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Like electrons, atomic nuclei have spin, and Cappellaro and Hirose use the spin state of the nitrogen nucleus to control the NV center’s electronic spin. First, a dose of microwaves puts the electronic spin into superposition. Then a burst of radio-frequency radiation puts the nitrogen nucleus into a specified spin state.
A second, lower-power dose of microwaves “entangles” the spins of the nitrogen nucleus and the NV center, so that they become dependent on each other. At this point, the NV qubit could, together with other qubits, be enlisted to perform a computation. But in their experiments, Cappellaro and Hirose were evaluating a single qubit, so they could test only the most rudimentary computational operation: the not gate, which flips a bit’s value.
Because the spins of the nitrogen nucleus and the NV center are entangled, if anything goes wrong during the computation, it will be reflected in the spin of the nitrogen nucleus.
After the computation is performed, a third dose of microwaves — whose polarization is rotated relative to that of the second — disentangles the nucleus and the NV center. The researchers then subject the system to a final sequence of microwave exposures. Those exposures are calibrated, however, so that their effect on the NV center depends on the state of the nitrogen nucleus. If an error crept in during the computation, the microwaves will correct it; if not, they’ll leave the NV center’s state unaltered.
In experiments, the researchers found that, with their feedback-control system, an NV-center quantum bit would stay in superposition about 1,000 times as long as it would without it.
“[Cappellaro] sheds light on a method, coherent feedback, which was discussed in the literature, in theory, a while back but has never been experimentally explored,” says Jörg Wrachtrup, a physics professor at the University of Stuttgart, in Germany. “What is extremely good in there is that she’s showing — once you do it right, and once you find the right algorithm, which she did — how easy it is in the end to protect the electron spin against spin flip or dephasing.”
“The main advantage of this technique compared to previously reported results, like protection of spin using echoes, is robustness against noise,” adds Fedor Jelezko, a physics professor at Ulm University, in Germany. “The technique demonstrated by the Cappellaro group is less sensitive to the time scale of the noise. I believe that applications of this technique will appear soon, as demonstrations of new protocols applied to quantum metrology and quantum computing.”
