  We prove a quantitative unique continuation principle for infinite
dimensional spectral subspaces of Schr\"odinger operators. Let $\Lambda_L =
(-L/2,L/2)^d$ and $H_L = -\Delta_L + V_L$ be a Schr\"odinger operator on $L^2
(\Lambda_L)$ with a bounded potential $V_L : \Lambda_L \to \mathbb{R}^d$ and
Dirichlet, Neumann, or periodic boundary conditions. Our main result is of the
type
  \[
  \int_{\Lambda_L} \lvert \phi \rvert^2 \leq C_{\mathrm{sfuc}} \int_{W_\delta
(L)} \lvert \phi \rvert^2,
  \] where $\phi$ is an infinite complex linear combination of eigenfunctions
of $H_L$ with exponentially decaying coefficients, $W_\delta (L)$ is some union
of equidistributed $\delta$-balls in $\Lambda_L$ and $C_{\mathrm{sfuc}} > 0$ an
$L$-independent constant. The exponential decay condition on $\phi$ can
alternatively be formulated as an exponential decay condition of the map
$\lambda \mapsto \lVert \chi_{[\lambda , \infty)} (H_L) \phi \rVert^2$. The
novelty is that at the same time we allow the function $\phi$ to be from an
infinite dimensional spectral subspace and keep an explicit control over the
constant $C_{\mathrm{sfuc}}$ in terms of the parameters. Moreover, we show that
a similar result cannot hold under a polynomial decay condition.
