Publications
Research interests:
Quantum computation in semiconductor devices
Work on the first semiconductor quantum-dot devices was published by the Semiconductor Physics group in 1983 [1]. These devices now form the building blocks for many ideas for realizing quantum information processing architectures. We were also the first group to develop a technique for measuring single electron movement in and out of a semiconductor quantum dot [2, 3]. This technique is regularly proposed for the readout mechanism for quantum computational devices. For quantum computation to become a reality, it is essential that dephasing from the surrounding environment can be reduced to a minimum. We have been investigating dephasing in a single [4] and coupled quantum dots strongly electrically isolated from the environment [5] where electron-electron scattering from co-tunneling events are greatly reduced. We have recently demonstrated that when two pairs of quantum dots are placed next to each other when we polarize the first pair of dots by moving a single electron from one dot to its neighbour, the resulting dipole field can be used to polarize the next double dot pair. Such a structure forms the basis of a quantum cellular automata computing architecture [6]. More recently we have been using RF capacitance readout of single-electron processes in quantum dots [17] as well as Au nanoparticles of nm dimensions [18].
Imaging and probing quantum phenomena using novel low-temperature probes
Over the past ten years we have built up a unique low-temperature scanning probe. A conducting atomic force microscope tip can be placed with nm accuracy anywhere on a 1 mm by 1 mm area at 100 mK temperatures and in magnetic fields up to 10 Tesla. We use the tip like a scanning gate to modify the behaviour of sub-micron devices. The images we create come from the change in conductance of the sub-micron devices as a function of the tip position. We have used this technique to image the probability densities for electrons in a 1D ballistic channel [7]. Recently we were able to demonstrate that we could use the tip to deposit charge on a semiconductor device at low temperatures. The charge was very stable over time and was used to deplete out a conducting layer bellow the surface enabling current paths to be defined at low temperatures. Later we could remove the charge either with the tip or by illuminating with a red LED [8]. We were then able to draw a different quantum structure. We call this technique Erasable Electrostatic Lithography. This will become an extremely important tool for rapidly prototyping quantum devices for quantum information processing.
This probe is ideally suited to study the transport properties of 2D materials such as graphene where we can use the tip to map out disorder in graphene Hall bars in the Quantum Hall regime for metrology applications [19] or to image electron trajectories in a magnetic field in high mobility graphene, boron nitride heterostructures [20].
Investigating the physics of nano-MEMS devices
I started my research by initiating an investigation into the quantization of 1D phonons in sub-micron free-standing wires [9]. During this time I started exploring the properties of sub-micron MEMS devices [10], research which later resulted in the generation of IP and the formation of the spin-out company, Cavendish kinetics Ltd. (www.cavendish-kinetics.com) which was recently acquired by Qorvo Inc (https://www.qorvo.com/newsroom/news/2019/qorvo-acquires-cavendish-kinetics). [11].
Spin injection in semiconductors
We have pioneered research on transport effects resulting from injection of spin-polarized electrons into semiconductor devices. In this work, we observed changes in resistance of an InAs quantum well with two ferromagnetic contacts having different coercive fields [12]. Many quantum computation proposals involve controlling electron spin in semiconductor devices.
Carbon nanotubes and graphene
We are working on a project funded by the QIP IRC with the aim of demonstrating entanglement in doped carbon nanotube material. We have also investigated the use of surface acoustic waves (SAW) combined with carbon nanotubes to investigate whether such structures can be used to produce a current standard. We have pioneered using SAW technology to drag single electrons through ballistic 1D channels to generate a well defined current I=ef, where f is the SAW frequencies [13]. We have demonstrated single-electron pumping at GHz frequencies in coupled graphene quantum dots in a project in collaboration with NPL [18].
Laboratory on a chip micro-fluidic pumping
We have developed a novel technique for pumping fluids in narrow channels using a low voltage alternating signal applied to two patterned electrodes. This technology has led to the formation of the spin out company Cambridge Lab on Chip Ltd. We are investigating the physical origin of this effect, which has many applications in the growing field of micro-fluidic pumping and mixing [14, 15].
Self-assembly
We have been studying how you can use semiconductor processing techniques to make micron sized colloidal particles with different shapes or material make up. These particles are then floated off into solution and we investigate how they coalesce [16]. By changing the shape and surface chemistry we will try to assemble sub-micron semiconductor devices into a three-dimensional lattice.
Current grants
PI EPSRC (EP/S019324/1) “Multiplexed Quantum Integrated Circuits” (£940,489). 1/3/2019 – 28/2/2022
CI (Local PI) (EP/R029075/1) Programme grant “Non-Ergodic Quantum Manipulation” (£7,032,540). 1/01/2019 – 31/12/2023
193 journal articles, 67 conference papers, 7 chapters, and 17 patents
Selected Publications
1. C. G. Smith, M. Pepper, H. Ahmed J. E. Frost, D. G. Hasko, D. C. Peacock, D. A. Ritchie, G. A. C. Jones, J. Phys. C: Solid State Phys. 21, L893, (1988) "The Transition From One- to Zero-Dimensional Ballistic Transport"
2. M. Field, C. G. Smith, M. Pepper, D. A. Ritchie, J. E. F. Frost, G. A. C. Jones, D. G. Hasko. Phys. Rev. Lett. 70, 1311, (1993) "Measurement of Coulomb Blockade with a Non-Invasive Voltage Probe"
3. M. Field, C. G. Smith, M. Pepper, K. M. Brown, E. H. Linfield, M. P. Grimshaw, D. A. Ritchie and G. A. C. Jones, Phys. Rev. Lett. 77, 350 (1996) "Coulomb Blockade as a Noninvasive Probe of the Local Density of States"
4. J. Cooper, C. G. Smith, D. A. Ritchie, E. H. Linfield, Y. Jin, H. Launois, Physica E, 6, 457 (2000) "Direct observation of single-electron decay from an artificial nucleus"
5. S. Gardelis, C. G. Smith, J. Cooper, D. A. Ritchie, E. H. Linfield, Y. Jin, and M. Pepper, Phys. Rev. B 67, 073302 (2003) "Dephasing in an isolated double-quantum-dot system deduced from single-electron polarization measurements"
6. S. Gardelis, C. G. Smith, J. Cooper, D. A. Ritchie, E. H. Linfield, and Y. Jin, Phys. Rev. B 67, 033302 (2003 "Evidence for transfer of polarization in a quantum dot cellular automata cell consisting of semiconductor quantum dots" and F. Perez-Martinez, I Farrer, D. Anderson, G.A.C. Jones, D. A. Ritchie, S. J. Chorley, and C. G. Smith 2007, Appl. Phys. Lett 91 032102 (2007) "Demonstration of a quantum cellular automata cell in a GaAs/GaAlAs heterostructure"
7. R Crook, C. G. Smith, M. Y. Simons, D. A. Ritchie J. Phys. Condense. Matter (2000) 12, L735-L740. "1D probability density observed using scanned gate microscopy"
8. R. Crook, A. C. Graham, C. G. Smith, I. Farrer, H. E. Beere, D. A. Ritchie. NATURE424 (6950): 751-754 AUG 14 2003 "Erasable electrostatic lithography for quantum components"
9. C. G. Smith and M. N. Wybourne, Solid State Comm. 57, 411, (1986) "Electric Field Heating in Supported and Free-Standing AuPd Fine Wires"
10. D. G. Hasko, C. G. Smith, J. K. Lucek, J. R. A. Cleaver and H Ahmed, Microelectron. Eng. 9, 337 (1989) "Fabrication and electrical , mechanical and thermal properties of sub-micron free-standing devices"
11. W. H. Teh, C. G. Smith, T. K. B Teo, R. G. Lacerda, G. A. J. Amaratunga, W. L. Milne, M. Castignolles, A. Loiseau. BOSTON TRANSDUCERS'03: DIGEST OF TECHNICAL PAPERS, VOLS 1 AND 2: 190-193 2003 "Uniform patterned synthesis of vertically-aligned carbon nanotubes on low-stress micromechanical structures"
12. S. Gardelis, C. G. Smith, C. H. W. Barnes, E. H. Linfield, and D. A. Ritchie, Phys. Rev. B 60, 7764-7767 (1999) "Spin-valve effects in a semiconductor field-effect transistor"
13. J. N. Shilton, V. I. Talyanskii, M. Pepper, D. A. Ritchie, J. E. F. Frost, C. J. B. Ford, C. G. Smith, and G. A. C. Jones J. Phys. C. 8, L531-L539 (1996) "Single electron transport in a quasi one-dimensional GaAs channel induced by high frequency surface acoustic waves"
14. A. B. D. Brown, C. G. Smith, A. R. Rennie Phys. Rev. E 63 (2001) 016305 "Pumping of water with an ac electric field applied to asymmetric pairs of microelectrodes"
15. M. Mpholo, C. G. Smith, A. B. D. Brown Sensors and Actuators B 92, 262-268 (2003) "Low voltage plug flow pumping using electrode arrays"
16. A. B. D. Brown, C. G. Smith, A. R. Rennie Phys. Rev. E 62, 951-960 (2000) "Fabricating colloidal particles with photolithography and their interactions at an air-water interface"
17. Petersson K., Smith C. G., Anderson D., Atkinson P., Jones G. A. C., and Ritchie D. A. , Nano Letts, 10, 2789–2793, (2010). “Charge and spin state readout of a double quantum dot coupled to a resonator”
18. M. R. Connolly, K. L. Chiu, S. P. Giblin, M. Kataoka, J. D. Fletcher, C. Chua, J. P. Griffiths, G. A. C. Jones, V. I. Fal'ko, C. G. Smith, T. J. B. M. Janssen Nature Nanotechnology 8, 417–420, (2013). “Gigahertz quantized charge pumping in graphene quantum dots”
19. Cassandra Chua, Malcolm Connolly, Arseniy Lartsev, Tom Yager, Samuel Lara-Avila, Sergey Kubatkin, Sergey Kopylov, Vladimir Fal’ko, Rositza Yakimova, Ruth Pearce, TJBM Janssen, Alexander Tzalenchuk, Charles G Smith. Nano Lett., 14 (6), pp 3369–3373 (2014). “Quantum Hall Effect and Quantum Point Contact in Bilayer-Patched Epitaxial Graphene“
20. Sei Morikawa, Ziwei Dou, Shu-Wei Wang, Charles G Smith, Kenji Watanabe, Takashi Taniguchi, Satoru Masubuchi, Tomoki Machida, Malcolm R Connolly. Applied Physics Letters 107 (24), 243102 (2015). “Imaging ballistic carrier trajectories in graphene using scanning gate microscopy”
Teaching and Supervisions
"The Physics of Nano-electronics systems".
Project Coordinator.
