Qubits and Superposition

A qubit is the smallest unit of quantum information. Unlike a classical bit, its state is described by amplitudes that can interfere with one another.

Learning objectives

Describe a qubit state as a combination of two basis states.
Explain why superposition is not the same as 'both values at once'.
Interpret measurement as a probabilistic readout.

From bits to amplitudes

A classical bit is 0 or 1. A qubit carries two complex amplitudes, one for each basis state. The squared magnitudes of those amplitudes give the probability of each measurement outcome, so they must sum to one.

Why superposition matters

Amplitudes can be negative or complex, so contributions to an outcome can cancel. Quantum algorithms are designed so that unwanted answers cancel and useful answers reinforce. Superposition alone provides no advantage; interference does.

Measurement destroys the description

Reading a qubit yields one classical value and collapses the amplitudes. Any algorithm must therefore arrange for the desired information to be visible in a single measurement statistic, repeated over many runs.

Check your understanding

Next moduleEntanglement and Correlation