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What is Spin qubit?

An approach that uses the spin direction of electrons or atomic nuclei as qubits. In silicon, single electrons are confined in quantum dots formed by gate electrodes. The qubits are small and are expected to be integrated using CMOS fabrication technology. To reduce noise from nuclear spins, silicon enriched in ²⁸Si is used.

Japanese name: スピン量子ビット

Spin qubit appears in 2 practice questions across 1 topic, including Quantum Computing Hardware.

Field: Quantum Computing Hardware

Try it: questions about Spin qubit

What is the main reason silicon spin qubits are expected to suit large-scale systems?

  1. AThey work at room temperature, so no refrigerator is needed at all
  2. BTheir qubits are photons, so they can be sent far away directly over optical fibers
  3. CEach qubit is small, about tens to 100 nm, and many of them might be integrated by drawing on CMOS fabrication technology and 300 mm wafer equipment
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Answer: C. Each qubit is small, about tens to 100 nm, and many of them might be integrated by drawing on CMOS fabrication technology and 300 mm wafer equipment

A silicon spin qubit confines single electrons in quantum dots formed in silicon by the voltages on gate electrodes and uses the direction of each electron's spin as a qubit. Each qubit is about tens to 100 nm in size, far smaller than a superconducting qubit of about several hundred µm, and the fine patterning of semiconductor fabs, manufacturing on 300 mm wafers and wafer-scale characterization are expected to be applicable. Operation at relatively high temperatures of around 1 K has also been reported, which may make it easier to place control circuits nearby. On the other hand, variation between quantum dots and wiring to many densely packed qubits are challenges.

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.

Spin qubit in the explanations

Questions that mention Spin qubit (2)

  1. What is the main reason silicon spin qubits are expected to suit large-scale systems?
  2. Why is silicon enriched in the isotope ²⁸Si (isotopically purified silicon) used for silicon spin qubits?

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Related terms

Qubit Decoherence Dilution refrigerator Ion trap Josephson junction NISQ Post-quantum cryptography Quantum entanglement Quantum error correction Quantum gate

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