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Abstract The spin-Hall effect underpins some of the most active topics in modern physics, including spin torques and the inverse spin-Hall effect, yet it lacks a proper theoretical description. This makes it difficult to differentiate the SHE from other mechanisms, as well as differentiate band structure and disorder contributions. Here, by exploiting recent analytical breakthroughs in the understanding of the intrinsic spin-Hall effect, we devise a density functional theory method for evaluating the conserved (proper) spin current in a generic system. Spin non-conservation makes the conventional spin current physically meaningless, while the conserved spin current has been challenging to evaluate since it involves the position operator between Bloch bands. The novel method we introduce here can handle band structures with arbitrary degeneracies and incorporates all matrix elements of the position operator, including the notoriously challenging diagonal elements, which are associated with Fermi surface, group velocity, and dipolar effects but often diverge if not treated correctly. We apply this method to the most important classes of spin-Hall materials: topological insulators, 2D quantum spin-Hall insulators, non-collinear antiferromagnets, and strongly spin-orbit coupled metals. We demonstrate that the torque dipole systematically suppresses contributions to the conventional spin current such that, the proper spin current is generally smaller in magnitude and often has a different sign. Remarkably, its energy-dependence is relatively flat and featureless, and its magnitude is comparable in all classes of materials studied. These findings will guide the experiment in characterizing charge-to-spin interconversion in spintronic and orbitronic devices. We also discuss briefly a potential generalization of the method to calculate extrinsic spin currents generated by disorder scattering.

Abstract Spins of electrons in silicon MOS quantum dots combine exquisite quantum properties and scalable fabrication. In the age of quantum technology, however, the metrics that crowned Si/SiO2 as the microelectronics standard need to be reassessed with respect to their impact upon qubit performance. We chart spin qubit variability due to the unavoidable atomic-scale roughness of the Si/SiO2 interface, compiling experiments across 12 devices, and develop theoretical tools to analyse these results. Atomistic tight binding and path integral Monte Carlo methods are adapted to describe fluctuations in devices with millions of atoms by directly analysing their wavefunctions and electron paths instead of their energy spectra. We correlate the effect of roughness with the variability in qubit position, deformation, valley splitting, valley phase, spin-orbit coupling and exchange coupling. These variabilities are found to be bounded, and they lie within the tolerances for scalable architectures for quantum computing as long as robust control methods are incorporated.

Spin qubits in Si/SiGe quantum dots suffer from variability in the valley splitting which will hinder device scalability. Here, by using 3D atomic characterization, the authors explain this variability by random Si and Ge atomic fluctuations and propose a strategy to statistically enhance the valley splitting

Quantum-dot spin qubits in Si/SiGe quantum wells require a large and uniform valley splitting for robust operation and scalability. Here the authors introduce and characterize a new heterostructure with periodic oscillations of Ge atoms in the quantum well, which could enhance the valley splitting.

Abstract Multi-donor quantum dots have been at the forefront of recent progress in Si-based quantum computation. Among them, 2P: 1P spin qubits have a built-in dipole moment, making them ideal for electron dipole spin resonance (EDSR) using the donor hyperfine interaction, and thus all-electrical spin operation. We report fast EDSR, with T π ~ 10 − 50 ns and a Rabi ratio (T 1/T π ) ~ 106. The fastest EDSR time T π occurs when the 2P: 1P axis is ∥ [111], while the best Rabi ratio occurs when it is ∥ [100]. Sensitivity to random telegraph noise due to nearby charge defects depends strongly on the location of the nearby defects. The qubit is robust against 1/f noise provided it is operated away from the charge anti-crossing. Entanglement via exchange is several orders of magnitude faster than dipole-dipole coupling. These findings pave the way towards fast, low-power, coherent and scalable donor dot-based quantum computing.

Modelling the quantum transport properties of qubit arrays and the electronic properties of dopant qubits is computationally challenging yet crucial for device optimization in quantum computing. Here, the authors compare different DFT-based methods to describe the properties of shallow donor-based qubits in silicon.

Quantum computers will require a large network of coherent qubits, connected in a noise-resilient way. Tosi et al. present a design for a quantum processor based on electron-nuclear spins in silicon, with electrical control and coupling schemes that simplify qubit fabrication and operation.