**Edit**: I have changed the nature of the question, but in order to have a better idea of what I can expect for the original problem (see below).

Given $T>0$ and $n \in \bf Z$, consider the following heat equation with non-zero boundary conditions: \begin{equation} \begin{cases} \psi_t - \psi_{xx} + n^2 \psi = 0 &\text{ in } (0, T) \times (-1, 1) \\ \psi(t, -1) = 0, \quad \psi(t, 1) = 1 &\text{ in } (0, T) \\ \psi(0, x) = 0 &\text{ in } (-1, 1). \end{cases} \end{equation} For any $n \in \bf Z$, the above equation admits a unique weak solution $$\psi_n \in L^2(0, T; H^1(-1,1)) \cap C^0([0, T]; L^2(-1,1)).$$

My question is the following.

Let $\omega = (a, b) \subset (-1, 0)$. Can it happen that there exist $C, \alpha>0$ and $\beta \in \bf R$ such that \begin{equation} \int_0^T \int_\omega |\psi_n|^2 dx dt \leq \frac{C}{n^\beta}e^{-\alpha n}, \end{equation} at least for $|n|\gg1$ large enough?

In other words, I want to guarantee that the decay as $n\to \infty$ of the solution $\psi_n$ is **at most polynomial**.

I managed to obtain polynomial decay, but I am not sure whether this is optimal. I proceeded as follows.
For $n \in \bf Z$, let $\eta_n$ be the solution to the associated stationary problem
\begin{equation}
\begin{cases}
-\eta_{xx} + n^2 \eta = 0 &\text{ in } (-1, 1) \\
\eta(-1) = 0, \quad \eta(1) = 1,
\end{cases}
\end{equation}
and it may be checked that
\begin{equation}
\eta_n(x) = \frac{e^{-3n}}{1-e^{-4n}}(e^{2n}e^{nx} - e^{-nx}).
\end{equation}
The stationary solution decays exponentially **away from $x=1$**.
We set $\varphi_n = \psi_n - \eta_n$, and it may be checked that
\begin{equation}
\varphi_n = e^{-n^2 t} \zeta_n
\end{equation}
where $\zeta_n$ solves
\begin{equation}
\begin{cases}
\zeta_t - \zeta_{xx} = 0 &\text{ in } (0, T) \times (-1, 1) \\
\zeta(t, \pm1) = 0 &\text{ in } (0, T)\\
\zeta(0, x) = -\eta_n(x) &\text{ in } (-1, 1).
\end{cases}
\end{equation}
We can thus write
\begin{equation}
\int_0^T \int_a^b |\psi_n|^2 dxdt \leq \int_0^T \int_a^b |\varphi_n|^2 dxdt + T \int_a^b |\eta_n|^2 dx.
\end{equation}
For the rightmost integral, we have
\begin{equation}
\int_a^b |\eta_n|^2 dx \leq \int_{-1}^0 |\eta_n|^2 dx = \frac{e^{-2n}}{2n}(1-e^{-4n}-4ne^{-2n}) \lesssim \frac{e^{-2n}}{2n}
\end{equation}
for $|n|\gg1$.
For the second integral, we expand $\varphi_n$ in Fourier series:
\begin{equation}
\int_0^T \int_a^b |\varphi_n|^2 dx dt = \int_0^T e^{-2n^2 t} \int_a^b \left| \sum_{k=1}^\infty \langle -\eta_n, \phi_k\rangle e^{-\lambda_k t} \phi_k \right|^2 dxdt
\end{equation}
where $\phi_k(x) = \sin(k\pi(x+1)/2)$ denote the Dirichlet eigenfunctions forming an ONB of $L^2(-1,1)$ with associated eigenvalues $\lambda_k = \frac{k^2\pi^2}{4}$ for $k\in \bf N$.
It can be checked that
\begin{equation}
\langle -\eta_n, \phi_k\rangle = (-1)^k\frac{ 2k\pi}{k^2\pi^2+4n^2}.
\end{equation}
Hence in view of the previous 2 identities (unless I am mistaken),
\begin{equation}
\int_0^T \int_a^b |\varphi_n|^2 \lesssim \int_0^T e^{-2n^2 t} \sum_{k=1}^\infty \frac{k^2}{(k^2\pi^2+4n^2)^2} e^{-2\lambda_kt}.
\end{equation}
After exchanging the sum and the integral, integrating in time, we can see that we have **at most** a polynomial decay (I got $1/n^2$ but it's likely better).
But maybe this strategy is not the optimal one, and an exponential decay can be obtained.

My original question was the following.

Given $\omega := (a, b) \subset (-1, 0)$, can we obtain a lower bound of the form \begin{equation} \int_0^T \int_\omega |\psi_n|^2 dx dt \geq a_n \end{equation}

where the behavior of $a_n$ is explicit (at least when $|n|\gg 1$ is large)?

By the last sentence, I mean that $a_n = C/n^2$ (for instance), for some $C$ independent of $n$.

All comments are welcome.

observability inequalities. My end objective is to provide the lower estimate, but I don't even know what decay I can expect. $\endgroup$ – bgsk Nov 6 '19 at 8:21