Personal Homepages

Design and realization of architectures for new forms of information processing, especially quantum computing. Theoretical work relating to the design and characterization of solid state nanostructures for computation, with particular current emphasis on (a) quantum dots systems (b) fullerene systems (nanotubes, endohedral C60, etc.) (c) defects in diamond. In particular, I do calculations of how to control and manipulate interactions between nanostructures such that entangled states might be produced between them.

Energy harvesting; in particular understanding how excitons in networks of coupled chromophores can be made to move to a target site most efficiently. Applications include understanding photosynthesis and solar energy harvesting.

All of these studies involve work on open quantum system theory to understand the nature of environmental interactions in solid state and molecular systems.

Nanomaterials and quantum computing
Dr. B.W. Lovett*, Dr. S.C. Benjamin, Dr. E.M. Gauger
We are looking at how certain nanomaterials (such as quantum dots or crystal defects) can be used to implement quantum gate operations. We have developed methods for coherent quantum control of systems with a range of Hamiltonians. We are also interested in modelling decoherence, which is caused by the interaction of a system with its environment, and employs the theory of open quantum systems. We look at both Markovian and non-Markovian models of such open systems. We aim to provide experimental tests of the different theories working closely with the Low Dimensional Structures and Devices group at the University of Sheffield http://ldsd.group.shef.ac.uk/. (*Heriot-Watt University)

Coherent Control of Spin Systems
Dr. B.W. Lovett*, Dr. S.C. Benjamin, Dr. E.M. Gauger
We are studying the quantum properties of nuclear and electron spins, primarily in molecular systems. Our aim is to provide theory that will allow for the control small numbers of spins, such that the quantum coherence is preserved for as long as possible. We collaborate with the Quantum Spin Dynamics experimental group (http://qsd.physics.ox.ac.uk/), and together we demonstrated that the quantum state of an electron spin can be transferred coherently to a nuclear spin, thus increasing the coherence time. We are now working on optical methods for further improving coherence, and for coupling several spins together. (*Heriot-Watt University)

Energy harvesting in biomimetic systems
Dr. B.W. Lovett*, Dr. A.A.R.Watt
Light is converted to chemical energy with extremely high efficiency in photosynthetic systems. Part of the reason for this is that light can create exciton states in protein antenna structures, and these antenae are arranged such that the exciton energy can be transferred to a specific site quickly and efficiently. In my work, I aim to understand this efficiency as the result of an interplay between exciton trasnfer interactions and environmental coupling to phonons. In particular, I aim to design synthetic systems that can mimic biological efficiencies, thus providing a route to optimized solar cells. (*Heriot-Watt University)

Carbon nanostructures
Dr. B.W. Lovett*, Dr. G. Giavaras
We are studying various aspects of carbon-based nanomaterials, specifically fullerenes and nanotubes. Two main themes are currently being pursued. First, a carbon nanotube can be filled with spin-possessing endohedral fullerenes making a so-called peapod material. Certain devices require that the peapod spins interact with one another, and one such interaction could be through the indirect exchange coupling via the conduction electrons of the nanotubes. We are developing various many-body techniques by which this coupling can be calculated. Second, we are interested in the transport properties of carbon nanotubes and peapods. Using master equation techniques, we aim to find methods by which magnetic resonance might be detected in such systems by measuring the electrical current through them. These transport calculations have already revealed that a carbon nanotube can act as a sensitive spin measurement device or as a spin characterization tool. This work is carried out in collaboration with the experimental semiconductor group of Prof Charles Smith at the University of Cambridge http://www.sp.phy.cam.ac.uk/SPWeb/home/cgs4.html. (*Heriot-Watt University)

Measurement Based Quantum Computing
Dr. B.W. Lovett*, Dr. S.C. Benjamin, Dr. J. Fitzsimons, Y. Matsuzaki
One can regard quantum entanglement as the fundamental resource needed in order to execute quantum algorithms. Certain kinds of entangled states exist which are universal resources, in the sense that any quantum algorithm can be performed simply by performing a prescribed series of quantum measurements. Moreover, even the entangled state itself can by created by making measurements. These insights have led to many new possible implementations of quantum computers, for example: one that uses only photons, one exploiting crossed atomic beams and others based on optical measurements on colour centres in diamond. Specific topics are: first principles physics of measurement, implementation of error correction or avoidance and entanglement creation by measurement. (*Heriot-Watt University)

Colour centres in diamond as solid state single photon sources and quantum spin registers
F. Grazioso, P. Dolan, Dr. E. Abe, Dr. J.J.L. Morton, Dr. S.C. Benjamin, Dr. J.M. Smith
Diamond colour centres have demonstrated exquisite properties as single photon sources and quantum spin registers that operate even at room temperature, providing great opportunities for quantum communications and information technologies. This project involves using optical microscopy and spin resonance techniques to characterise the underlying physics and properties of single colour centres in new ultra-pure synthetic diamond material. Principal collaborations are with Element Six Ltd and the Diamond Trading Company.

6 public active projects

 

Book:

Introduction to Optical Quantum Information Processing, P. Kok and B. W. Lovett, Cambridge University Press (2010)

 

Papers:

1.  Phonon induced Rabi frequency renormalization of optically driven single InGaAs/GaAs quantum dots, A. J. Ramsay, T. M. Godden, S. J. Boyle, E. M. Gauger, A. Nazir, B. W. Lovett, A. M. Fox, M. S. Skolnick, Phys. Rev. Lett., 105 177402 (2010)
   

2. Entangling remote nuclear spins linked by a chromophore, M. Schaffry, V. Filidou, S. D. Karlen, E. M. Gauger, S. C. Benjamin, H. L. Anderson, A. Ardavan, G. A. D. Briggs, K. Maeda, K. B. Henbest, F. Giustino, J. J. L. Morton and B. W. Lovett, Phys. Rev. Lett. 104 200501 (2010)

3. Excitation-induced-dephasing of quantum dot excitonic Rabi rotations, A. J. Ramsay, Achanta Venu Gopal, E. M. Gauger, A. Nazir, B. W. Lovett, A. M. Fox and M. S. Skolnick, Phys. Rev. Lett. 104 017402 (2010)

4. Spin detection at elevated temperatures using a driven double quantum dot, G. Giavaras, J. Wabnig, B. W. Lovett, J. H. Jefferson, and G. A. D. Briggs, Phys. Rev. B 82 085410 (2010)
   

5. Quantum metrology with molecular ensembles, M. Schaffry, E. M. Gauger, J. J. L. Morton, J. Fitzsimons, S. C. Benjamin, and B. W. Lovett,  Phys. Rev. A 82, 042114 (2010)

6. Spin lifetimes in quantum dots from noise measurements, J. Wabnig, B. W. Lovett, J. H. Jefferson and G. A. D. Briggs, Phys. Rev. Lett. 102 016802 (2009)
   

7. Comment on Multipartite Entanglement Among Single Spins in Diamond, B. W. Lovett and S. C. Benjamin, Science 323 DOI 10.1126/science.1168458 (2009)

8. Prospects for measurement-based quantum computing with solid state spins, S. C. Benjamin, B. W. Lovett and J. M. Smith, Laser and Photonics Reviews, 3 556 (2009)

9. Aspects of quantum coherence in nanosystems, B. W. Lovett and A. Nazir, European Journal of Physics 30 S89 (2009)

10. Large spin entangled current from a passive device, A. Kolli, S. C. Benjamin, J. Garcia Coello, S. Bose, and B. W. Lovett, New J. Phys. 11 013018 (2009)
    

11. Measurement-based approach to entanglement generation in coupled quantum dots, A. Kolli, S. C. Benjamin, B. W. Lovett and T. M. Stace, Phys. Rev. B 79 035315 (2009)

12. Solid state quantum memory using the 31P nuclear spin, J. J. L. Morton, A. M. Tyryshkin, R. M. Brown, S. Shankar, B. W. Lovett, A. Ardavan, T. Schenkel, E. E. Haller, J. W. Ager and S. A. Lyon, Nature 455 1085 (2008)
    

13. High fidelity all-optical control of quantum dot spins: Detailed study of the adiabatic approach, E. M.Gauger, S. C. Benjamin, A. Nazir and B. W. Lovett, Phys. Rev. B 77 115322 (2008)
    

14. Freezing distributed entanglement in spin chains, I. D’Amico, B. W. Lovett and T. P. Spiller Phys. Rev. A 76 030302 (2007)

15. All-optical measurement-based quantum-information processing in quantum dots, A. Kolli, B. W. Lovett, S. C. Benjamin and T. M. Stace, Phys. Rev. Lett. 97 250504 (2006)

16. Materials science - Qubits in the pink, P. Kok and B. W. Lovett, Nature 444, 49 (2006)

17. Quantum computing with spin qubits interacting through delocalized excitons: Overcoming hole mixing, B. W. Lovett, A. Nazir, E. Pazy and G. A. D. Briggs, Phys. Rev. B 72 115324 (2005)

18. Anticrossings in Foerster coupled quantum dots, A. Nazir, B. W. Lovett, S. D. Barrett, J. H. Reina, and G. A. D. Briggs, Phys. Rev. B 71 045334 (2005)

19. Selective spin coupling through a single exciton, A. Nazir, B. W. Lovett, S. D. Barrett, T. P. Spiller and G. A. D. Briggs, Phys. Rev. Lett. 93, 150502 (2004)

20. Controlling excitonic entanglement in quantum dots through the optical Stark effect, A. Nazir, B.W.Lovett and G. A. D. Briggs, Phys. Rev. A 70, 052301 (2004)

21. All-optical control of perpetually coupled qubits, S.C.Benjamin, B.W.Lovett and J.H.Reina, Phys.Rev. A 70 060305 (2004)

22. Optical schemes for quantum computing in quantum dot molecules, B. W. Lovett, J. H. Reina, A. Nazir and G. A. D. Briggs, Phys. Rev. B 68 205319 (2003)

Quantum Information Processing
B W Lovett / S C Benjamin

The Quantum and Nanotechnologies Group (www.qunat.org) anticipates that they will be able to offer one or more doctoral studentships in the area of quantum information processing. The group has broad interests, ranging from detailed modelling of semiconductor structures through more abstract ideas related to designs for quantum computer architectures and extending to fundamental questions about the nature of quantum information and measurement. At the time of writing the following are active projects:

i) Measurement based quantum computing. One can regard quantum entanglement as the fundamental resource needed in order to execute quantum algorithms. Certain kinds of entangled states exist which are universal resources, in the sense that _any_ quantum algorithm can be performed simply by performing a prescribed series of quantum measurements. Moreover, even the entangled state itself can by created by making measurements. These insights have led to many new possible implementations of quantum computers, for example: one that uses only photons, one exploiting crossed atomic beams and others based on optical measurements on colour centres in diamond.

Specific topics are: first principles physics of measurement, implementation of error correction or avoidance and entanglement creation by measurement.

ii) Nanomaterials and quantum computing. We are looking at how certain nanomaterials (such as quantum dots, molecules or crystal defects) can be used to implement quantum gate operations. We have developed methods for coherent quantum control of systems with a range of Hamiltonians. We are also interested in modelling decoherence, which is caused by the interaction of a system with its environment, and employs the theory of open quantum systems.

iii) Spin chains. One of the most important questions in quantum information processing is how we might transmit information from one computer to another. We have been looking at at this might be done using one (or higher) dimensional arrays of interacting spins (or similar quantum two level systems). An important theme is to achieve is much as possible with minimal external control --- in other words, to exploit the 'natural' dynamics of the spin system as completely as possible.

Another potential application of a spin chain is as a globally controlled quantum memory element. We are interested in developing the theory of molecular quantum memories, for both interacting and independent molecular systems.

There are several collaborators on these projects, including Dr Tom Stace (University of Queensland), Prof Sougato Bose (University College London), and Prof Leong Chuan Kwek (National University of Singapore). Currently no specific funding is in place; however, a number of funding routes exist and we would be happy to advise strong students about how to explore these.

Also see homepages: Simon Benjamin Brendon Lovett

A quantum model for photosynthesis: towards efficient solar cells
B. W. Lovett, S. C. Benjamin

Many living systems harvest the sun's energy through photosynthesis, a process that has three main steps. First, sunlight is absorbed by a protein-pigment complex, and converted into electronic excitation. Second, the electronic excitation moves through a network of excitation sites in a directed fashion, until it reaches a reaction centre. Third, charge separation at the reaction centre facilitates the production of ATP, which leads to the conversion of carbon dioxide to sugar.

This project will focus on developing models of the second of these steps, and will investigate the role of the protein's vibrational modes (phonons) in determining the efficiency of energy transport through the system.

There is a strong parallel between how photosynthesis works and how organic heterostructure solar cells operate. By exploiting the results on photosynthetic systems, we aim to develop new design principles for artificial energy harvesting devices.

Also see homepages: Brendon Lovett

Also see a full listing of New projects available within the Department of Materials.