Carbon Nanotube Double Quantum Dot Spin Qubits in Research

Short answer: Carbon nanotube double quantum dots are a research platform for spin and valley qubits, attractive because carbon's dominant nuclear species is spinless. A 2013 experiment reported a qubit with a measured decoherence time of 65 ns, and a 2025 paper reported coherence times of the order of 1.3 microseconds inside a microwave cavity. These are laboratory results from single devices.

Why carbon nanotubes for spin qubits?

According to the 2012 arXiv preprint, later published in Nature Nanotechnology in 2013, electron spins in III-V semiconductor quantum dots suffer unavoidable hyperfine decoherence. In group IV materials the dominant nuclear species are spinless, and the authors note that qubit coherence times have been extended up to seconds in diamond and silicon. They describe carbon nanotubes as an attractive host because spin-orbit interaction with the valley degree of freedom allows electrical manipulation of the qubit.

What did the 2013 device show?

The authors report a qubit in a nanotube double quantum dot, encoded in two valley-spin states. Coherent manipulation used electrically driven spin resonance mediated by a bend in the nanotube, and readout was done by measuring the current in Pauli blockade. They demonstrate arbitrary qubit rotations and measure the coherence time with a Hahn echo. The measured decoherence time was only 65 ns in that device. The authors say the work offers the possibility of a qubit in which hyperfine interaction can be virtually eliminated, which is a prospect and not a demonstrated result.

What changed with microwave cavities?

A 2019 paper in npj Quantum Information used a circuit QED spin-photon interface to drive a single electronic spin in a nanotube double quantum dot using cavity photons. Microwave spectroscopy identified an electrically controlled spin transition with a decoherence rate that can be tuned as low as 250 kHz, which the authors say is consistent with the expected hyperfine coupling in carbon nanotubes. They attribute the coherence properties to pristine nanotubes stapled inside the cavity, and suggest purified 12C growth could improve coherence further. They also state that coupling distant spins through virtual photons for two-qubit gates still remains to be demonstrated with spin qubits.

A 2025 paper in Nature Communications reports a suspended carbon nanotube double quantum dot with ferromagnetic contacts embedded in a microwave cavity. The abstract notes that integrating quantum dots into cavities has been shown to reduce their coherence compared with cavity-free implementations. By performing quantum manipulations via the cavity photons, the authors report coherence times of the order of 1.3 microseconds. They describe this as two orders of magnitude larger than those measured so far in any carbon quantum circuit and one order of magnitude larger than silicon-based quantum dots in a comparable environment. Those comparisons are the authors' own.

What should readers keep in mind?

Coherence numbers across these three papers come from different devices, methods and definitions, so they should not be read as one steady trend. The 65 ns figure is a decoherence time from a Hahn echo, the 250 kHz figure is a decoherence rate from spectroscopy, and the 1.3 microsecond figure is described as a coherence time. This post relies on the abstracts and visible summary text of the papers and has not checked the full texts. Nothing here describes a product or a deployed system.

FAQ

Why is hyperfine interaction a problem for quantum dot spin qubits?

The 2012 preprint says hyperfine decoherence is unavoidable in III-V semiconductor quantum dots. In group IV materials the dominant nuclear species are spinless, which is why the authors pursue carbon nanotubes.

How long did the first nanotube qubit stay coherent?

The authors of the 2013 Nature Nanotechnology work measured a decoherence time of 65 ns in their device using a Hahn echo.

What did the 2025 cavity experiment report?

Coherence times of the order of 1.3 microseconds for a suspended carbon nanotube double quantum dot embedded in a microwave cavity, which the authors compare favourably with other carbon circuits and silicon quantum dots.

Has a two-qubit gate between nanotube spins been shown?

Not in these abstracts. The 2019 authors say coupling distant spins through cavity photons for two-qubit gates remains to be demonstrated with spin qubits.