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Nature Communications

Coulomb-driven single defect engineering for scalable qubits and spin sensors in diamond

Tobias Lühmann, Roger John, Ralf Wunderlich, Jan Meijer, Sébastien Pezzagna

Published Oct 31, 2019• 130 citations
Publisher

Abstract

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.

DOI
10.1038/s41467-019-12556-0

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Journal of Physics D Applied Physics•Sep 7, 2018•92 citations
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.
Owing to its unique optical and spin properties, the nitrogen‐vacancy (NV) center holds the promise of a diamond‐based room‐temperature scalable quantum computer, but the numerous technical and physical issues are not yet overcome, especially the poor N to NV conversion. The NV and other color centers are, however, successfully used as multitasking quantum sensors, and open new routes in quantum sensing technologies. Most recently, several breakthroughs and demonstrations were achieved, shedding a new light on the feasibility of a NV‐based quantum computer. Herein, the recently developed method of charge‐assisted single defect engineering is reviewed. With a controlled charging of the involved species, it modifies the diffusion or kinetics of defect formation and acts as a catalyzer of the NV creation while hindering the formation of competing and perturbing defects such as divacancies or NVH. Very high NV creation yields up to 75% are obtained and the method is suitable to other impurity‐vacancy defects. Furthermore, it has positive consequences on the charge state stability and coherence time of the NV centers, which is as well discussed. Together with the possibility to nowadays deterministically implant single ions, this powerful method brings the scalability of NV qubits closer.
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 The nitrogen-vacancy (NV) centre in diamond is a premier solid-state defect for quantum information processing and metrology. An integrated diamond quantum device harnesses the collective properties of multiple NV centres, enabling room-temperature quantum computing and sensing. While large-scale devices are poised to fill an important gap in the burgeoning quantum technology landscape, their practical realisation has not been achieved using current top-down fabrication techniques such as ion implantation. Consequently, this necessitates the development of a bottom-up fabrication technique, which is scalable, deterministic, and possesses atomic-scale precision. Informed by existing methods for fabricating phosphorous defect qubits in silicon, we envision a hydrogen depassivation lithography technique for atomically-precise manufacturing of nitrogen-vacancy centres in diamond. This perspective article outlines a viable multi-step procedure for realising scalable fabrication of diamond quantum devices and identifies the key challenges in its development.
Among hundreds of optically active solid-state defects, very few of them hold the promise of spin qubits coherently operating at room temperature. Besides the prominent nitrogen-vacancy (NV) center in diamond, the more recently discovered ST1 center possesses advantageous electronic and spin properties such as a spin-free ground state and a large spin readout contrast (>50%) at 300 K. However, because of its unknown nature, the ST1 center was never created on demand or computed theoretically. Here, we show that both oxygen and the lattice vacancy are constituents of the ST1 center. The involvement of oxygen is unambiguously proved from two independent experiments conducted on two different samples. The creation efficiency of ST1 at 1200 °C annealing is about 10 –5 for oxygen implantation of energy 6–50 keV. The addition of vacancies followed by a thermal treatment activate further oxygen into ST1 centers, as it does for native nitrogen into NV centers. Possible ST1 configurations are discussed. The ST1 creation competes with the likely more efficient formation of OV and OVH defects. The controlled fabrication of ST1 and NV centers opens up the way to scalable room-temperature hybrid qubit systems and universal nuclear spin gates.
Abstract We present a high throughput and systematic method for the screening of colour centres in diamond with the aim of searching for and reproducibly creating new optical centres down to the single defect level, potentially of interest for a wide range of diamond-based quantum applications. The screening method presented here should, moreover, help to identify some already indexed defects among hundreds in diamond (Zaitsev 2001 Optical Properties of Diamond (Berlin: Springer)) but also some promising defects of a still unknown nature, such as the recently discovered ST1 centre (Lee et al 2013 Nat. Nanotechnol . 8 487; John et al 2017 New J. Phys . 19 053008). We use ion implantation in a systematic manner to implant several chemical elements. Ion implantation has the advantage of addressing single atoms inside the bulk with defined depth and high lateral resolution, but the disadvantage of producing intrinsic defects. The implanted samples are annealed in vacuum at different temperatures (between 600 °C and 1600 °C with 200 °C steps) and fully characterised at each step in order to follow the evolution of the defects: formation, dissociation, diffusion, re-formation and charge state, at the ensemble level and, if possible, at the single centre level. We review the unavoidable ion implantation defects (such as the GR1 and 3H centres), discuss ion channeling and thermal annealing and estimate the diffusion of the vacancies, nitrogen and hydrogen. We use different characterisation methods best suited for our study (from widefield fluorescence down to sub-diffraction optical imaging of single centres) and discuss reproducibility issues due to diamond and defect inhomogeneities. Nitrogen is also implanted for reference, taking advantage of the considerable knowledge on NV centres as a versatile sensor in order to retrieve or deduce the conditions and local environment in which the different implanted chemical elements are embedded. We show here the preliminary promising results of a long-term study and focus on the elements O, Mg, Ca, F and P from which fluorescent centres were found.