 To see why, let us suppose we have two qubits at our disposal. If these were classical bits, then they could be in four possible states (00, 01, 10, 11). Correspondingly, a pair of qubits has four computational basis states (\(\lvert 00\rangle\), \(\lvert 01\rangle\), \(\lvert 10\rangle\), \(\lvert 11\rangle)\). But while a single classical two-bit register can store these numbers only one at a time, a pair of qubits can also exist in a superposition of these four basis states, each with its own complex coefficient (whose mod square, being interpreted as a probability, is normalised). For example, using a “Hadamard gate”—which unitarily transforms a single qubit to the state \(\frac{\lvert 0\rangle + \lvert 1\rangle}{\sqrt 2}\) whenever it is in the state \(\lvert 0\rangle\), and to the state \(\frac{\lvert 0\rangle - \lvert 1\rangle}{\sqrt 2}\) whenever it is in the state \(\lvert 1\rangle\)—we can transform the \(n\)-qubit state \(\lvert 0...01 \rangle\) as follows: