The expression "O(n²)" in this quote is referring to the time complexity of the quantum Fourier transform algorithm. It means that the number of operations required to perform the transform grows proportionally to the square of the number of qubits (n).
Here's a breakdown:
In simpler terms: The quantum Fourier transform is relatively efficient, especially compared to classical algorithms for the Fourier transform. However, its efficiency does decrease slightly as the number of qubits increases, but not as drastically as some other algorithms that might require O(n³) or even higher complexities.