Computational basis

For $n$ qubits, the computational basis is the orthonormal basis $\{|x⟩:x\in \{0,1{\}}^n\}$ labelled by bit strings.

Measuring ${\sum }_x{a}_x|x⟩$ in the computational basis returns $x$ with probability $|a_x{|}^2$ and leaves the measured register in $|x⟩$.

A computational-basis state of $n$ qubits is $|x⟩$ for a bit string $x\in \{0,1{\}}^n$.