Gate-based quantum computer

A register of qubits — typically superconducting transmons cooled to ~10 mK — whose state is manipulated by sequences of microwave pulses implementing one- and two-qubit unitary gates. Any computation is a product of these gates, forming a universal gate set. Superposition lets a qubit represent 0 and 1 simultaneously; entanglement correlates qubits non-classically; interference is used to amplify correct answers and cancel wrong ones. Shor's algorithm factors n-bit integers in O(n³) gate operations vs. exponential classically; Grover's algorithm searches an unsorted list in O(√N). Current NISQ (noisy intermediate-scale quantum) devices have 100–1000 physical qubits with limited coherence; fault-tolerant quantum computing requires ~1000 physical qubits per logical qubit. Google's 2019 Sycamore experiment claimed quantum supremacy on a sampling task in 200 seconds vs. ~10,000 years classically. Speed: nanosecond gate times; microseconds coherence (NISQ era). Capacity: 53–1121 physical qubits (current hardware); fault-tolerant QC requires orders of magnitude more.