Roadmap on quantum nanotechnologies.

Quantum phenomena are typically observable at length and time scales smaller than those of our everyday experience, often involving individual particles or excitations. The past few decades have seen a revolution in the ability to structure matter at the nanoscale, and experiments at the single particle level have become commonplace. This has opened wide new avenues for exploring and harnessing quantum mechanical effects in condensed matter.

Scanned Single-Electron Probe inside a Silicon Electronic Device.

Solid-state devices can be fabricated at the atomic scale, with applications ranging from classical logic to current standards and quantum technologies. Although it is very desirable to probe these devices and the quantum states they host at the atomic scale, typical methods rely on long-ranged capacitive interactions, making this difficult. Here, we probe a silicon electronic device at the atomic scale using a localized electronic quantum dot induced directly within the device at a desired location, using the biased tip of a low-temperature scanning tunneling microscope.

Readout and control of the spin-orbit states of two coupled acceptor atoms in a silicon transistor.

Coupling spin qubits to electric fields is attractive to simplify qubit manipulation and couple qubits over long distances. Electron spins in silicon offer long lifetimes, but their weak spin-orbit interaction makes electrical coupling challenging. Hole spins bound to acceptor dopants, spin-orbit-coupled = 3/2 systems similar to Si vacancies in SiC and single Co dopants, are an electrically active spin system in silicon.

Towards visualisation of central-cell-effects in scanning tunnelling microscope images of subsurface dopant qubits in silicon.

Atomic-scale understanding of phosphorus donor wave functions underpins the design and optimisation of silicon based quantum devices. The accuracy of large-scale theoretical methods to compute donor wave functions is dependent on descriptions of central-cell corrections, which are empirically fitted to match experimental binding energies, or other quantities associated with the global properties of the wave function. Direct approaches to understanding such effects in donor wave functions are of great interest.

Charge-Insensitive Single-Atom Spin-Orbit Qubit in Silicon.

High fidelity entanglement of an on-chip array of spin qubits poses many challenges. Spin-orbit coupling (SOC) can ease some of these challenges by enabling long-ranged entanglement via electric dipole-dipole interactions, microwave photons, or phonons. However, SOC exposes conventional spin qubits to decoherence from electrical noise.

Quantum computing with acceptor spins in silicon.

The states of a boron acceptor near a Si/SiO2 interface, which bind two low-energy Kramers pairs, have exceptional properties for encoding quantum information and, with the aid of strain, both heavy hole and light hole-based spin qubits can be designed. Whereas a light-hole spin qubit was introduced recently (arXiv:1508.04259), here we present analytical and numerical results proving that a heavy-hole spin qubit can be reliably initialised, rotated and entangled by electrical means alone.

Donor hyperfine Stark shift and the role of central-cell corrections in tight-binding theory.

Atomistic tight-binding (TB) simulations are performed to calculate the Stark shift of the hyperfine coupling for a single arsenic (As) donor in silicon (Si). The role of the central-cell correction is studied by implementing both the static and the non-static dielectric screenings of the donor potential, and by including the effect of the lattice strain close to the donor site. The dielectric screening of the donor potential tunes the value of the quadratic Stark shift parameter (η2) from -1.

Probing the spin states of a single acceptor atom.

We demonstrate a single-hole transistor using an individual acceptor dopant embedded in a silicon channel. Magneto-transport spectroscopy reveals that the ground state splits as a function of magnetic field into four states, which is unique for a single hole bound to an acceptor in a bulk semiconductor. The two lowest spin states are heavy (|m(j)| = 3/2) and light (|m(j)| = 1/2) hole-like, a two-level system that can be electrically driven and is characterized by a magnetic field dependent and long relaxation time, which are properties of interest for qubits.

Electrical characteristics and photocurrent spectral response of Si nanowires p-i-n junctions.

P-i-n junctions were fabricated along Si nanowires (SiNWs) via the conventional top-down approach using optical lithography. Each device comprises 500 identical SiNWs connected in parallel, and each SiNW has triangular cross-section with dimensions of ~6 nm (base) by ~8 nm (height). The photodiodes exhibit very good rectifying electrical characteristics with a low reverse bias current of ~0.

Probing the gate--voltage-dependent surface potential of individual InAs nanowires using random telegraph signals.

We report a novel method for probing the gate-voltage dependence of the surface potential of individual semiconductor nanowires. The statistics of electronic occupation of a single defect on the surface of the nanowire, determined from a random telegraph signal, is used as a measure for the local potential. The method is demonstrated for the case of one or two switching defects in indium arsenide (InAs) nanowire field effect transistors at temperatures T=25-77 K.