Title: Advanced Quantum Electronic and Spin Systems
Description: When nature is observed at the nanoscale, quantum physics is typically the most accurate model to describe and predict its behavior. Furthermore, quantum effects are increasingly at the core of the operation of new advanced electronic and photonic devices, which, in some cases, are designed on the basis of controlling quantum systems. This thesis focuses on two such systems, united by the methods used to realize them. These methods represent the cutting-edge of nanofabrication, which is the structuring of matter at ultra-small dimensions with a degree of precision and control that has not been previously attained. Pushing these methods to their limits enables the emergence of unique phenomena in the quantum systems explored here. The first system involves the realization of artificial graphene in an AlGaAs/GaAs quantum heterostructure. The appearance of massless charge carriers in graphene, which are described by the relativistic Dirac equation, originates from the linear energy-momentum dispersion of the electronic states in proximity to the K and K’ points of the hexagonal Brillouin zone.

This unique quantum behavior is a direct result of the honeycomb symmetry of the graphene lattice. The prospect of reproducing this physics in an adjustable, artificial honeycomb lattice, known as artificial graphene, offers a platform for the exploration of novel quantum regimes of massless Dirac fermions beyond the limits imposed by the inability to manipulate the lattice of the natural material. The electronic properties of a two-dimensional electron gas whose density is modulated by a periodic potential with honeycomb symmetry have been predicted to generate massless Dirac-fermions with tunable Fermi velocity. This thesis reports the observation of a graphene-like band structure in a modulation-doped AlGaAs/GaAs quantum well engineered with a honeycomb lateral surface superlattice. This was accomplished by reactive ion etching of the surface to within a few tens of nanometers from the quantum well. A metal hard-mask, patterned by electron beam lithography combined with metal deposition and lift-off, was used to form a honeycomb artificial lattice with a variable lattice period, down to 40 nm.

This is a three-fold reduction with respect to the smallest reported to date in pertinent literature. The BCl3-based shallow etching produces cylindrical pillars below which the two-dimensional electron gas is expected to form quantum dots, where the electron density is higher than in the surrounding etched regions. Low-temperature resonant inelastic light scattering measurements reveal new electronic transitions. An accurate interpretation of these can be found in the joint density of states derived from the calculated graphene-like linearly-dispersed energy bands, induced by the honeycomb potential modulation. The second system comprises the nanoscale engineering of individual electron spin qubits in diamond. Spin systems in solid-state have been intensively investigated as an outstanding pathway towards quantum information processing. One of the advantages of solid-state spintronics is the possibility of applying nanofabrication techniques commonly used by the semiconductor industry to produce and integrate spin qubits.

The negatively charged nitrogen-vacancy (NV-) center in diamond stands out because of its optically addressable spin, which shows long coherence time and viable spin initiation, manipulation and read-out. A central challenge has been the positioning of NV- centers with nanometer scale control, that would allow for efficient and consistent dipolar coupling and the integration within an optoelectronic device. I demonstrate a method for chip-scale fabrication of arrays of closely-spaced NV- centers with record spatial localization of approximately 10 nm in all three dimensions and controllable inter-NV spacing as small as 40 nm. This is the highest spatial resolution realized to date in positioning NV- centers at the nanoscale with high throughput, and approaches the length scale of strong dipolar coupling. This method used masked implantation of nitrogen in an ultra-pure CVD-grown diamond substrate through nano-apertures in a thin gold film, patterned via electron-beam lithography and dry etching.

The high-density and high-atomic weight of gold results in a mask which is significantly thinner than polymer films used in other works, whilst still successfully impeding ion penetration, with a mask contrast of more than 40 dB. This process allows for the creation of apertures with lower aspect ratio which are therefore easier to pattern in close proximity to one another, i.e., within the dipolar coupling range. The position and spin coherence properties of the resulting near-surface NVs were measured through wide-field super-resolution optically detected magnetic resonance imaging, Hahn echo and CPMG pulsed microwave spectroscopy. The patterning methodology demonstrated here is optimally suited to functional integration with plasmonic nanostructures, which can enhance our ability to control single-photon emission with the prospect of creating near-surface nanoscale sensors of electric or magnetic fields and quantum optoelectronic devices.