Short answer: The neutral divacancy defect in silicon carbide hosts an electron spin that can be controlled with light and microwaves, similar to the nitrogen-vacancy center in diamond. A 2011 Nature paper showed optical addressing and coherent control of such spins from 20 to 300 kelvin, a 2016 Nature Communications paper reports a Hahn-echo coherence time of 1.3 ms, and a 2020 Science paper reports a protected qubit with a Hahn-echo coherence time approaching 64 ms. These are laboratory results on one material.
2011: the spins can be addressed
The Nature abstract notes that the nitrogen-vacancy center in diamond has attracted interest as an individually addressable solid-state qubit, even at room temperature, and that researchers have sought similar defects in other semiconductors. It names silicon carbide as a candidate. The authors demonstrate that several defect spin states in the 4H polytype can be optically addressed and coherently controlled in the time domain at temperatures from 20 to 300 kelvin. They study the spin-1 ground state of each of four inequivalent forms of the neutral carbon-silicon divacancy, as well as a pair of defect spin states of unidentified origin. They say these defects are optically active near telecommunication wavelengths and sit in a host material for which industrial-scale crystal growth already exists. The rest of the abstract was truncated here and is not described.
2016: why the coherence is long
The Nature Communications paper reports, experimentally and theoretically, that the Hahn-echo coherence time of electron spins associated with divacancy defects in 4H-SiC reaches 1.3 ms. The authors call this one of the longest Hahn-echo coherence times of an electron spin in a naturally isotopic crystal. With a first-principles quantum-bath model, they identify two factors. At moderate magnetic fields, 30 mT and above, the silicon-29 and carbon-13 nuclear spin baths are decoupled. And because SiC is a binary crystal, homonuclear spin pairs are diluted and cannot form strongly coupled nearest-neighbour pairs. Longer neighbour distances mean fewer nuclear spin flip-flops, a less fluctuating magnetic environment and longer coherence. They say the results point to polyatomic crystals as promising hosts.
2020: a protected qubit
The Science paper constructs a qubit in a decoherence-protected subspace by applying microwave dressing to a clock transition of the ground-state electron spin of a divacancy defect. The authors say the qubit is universally protected from magnetic, electric and temperature fluctuations, which they say account for nearly all relevant solid-state decoherence channels. They report that the inhomogeneous dephasing time increases by more than four orders of magnitude, to over 22 milliseconds, while the Hahn-echo coherence time approaches 64 milliseconds. They suggest that few, platform-independent components are needed, so similar improvements may be possible in other architectures. That suggestion is their outlook.
Limits
These are single-defect or small-ensemble spin measurements in one host material. Coherence time is one property of a qubit. The abstracts do not report two-qubit gates, readout fidelity at scale, or a working processor. The different figures also measure different things, such as Hahn-echo coherence and inhomogeneous dephasing, so they should not be compared as one improving number.
FAQ
What is a divacancy?
A defect formed by a missing carbon and silicon atom pair in silicon carbide, whose electron spin can serve as a qubit.
Why is silicon carbide interesting?
The 2011 authors point to optical activity near telecommunication wavelengths and an existing industrial crystal growth base.
How long did the spin stay coherent?
1.3 ms Hahn-echo in 2016, and approaching 64 ms in 2020 with microwave dressing, as reported by the authors.
What explains the long coherence in 4H-SiC?
Decoupled nuclear spin baths at moderate field and the dilution of like-nucleus pairs in a binary crystal.
Is this a quantum computer?
No. These are qubit-level spin experiments.
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