Atomic-size spin defects in solids are unique quantum systems. Most applications require nanometre positioning accuracy, which is typically achieved by low-energy ion implantation. A drawback of this technique is the significant residual lattice damage, which degrades the performance of spins in quantum applications. Here we show that the charge state of implantation-induced defects drastically influences the formation of lattice defects during thermal annealing. Charging of vacancies at, for example, nitrogen implantation sites suppresses the formation of vacancy complexes, resulting in tenfold-improved spin coherence times and twofold-improved formation yield of nitrogen-vacancy centres in diamond. This is achieved by confining implantation defects into the space-charge layer of free carriers generated by a boron-doped diamond structure. By combining these results with numerical calculations, we arrive at a quantitative understanding of the formation and dynamics of the implanted spin defects. These results could improve engineering of quantum devices using solid-state systems.
Spins of negatively charged nitrogen-vacancy (NV${}^{\ensuremath{-}}$) defects in diamond are among the most promising candidates for solid-state qubits. The fabrication of quantum devices containing these spin-carrying defects requires position-controlled introduction of NV${}^{\ensuremath{-}}$ defects having excellent properties such as spectral stability, a long spin coherence time, and a stable negative charge state. Nitrogen ion implantation and annealing enable the positioning of NV${}^{\ensuremath{-}}$ spin qubits with high precision, but to date, the coherence times of qubits produced this way are short, presumably because of the presence of residual radiation damage. In the present work, we demonstrate that a high-temperature annealing at 1000 ${}^{\ensuremath{\circ}}$C allows 2 ms coherence times to be achieved at room temperature. These results were obtained for implantation-produced NV${}^{\ensuremath{-}}$ defects in a high-purity, 99.99%
12C-enriched single crystal chemical vapor deposited diamond. We discuss these long coherence times in the context of the thermal behavior of residual defect spins.
Abstract Qubits based on colour centres in diamond became a prominent system for solid-state quantum information processing and sensing. But the deterministic creation of qubits and the control of their environment are still critical issues, preventing the development of a room-temperature quantum computer. We report on the high creation yield of NV centres of 75% (a tenfold enhancement) by charge-assisted defect engineering, together with an improvement of their spin coherence. The method strongly favours the formation and negative charge state of the NV centres with respect to intrinsic diamond, while it hinders the formation of competing and perturbing defects such as di-vacancies or NVH complexes. We evidence spectrally the charge state tuning of the implantation-induced vacancies from V0 to V−, key element of this Coulomb-driven engineering. The generality of the method is demonstrated using several donors (phosphorous, oxygen and sulphur) and applying it to other centres (SnV and MgV) in diamond.
The Nitrogen-Vacancy (NV) color center is a defect of diamond which behaves as an artificial atom hosted in a solid-state matrix. Due to its electron spin properties which can be read-out and manipulated as an elementary quantum system, the NV center has found a wide panel of applications as a qubit for quantum information and as a magnetic field sensor. However these applications require to control the properties of the NV centers and their localization. This doctoral thesis investigates methods allowing us to tailor the properties of NV centers by combining techniques for the implantation of nitrogen atoms and the plasma-assisted (CVD) synthesis of diamond.The manuscript is divided into six chapters. The first chapter summarizes the properties of the NV center which will set our objectives for the NV engineering. The second chapter will describe how arrays of NV centers can be created using Focused Ion Beam implantation. The results open a wide range of applications for the targeted creation of NV centers in diamond structures such as photonic crystals and tips. However the low kinetic energy which is required for achieving implantation within a spot of 10-nm diameter leads to shallow defects which properties are strongly affected by the sample surface. The third chapter investigates how the overgrowth of a diamond layer over implanted NV centers can remove the detrimental influence of the surface. The fourth and fifth chapters describe effective methods for NV center fabrication through the control of the CVD growth conditions of the hosting crystal. Thin layers with high NV doping can be grown and almost perfect control of the orientation of the NV axis can be achieved. With the goal to optimize the spin coherence properties, the sixth chapter investigates how the electron spin of the NV center can be protected from decoherence effects induced by magnetic noise due to the unpolarized spins in the diamond lattice.
Coherently coupled pairs or multimers of nitrogen-vacancy defect electron spins in diamond have many promising applications especially in quantum information processing (QIP) but also in nanoscale sensing applications. Scalable registers of spin qubits are essential to the progress of QIP. Ion implantation is the only known technique able to produce defect pairs close enough to allow spin coupling via dipolar interaction. Although several competing methods have been proposed to increase the resulting resolution of ion implantation, the reliable creation of working registers is still to be demonstrated. The current limitation are residual radiation-induced defects, resulting in degraded qubit performance as trade-off for positioning accuracy. Here we present an optimized estimation of nanomask implantation parameters that are most likely to produce interacting qubits under standard conditions. We apply our findings to a well-established technique, namely masks written in electron-beam lithography, to create coupled defect pairs with a reasonable probability. Furthermore, we investigate the scaling behavior and necessary improvements to efficiently engineer interacting spin architectures.
Nitrogen-vacancy (NV) colour centres in diamond are attracting growing attention due to potential applications in solid-state quantum information processing and magnetometry. Although proof-of-principle experiments have been demonstrated, further development requires the controllable production of defects with a high yield. In this paper, we experimentally show that the production efficiency of NV defects strongly depends on the ion implantation energy. This can be explained in terms of the number of vacancies produced per implanted ion and surface proximity. The dependence on ion fluence is also underlined, revealing a nonlinear regime and showing how the diamond lattice is damaged at higher fluences.
Dense and shallow ensembles of negatively charged nitrogen-vacancy centers (NV
−) with good optical and spin properties play a key role in the performance enhancement of diamond-based quantum sensors. Ion implantation enables precise control of NV
− depth and density. However, at high ion fluence, this method is limited by low NV
− creation yields and sample amorphization. Additionally, shallow NV
− spin properties deteriorate due to surface proximity. In this paper, we study N
2+ ion implantation at energies between 10 and 15 keV with fluences as high as 1e15 ions/cm2 at temperatures of 20, 400 and 800°C to investigate the influence of implantation temperature on lattice damage, NV
− creation yield and NV
− spin properties. Our results show that diamond maintains structural integrity at 800°C with fluences up to 1e15 ions/cm2 without amorphization. Furthermore, high-temperature implantation improves NV
− creation yields up to five times without compromising T2
∗, T2 and T1, making it a promising approach to enhance the magnetic field sensitivity of NV
− ensembles.