A qubit (quantum bit) is the fundamental unit of information in quantum computing, just as the bit is the fundamental unit in classical computing. Unlike a classical bit, which must be either 0 or 1, a qubit can exist in a superposition of both states at once, described mathematically as a linear combination α|0⟩ + β|1⟩ where α and β are complex amplitudes whose squared magnitudes sum to 1. When measured, the qubit collapses to |0⟩ with probability |α|² or |1⟩ with probability |β|².
Qubits can be physically realized in many ways — superconducting circuits (transmon qubits used by IBM and Google), trapped ions (Quantinuum, IonQ), photons (PsiQuantum, Xanadu), neutral atoms (QuEra, Pasqal), electron spins in semiconductors (Intel), nitrogen-vacancy centers in diamond (Quantum Brilliance), and topological states of matter (Microsoft). Each implementation has different tradeoffs in coherence time, gate fidelity, connectivity, and scalability.
The power of quantum computing comes not from individual qubits but from the exponential growth of the state space — n qubits can represent 2ⁿ states simultaneously through superposition and entanglement. This enables quantum algorithms to explore solution spaces in ways that are fundamentally impossible for classical computers, though extracting useful answers requires carefully designed interference patterns that amplify correct solutions and cancel incorrect ones.