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arXiv:1311.0978 · Physical Review B

Strongly coupled diamond spin qubits by molecular nitrogen implantation

T. Yamamoto, Christoph Müller, Liam Paul McGuinness, Tokuyuki Teraji, Boris Naydenov, Shinobu Onoda, Takeshi Ohshima, Jörg Wrachtrup, Fedor Jelezko, Junichi Isoya

Published Nov 4, 2013• 52 citations
arXivHTMLPDF

Abstract

Ionized nitrogen molecules (${}^{15}$N${}_{2}^{+}$) are used as efficient point sources for creating NV${}^{\ensuremath{-}}$ pairs in diamond with nanoscale spatial separation and up to 55 kHz magnetic coupling strength. Co-implantation of ${}^{12}$C${}^{+}$ increased the yield of pairs, and a ${}^{13}$C-depleted diamond allowed 0.65 ms coherence times to be obtained. Further coupling to a third dark spin provided a strongly coupled three-spin register. These results mark an important step towards realization of multiqubit systems and scalable NV${}^{\ensuremath{-}}$ quantum registers.

DOI
10.1103/physrevb.88.201201

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Ionized nitrogen molecules (15{}^{15}15N2+{}_{2}^{+}2+​) are used as efficient point sources for creating NV\ensuremath−{}^{\ensuremath{-}}\ensuremath− pairs in diamond with nanoscale spatial separation and up to 55 kHz magnetic coupling strength. Co-implantation of 12{}^{12}12C+{}^{+}+ increased the yield of pairs, and a 13{}^{13}13C-depleted diamond allowed 0.65 ms coherence times to be obtained. Further coupling to a third dark spin provided a strongly coupled three-spin register. These results mark an important step towards realization of multiqubit systems and scalable NV\ensuremath−{}^{\ensuremath{-}}\ensuremath− quantum registers.
The coupled negatively charged nitrogen-vacancy (NV − ) center system is a promising candidate for scalable quantum information techniques. In this work, ionized nitrogen molecules are implanted into bulk diamond to generate coupled NV − center pairs. The two-photon autocorrelation measurement and optically detected magnetic resonance measurement are carried out to confirm the production of the NV − center pair. Also, both 1.3 μs decoherence time and 4.9 kHz magnetic coupling strength of the NV − center pair are measured by controlling and detecting the spin states. Along with nanoscale manipulation and detection methods, such coupled NV − centers through short distance dipole-dipole interaction would show high potential in scalable quantum information processes.
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.
A nitrogen vacancy (NV) center in diamond is known as a solid-state spin quantum bit (qubit) at room temperature. The fabrication of scalable quantum registers based on NV centers is a long-standing issue. In 2005, spin-spin interaction between NV-N pairs, which were fabricated by N2 ion implantation, was reported. An entanglement between two electron-spin qubits becomes an excellent quantum information processing component, as accomplished by an NV-NV pair with a nanometer-scale distance in 2010 and 2013. More recently,strongly dipolar coupled NV-NV pairs were realized by 14N 30 keV implantationvia nanoholes in PMMA resist mask. The creation yields of NV-NV pairs dramatically improved, and the nanohole implantation had been thought to be the most promising technique to integrate NV centers. However, the problem of the technique is the finite size of nanoholes. The distribution of nitrogen atoms is determined by both ion straggling and nanohole diameter. The scaling of NV centers is stagnant up to two for several years. To overcome the barrier of triple NV centers, our previous research gave a ground-breaking idea of a highly populated NV center complex by introducing a molecular ion implantation technique in 2019. Since the molecular ion is regarded as an idea point source, the distribution of nitrogen atoms is determined by only ion straggling. Therefore, our proposed method is more effective than other implantation techniques including nanohole implantation. Instead of a pure nitrogen cluster which is commonly unstable, the stable organic molecular ions (adenine; C5N5H5) were utilized as the ion source. We successfully implanted C5N4Hn ions into single crystalline diamonds. Three NV centers were integrated, and the dipolar interactions among them were successfully observed. In 2022, we succeeded in creating more than 5 NV centers by phthalocyanine (C32H18N8) ion implantation. The next challenge is the implantation of isotopically enriched molecular ions. The isotopically enriched ion contains 13C and 15N. The implantation of isotopically enriched molecular ions is able to create 15NV-13C hybrid quantum registers. In this presentation, we will show the development of the isotopically enriched molecular ion beam and the characterization of created NV centers by laboratory-built confocal microscopy (CFM) and optically detected magnetic resonance (ODMR).
Scaling of diamond defect center based quantum registers relies on the ability to position nitrogen vacancy (NV) centers with high spatial resolution. Using ion implantation, shallow (<10 nm) NVs can be placed with accuracy below 20 nm, but generally show reduced spin properties compared to bulk NVs. We demonstrate the enhancement of spin properties for shallow implanted NVs using a diamond overgrowth technique. An increase of coherence times up to an order of magnitude (T2 = 250 μs) was achieved, ms decoherence times were realized using dynamical decoupling. This marks a further step towards achieving strong coupling among defects positioned with nm precision.
We demonstrate three-dimensional localization of a single nitrogen-vacancy (NV) center in diamond by combining nitrogen doping during growth with a post-growth 12C implantation technique that facilitates vacancy formation in the crystal. We show that the NV density can be controlled by the implantation dose without necessitating increase of the nitrogen incorporation. By implanting a large 12C dose through nanoscale apertures, we can localize an individual NV center within a volume of (∼180 nm)3 at a deterministic position while repeatedly preserving a coherence time (T2) > 300 μs. This deterministic position control of coherent NV centers enables integration into NV-based nanostructures to realize scalable spin-sensing devices as well as coherent spin coupling mediated by photons and phonons.