Professor Andrew Briggs
Department of Materials
Tel: +44 1865 273700 (switchboard)
Fax: +44 1865 273730
QIPIRC HomepageQuantum Nanomaterials@Oxford
Summary of Interests
Whenever a fundamental new principle of science is discovered, the chances are that sooner or later a way will be found to use it for a new technology. The quantum mechanical principles of superposition and entanglement, identified nearly a century ago, are now understood to offer spectacular potential for technological applications. Superposition describes how an object can be in two states at once, as it were "here" and "there" at the same time. Two or more objects in superposition states can be entangled, so that measurements on each of them are correlated in a way that goes beyond anything we would expect from everyday intuition. Exploiting these effects in practical devices would provide new capabilities for fields such as molecular light harvesting and for molecular quantum technologies such as sensors, simulators, and quantum computers.
Successful laboratory experiments have shown that molecules of various kinds can exhibit these crucial quantum properties. Molecules are composed of electrons and atomic cores or "nuclei". Both electrons and nuclei can have a property called spin associated with them that makes them behave like tiny bar magnets. We have confirmed that electron and nuclear spins can be put into superpositions or entangled, and they can last for a long time in that condition. Most of the experiments so far have been in small test tubes. The crucial step now is to implement the same effects in nanometre scale electrical devices, such as single electron transistors consisting of single sheets of carbon rolled up as nanotubes or flat as sheets of graphene. By making hybrid technologies that combine molecules with nanoelectronics, we will lay the foundation for scaling up to more complex systems.
At this very small size, different atoms or molecules in different places affect the behaviour of the device. A breakthrough in the past few years enables us to see the positions of individual atoms in the materials which we want to use in our devices. The technique is aberration-corrected electron microscopy, and provided the electrons are not too energetic it is possible to look at the structures which we have made without damaging them. In this way we shall be able to relate the device performance to the atomic resolution microscopy of the component materials. Individual components of this vision include the following.
- design the devices to build, based on a deep understanding of how to control their quantum states;
- produce the materials, such as molecules with suitable spin states with carbon nanotubes and graphene for electrical substrates;
- make nanoscale devices and examine them in a microscope to see where the individual atoms and molecules are;
- perform the experiments to develop the quantum control and measurement for the effects which we aim to exploit;
- undertake theoretical modelling to understand the electron behaviour and to design new materials systems for improved performance.
Prizes and Awards
- Holliday Prize, Institute of Materials, 1984
- Metrology award for World Class Manufacturing, 1999
- Honorary Fellow of Royal Microscopical Society, 2000
- "The Search for Evidence-based Reality" - Video presentation, Heidelberg, October 2012
- "Experimental Implementations of Quantum Paradoxes" - Video presentation, Chapman University, California, August 2012
Synthesis and characterization of dimers for quantum entanglement
B.J. Farrington, Dr. K. Porfyrakis, Dr. A.N. Khlobystov*, Dr. A. Ardavan**, Professor G.A.D. Briggs
The fabrication of asymmetric dimers of endohedral fullerenes containing nitrogen atoms will enable spin resonance experiments to be performed that demonstrate entanglement between electron spins. This will constitute a crucial proof of principle of controlled entanglement in carbon nanomaterials. Chemical synthesis routes will be used that preserve the delicate endohedral nitrogen species, including the use of metal atoms to join the fullerenes through metal coordination bonds or via functionalising groups. (*University of Nottingham; **Clarendon Laboratory, Department of Physics)
Endohedral fullerene derivatives for Quantum Information Processing
B.J. Farrington, Dr. A. Ardavan**, Professor G.A.D. Briggs, Dr. K. Porfyrakis
Using new high-throughput purification technologies it is now possible to produce high purity N@C60 samples on a sufficiently large scale to allow chemical functionalization of the fullerene cage. We are developing the chemistry of endohedral fullerenes with the aim of synthesizing initially dimeric and subsequently oligomeric chains of spin-active molecules. We shall control the distance between endohedral fullerenes by designing appropriate bridge molecules with varying length. We shall develop the ability to engineer and manipulate spin-spin interactions along these chains. A molecular structure of this kind could constitute a key building block for any technology based on information processing with electron spins. (**Clarendon Laboratory, Department of Physics)
Electron spin ensemble based multimode quantum memory
Dr. A. Ardavan *, Professor G.A.D. Briggs
Ensembles of electron spin could be used as the media for quantum memory by utilizing the principle of holographic information storage. Multiple spatial phase modes are created by applying magnetic field gradient to the spin ensembles, in which multiple units of information are stored. The use of pulsed magnetic field gradients allows us to get access to the stored information selectively. This type of multimode quantum memory, in combination with superconducting qubit and cavity, could be used to develop a hybrid model of quantum computer. (*Clarendon Laboratory, Department of Physics)
Dimers for quantum computing
Dr. K. Porfyrakis, Dr. A. Ardavan*, Professor G.A.D. Briggs
Electron spin active dimers could be used to realize a two-qubit system for quantum computing. We have synthesized directly bonded empty fullerene dimers by high speed vibration milling. The same method can be used to synthesize endohedral fullerene dimers to realize a two-qubit system. We are investigating the switchable dimers for a controllable two-qubit system. We are synthesizing azobenzene bridged nitroxyl free radical dimers and fullerene dimers. UV/Vis light can be used to switch the bridge from trans- to cis-, or vice verse, in order to control the qubit interaction, which can be probed by ESR method. (*Clarendon Laboratory, Department of Physics)
Endohedral Fullerenes for Quantum Information Processing
Dr. K. Porfyrakis, Dr. A.M. Khlobystov*, Dr. A. Ardavan**, Professor G.A.D. Briggs
One of the most remarkably robust examples of an unpaired electron spin within a molecule is that of a nitrogen atom trapped inside a spherical fullerene (termed N@C60). We have measured the coherence time of a qubit encoded within this electron spin system and performed single qubit operations using pulsed electron paramagnetic resonance (EPR). We are investigating the synthesis of several types of endohedral fullerene dimers including directly-bonded and oxygen-bridged dimers. These multi-qubit systems will then be characterised by EPR. We shall study the ability to control qubit interactions through the inter-fullerene bridge, and move on to investigate larger qubit arrays. (*University of Nottingham; **Clarendon Laboratory, Department of Physics)
Quantum superposition in large systems
Dr. S.C. Benjamin, E. Gauger, Professor G.A.D. Briggs, G. Knee
This is a theoretical project looking at the possibilities inherent in creating quantum superpositions of large objects such as massive molecules or SQUIDs and similar. A key theoretical tool is be the Leggett-Garg inequality, which tests to see if a system needs quantum physics to describe its behavoir. We are now buildings on the early success of this project, which we reported in this open Nature Communications paper: http://www.nature.com/ncomms/journal/v3/n1/full/ncomms1614.html
6 public active projects
Publications since 2010 are listed below. For more information see Full List of Publications.
Ultrasonic force and related microscopies. In Advances in Acoustic Microscopy and High Resolution Imaging (ed R.G. Maev), Weinheim: Wiley-VCH (2013). G.A.D. Briggs and O.V. Kolosov.
Opening up three quantum boxes causes classically undetectable wavefunction collapse. Proc. Natl. Acad. Sci. USA 110, 3777-3781 (2013); DOI: 10.1073/pnas.1208374110. R.E. George, L. Robledo, O.J.E. Maroney, M. Blok, H. Bernien, M.L. Markham, D.J. Twitchen, J.J.L. Morton, G.A.D. Briggs and R. Hanson.
Quantum sensors based on weak-value amplified cannot overcome decoherence. Phys. Rev. A 87 012115 (2013); DOI: 10.1103/PhysRevA.87.012115. G.C. Knee, G.A.D. Briggs, S.C. Benjamin and E.M. Gauger.
Comment on "A scattering quantum circuit for measuring Bell's time inequality: a nuclear magnetic resonance demonstration using maximally mixed states". New J. Phys. 14 058001 (2012). G.C. Knee, E.M. Gauger, G.A.D. Briggs and S.C. Benjamin.
Formation mechanism for a hybrid supramolecular network involving cooperative interactions. Phys. Rev. Lett. 108, 176103 (2012). M. Mura, F. Silly, V.M. Burlakov, M.R. Castell, G.A.D. Briggs and L. Kantorovich.
N@C60-porphyrin: a dyad of two radical centers. J. Amer. Chem. Soc. 134, 1938-1941 (2012). G. Liu, A.N. Khlobystov, G. Charalambidis, A. Coutsolelos, G.A.D Briggs and K. Porfyrakis.
Catalytic and non-catalytic roles of pendant groups in the decomposition of N@C60: a DFT investigation. Chem. Commun. 48, 5148-5150 (2012). G. Liu, A.N. Khlobystov, G.A.D. Briggs and K. Porfyrakis.
Chemistry at the nanoscale: synthesis of an N@C60-N@C60 endohedral fullerene dimer. Angew. Chem. Int. Ed. 51, 3587-3590 (2012). B.J. Farrington, M. Jevric, G.A. Rance, A. Ardavan, A.N. Khlobystov, G.A.D. Briggs and K. Porfyrakis.
Violation of a Leggett-Garg inequality with ideal non-invasive measurements. Nat. Commun. 3, 606 (2012). G.C. Knee, S. Simmons, E.M. Gauger, J.J.L. Morton, H. Riemann, N.V. Abrosimov, P. Becker, H-J. Pohl, K.M. Itoh, M.L. Thewalt, G.A.D. Briggs and S.C. Benjamin.
Resolving strain in carbon nanotubes at the atomic level. Nature Materials (2011). J.H. Warner, N.P. Young, A.I. Kirkland and G.A.D. Briggs.
Photostability of N@C60 in common solvents. ECS Transactions 35, 113-117 (2011). B.J. Farrington, T.J. Hingston, G.A.D. Briggs, M.R. Sambrook and K. Porfyrakis.
Functionalized fullerenes in self-assembled monolayers. Langmuir 27, 10977-10985 (2011). M.D. Gimenez-Lopez, M.T. Räisänen, T.W. Chamberlain, U. Weber, M. Lebedeva, G.A. Rance, G.A.D. Briggs, D.G. Pettifor, V.M. Burlakov, M. Buck and A.N. Khlobystov.
Quantum control in spintronics. Phil. Trans. R. Soc. Lond. A 369, 3229-3248 (2011). A. Ardavan & G.A.D. Briggs.
Transport spectroscopy of an impurity spin in a carbon nanotube double quantum dot. Phys. Rev. Lett. 106, 206801 (2011). S.J. Chorley, G. Giavaras, J. Wabnig, G.A.C. Jones, C.G. Smith, G.A.D. Briggs and M.R. Buitelaar.
Photochemical stability of N@C60 and its pyrrolidine derivatives. Chem. Phys. Lett. 508, 187-190 (2011). K. Porfyrakis, G. Liu, A.N. Khlobystov, A. Ardavan and G.A.D. Briggs. Selected as Editor's Choice.
Carbon nanotube nanoelectronic devices compatible with transmission electron microscopy. Nanotechnology 22, 245305 (2011). H.L. Wang, J. Luo, F. Schäffel, M. Rümmeli, G.A.D. Briggs and J.H. Warner.
Utilizing boron nitride sheets as thin supports for high resolution imaging of nanocrystals. Nanotechnology, 22, 195603 (2011). Y.A. Wu, A.I. Kirkland, F. Schäffel, K. Porfyrakis, N.P. Young, G.A.D. Briggs and J.H. Warner.
Atomic resolution imaging of the edges of catalytically etched suspended few layer graphene. ACS Nano 5, 1975-1983 (2011). F. Schäffel, A. Bachmatiuk, M. Rümmeli, U. Queitsch, B. Rellinghaus, G.A.D. Briggs and J.H. Warner.
Coherent state transfer between an electron- and nuclear spin in 15N@C60. Phys. Rev. Lett. 106, 110504 (2011). R.M. Brown, A.M. Tyryshkin, K. Porfyrakis, E.M. Gauger, B.W. Lovett, A. Ardavan, S.A. Lyon, G.A.D. Briggs and J.J.L. Morton.
Response to 'Comment on Ultrahigh secondary electron emission of carbon nanotubes' Appl. Phys. Lett. 98, 66101 (2011). J. Luo, J.H. Warner and G.A.D. Briggs.
Atomic scale growth dynamics of nanocrystals within carbon nanotubes. ACS Nano, 5, 1410-1417 (2011). J.H. Warner, S.R. Plant, N.P. Young, K. Porfyrakis, A.I. Kirkland and G.A.D. Briggs.
Electron paramagnetic resonance investigation of purified catalyst-free single-walled carbon nanotubes. ACS Nano 4, 7708-7716 (2010). M. Zaka, Y. Ito, H. Wang, W. Yan, A. Robertson, Y. Wu, M. Rümmeli, D. Staunton, T. Hashimoto, J.J.L. Morton, A. Ardavan, G.A.D. Briggs and J.H. Warner.
Synthesis and magnetic properties of a nitrogen containing fullerene dimer. Eur. J. Org. Chem. 1, 117-121 (2010). F. Hörmann, A. Hirsch, K. Porfyrakis and G.A.D. Briggs.
High performance field effect transistors from solution processed carbon nanotubes. ACS Nano 4, 6659-6664 (2010). Correction to references 12 and 28 ACS Nano 5, 3400 (2011). H.L. Wang, J. Luo, A. Robertson, Y. Ito, W. Yan, V. Lang, M. Zaka, F. Schäffel, M. Rümmeli, G.A.D. Briggs and J.H. Warner.
Storage of multiple coherent microwave excitations in an electron spin ensemble. Phys. Rev. Lett. 105, 140503 (2010). H. Wu, R.E. George, A. Ardavan, J.H. Wesenberg, K. Mølmer, D.I. Schuster, R.J. Schoelkopf, K.M. Itoh, J.J.L. Morton and G.A.D. Briggs. Featured in Viewpoint PhysRevLett.105.140503; News & Views, Nature 468, 44-45 (04 November 2010).
High cooperativity coupling of electron-spin ensembles to superconducting cavities. Phys. Rev. Lett. 105, 140501 (2010). D.I. Schuster, A.P. Sears, E. Ginossar, L. DiCarlo, L. Frunzio, J.J.L. Morton, H. Wu, G.A.D. Briggs and R.J. Schoelkopf. Featured in Viewpoint PhysRevLett.105.140503; News & Views, Nature 468, 44-45 (04 November 2010).
Book of the Week: Science vs. Religion: What Scientists Really Think. Times Higher Education (16 September 2010);
www.timeshighereducation.co.uk/story.asp?sectioncode=26&storycode=413457&c=1 . G.A.D. Briggs.
Spin detection at elevated temperatures using a driven double quantum dot. Phys. Rev. B 82, 085410 (2010). G. Giavaras, J. Wabnig, B.W. Lovett, J.H. Jefferson and G.A.D. Briggs.Selected for the August 23, 2010 issue of Virtual Journal of Nanoscale Science & Technology, www.vjnano.org.
Direct imaging and chemical identification of the encapsulated metal atoms in bimetallic endofullerene peapods. ACS Nano 4, 3943-3948 (2010). R.J. Nicholls, K. Sader, J.H. Warner, S.R. Plant, K. Porfyrakis, P.D. Nellist, G.A.D. Briggs and D.J.H. Cockayne.
Electron spin coherence in metallofullerenes: Y, Sc and La@C82. Phys. Rev. B 82, 033410 (2010). R.M. Brown, Y. Ito, J.H. Warner, A. Ardavan, H. Shinohara, G.A.D. Briggs and J.J.L. Morton.
Single shot measurement in silicon single electron transistors. 2008 IEEE Silicon Electronics Workshop, 32-33 (2008); IDS Number: BPK87; ISBN: 978-1-4244-2071-1. T. Ferrus, D.A. Williams,D.G. Hasko, L. Creswell, R.J. Collier, A. Lam, Q.R. Morrissey, S.R. Burge, M.J. French and G.A.D. Briggs .
Ultrahigh secondary electron emission of carbon nanotubes. Appl. Phys. Lett. 96, 213113 (2010). J. Luo, J.H. Warner, C. Feng, Y. Yao, Z. Jin, H. Wang, C. Pan, S. Wang, L. Yang, Y. Li, J. Zhang, A.A.R. Watt, L.M. Peng, J. Zhu and G.A.D. Briggs.
Entangling remote nuclear spins linked by a chromophore. Phys. Rev. Lett. 104, 200501 (2010). 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.
Experimental and theoretical analysis of H-bonding supramolecular assemblies of PTCDA molecules. Phys. Rev. B 81, 195412 (2010). M. Mura, X. Sun, F. Silly, H.T. Jonkman, G.A.D. Briggs, M.R. Castell and L.N. Kantorovich.
Magnetic field sensing using a driven double quantum dot. Physica E 42, 895-898 (2010). G. Giavaras, J. Wabnig, B.W. Lovett, J. H. Jefferson and G.A.D. Briggs.
Intricate hydrogen-bonded networks: binary and ternary combinations of uracil, PTCDI and melamine. J. Phys. Chem. C 114, 5859-5866 (2010). J.A. Gardener, O.Y. Shvarova, G.A.D. Briggs and M.R. Castell.
Exchange interactions of spin-active metallofullerenes in solid-state carbon networks. Phys. Rev. B 81, 075424 (2010). M. Zaka, J.H. Warner, Y. Ito, J.J.L. Morton, M.H. Rümmeli, T. Pichler, A. Ardavan, H. Shinohara and G.A.D. Briggs.
Nanoethics: Big Ethical Issues with Small Technology , By Dónal P. O’Mathúna. Times Higher Education 1935, 48-49 (18-24 February 2010); www.timeshighereducation.co.uk/story.asp?storycode=410398. G.A.D. Briggs.
Controlling intermolecular spin interactions of La@C82 in empty fullerene matrices. Phys. Chem. Chem. Phys. 12, 1618-1623 (2010). Y. Ito, J.H. Warner, R. Brown, M. Zaka, R. Pfeiffer, T. Aono, N. Izumi, H. Okimoto, J.J.L. Morton, A. Ardavan, H. Shinohara, H. Kuzmany, H. Peterlik and G.A.D. Briggs.
Endohedral metallofullerenes in self-assembled monolayers. Phys. Chem. Chem. Phys. 12, 123-131 (2010). M.C. Gimenez-Lopez, J. Gardener, A. Iwasiewicz-Wabnig, K. Porfyrakis, C. Balmer, G. Dantelle, A.Q. Shaw, M. Hadjipanayi, A. Crossley, N.R. Champness, M.R. Castell, G.A.D. Briggs and A.N. Khlobystov.
Nanomaterials for quantum technologies
Professor G. A. D. Briggs, Dr K. Porfyrakis, Dr J. H. Warner and Dr E. A. Laird
Quantum information processing offers one of the most exciting challenges in the study and development of nanomaterials. It is at the cutting edge of quantum nanoelectronics, and Oxford is part of the world-wide endeavour to develop scalable quantum computers. Instead of classical bits of information, these will work with qubits (quantum bits). We need materials with quantum states that can be individually controlled and measured, and yet which are sufficiently robust against decoherence that they can sustain a sequence of quantum manipulations and interactions. We lead the world in using the new family of fullerene materials (popularly known as Bucky balls), which can be used to contain atomic species inside a cage that separates them from the environment. We can store the quantum information in an electron or nuclear spin, and exchange it between the two. We can manipulate and characterize the spin states by electron paramagnetic resonance and also optically. By creating entanglement between several spins, it is possible to develop sensors that exceed the standard quantum limit. A core thrust of our research is to incorporate molecular materials in working devices for practical quantum technologies. There will be several projects with these nanomaterials, ranging from synthesis and microscopy to experimental implementation of candidate schemes for quantum computing. The research is highly interdisciplinary, and there is scope for a range of skills and interests from materials science and chemistry to experimental quantum physics. There may be possibilities for industrial support and for international travel and collaboration.
A carbon nanotube quantum computer
Dr E. A. Laird and Professor G. A. D. Briggs
A computer based on the quantum states of single electrons has the potential to be exponentially more powerful for some tasks than existing classical computers. Creating such a computer is an extremely difficult challenge, because quantum states usually decohere rapidly due to interactions with their environment. One leading approach is to use the spin states of an electron in a semiconductor. Carbon nanotubes are a particularly attractive material for this purpose, because nuclear spins, which cause decoherence in some other semiconductors, can be virtually eliminated. In this project, you will create a proof-of-principle two-bit quantum computer in an isotopically purified carbon nanotube. Very recently, a single-qubit gate was demonstrated in a nanotube quantum dot. You will extend this work by using a radio-frequency measurement setup for high-fidelity qubit readout and implementing a two-qubit gate in a pair of quantum dots. The goal is to demonstrate simple quantum algorithms in a carbon nanotube device. This project will involve training in nanofabrication, as well as low- and high-frequency electronics at millikelvin temperatures.
Also see homepages: Andrew Briggs
Carbon-based quantum devices characterised electrically and by imaging
Dr E. A. Laird, Dr J. H. Warner and Professor G. A. D. Briggs
Carbon nanomaterials are attractive materials for electronic devices, especially for quantum computers using the spin of single electrons. One challenge in realising these applications is that the detailed device structure cannot be fully controlled during fabrication. The aim of this project is to characterize the same devices both through electrical measurements and by high-resolution electron microscopy, with the aim of understanding the electronic properties in terms of the atomic structure.
The goal of project will be to determine how the bandstructure and spin-orbit coupling of carbon nanotubes depend on the atomic structure – in particular on the chirality, which describes how the carbon atoms are aligned. Through aberration-corrected high-resolution transmission electron microscopy (AC-HRTEM), it is possible to measure the chiral indices of individual nanotubes, but so far this has not been performed on nanotubes integrated into electronic devices. You will combine these two kinds of measurement by incorporating a TEM window into existing fabrication technology for ultra-low-disorder nanotubes. You will then make electrical measurements of selected devices in a dilution refrigerator, with the aim of correlating the measured spin-orbit coupling to the previously determined chirality and comparing with existing theoretical predictions.
Coupling valley-spin qubits in nanotubes by superconducting cavities
Professor G. A. D. Briggs, Dr E. A. Laird and Dr P. J. Leek*
Carbon-based quantum technologies will use electron spin states in nanotubes as a fundamental resource. A single electron in a confined region (sometimes referred to as a quantum dot) in a nanotube can form a valley-spin qubit, through the interaction between the valleys in the band structure and the spin of the electron. The spin-orbit coupling provides a means of linking the qubit to an external system via an electric dipole.
Quantum technologies will demand the ability to transfer quantum information from one device to another. The magnetic dipole moment of a single electron spin is too weak for this purpose, but by converting it to an electric dipole moment the coupling can be significantly increased. Superconducting cavities, fabricated either in the form of on-chip resonators or as a 3-D box, provide the means to couple two nanotube quantum devices together, or to couple a nanotube device to collective spin states as a quantum information register. The goal of this project will be to learn how to couple a valley-spin qubit to a superconducting cavity, and then to show how that will useful for scalable quantum technologies.
* Department of Physics
Also see homepages: Andrew Briggs
Coupling molecular spins to valley-spin qubits in carbon nanotubes
Professor G. A. D. Briggs, Dr J. H. Warner, Dr K. Porfyrakis and Dr E. A. Laird
Carbon-based quantum technologies require the ability to transfer quantum information from one form to another. Valley-spin qubits (so called because of the hybridisation between electron spin states and the valleys in the band structure of the nanotube) enable single states to be manipulated and measured in electron dipole spin resonance, but the coherence times are not long enough for scaleable quantum computing. Molecular qubits are known from ensemble experiments to have useful quantum coherence times, but to exploit these in useful devices we must have ways to measure them individually. By attaching molecules to the nanotube, and transferring quantum states between the nanotube and the molecule, it should be possible to exploit the best of each. The storage time can be increased a further thousandfold by using molecular nuclear spins as a further resource.
Successful development of this scheme will require nanofabrication of the devices, attachment of the spin-bearing molecules, microscopy of the resulting structures, and magnetic resonance at cryogenic temperatures. This project will involve training in nanofabrication, together with electron microscopy and low-temperature electronic measurements. Aspects of the project will be undertaken in collaboration with other members of the laboratory. The goal will be to show that quantum information can be effectively transferred between the nanotube device and the spin states of the attached molecules. This will be achieved by entangling quantum mechanically the molecular spin with an electron spin on the nanotube, and measuring the molecular spin state through its effect on the electron.
Quantum superposition in a vibrating carbon nanotube
Professor G. A. D. Briggs, Dr E. A. Laird and Dr N. Ares
There are two motives for extending the amount of ‘quantumness’ in our materials and devices. One is to develop technologies which exploit uniquely quantum phenomena, to perform tasks which are not feasible using classical physics alone. The other is to investigate ways in which classical realism can be ruled out in interpreting quantum measurements. A well-established test of ‘macrorealism’ is provided by the Leggett-Garg inequality (LGI), which enables you to tell whether or not a system is in one and only one state at any given time. We have demonstrated violations of the LGI in atomic scale systems, and the time is now ripe to stretch the limits of ‘macroscopicity’ which can be tested.
This project will test the LGI in different states of a vibrating single-walled carbon nanotube. Nanotubes will be mounted so that their vibrational frequency can be tuned electrically at frequencies of tens of gigahertz. They will have a high quality factor, so that their quantum states decay only slowly, typically over 10,000 or more oscillations, with large level spacing, so that they can be cooled to the quantum ground state in a dilution refrigerator. A pair of valley-spin qubits in the nanotube will provide means of excitation one quantum at a time, and also the means of measuring resulting states. A carefully designed experiment will make it possible to test whether the LGI is violated, and hence whether a macrorealistic interpretation of the experiment can be ruled out. This will be an important practical demonstration of how to increase size of systems that manifest fully quantum behaviour.
Also see homepages: Andrew Briggs
Quantum interference in single-molecule devices
Professor G. A. D. Briggs, Dr E. A. Laird, Dr J. A. Mol and Professor H. L. Anderson*
Quantum interference offers a rich resource which could be exploited in molecular devices. If there are multiple pathways for energy transport through a molecule, or if electrical transport is subject to resonances within a molecule, then these effects could be exploited for practical technologies. For example, it may be possible to make transistors with much lower power consumption than current silicon CMOS, and it may be possible to exceed the current limits of thermovoltaic materials for scavenging heat that would otherwise be wasted. Understanding such phenomena may also shed light on postulated quantum coherent processes in biology, ranging from photosynthesis to bird navigation.
The project will require nanofabrication of carbon-based devices into which individual molecules can be inserted. The current through the molecules will be measured with a view to discovering mechanisms of quantum interference. A major challenge will be to devise and fabricate geometries with additional gates to control the quantum interference. The project will involve nanofabrication, chemical attachment of the molecules, and electrical measurements over a range of temperatures and frequencies, with especial regard to discovering the conditions under which quantum coherence can be found. A successful outcome will be to find regimes in which quantum coherence gives enhanced device performance.
* Department of Chemistry
Quantum read-out of spin resonance in a silicon transistor
Dr J. A. Mol, and Professor G. A. D. Briggs
If quantum computers could be made using silicon materials, they would benefit from the vast investment that has been made in the purification of the materials and the nanofabrication of devices. Recent work in collaboration with Oxford has shown that the quantum coherence time of spin qubits can be more than three hours. The great challenges now are to learn how to control the interactions and measure the states of a small numbers of spins in silicon. This project will seek to detect electron spin resonance in a nanoscale silicon transistor through the resulting change in current.
The project will study electrically detected magnetic resonance (EDMR) in state-of-the-art silicon transistors. The size and geometry of these devices allows for the detection of spin states down to level of a single donor atom. The project will combine device fabrication and measurement with cavity-based electron and nuclear magnetic resonance. This will open the way to exploring phenomena usually associated with atomic physics for quantum information processing. The project will demand good knowledge of physics and/or electrical engineering as well as a deep understanding of semiconductor materials, and will benefit from new facilities currently being established at Oxford for experiments on quantum devices.
Quantum control of donor spins in silicon devices
Dr J. A. Mol and Professor G. A. D. Briggs
Quantum information embodied in spins in silicon can be manipulated very precisely, and can persist for over three hours. Quantum computers made from silicon materials would benefit from the great industrial development of silicon processing and nanofabrication. Much of previous research in silicon for quantum computing has used donor impurities in bulk crystals. The great challenges now are to learn how to measure the states and control the interactions of a small numbers of spins in silicon. This project will seek to develop control techniques that will be sufficiently precise to allow fault-tolerant error-correction in silicon devices with small numbers of spins.
The long lifetimes of spins in bulk silicon place a very high upper bound on the possible quality of quantum control. Donor spin qubits located close to a material interface in a silicon device, such as a transistor, experience a nonuniform environment which can affect the quality of quantum control. Scalable quantum technologies will need to meet and surpass certain accuracy thresholds to demonstrate quantum error correction. The goal of the project will be to exceed these thresholds using interface spins in silicon devices. For this you will investigate magnetic resonance and interface effects in both many-spin and single-spin regimes. You will need to have, or to acquire early in the project, good knowledge of physics, magnetic resonance, quantum information, and semiconductor materials. New cryogenic and pulsed microwave facilities are currently being established at Oxford for experiments on quantum devices.
Also see a full listing of New projects available within the Department of Materials.