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Substitutional donor atoms in silicon are promising qubits for quantum computation with extremely long relaxation and dephasing times demonstrated. One of the critical challenges of scaling these systems is determining inter-donor distances to achieve controllable wavefunction overlap while at the same time performing high fidelity spin readout on each qubit. Here we achieve such a device by means of scanning tunnelling microscopy lithography. We measure anti-correlated spin states between two donor-based spin qubits in silicon separated by 16 ± 1 nm. By utilising an asymmetric system with two phosphorus donors at one qubit site and one on the other (2P-1P), we demonstrate that the exchange interaction can be turned on and off via electrical control of two in-plane phosphorus doped detuning gates. We determine the tunnel coupling between the 2P-1P system to be 200 MHz and provide a roadmap for the observation of two-electron coherent exchange oscillations. [ABSTRACT FROM AUTHOR]

Electron and nuclear spins of donor ensembles in isotopically pure silicon experience a vacuum-like environment, giving them extraordinary coherence. However, in contrast to a real vacuum, electrons in silicon occupy quantum superpositions of valleys in momentum space. Addressable single-qubit and two-qubit operations in silicon require that qubits are placed near interfaces, modifying the valley degrees of freedom associated with these quantum superpositions and strongly influencing qubit relaxation and exchange processes. Yet to date, spectroscopic measurements have only probed wavefunctions indirectly, preventing direct experimental access to valley population, donor position and environment. Here we directly probe the probability density of single quantum states of individual subsurface donors, in real space and reciprocal space, using scanning tunnelling spectroscopy. We directly observe quantum mechanical valley interference patterns associated with linear superpositions of valleys in the donor ground state. The valley population is found to be within 5% of a bulk donor when 2.85 ± 0.45 nm from the interface, indicating that valley-perturbation-induced enhancement of spin relaxation will be negligible for depths greater than 3 nm. The observed valley interference will render two-qubit exchange gates sensitive to atomic-scale variations in positions of subsurface donors. Moreover, these results will also be of interest for emerging schemes proposing to encode information directly in valley polarization. [ABSTRACT FROM AUTHOR]

The ability to build structures with atomic precision is one of the defining features of nanotechnology. Achieving true atomic-level functionality, however, requires the ability to control the wavefunctions of individual atoms. Here, we investigate an approach that could enable just that. By collecting and analysing transport spectra of a single donor atom in the channel of a silicon FinFET, we present experimental evidence for the emergence of a new type of hybrid molecule system. Our experiments and simulations suggest that the transistor’s gate potential can be used to control the degree of hybridization of a single electron donor state between the nuclear potential of its donor atom and a nearby quantum well. Moreover, our theoretical analysis enables us to determine the species of donor (arsenic) implanted into each device as well as the degree of confinement imposed by the gate. [ABSTRACT FROM AUTHOR]

The Kane concept of quantum computation using single phosphorus donors in silicon has revolutionized the way we think about P-doped silicon devices. However, because of the enormous effort required in fabricating such devices using single atom techniques, detailed modeling and assessment of fault-tolerant operation is required to determine and optimize the scalability, and indeed feasibility, of such a vision. [ABSTRACT FROM AUTHOR]

Epitaxial circuitry offers a revolution in silicon technology, with components that can be fabricated on atomic scales. We perform the first ab initio calculation of atomically thin epitaxial nanowires in silicon, investigating the fundamental electronic properties of wires two P atoms thick, similar to those produced this year by Weber et al. For the first time, we catch a glimpse of disorder-related effects in the wires—a prerequisite for understanding real fabricated systems. Interwire interactions are made negligible by including 40 ML of silicon in the vertical direction (and the equivalent horizontally). Accurate pictures of band splittings and the electronic density are presented, and for the first time the effective masses of electrons in such device components are calculated. [ABSTRACT FROM AUTHOR]

Recent breakthroughs in single-atom fabrication in silicon have brought the exciting prospect of monolayer-based nanoelectronics and theoretical understanding of such systems into sharp focus. Of particular interest is the effect of such sharp two-dimensional Coulomb array confinement on electronic properties of these donor-based semiconducting systems such as valley splitting, which is critical to quantum electronic applications. In this paper we apply ab initio techniques to these high-density donor systems specifically in order to investigate the approach to monolayer confinement. An optimized basis set is developed for Si:P and validated against our previous work on single δ-doped layers. A systematic study is then conducted wherein the effect of multiple adjacent phosphorus δ layers on the electronic properties of the material is explored. We find nonmonotonic electronic behavior as we approach the monolayer confinement limit, with potentially far-reaching implications for large-scale fabrication techniques. [ABSTRACT FROM AUTHOR]

We report the observation of lifetime-enhanced transport (LET) based on perpendicular valleys in silicon by transport spectroscopy measurements of a two-electron system in a silicon transistor. The LET is manifested as a peculiar current step in the stability diagram due to a forbidden transition between an excited state and any of the lower energy states due to perpendicular valley (and spin) configurations, offering an additional current path. By employing a detailed temperature dependence study in combina- tion with a rate equation model, we estimate the lifetime of this particular state to exceed 48 ns. The two- electron spin-valley configurations of all relevant confined quantum states in our device were obtained by a large-scale atomistic tight-binding simulation. The LET acts as a signature of the complicated valley physics in silicon: a feature that becomes increasingly important in silicon quantum devices. [ABSTRACT FROM AUTHOR]