Optically active point defects in crystals have gained widespread attention as photonic systems that can find use in quantum information technologies. However challenges remain in the placing of individual defects at desired locations, an essential element of device fabrication. Here we report the controlled generation of single nitrogen-vacancy (NV) centres in diamond using laser writing. The use of aberration correction in the writing optics allows precise positioning of vacancies within the diamond crystal, and subsequent annealing produces single NV centres with up to 45% success probability, within about 200 nm of the desired position. Selected NV centres fabricated by this method display stable, coherent optical transitions at cryogenic temperatures, a pre-requisite for the creation of distributed quantum networks of solid-state qubits. The results illustrate the potential of laser writing as a new tool for defect engineering in quantum technologies.Comment: 21 pages including Supplementary informatio

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...

We demonstrate the tunable enhancement of the zero phonon line of a single nitrogen-vacancy colour centre in diamond at cryogenic temperature. An open cavity fabricated using focused ion beam milling provides mode volumes as small as 1.24 μm 3 (4.7 3 l ) and quality factor Q 3000.  In situ tuning of the cavity resonance is achieved with piezoelectric actuators. At optimal coupling to a TEM 00 cavity mode, the signal from individual zero phonon line transitions is enhanced by a factor of 6.25 and the overall emission rate of the NV − centre is increased by 40% compared with that measured from the same centre in the absence of cavity field confinement. This result represents a step forward in the realisation of efficient spin-photon interfaces and scalable quantum computing using optically addressable solid state spin qubits.

Atomically flat semiconducting materials such as monolayer WSe2 hold great promise for novel optoelectronic devices. Recently, quantum light emission has been observed from bound excitons in exfoliated WSe2. As part of developing optoelectronic devices, the control of the radiative properties of such emitters is an important step. Here we report the coupling of a bound exciton in WSe2 to open microcavities. We use a range of radii of curvature in the plano-concave cavity geometry with mode volumes in the λ 3 regime, giving Purcell factors of up to 8 while increasing the photon flux five-fold. Additionally we determine the quantum efficiency of the single photon emitter to be η = 0.46 ± 0.03. Our findings pave the way to cavity-enhanced monolayer based single photon sources for a wide range of applications in nanophotonics and quantum information technologies.Single photon emission has been observed from a range of systems such as single atoms, quantum dots and localised excitons in a multitude of materials. Recently, two-dimensional semiconductors have attracted increased attention because of their strong interaction with light owed to a direct bandgap transition with a strong transition dipole moment of the delocalised exciton [1, 2]. Of these, the transition metal dichalcogenide WSe 2 has been found to contain localised excitons, stable at cryogenic temperatures below 15 K [3-8], emitting quantum light with impressive brightness [9] and stability [10]. In particular the localised excitons can be created with nanometric precision [11][12][13], exhibit strain tunability [8] and the hosting two-dimensional material allows for integration into ultra-compact, charge tunable devices [2,14]. * lucas.flatten@materials.ox.ac.uk

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.