In this paper, we analyse the prospects for using nitrogen-vacancy centre (NV) containing diamond as a laser gain material by measuring its key laser related parameters. Synthetic chemical vapour deposition grown diamond samples with an NV concentration of ~1 ppm have been selected because of their relatively high NV concentration and low background absorption in comparison to other samples available to us. For the samples measured, the luminescence lifetimes of the NV- and NV0 centres were measured to be 8±1 ns and 20±1 ns respectively. The respective peak stimulated emission cross-sections were (3.6±0.1)×10-17 cm2 and (1.7±0.1)×10-17 cm2. These measurements were combined with absorption measurements to calculate the gain spectra for NV- and NV0 for differing inversion levels. Such calculations indicate that gains approaching those required for laser operation may be possible with one of the samples tested and for the NV- centre

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.

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