Electron Exchange Coupling for Single Donor Solid-State Qubits

Inter-valley interference between degenerate conduction band minima has been shown to lead to oscillations in the exchange energy between neighbouring phosphorus donor electron states in silicon \cite{Koiller02,Koiller02A}. These same effects lead to an extreme sensitivity of the exchange energy on the relative orientation of the donor atoms, an issue of crucial importance in the construction silicon-based spin quantum computers. In this article we calculate the donor electron exchange coupling as a function of donor position incorporating the full Bloch structure of the Kohn-Luttinger electron wavefunctions. It is found that due to the rapidly oscillating nature of the terms they produce, the periodic part of the Bloch functions can be safely ignored in the Heitler-London integrals as was done by Koiller et. al. [Phys. Rev. Lett. 88,027903(2002),Phys. Rev. B. 66,115201(2002)], significantly reducing the complexity of calculations. We address issues of fabrication and calculate the expected exchange coupling between neighbouring donors that have been implanted into the silicon substrate using an 15keV ion beam in the so-called 'top down' fabrication scheme for a Kane solid-state quantum computer. In addition we calculate the exchange coupling as a function of the voltage bias on control gates used to manipulate the electron wavefunctions and implement quantum logic operations in the Kane proposal, and find that these gate biases can be used to both increase and decrease the magnitude of the exchange coupling between neighbouring donor electrons. The zero-bias results reconfirm those previously obtained by Koiller.Comment: 10 Pages, 8 Figures. To appear in Physical Review

Quantum cellular automata quantum computing with endohedral fullerenes

We present a scheme to perform universal quantum computation using global addressing techniques as applied to a physical system of endohedrally doped fullerenes. The system consists of an ABAB linear array of Group V endohedrally doped fullerenes. Each molecule spin site consists of a nuclear spin coupled via a Hyperfine interaction to an electron spin. The electron spin of each molecule is in a quartet ground state $S=3/2$. Neighboring molecular electron spins are coupled via a magnetic dipole interaction. We find that an all-electron construction of a quantum cellular automata is frustrated due to the degeneracy of the electronic transitions. However, we can construct a quantum celluar automata quantum computing architecture using these molecules by encoding the quantum information on the nuclear spins while using the electron spins as a local bus. We deduce the NMR and ESR pulses required to execute the basic cellular automata operation and obtain a rough figure of merit for the the number of gate operations per decoherence time. We find that this figure of merit compares well with other physical quantum computer proposals. We argue that the proposed architecture meets well the first four DiVincenzo criteria and we outline various routes towards meeting the fifth criteria: qubit readout.Comment: 16 pages, Latex, 5 figures, See http://planck.thphys.may.ie/QIPDDF/ submitted to Phys. Rev.

Charge Transfer Doping of Silicon

We demonstrate a novel doping mechanism of silicon, namely n type transfer doping by adsorbedorganic cobaltocene CoCp2 molecules. The amount of transferred charge as a function of coverage is monitored by following the ensuing band bending via surface sensitive core level photoelectron spectroscopy. The concomitant loss of electrons in the CoCp2 adlayer is quantified by the relative intensities of chemically shifted Co2p components in core level photoelectron spectroscopy which correspond to charged and neutral molecules. Using a previously developed model for transfer doping, the evolution in relative intensities of the two components as a function of coverage has been reproduced successfully. A single, molecule specific parameter, the negative donor energy of amp; 8722; 0.50 0.15 eV suffices to describe the self limiting doping process with a maximum areal density of transferred electrons of 2 1013 cm amp; 8722;2 in agreement with the measured downward band bending. The advantage of this doping mechanism over conventional doping for nanostructures is addresse

Semi-endogenous versus Schumpeterian growth models: testing the knowledge production function using international data

Schumpeterian growth theory, Semi-endogenous growth theory, O3, O4,

R&D, International Trade and Creative Destruction—Empirical Findings from Finnish Manufacturing Industries*

The determinants of productivity-enhancing micro-level restructuring are examined empirically with a panel of the twelve Finnish manufacturing industries. It is hypothesized that R&D leads to productivity diversity among plants, which in turn leads to the gradual reshuffling of input shares in the presence of dynamic competitive pressure. The effect of the “creative destruction” on industry productivity growth is measured with the between-component of productivity decomposition. Econometric results indicate with reasonable robustness that R&D generates creative destruction with a lag of several years. Some evidence is found that imports stimulate productivity-enhancing restructuring, especially when domestic R&D is low. Copyright Springer Science + Business Media, Inc. 2005R&D, competition, international trade, productivity, plant-level restructuring,