Let $\mathcal{L}=(L^2(\mathbb{R}^3))^N$ be the product space with the associated norm $$ \Vert U\Vert_0=\left(\sum^N_{i=1}\Vert u_i\Vert_0^2\right)^{1/2} $$ where $U=(u_1,u_2,...,u_N)\in\mathcal{L}$. Let $$ \mathcal{K}=\{U\in\mathcal{L};\int_{\mathbb{R}^3}u_iu_j=\delta_{i,j},\forall 1\leq i,j\leq N\} $$ and for any matrix $A\in\mathbb{R}^{N\times N}$ denote $$ AU=(...,\sum_{j=1}^N A_{ij}U_j,...,)^T $$ For any $U_1,U_2\in\mathcal{K}$, is the following minimization problem have a unique solution $$ \min\limits_{O\in\mathcal{O}^{N\times N}}\Vert OU_1-U_2\Vert_0 $$ where $\mathcal{O}^{N\times N}$ is the set of all unitary matrix.

Thank you very much.