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One can rattle off names of implementations of Superconducting Qubits, transmon qubits, etc

For example, see Physical realizations used by Google

But NV centre Qubits as in this question, Is there any company that backs and implements diamond vacancy quantum computers? have no large company spending money on it.

Any research on it is relegated to academia or the companies in stealth mode.

What about its implementation makes it unattractive?

One would think that since it operates at near room temperatures should make it more appealing? For reference See Quantum computing book by Nielsen and Chuang

Disclaimer This question is completely produced by a comment in this answer.

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I suspect that fabrication challenges play a significant role in limiting broader interest. Growing single crystal diamonds is notoriously difficult, but not the primary impediment here. Rather, controlling the spatial distribution of NV centers in a diamond lattice with sufficient precision to produce a scalable and controllable network of qubits seems to be currently out of reach. David Awschalom discusses this briefly here.

In addition to spatial distribution, orientation of the NV center relative to the nitrogen impurity may also need to be controlled for certain applications. Complicating the matter further, the final position and orientation of the NV center in the lattice is only fixed after an annealing process during which the lattice damage (from ion implantation) is repaired and the NV center is allowed to migrate to one of the nitrogen impurity's four nearest neighbors.

Despite the difficulties, it seems highly likely that NV centers, or similar semiconductor impurities (there are at least seven other possibilities, see, e.g., here), will play an important role in quantum computing. These systems have some very attractive qualities that will likely be leveraged more as fabrication processes improve.

One of the most striking qualities is extremely long coherence times made possible by coupling NV center electron spins to surrounding nuclear spins through hyperfine interactions. Relatively recently, Tim Taminiau's group showed a 75 second coherence time for a single qubit state here. The system described in that paper uses the NV center plus 9 surrounding nuclear spins (one ${}^{14}$N plus eight ${}^{13}$C) to create a fully connected 10-qubit register. That's 10 connected qubits from a single NV center, very impressive!

As a final note, Awschalom (from the videos linked above) is a leading expert on this topic and runs a group at Chicago doing great work in this space. Awschalom is also Director of Q-Next, which just received commitments for \$115M in funding from the DOE, plus \$93M in private money. Hopefully with Awschalom at the helm this funding will help drive NV Center technology towards maturity.

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