Beyond Qubit Count: A Comparative Review of Quantum Chip Architectures, Error Correction, and the Path to Fault-Tolerant Computing

Quantum processors are often compared by the number of physical qubits they contain. That number, however, does not by itself indicate whether a processor can execute a useful computation reliably. Qubit coherence, gate and measurement fidelity, connectivity, control infrastructure, fabrication yield, and the overhead of quantum error correction all influence practical capability. This paper presents a structured comparative review of six major quantum-computing modalities: superconducting circuits, trapped ions, photonic systems, silicon-spin qubits, neutral atoms, and topological approaches. It examines their physical implementations, operational strengths, engineering constraints, and prospects for scaling. Particular attention is given to surface-code error correction and the 105-qubit Willow processor result reported by Google Quantum AI, which demonstrated below-threshold surface-code memory and a logical-error suppression factor of 2.14 ± 0.02 when code distance increased by two. The paper also considers cryogenic control, wiring, modular interconnects, fabrication variability, real-time decoding, and the Indian National Quantum Mission. The review finds that progress towards fault tolerance is best assessed through logical performance and system-level engineering rather than physical-qubit count alone. No modality currently removes the need to address scaling overhead, reliable operations, and application-level advantage