Spin qubits encode quantum information in the spin of individual electrons (or holes) trapped in semiconductor quantum dots — nanoscale potential wells created by patterning electrodes on silicon or silicon-germanium substrates. The qubit states correspond to spin-up and spin-down orientations in a magnetic field. Intel is the most prominent commercial developer, leveraging its advanced semiconductor manufacturing capabilities to fabricate spin qubits using processes adapted from classical chip production.
The primary appeal of spin qubits is their compatibility with existing semiconductor fabrication infrastructure. Quantum dots can be fabricated using lithographic techniques similar to those used for classical transistors, potentially enabling the production of millions of qubits using established foundry processes. Spin qubits are also extremely small (tens of nanometers, compared to hundreds of micrometers for transmon qubits), offering higher qubit density. Gate times are fast (1-100 nanoseconds for single-qubit gates), and recent results in silicon have demonstrated two-qubit gate fidelities above 99%.
The challenges for spin qubits include the need for precise control of individual electrons (voltage noise at the millivolt level can displace the electron from the quantum dot), achieving reliable two-qubit coupling (qubits must be physically adjacent, separated by only ~100 nm, to interact via exchange coupling), and operating at millikelvin temperatures like superconducting qubits. Valley splitting — an unwanted energy splitting arising from silicon's crystal structure — must be carefully controlled. Despite these challenges, spin qubits remain a compelling long-term bet due to their potential for high-density integration using semiconductor manufacturing.