optical cavities and waveguides on a photonic-bandgap material platform. In particular, it will focus on the influence of photonic density of states, slow light, and light localization on the emission of photons in an optical cavity coupled to a waveguide channel.

[1] P. Yao, V.S.C. Manga Rao, and S. Hughes, "On-chip single photon sources using planar photonic crystals and single quantum dots", Laser and Photonics Reviews 4, 499-516 (2010).
[2] Y. Akahane, T. Asano, B. Song, S. Noda, "High-Q photonic nanocavity in a 2-dimensional photonic crystal”, Nature  425, 944 (2003).
[3] M. Florescu, S. Scheel, H. Haeffner, H. Lee, P. L. Knight, J. P. Dowling, "Single photons on demand from 3D photonic band-gap structures", Europhysics Letters 69 (6), 945 (2005).
[4] M. Florescu, S. Scheel, H. Lee, P. L. Knight, J. P. Dowling, “Nonlinear tuning of 3D photonic band gap structures for single-photon on demand sources”, Physica E 32, 484 (2006).

Superconducting quantum circuits with majorana fermions

Supervisor

Eran Ginossar

Research Group

Theory and Advanced Computation

Type

Theoretical

Techniques used

Analytical calculations.

Student will require

Strong background in quantum physics and interest in the science behind quantum technology.

Objectives

  • Develop methods for dynamical control of Majorana hybrid qubits with (1) coherent microwave photons for single qubit gates, (2) adiabatic control of the system parameters
  • Perform analysis of parity state readout and control of several qubit devices. Perform analysis of decoherence processes in hybrid devices
  • Explore prospects for unique detection of Majorana fermions such as by probing their self-adjoint character in these devices.

Project description

Topological states of matter are fascinating physicists for decades and the search to discover particles or quasi-particles which demonstrate anyonic exchange statistics is ongoing in condensed matter physics and particle physics. There is a concentrated effort today on detection and manipulation of Majorana quasi-particles with experiments under way after an initial breakthrough two years ago [Mourik et al., Science 336, 6084 (2012)] and this year [Nadj-Perge et al., Science 346, 602 (2014)]. We have recently shown [Nat. Comm. 5, 4772 (2014)] that a topological superconductor embedded within a transmon superconducting circuit possesses spectroscopic features compatible with Majorana fermions which can be probed by inducing microwave transitions. In this project we will propose and analyse hybrid light-matter majorana-superconducting circuit systems with the aim to propose a viable architecture where the topological exchange properties of the Majorana are detected and utilised. 

Thermal radiation control with microstructured photonic materials

Supervisor

Marian Florescu

Research Group

Theory and Advanced Computation

Type

Theoretical

Collaborations

Institut fuer Teoretische Festkorperphysik, Universitataet Karlsruhe.

Techniques used

Computer simulations and/or analytical calculations.

Student will require

Potential students should have a strong interest in modeling the physics of micro and nanostructured photonic structures and devices.

Objectives

  • To investigate the thermalization of the electromagnetic radiation in absorptive and dispersive microstructured photonic materials, necessary to describe a series of experimental studies and to design new applications.
  • To develop a fundamental understanding of how to tailor and manipulate the photonic characteristics in periodic and aperiodic photonic band gap materials to achieve spectral compression for incident broadband solar radiation.
  • To explore mechanisms that enable efficient extraction and transfer of the thermal energy by leveraging evanescent-field coupling to achieve super-Planckian regimes.

Project description

The ability of micro and nanostructured photonic materials to facilitate significant alteration of thermal radiation processes is receiving considerable attention due to both the scientific relevance and potential for technological applications [1], [2]. The major losses in conventional thermophotovoltaic (TPV) energy conversion are due to the loss of the photons with smaller frequencies than the band gap of the photovoltaic cell and the thermalization of electron-hole pairs produced by the photons with larger frequencies than the electronic band gap. By using photonic band gap materials as an intermediate radiator, it becomes possible to engineer the spectral and directional characteristics of the thermal radiation such that the emission can be suppressed or enhanced depending on the frequency and the propagation direction [3], [4] and the above mentioned losses can be minimized.

The project focuses on the theoretical and numerical analysis of the thermal radiation generation and transfer, and the re-thermalization mechanisms inside a photonic-crystal based thermal source. This investigation will further our understanding of the physical thermal phenomena in microstructures photonic materials and will provide indispensable tools for identifying suitable choices of the materials and the structures with high energy conversion efficiency and maximum power extraction.

[1] M. Florescu, K. Busch and J. Dowling, “Thermal radiation in photonic crystals”, Physical Review B, 75, 201101 (2007).
[2] M. Florescu, H. Lee, I. Puscasu, M. Pralle, L. Florescu,  D. Ting and J. Dowling, “Improving Solar Cell Efficiency Using Photonic Band-Gap Materials”, Solar Energy Materials and Solar Cells 91 1599 (2007).
[3]  Marian Florescu  and Kurt Busch, “Properties of thermal radiation in photonic crystals”,  Journal of Optics A: Pure and Applied Optics 11, 114005 (2009).
[4] Christian Schuler, Christian Wolff, Kurt Busch and Marian Florescu, “Thermal radiation in finite photonic crystals”,  Applied Physics Letters  95, 241103 (2009).

Topological-insulator surface states in hybrid superconducting quantum circuits

Supervisor

Eran Ginossar

Research Group

Theory and Advanced Computation

Type

Theoretical

Collaborations

National Physical Laboratory, Ben-Gurion University

Student will require

A keen interest in quantum optics and condensed matter theory.

Objectives

We identify the following main theoretical objectives of the project: (1) Development of theoretical models to describe hybrid devices of superconducting circuits and topological insulators (2) Evaluate optimal materials and device design to enable the experimental realisation and probing of hybrid TI devices, (3) Explore the application of topological insulators for quantum computation and quantum metrology. (4) Inspire and support experimental efforts on hybrid systems carried out by collaborators.

Funding source

A university funded scholarship is available, depending on the applicant qualifications. Full funding (stipend plus fees) is possible for UK/EU citizens. Non-EU citizens will only be considered with proof of part sponsorship for the additional fee component.

You should hold the equivalent of a UK first-class or 2:1 BSc/MPhys degree or equivalent. Preference will be given to candidates with higher qualifications and evidence of research experience.

In order to enhance the new Partnership between The University of Surrey and NPL, and to enable the creation of new research programmes that exploit the complementary facilities available at the two sites, Surrey is funding 25 new PhD positions with full scholarships. Five scholarships will be awarded for start in Oct 2015 or Jan 2016, and applicants must choose from 11 possible projects, of which this is one.

If you would like to apply for this project, you must email Dr Ginossar (e.ginossar@surrey.ac.uk) to discuss your application and arrange an interview.

Prior to your interview you will need to make a formal application through the Faculty Graduate School web application system, with your CV (by the deadline of August 31, 2015)
 

If your interview is successful and you are put forwards by the supervisor, the final stage is selection by the NPL Scholarship Panel, in competition with the other projects from elsewhere in the ATI and the rest of the Faculty. The five best applicants out of the 11 put forward will be offered scholarships.

The process is expected to complete by mid-September.

Project description

The search for universal physical effects, independent of the details and defects of the material, is a basic requirement for metrology. The requirement for most other quantum technological applications lies in systems exhibiting robust quantum coherence, the hallmark of the principle of superposition. Topological insulators (TI) are materials which promise to satisfy both of these requirements.This project focuses on theoretical modelling of devices which will be used for probing and utilizing the unique properties of these materials. 

Topological insulators are a class of semiconductors discovered in recent years where the spin-orbit interaction has a dramatic effect as its presence induces a new electronic phase which is characterised and explained by a topological property of the band structure. These discoveries created a new field in which theoretical predictions are playing a pivotal role. The hallmark of the topological insulating phase is the existence of gapless electronic surface states. The surface states have inherent robustness against perturbations and hence are predicted to be useful in realising topological quantum computation. The topological insulating state also bears a resemblance to the quantum Hall state which is also a manifestation of topological order and which serves as standard of electrical resistance. It is therefore envisaged that TIs could find metrological application as a magnetic-field free resistance standard. Although experiments with TI materials have been very promising so far, exploiting TIs for applications is challenging due to the need to develop advanced probing and state manipulation capabilities that separate the bulk from the surface response. In this project we will develop physical models that describe hybrid quantum systems involving TIs and superconducting circuits and use these models to explore the application of TIs for topologically protected quantum computation and quantum metrology as well as collaborate with experiments.

[1] Colloquium: Topological insulators, M. Z. Hasan and C. L. Kane , Rev. Mod. Phys. 82, 3045 (2010)
( http://arxiv.org/pdf/1002.3895.pdf  -- and references within)

[2] Eran Ginossar and Eytan Grosfeld, Nature Communications 5, 4772 (2014)

http://arxiv.org/pdf/1307.1159)

[3] http://www.npl.co.uk/science-technology/quantum-detection/

For more information on these and other projects see the ATI web pages and the relevant PhD descriptors:

http://www.surrey.ac.uk/ati/activity/pg/phd_projects/index.htm