Superposition

Superposition is the quantum-mechanical principle by which a system occupies a weighted combination of its basis states at once. A Qubit in superposition holds both 0 and 1, described by two complex amplitudes whose squared magnitudes give the probability of each result when the qubit is measured. Measurement collapses the superposition to a single classical value.

#Why it matters for computation

Superposition lets an n-qubit register represent a combination of all 2^n basis states simultaneously. This is not the same as evaluating every answer in parallel and reading them all out, because a measurement returns only one result. Quantum algorithms instead shape the amplitudes so that, through interference, the amplitude of correct answers grows while wrong answers cancel. Shor's algorithm and Grover's algorithm both depend on this effect, as does the correlation between qubits known as Entanglement. Holding a superposition long enough to compute is the central engineering challenge, since contact with the environment destroys it through Decoherence.