Atomic defects in wide band gap materials show great promise for development of a new generation of quantum information technologies, but have been hampered by the inability to produce and engineer the defects in a controlled way. The nitrogen-vacancy (NV) color center in diamond is one of the foremost candidates, with single defects allowing optical addressing of electron spin and nuclear spin degrees of freedom with potential for applications in advanced sensing and computing. Here we demonstrate a method for the deterministic writing of individual NV centers at selected locations with high positioning accuracy using laser processing with online fluorescence feedback. This method provides a new tool for the fabrication of engineered materials and devices for quantum technologies and offers insight into the diffusion dynamics of point defects in solids. Main Text:The engineering of materials at the scale of individual atoms has long been viewed as a holy grail of technology. With the extreme miniaturization of modern semiconductor technology to sub-10 nm feature sizes and the emerging promise of quantum technologies that rely inherently on the principles of quantum physics, the ability to fabricate and manipulate atomic-scale systems is becoming increasingly important.One promising approach to quantum technologies is the use of 'color center' point defects in wide band gap materials that display strong optical transitions, allowing the addressing of single atoms using optical wavelengths within the transparency window of the solid. Fluorescence from single color centers displays quantum statistics with potential for use in communications, sensingWe would like to acknowledge DeBeers and Element Six for providing suitably characterized diamond samples for this work, and in particular Daniel Twitchen and David Fisher for their comments on the manuscript. Data reported in the paper are presented in the Supplementary Materials and are archived at (tbc). The work was funded by the UK Engineering and Physical Sciences Research Council (EPSRC) through the UK hub in Networked Quantum Information Technologies (NQIT), grant # EP/M013243/1. Y-CC, B Griffiths and SN carried out the experiments and performed the data analysis with supervision from JS and PS; B Griffiths, LW, SJ and PS constructed the laser writing and fluorescence feedback apparatus; SI, YL, CJS and BLG carried out the Hahn echo and spatial localization measurements under the supervision of GM and MN; and JS, YCC, PS and MB conceived the experiment; all authors contributed to writing the manuscript. Supplementary Materials: Materials and Methods:The samples used were single-crystal type 1b diamond with nitrogen concentration of 1.8 ppm, produced by a High Pressure High Temperature (HPHT) technique. The diamond was cut and polished with flat surfaces parallel to the (110) plane of the cubic crystal.The optical layout for the combined laser processing and fluorescence feedback apparatus is shown in Figure S1. The laser processing was performed using a regenerat...

Three-dimensional arrays of silicon transistors increase the density of bits 1 . Solid-state qubits are much larger so could benefit even more from using the third dimension given that useful fault-tolerant quantum computing will require at least 100,000 physical qubits and perhaps one billion 2 . Here we use laser writing to create 3D arrays of nitrogen-vacancy centre (NVC) qubits in diamond. This would allow 5 million qubits inside a commercially available 4.5x4.5x0.5 mm diamond based on five nuclear qubits per NVC 3,4 and allowing (10 µm) 3 per NVC to leave room for our laser-written electrical control. The spin coherence times we measure are an order of magnitude longer than previous laser-written qubits 5 and at least as long as non-laser-written NVC 6 . As well as NVC quantum computing 3,4,6-8 , quantum communication 7,9,10 and nanoscale sensing 11-14 could benefit from the same platform. Our approach could also be extended to other qubits in diamond 15-18 and silicon carbide 19,20 .Demonstrated qubit fidelities 8 for a single negatively-charged nitrogen vacancy centre (NVC) and its nearby nuclear spins are above the required thresholds for quantum computing 2 . Two NVCs in different diamonds, in separate cryostats, have been optically entangled faster than the decoherence of this entanglement 7 , but it will not be practical to have 10 6 cryostats for 10 6 NVCs. In the transparent lattice of wide-band-gap diamond, individual opticallyaddressable qubits can fill a volume rather than be restricted to the surface. For computation, a 3D array spanning the upper 50 µm of a commercially-available electronic (EL) grade 4.5×4.5×0.5 mm diamond could contain 10 6 NVCs with (10 µm) 3 for each NVC. Each NVC has, on average, five individually-addressable 13 C nuclear spin qubits 3,4 . For communications, having an array of NVCs will provide many spin-photon interfaces within one cryostat 10 , increasing data rates and allowing multiplexing. Sensing with 2D arrays of NVCs will combine the high resolution of single NVC sensing 11 with the simultaneous imaging achieved with wide-field microscopy 13 . Stacking two of these 2D arrays will then permit gradiometry which will increase the sensitivity by subtracting the background noise measured by the array that is further from the sample of interest.

Nanodiamonds containing negatively charged nitrogen vacancy centers (NV − ) have applications as localized sensors in biological materials and have been proposed as a platform to probe the macroscopic limits of spatial superposition and the quantum nature of gravity. A key requirement for these applications is to obtain nanodiamonds containing NV − with long spin coherence times. Using milling to fabricate nanodiamonds processes the full 3D volume of the bulk material at once, unlike etching pillars, but has, up to now, limited NV − spin coherence times. Here, we use natural isotopic abundance nanodiamonds produced by Si 3 N 4 ball milling of chemical vapor deposition grown bulk diamond with an average single substitutional nitrogen concentration of 121 ppb. We show that the electron spin coherence times of NV − centers in these nanodiamonds can exceed 400 μs at room temperature with dynamical decoupling. Scanning electron microscopy provides images of the specific nanodiamonds containing NV − for which a spin coherence time was measured.

No abstract