This article presents the results of studies of the linear antiferromagnetic spin chain with uniaxial anisotropy (XXZ– model) placed in a transverse magnetic field directed along the anisotropy axis of the chain with a constant gradient along the chain. A spin Hamiltonian for the states close to the ground was written in the spin-wave approximation. It was diagonalized by an asymptotic expansion in the small parameter describing the magnitude of the field gradient. Local energy of the ground state and the reduced density matrix of the quantum states for a selected pair of spins in the chain have been considered in the gapped and gapless cases. Local spin polarization and the local toroidal magnetic moment have also been found. Local pair entanglement of quantum states as a characteristic of quantum correlations in the spin chain was studied.
It is given the refinements and further development of NMR model of quantum register based on easy-axis
nuclear spin-free antiferromagnet, which was presented previously in paper. The model suggests that external
magnetic field is directed along the easy axis, normally to the plate surface and has a weak constant gradient
along the nuclear spin chain of quantum register.
In present paper it is evaluated the general expression for indirect coupling between nuclear spins due to
hyperfine nuclear-electron coupling in substituted atoms and spin-wave propagation in antiferromagnet close to
critical point of quantum phase transition in antiferromagnet of spin-flop type. It is given also an estimation
of quantum states decoherence rate, which is caused by the interaction of nuclear spins with spin waves in
antiferromagnet.
As a large-scale quantum register, the one-dimensional chain of the magnetic atoms with nuclear spins 1/2 in thin plate of nuclear spin-free easy-axis 3D antiferromagnet is considered. The external magnetic field is directed along the easy axis, normally to the plate surface and has a week constant gradient along the nuclear spin chain. The expression for indirect inter-spin coupling, which is due to hyperfine nuclear electron coupling in atoms and spin-wave propagation in antiferromagnet, was evaluated. It was shown that near critical point of spin-flop quantum phase transition in antiferromagnet indirect nuclear spin coupling in inhomogeneous external magnetic field might have both long-range damping and oscillating dependence from interspin distance. The external magnetic field and its gradient play the role of control parameters.
It is considered the indirect inter-qubit coupling in 1D chain of atoms with nuclear spins 1/2 which plays role of qubits fin the quantum register. This chain of the atoms is placed by regular way in easy-axis 3D antiferromagnetic thin plate substrate which is cleaned from the other nuclear spin containing isotopes. It is shown that the range of indirect inter-spin coupling may run to a great number of lattice constants both near critical point of quantum phase transition in antiferromagnet of spin-flop type (control parameter is external magnetic field) and/or near homogeneous antiferromagnetic resonance (control parameter is microwave frequency).
KEYWORDS: Chemical species, Quantum computing, Quantum communications, Computing systems, Magnetism, Modulation, Solid state physics, Silicon, Information operations, Correlation function
It is discussed the decoherence problems in ensemble large-scale solid state NMR quantum computer based on the array of 31P donor atoms having nuclear spin I=1/2. It is considered here, as main mechanisms of decoherence for low temperature (<0.1K), the adiabatic processes of random modulation of qubit resonance frequency determined by secular part of nuclear spin interaction with electron spin of the basic atoms, with impurity paramagnetic atoms and also with nuclear spins of impurity and of spin temperature whereby the required decoherence suppression is obtained. It is discussed the random phase error suppression in the ensemble quantum register basic states.
When threshold voltage of CMOS-cell is corrected by low-dose ion- implantation method a shallow p-n junction is formed near oxide- semiconductor interface with junction depth approximately equal to Debye length. Known methods of electro-physical parameters control by C-V characteristic cannot be applied for these MOS structures directly. So, for the task of the determination of electro-physical parameters and doping profile in nonuniform substrate MOS structure the numerical solution of Poisson equation is commonly used. The approach to the designing of a low frequency C-V characteristic is considered in this paper for MOS structures with substrate doping impurity of different types. The mathematical model is based on approximate solution of Poisson equation and takes into account the majority and minority carriers. This model can be simply tune for any modeling impurity profile and allows to extract parameters such as dose of the implanted impurity, substrate impurity concentration, p-n junction depth, effective flat-band voltage, oxide charge density, oxide thickness, profile doping parameters. The cases of abrupt and gradual impurity distribution were considered. The parameters of MOS structure and impurities profile for different doping doses were extracted from experimental C-V curves. The comparison of experimental and modeling C-V characteristics shows good agreement (5-10%).
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