Atomistic tight-binding
Atomistic tight-binding
A solid-state analog of stimulated Raman adiabatic passage can be implemented in a triple-well solid-state system to coherently transport an electron across the wells with exponentially suppressed occupation in the central well at any point of time. Termed coherent-tunneling adiabatic passage CTAP, this method provides a robust way to transfer quantum information encoded in the electronic spin across a chain of quantum dots or donors. Using large-scale atomistic tight-binding simulations involving over 3.5106 atoms, we verify the existence of a CTAP pathway in a realistic solid-state system: gated triple donors in silicon. Realistic gate profiles from commercial tools were combined with tight-binding methods to simulate gate control of the donor to donor tunnel barriers in the presence of crosstalk. As CTAP is an adiabatic protocol, it can be analyzed by solving the time-independent problem at various stages of the pulse justifying the use of time-independent tight-binding methods to this problem. This work also involves the first atomistic treatment to translate the three-state-based quantum-optics type of modeling into a solid-state description beyond the ideal localization assumption. Our results show that a three-donor CTAP transfer, with interdonor spacing of 15 nm can occur on time scales greater than 23 ps, well within experimentally accessible regimes. The method not only provides a tool to guide future CTAP experiments but also illuminates the possibility of system engineering to enhance control and transfer times.
Quantum transport
A key feature behind the remarkable progress in solidstate electronics over the past years has been the ability to modulate the conductivity of semiconductor devices at will by using ensembles of dopants. As we approach the era of nano-scale electronics, dopants have yet another interesting role to play. Individual dopants at low temperatures provide 3D confinement to electrons and holes on length scales that are greater than individual atoms but usually less than that of quantum dots. These naturally occurring carrier traps not only provide access to a number of quantum phenomena typically associated with natural or artificial atoms, but also provide possibilities of wave-function engineering [1, 2] by classical control mechanisms with electric and magnetic fields. The homogeneity of the confining potential from one dopant to another of the same species is an added advantage over quantum dots, which are usually not identical in practice. On the other hand, the small length scales associated with dopants can make individual donor gate control difficult to achieve. Among other factors, developments in this area rely on a boost in the ability to scale down gate lengths to tens of nanometers.
A schematic of a single-donor device. An electric field perpendicular to the oxide interface generates a potential well at the surafce, which can can couple to the Coulombic potential well produced by a donor. The electronic structure of the whole system is sensitive to the donor depth D and the applied field F.