Possibilities of Controlling the Quantum States of Hole Qubits in an Ultrathin Germanium Layer Using a Magnetic Substrate: Results from ab Initio Calculations.

Using density functional theory in the noncollinear approximation, the behavior of quantum states of hole qubits in a Ge/Co:ZnO system was studied in this work. A detailed analysis of the electronic structure and the distribution of total charge density and hole states was carried out. It was shown that in the presence of holes, the energetically more favorable quantum state is the state |0˃, in contrast to the state |1˃ when there is no hole in the system. The favorability of hole states was found to be dependent on the polarity of the applied electric field.

Quantum mechanical modelling of phosphorus qubits in silicene under constrained magnetization.

A non-collinear density functional theory calculation of the electronic and magnetic structure of phosphorus-doped silicene was performed using atomic constrained magnetization. The antiferromagnetic state for the local magnetic moments of a pair of phosphorus atoms was found to be preferable both without and with constrained magnetization. A spatial change in the charge densities in the regions of substituting phosphorus atoms was shown. It was found that upon rotation from the |0〉 state to the |1〉 state, the charge density in the intermediate state changes asymmetrically relative to the bonds of the P atom with the neighbouring Si atoms.

Ab Initio Prediction of Noncollinear Magnetic States of the Quantum Phosphorus Qubit in a Silicon Lattice.

The problem of the practical implementation of quantum computers is an important scientific and technological task at the present time. In this work, using first-principles calculations, a quantum qubit behavior based on a doped phosphorus atom in a Si lattice was theoretically investigated. The local magnetic field B(r), the local magnetization m(r), and the spin current density created by the excess electron from the phosphorus atom were calculated. It was shown that the P atom spin equilibrium orientation in the Bloch sphere corresponds to the following polar coordinates (θ; φ) = 176°; 102° and the |1⟩ quantum state. For the different |0⟩ and |1⟩ quantum states, there is a different direction of the spin current densities. The obtained novel results have a prospective significance for the design technology of future quantum computers.