Endo-fullerenes and Doped Bucky Onions as Seed Materials for Solid State Quantum Bits
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Endo-fullerenes and Doped Bucky Onions as Seed Materials for Solid State Quantum Bits 1
Seongjun Park, 2Deepak Srivastava and 3Kyeongjae Cho 1 Department of Chemical Engineering, Stanford University, Stanford, CA 94305-5025 2 Computational Nanotechnology, NASA Ames Research Center, Moffett Field, CA 94035-1000 3 Department of Mechanical Engineering, Stanford University, Stanford, CA 94305-4040 ABSTRACT Two different models for solid-state quantum bits have been investigated. Both are based on the nuclear spin of doped atoms in endo-fullerenes or bucky-onions. 1H or 31P have been tested as suitable dopant atoms because they have half nuclear spins. The thermal stability and electronic properties of the dopant atoms and the encapsulating cages have been examined with ab-initio pseudo potential density functional methods, and the results show that both models are suitable for single qubit applications.
INTRODUCTION Recently a conceptual design of solid-state quantum computer, based on fabricating arrays of P atoms in bulk Si, has been proposed. [1] When a 31P donor atom is doped at a substitutional site in bulk Si, four out of five valance electrons of the 31P atom form four tetrahedral covalent bonds with the Si lattice. Kane has argued that the weakly bound fifth electron of 31P donor atom can be used for controlling the nuclear spin state of the donor atom, via hyper-fine interaction, for a solid-state qubit application. The main problem with this proposal, however, is the experimental difficulty involved in fabricating precise arrays of dopant atoms, 31P, in bulk Si layers. Moreover, even if such arrays are fabricated, the individual donor atoms may diffuse away from their intended locations via transient induced diffusion mechanisms. To overcome the above difficulties, we have investigated encapsulating ½ nuclear spin atoms in endo-fullerenes and bucky-onions for the above described solid-state qubit applications. If the doped endo-fullerene and bucky-onions are stable and have suitable electronic behavior for qubit applications, it may be easier to make the required arrays for solid-state quantum computer applications. The 1H and 31P atoms with ½ nuclear spin have been tried as suitable dopant atoms because both have ½ nuclear spin and one isotope except deuterium 2D. Two different models have been explored for solid-state qubit applications: One model involves encapsulating 1H atom in a fullerene, and the second model involves encapsulating 31P atom in a few nanometer sized diamond nanocrystallite. The fabrication pathway for encapsulating a 31P atom in a diamond nanocrystallite involves first encapsulating a 31P atom in a fullerene at the core of a bucky-onion, and then converting the core to a diamond nanocrystallite by e-beam irradiation and annealing. [2] All the results in this work are calculated using total energy pseudo-potential density functional theory, [3, 4] where geometry is optimized and the atomic positions are relaxed until the forces become smaller than 0.05 eV/Å. 31
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