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Here is the first article I could find on this idea in 2016:

https://arxiv.org/abs/1611.07690

And here is a patent in 2017 for a quantum electronic device developed with one of the authors of the paper, Mohammad Choucair along with Martin Fuechsle (who invented a single atom transistor):

https://patentscope.wipo.int/search/en/detail.jsf?docId=WO2017091870

The two are now working at Archer Materials to commercialize this idea.

Fuechsle is known for inventing a single-atom transistor, which has applications to the mentioned quantum device:

https://www.researchgate.net/publication/221840938_A_single-atom_transistor

This leads me to my questions:

  1. How promising is a carbon-based qubit? Any disadvantages to this approach?
  2. If topological quantum computing prevails, could a room temperature qubit based on carbon still be beneficial to topological quantum computing?
  3. Is anyone outside of Archer Materials researching this approach?
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In my opinion this is not a very promising qubit for quantum computing, though it may hold more promise for quantum sensing or communication.

Making a qubit, a two-level quantum system, is not that hard, but making a good qubit is very hard. David DiVincenzo laid out 5 criteria on which you could gauge how good a qubit is for quantum computing. https://en.m.wikipedia.org/wiki/DiVincenzo%27s_criteria Going through those criteria it becomes obvious where the system demonstrated in the first paper falls short.

First what they did right, they developed and characterized a new spin qubit and demonstrated that they can manipulate it with microwaves in a magnetic field. (Somewhat fulllfilling criteria 1 and 4)

They also demonstrated long, for this type of system, coherence times (175 ns). However, if you consider their minimum gate times, about 16 ns, those coherence times really aren't that long. And just as an example other organic radicals (which could be considered qubits) can exceed 10 us at room temperature. https://doi.org/10.1021/acs.jpcb.5b03027

Next the biggest problem comes from scaling the systems, both down to the single qubit level(criteria 5) and to multi-qubit systems (criteria 1 and 3).

They were working with ensembles of qubits, I'd you want to use those qubits in a fashion similar to topological QC's, you ideally need to work with single qubits. Single spin magnetic resonance is very hard and there are really only two solutions: a superconducting microwave resonator, which commonly require low temperature; or optical detection, which require very specific photophysical processes in order to read out the spinstate. Nitrogen vacancy centers are a good example of a spin system with optical detection.

That said, there are proposal about how to perform ensemble quantum computing, where you basically get your statistics out in one shot which would render that point moor.

Scaling up to multi-qubit devices also poses a challenge. One way to have qubits communicate is through spin spin interactions, but those tend to also destroy the coherence times. There might be other clever ways to enable communication between qubits so we can use two qubit gates but I'm unfamiliar with them.

Lastly, the biggest issue with spin qubits in in criteria 2, initialization. Unfortunately, many of the spin qubits systems rely on thermal Boltzmann population and T1 relaxation to provide polarization. In order to get close to a pure starting state one needs to go to very high fields(>3T) and very low temperature (<4K) Though, optically generated polarization is a thing but just like with optical readout, you need to satisfy very specific photophysical conditions.

Overcoming these challenges is not just unique to the paper you cited, but to the very diverse field of electron spin qubits(which includes solid state defects, and a huge range different sized and composition molecules).

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