Proving an inequality using Cauchy-Schwarz

I am supposed to use the Cauchy Schwarz inequality to solve this problem but I am stuck. I can't see how the square root of $N$ comes out. Could anyone please help me?

Answer

If $x_1,\ldots,x_N$ are a basis of orthonormal vectors and $\psi$ has a unit norm, $\psi$ can be decomposed as
$$ \psi = \sum_{k=1}^{N}\alpha_k x_k,\qquad\sum_{k=1}^{n}\alpha_k^2=1 $$
and
$$\begin{eqnarray*} \sum_{k=1}^{N}\|\psi-x_k\|^2 &=& \sum_{k=1}^{N}\left[(\alpha_k-1)^2+\sum_{j\neq k}\alpha_j^2\right]\\&=&\sum_{k=1}^{N}\left[2-2\alpha_k \right]=2N-2\sum_{k=1}^{N}\alpha_k\end{eqnarray*}$$

where by the Cauchy-Schwarz inequality
$$\left|\sum_{k=1}^{N}\alpha_k\right|^2\leq \sum_{k=1}^{N}1\sum_{k=1}^{N}\alpha_k^2 = N, $$
hence:
$$ \sum_{k=1}^{N}\|\psi-x_k\|^2 \geq 2N-2\sqrt{N} $$
as wanted.