Physical vs Logical Qubits

The key distinction and overhead cost of quantum error correction.

Physical Qubits	Real qubits present in quantum hardware (e.g. transmons, fluxoniums). They store states such as (|0rangle), (|1rangle), and (|psirangle) directly, but are highly noisy and error-prone.
Logical Qubits	Created using multiple physical qubits. QEC protects the encoded state (|psi_Lrangle = alpha|0_Lrangle + beta|1_Lrangle), making it error-corrected and reliable.
Overhead Ratio	(1) logical qubit (=) (10^2) to (10^3) physical qubits in many practical surface-code estimates. Many physical qubits work together to behave like a stable logical qubit even when individual physical qubits fail.