spinqubits

Spin qubits are two-level quantum systems where the computational basis states correspond to spin-up and spin-down states of a particle, typically an electron or nucleus. The state is represented on the Bloch sphere, with quantum gates implemented by controlled rotations using magnetic or electric fields. Entanglement between spin qubits enables universal quantum computation.

Major implementations include electron spins confined in quantum dots, and defect center spins such as the nitrogen-vacancy (NV) center in diamond, and donor spins in silicon (for example phosphorus donors). These platforms offer different trade-offs in scalability, operating temperature, and coupling to control fields. NV centers can operate at room temperature with optical readout; silicon donor spins benefit from mature semiconductor fabrication.

Control is typically achieved with microwave pulses for electron spins, or optical transitions for NV centers. Readout methods include spin-dependent fluorescence for NV centers or spin-to-charge readout and tunneling-based schemes in quantum dots. Two-qubit gates often rely on exchange interactions, dipolar couplings, or mediated couplers, with error rates continuing to improve.

Spin qubits are attractive due to potential compatibility with existing semiconductor technology and long coherence times in suitable materials. Challenges include precise qubit fabrication, integration and scaling, fast and high-fidelity readout, and maintaining coherence in solid-state environments. Ongoing research aims to develop scalable architectures and robust error correction.