  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.
