Abstract
Three-dimensional arrays of silicon transistors increase the density of bits. 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. 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 and allowing $(10 \mu m)^3$ 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 and at least as long as non-laser-written NVC. As well as NVC quantum computing, quantum communication and nanoscale sensing could benefit from the same platform. Our approach could also be extended to other qubits in diamond and silicon carbide.
Highlights
Demonstrated qubit fidelities for a single nitrogenvacancy center (NVC) and its nearby nuclear spins [1] are above the required thresholds for fault-tolerant quantum computing [2]
We focus on optical entanglement rather than the possibility of having NVCs so close to each other that they directly interact magnetically [12] because no scalable fabrication technique has been found for the latter approach
Looking at shallower NVCs would reveal the effects of the surface and provide clearer statistics on the NVCs that are 6-μm deep
Summary
Demonstrated qubit fidelities for a single nitrogenvacancy center (NVC) and its nearby nuclear spins [1] are above the required thresholds for fault-tolerant quantum computing [2]. Earlier work from our collaboration has shown that 2D arrays of NVCs (some single, some double, some triple) can be laser written with no mask, but the T2 time measured was typically only 30–80 μs [25] These short times may have been due to the creation of too much damage by overly energetic laser write pulses. Recent work from our collaboration describes preferential orientation and near 100% yield for 5 × 5 2D arrays of laser-written NVCs in diamond, but the T2 times are 170 μs or less [41] This reduced spin coherence may be due to the high concentration of nitrogen currently needed for the in situ annealing technique used or it may be that the in situ annealing does not heal the damage as well as traditional annealing in a furnace. Precisely calibrated laser-write-pulse energy to avoid creating unnecessary amounts of damage
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