Diamond for quantum applications

For many applications in the field of quantum information processing stationary qubits are required, providing long-lived spin coherence and suitable level schemes for coherent control and efficient optical read out. In addition, transferring the spin information to indistinguishable single photons is necessary e.g. to distribute entanglement in quantum networks. 
Colour centres in diamond, more specifically the group-IV-vacancy centres, have emerged as promising candidates among solid state qubits. They exhibit favourable properties such as individually addressable spins with long coherence times and bright emission of single, close to transform limited photons. Recent experiments have shown that the negatively charged tin-vacancy centre (SnV) combines long spin coherence times at conveniently achievable cryogenic temperatures (>1K) with truly lifetime-limited transition linewidths down to 20 MHz. We explore the charge transition cycle upon resonant excitation which leads to shelving in the dark SnV(2-) state and devise a method for charge state stabilization by illumination with a second light field. The charge-stabilized SnV(-) centre exhibits exceptional spectral stability with very small spectral diffusion (4 MHz on a homogenous linewidth of 25 MHz over 1 hour) and promising spin dephasing time (5 µs, measured via coherent population trapping). We discuss prospects of coherent spin manipulation and generation of indistinguishable single photons.

Novel developments in the ab initio theory of solid state defect spins are presented. The nature of the spin-phonon relaxation for the nitrogen-vacancy centre of diamond is identified as a second-order Raman scattering process which can be described as a double Orbach-process at elevated temperatures. Inversion symmetric colour centres identified as the negatively charged magnesium-vacancy and nickel-vacancy defects will be characterised. The magnesium-vacancy centre has a unique electronic structure with competing spin states whereas the predicted qubit properties of nickel-vacancy centre outperform those of the well-known silicon-vacancy centre in diamond. Nickel-related optical and electron paramagnetic resonance centres are associated with the nickel-vacancy defect in its various charged states.

Optically active spins in solids are often considered prime candidates for scalable and feasible quantum-optical devices. Numerous material platforms including diamond, semiconductors, and atomically thin 2d materials are investigated, where each platform brings their own advantages along with their challenges. Professor Atatüre will provide a snapshot of current progress on novel colour centres in diamond and discuss their imminent challenges.

Electron-nuclear spin systems based on optically active defects in diamond provide a promising platform for distributed quantum simulations and computation. In this approach, optically active defect spins are used to form multi-qubit processors that can be linked together in a network through photonic links. Quantum error correction and computations are then distributed over the network.

In this talk Dr Taminiau will introduce such spin-based distributed quantum computations and present their recent progress. In particular, they have recently shown that it is possible to control large numbers of nuclear spins around a single NV centre, and to use these qubits for quantum simulations of many-body physics and for encoding fault-tolerant logical qubits.

Defect centres in materials with large band gaps, especially in diamond and SiC, have proven to be excellent candidates for so-called qubits in quantum information and detection. Ion implantation is crucial here as it is the only way to fabricate qubits with high lateral resolution in materials such as diamond. In the talk, the techniques of deterministic single ion implantation will be discussed, and Professor Meijer will present a mobile quantum computer based on this technology.

The real world applications of diamond quantum sensors require optically active defects to be increasingly located within nanometres of the diamond surface, where their quantum properties such as coherence time, charge state stability and spectral width can suffer significant degradation. I will detail our efforts to explore the origins of these surface noise and band-bending effects, through a novel combination of surface spectroscopy and defect-based electromagnetic measurements. I will then present our efforts to engineer and exploit these phenomena to enhance the sensitivity of near-surface NV systems for biological systems.

The Nitrogen Vacancy centre in its negatively charged state (NV) became the most established quantum emitter for the development of proof-of-concept quantum devices, in particular in the field of quantum sensing. The performance of NV-based quantum sensors strictly depends on the concentration of colour centres in the sensing ensemble, making it fundamental the possibility to control such concentration. An emerging technique for the formation of NV ensembles on demand is femtosecond laser writing, in which static exposures allow the formation of vacancies and a consequent annealing permits the formation of NVs thanks to the nitrogen pre-existing in diamond. In this work, we will discuss how laser writing can be used to achieve high concentration of NVs in diamond, aligned to laser written photonics for enhanced interaction. In addition, we will introduce a hybrid method of shallow ion implantation of quantum emitters into laser written photonics for novel quantum sensing devices.

The development of a truly scalable quantum information processing (QIP) platform, so far, has been impeded by a lack of qubits with long coherence times and high-quality optical interfaces. Crystal defects in diamond, specifically the nitrogen vacancy (NV) and group-IV vacancy (SiV, GeV, SnV) centers have emerged as leading qubit candidates but face several challenges such as poor optical quality and susceptibility to spectral diffusion (NV), a requirement for dilution refrigeration to reach sufficiently long spin decoherence (SiV), low controllability (GeV, SnV), or a lack of deterministic creation (SiV, SnV, GeV).

Nevertheless, past detailed studies of these systems now enable a more targeted search for new defects in diamond with optimized properties for QIP applications. We here report on one of these candidates, the nickel vacancy (NiV) defect in diamond, and present first fluorescence spectroscopic measurements, demonstrating the NiV’s favorable optical properties. Through magneto-optical studies and group-theoretical modelling we confirm the center’s electronic properties and show first results hinting towards favorable spin properties. Finally, we will discuss the possibility of deterministic fabrication of NiVs via multiphoton laser writing in combination with growth of bespoke CVD material to further enable a future scalable QIP platform based on NiV defects in diamond.