Quantum hardware is the physical substrate for quantum computation. Main modalities: superconducting qubits (IBM, Google, Rigetti; ~1 μs coherence), trapped ions (IonQ, Honeywell; ~100 ms coherence, high fidelity), neutral atoms (Pasqal, Atom Computing; scalable but addressing challenges), photonic qubits (Xanadu; room temperature), and topological qubits (research stage). Near-term systems span 10–1000 qubits with error rates 0.1–1%; scaling and reducing errors below 10^-4 are open challenges.