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arXiv:1309.4316 · Physical Review B

Extending spin coherence times of diamond qubits by high-temperature annealing

Takashi Yamamoto, Takahide Umeda, Kenji Watanabe, Shinobu Onoda, Matthew Markham, Daniel J. Twitchen, Boris Naydenov, Liam Paul McGuinness, Tokuyuki Teraji, Satoshi Koizumi, Florian Dolde, Helmut Fedder, Jan Honert, Jörg Wrachtrup, Takeshi Ohshima, Fedor Jelezko, Junichi Isoya

Published Aug 23, 2013• 179 citations
arXivHTMLPDF

Abstract

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% ${}^{12}$C-enriched single crystal chemical vapor deposited diamond. We discuss these long coherence times in the context of the thermal behavior of residual defect spins.

DOI
10.1103/physrevb.88.075206

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Spins of negatively charged nitrogen-vacancy (NV\ensuremath−{}^{\ensuremath{-}}\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−{}^{\ensuremath{-}}\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−{}^{\ensuremath{-}}\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∘{}^{\ensuremath{\circ}}\ensuremath∘C allows 2 ms coherence times to be achieved at room temperature. These results were obtained for implantation-produced NV\ensuremath−{}^{\ensuremath{-}}\ensuremath− defects in a high-purity, 99.99% 12{}^{12}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.
Single defects in diamond and especially negatively charged nitrogen vacancy (NV) centers are very promising quantum systems with wide applications in physics and biology. It was shown that their coherence properties can be strongly improved by growing ultrapure diamond with low concentration of parasitic spins associated with nitrogen electron spins and nuclear spins related to 13^{13}13C carbon isotope. Here we report a high quality 12^{12}12C-enriched polycrystalline CVD diamond material with properties comparable with single crystals. We find single NVs in the grains of this material, which show extremely long electron spin coherence time T2>2 msT_2 > 2\,msT2​>2ms.
Nitrogen impurities help to stabilize the negatively-charged-state of ${\mathrm{NV}}^{\ensuremath{-}}$ in diamond, whereas magnetic fluctuations from nitrogen spins lead to decoherence of ${\mathrm{NV}}^{\ensuremath{-}}$ qubits. It is not known what donor concentration optimizes these conflicting requirements. Here we used 10-MeV 15N3+{}^{15}{\mathrm{N}}^{3+}15N3+ ion implantation to create ${\mathrm{NV}}^{\ensuremath{-}}$ in ultrapure diamond. Optically detected magnetic resonance of single centers revealed a high creation yield of $40\ifmmode\pm\else\textpm\fi{}3%$ from 15N3+{}^{15}{\mathrm{N}}^{3+}15N3+ ions and an additional yield of $56\ifmmode\pm\else\textpm\fi{}3%$ from 14N^{14}\mathrm{N}14N impurities. High-temperature anneal was used to reduce residual defects, and charge stable ${\mathrm{NV}}^{\ensuremath{-}}$, even in a dilute 14N^{14}\mathrm{N}14N impurity concentration of 0.06 ppb were created with long coherence times.
The negatively-charged nitrogen-vacancy (NV) center is the most studied optical center in diamond and is very important for applications in quantum information science. Many proposals for integrating NV centers in quantum and sensing applications rely on their tailored fabrication in ultra pure host material. In this study, we use ion implantation to controllably introduce nitrogen into high purity, low nitrogen chemical vapor deposition diamond samples. The properties of the resulting NV centers are studied as a function of implantation temperature, annealing temperature, and implantation fluence. We compare the implanted NV centers with native NV centers present deep in the bulk of the as-grown samples. The results for implanted NV centers are promising but indicate, at this stage, that the deep native NV centers possess overall superior optical properties. In particular, the implanted NV centers obtained after annealing at 2000 °C under a stabilizing pressure of 8 GPa showed an ensemble linewidth of 0.17 nm compared to 0.61 nm after annealing at 1000 °C. Over the same temperature range, the ensemble NV−/NV0 ratio increased by a factor of ∼5, although this was accompanied by an overall decrease in the NV count.
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.
Abstract Single negatively charged nitrogen vacancy (NV−) centers in diamond have emerged as promising platforms for quantum information science, where long coherence times are essential for advancing quantum technologies. However, traditional fabrication methods often introduce lattice damage during the irradiation process used to create vacancies, significantly impairing the spin coherence properties of NV− centers. In this study, we systematically investigate high-pressure/high-temperature (HPHT) annealing as a non-destructive approach to generate high-coherence single NV− centers and elucidate the underlying mechanisms. Our results reveal that the distortion of the lattice field significantly influences the preferential formation of NV− centers at the interface of the 〈111〉-growth sector during diamond crystallization. Moreover, HPHT annealing effectively mobilizes nitrogen and vacancies, significantly promoting the formation of single NV− centers. This process also mitigates the influence of the distortion of the lattice field, resulting in a marked enhancement of coherence properties, with T 2 times reaching 578 μs. Notably, a naturally formed single NV− center in a low-stress region achieves an exceptional T 2 time of 721 μs, representing one of the highest values reported for diamonds with natural 13C abundance. Building on these insights, we have successfully achieved controlled, large-scale fabrication of high-coherence single NV− centers, providing essential guidance for future efforts to further enhance their coherence and establishing a robust foundation for their application in quantum technologies.
Abstract The nitrogen-vacancy (NV) center in diamond is a promising quantum system for magnetometry applications exhibiting optical readout of minute energy shifts in its spin sub-levels. Key material requirements for NV ensembles are a high NV − concentration, a long spin coherence time and a stable charge state. However, these are interdependent and can be difficult to optimize during diamond growth and subsequent NV creation. In this work, we systematically investigate the NV center formation and properties in bulk chemical vapor deposition (CVD) diamond. The nitrogen flow during growth is varied by over four orders of magnitude, resulting in a broad range of single substitutional nitrogen concentrations of 0.2–20 parts per million. For a fixed nitrogen concentration, we optimize electron-irradiation fluences with two different accelerated electron energies, and we study defect formation via optical characterizations. We discuss a general approach to determine the optimal irradiation conditions, for which an enhanced NV concentration and an optimum of NV charge states can both be satisfied. We achieve spin–spin coherence times T 2 ranging from 45.5 to 549 μ s for CVD diamonds containing 168 to 1 parts per billion NV − centers, respectively. This study shows a pathway to engineer properties of NV-doped CVD diamonds for improved sensitivity.