SILICON-28

LITNatural silicon is about 4.7% ²⁹Si, the only stable isotope with a nuclear spin. Strip it out and the crystal becomes a "semiconductor vacuum". Saeedi et al. (Science 342, 830, 2013) stored quantum states in the nuclear spins of ionized ³¹P donors in 99.995% ²⁸Si: T₂ over 39 minutes at room temperature and 3 hours at cryogenic temperature (both with dynamical decoupling). Silicon quantum-dot qubits crossed the 99% two-qubit threshold in 2022: gates above 99.5% (Xue et al., Nature 2022) and a 99.5% two-qubit gate (Noiri et al., Nature 2022).

MODELA die-photo view of a (100) silicon plane: amber sites are spin-free ²⁸Si, flickering sites are ²⁹Si. A ³¹P donor sits at the centre. Each ²⁹Si within reach jostles the donor's phase; purify and watch the phase trace go quiet. The phase noise is a random-walk toy with variance ∝ the number of nearby ²⁹Si.

²⁹Si % enrich (÷10)

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