A logical qubit is a more reliable qubit created from one or more physical qubits using error mitigation or, more commonly, quantum error correction. Physical qubits are fragile: they drift, decohere, and produce measurement errors. By encoding one logical state across several physical qubits, the system can detect and sometimes correct faults without destroying the computation.
In cyber security, logical qubits matter because useful quantum attacks and defenses require stability. Breaking modern public-key cryptography at scale would need many high-quality logical qubits, not just raw physical qubits. That makes logical-qubit count a practical measure of quantum capability, helping defenders judge when to accelerate migration to post-quantum cryptography. In real systems, logical qubits appear in research demos, fault-tolerant roadmaps, and cloud quantum services where job isolation, access control, and audit logging must cover a highly sensitive compute workload.