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Why is silicon enriched in the isotope ²⁸Si (isotopically purified silicon) used for silicon spin qubits?

  1. A²⁸Si has a wider band gap than natural silicon, so less leakage current flows
  2. B²⁹Si, about 4.7% of natural silicon, has a nuclear spin, and fluctuations of its magnetic field disturb the phase of the electron spin
  3. C²⁸Si is lighter than ²⁹Si, so the electron mobility becomes 10 or more times higher
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Answer: B. ²⁹Si, about 4.7% of natural silicon, has a nuclear spin, and fluctuations of its magnetic field disturb the phase of the electron spin

Natural silicon is about 92% ²⁸Si, about 4.7% ²⁹Si and about 3.1% ³⁰Si, and only ²⁹Si has a nuclear spin. Nuclear spins act like tiny magnets, and when their orientations change randomly, the magnetic field felt by the electron spin fluctuates, disturbing its phase and shortening T2. Using isotopically purified silicon with ²⁹Si reduced to about 0.1% or less greatly extends the coherence time.

  • A. The band gap hardly changes between isotopes.
  • B. Correct. Reducing ²⁹Si, which has a nuclear spin, makes the phase of the electron spin less easily disturbed.
  • C. The change in mobility due to isotope mass is small and is not the reason it is used.
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