  The aim of this paper is to pursue the investigation of the phase retrieval
problem for the fractional Fourier transform $\ff\_\alpha$ started by the
second author. We here extend a method of A.E.J.M Janssen to show that there is
a countable set $\qq$ such that for every finite subset $\aa\subset \qq$, there
exist two functions $f,g$ not multiple of one an other such that $|\ff\_\alpha
f|=|\ff\_\alpha g|$ for every $\alpha\in \aa$. Equivalently, in quantum
mechanics, this result reformulates as follows: if
$Q\_\alpha=Q\cos\alpha+P\sin\alpha$ ($Q,P$ be the position and momentum
observables), then $\{Q\_\alpha,\alpha\in\aa\}$ is not informationally complete
with respect to pure states. This is done by constructing two functions
$\ffi,\psi$ such that $\ff\_\alpha\ffi$ and $\ff\_\alpha\psi$ have disjoint
support for each $\alpha\in \aa$. To do so, we establish a link between
$\ff\_\alpha[f]$, $\alpha\in \qq$ and the Zak transform $Z[f]$ generalizing the
well known marginal properties of $Z$.
