The basic unit of quantum information — a two-state quantum system (like an electron's spin or a photon's polarization) that can exist in a superposition of 0 and 1 rather than just one or the other.
Where a classical bit is definitively 0 or 1, a qubit exists in a probabilistic combination of both states until measured. This superposition is the source of quantum computing's potential power: n qubits can represent 2^n states simultaneously, giving exponential information density. Physical qubits are implemented in many ways — superconducting circuits (Google, IBM), trapped ions (IonQ, Quantinuum), photonic systems (PsiQuantum), and neutral atoms (QuEra) — each with different coherence, fidelity, and scaling tradeoffs.
Encoding the inputs to a Grover's search across 1,024 items into 10 qubits — searching the entire space simultaneously rather than one item at a time.
Qubit counts are the most-watched quantum hardware metric, but qubit quality (coherence time, gate fidelity) matters at least as much for actually useful computation.
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