USTC stores entanglement on silicon-29 nuclei for 240 microseconds at room temperature

About 5% of the silicon nuclei in silicon carbide are silicon-29, the isotope that carries a nuclear spin, and that spin couples to a color center's electron through the hyperfine interaction ^[5]. The coupling is what lets the electron write a state onto a nucleus. It is also weak enough that the nucleus is far better shielded from surface noise than the electron, at the cost of being hard to address directly ^[6].

That trade matters because the defects only become sensitive detectors when they sit a few nanometers under the surface, close enough to external signals while still being set and read with light and microwaves ^[7]. "These color centers are highly attractive for quantum sensing because bringing the spin close to the surface strengthens its interaction with external signals," said Jin-Shi Xu, a co-author ^[8]. "The drawback is that the electron spin is also more exposed to surface noise, so its coherence and entanglement can decay much faster," he said ^[9].

The team's answer was a SWAP-gate sequence, a train of control pulses that exchanges the states of two qubits ^[10]. They prepared entanglement between a color center's electron and one nearby nucleus, transferred it onto two silicon-29 nuclei, stored it there, and mapped it back to the electron to read out ^[11]. "This allowed us to use the electron for fast control while using the nuclear spins as the memory," Xu said ^[12]. Control and readout both still run through the electron ^[5].

The two transfers cost 1.5 percentage points of fidelity, the difference between the state as prepared and the state once it had reached the nuclei ^[2]. The measurement was made at room temperature; the conventional remedy for this kind of thermal noise is cooling the whole system to ultracold temperatures ^[17].

A sensing protocol now has to fit inside the storage window, and the figure of merit for a sensor is sensitivity per unit of measurement time, which depends on the full cycle: initialization, the two gates, the storage interval, the readback. The published account gives fidelities and lifetimes. It reports no field sensitivity ^[4]. The work establishes that entanglement can be stored on the nuclei at room temperature and retrieved with most of its fidelity intact ^[13][14].

The survey suggests the method generalizes past one lucky defect. More than 20 of the 200 color centers examined carried two or more strongly coupled nuclear spins, in material with no isotopic engineering at all ^[3]. "We found that more than 10% host two or more strongly coupled nuclear spins suitable for forming quantum registers," Xu said ^[19]. "This indicates that such registers are readily available in our system," he said ^[20].