Quantum engineering on the surface of silicon

ROM 2013-7
Author: S.R. Schofield et al.
Institute: London Centre for Nanotechnology, University College London, UK
Publication: S. R. Schofield, et al. Quantum engineering at the silicon surface using dangling bonds. Nature Communications 4, 1649 (2013).
Instrument: LT STM

Individual atoms and ions are now routinely manipulated using scanning tunnelling microscopes or electromagnetic traps for the creation and control of artificial quantum states. For applications such as quantum information processing, the ability to introduce multiple atomic-scale defects deterministically in a semiconductor is highly desirable. Here we use a scanning tunnelling microscope to fabricate interacting chains of dangling bond defects on the hydrogen-passivated silicon (001) surface. We image both the ground-state and the excited-state probability distributions of the resulting artificial molecular orbitals, using the scanning tunnelling microscope tip bias and tip-sample separation as gates to control which states contribute to the image. Our results demonstrate that atomically precise quantum states can be fabricated on silicon, and suggest a general model of quantum-state fabrication using other chemically passivated semiconductor surfaces where single-atom depassivation can be achieved using scanning tunnelling microscopy.

By introducing individual silicon atom ‘defects’ using a scanning tunnelling microscope, scientists at the London Centre for Nanotechnology have coupled single atoms to form quantum states. Published in Nature Communications, the study demonstrates the viability of engineering atomic-scale quantum states on the surface of silicon – an important step toward the fabrication of devices at the single-atom limit. Advances in atomic physics now allow single ions to be brought together to form quantum coherent states.  However, to build coupled atomic systems in large numbers, as required for applications such as quantum computing, it is highly desirable to develop the ability to construct coupled atomic systems in the solid state. Semiconductors, such as silicon, routinely display atomic defects that have clear analogies with trapped ions. However, introducing such defects deterministically to observe the coupling between extended systems of individual defects has so far remained elusive. Now, LCN scientists have shown that quantum states can be engineered on silicon by creating interacting single-atom defects.  Each individual defect consisted of a silicon atom with a broken, or “dangling”, bond. During this study, these single-atom defects were created in pairs and extended chains, with each defect separated by just under one nanometer. Importantly, when coupled together, these individual atomic defects produce extended quantum states resembling artificial molecular orbitals. Just as for a molecule, each structure exhibited multiple quantum states with distinct energy levels. The visibility of these states to the scanning tunneling microscope could be tuned through the variation of two independent parameters – the voltage applied to the imaging probe and its height above the surface.  Ongoing research at the LCN is exploring even more complex arrangements of these defects, including the incorporation of impurity atoms within the defect structures, which is expected to alter the symmetry of the defects (similar to the role of the nitrogen atom in the nitrogen-vacancy center defect in diamond).

S.R. Schofield1,2, P. Studer1,3, C.F. Hirjibehedin1,2,4, N.J. Curson1,3, G. Aeppli1,2 & D.R. Bowler1,2

(1) London Centre for Nanotechnology, University College London, London WC1H 0AH, UK
(2) Department of Physics and Astronomy, University College London, London WC1E 6BT, UK
(3) Department of Electronic and Electrical Engineering, University College London, London WC1E 7JE, UK
(4) Department of Chemistry, University College London, London WC1H 0AJ, UK

Name and email of corresponding author
Steven R. Schofield (1), steven.schofield@physics.org