Physical Implementation

Physical implementation of quantum gates depends on the quantum computing platform. Different platforms (superconducting, trapped ion, photonic, etc.) realize gates differently.

Gates via microwave pulses: drive qubits with resonant microwave fields.

• Single-qubit gates: RX, RY, RZ via microwave pulse duration and phase (~20–50 ns)
• Two-qubit gates: CNOT via flux pulse tuning (parametric interaction) (~20–100 ns)

Gate fidelity: 99.5–99.9% for single-qubit, 99–99.5% for CNOT (state-of-the-art).

Gates via laser pulses: cool ions, then drive transitions with laser.

• Single-qubit gates: resonant pulses targeting transition frequencies (~microseconds)
• Two-qubit gates: Mølmer-Sørensen or other entangling laser interactions

Gate fidelity: 99.9%+ (best current platform), but slower gate times.

Gates via optical components: beam splitters, phase shifters, nonlinear media.

• Single-qubit gates: optical rotations and phase shifts (~nanoseconds)
• Two-qubit gates: nonlinear effects or probabilistic schemes (challenging)

Gate fidelity: ~95–99%, variable depending on architecture.

Gates via resonance and coupling:

• Single-qubit: microwave resonance (~microseconds)
• Two-qubit: magnetic dipole coupling or cavity-mediated

Fidelity: 99–99.9%, long coherence times (milliseconds).

Fast gates accumulate errors quickly (off-resonance effects, spontaneous emission). Slow gates suffer decoherence. Optimal gate time balances these effects.

Typical gate times:

• Superconducting: 20–100 ns (fast, higher error rates)
• Trapped ion: 1–10 μs (slow, higher fidelity)
• Photonic: 1–100 ns (variable)