The basic unit of quantum information. It is 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, including 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, which searches 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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