  We constrain the rotation angle $\alpha$ of the linear polarization of CMB
photons using the large angular scale (up to $\sim 4^\circ$) signal in WMAP 7
year data. At these scales, the CMB polarization pattern probes mostly the
reionization era. A genuine rotation may be interpreted as cosmological
birefringence, which is a well known tracer of new physics, through the
breakdown of fundamental symmetries. Our analysis provides $\alpha =
-1.6^{\circ} \pm 1.7^{\circ} (3.4^{\circ})$ at 68% (95%) C.L. for the
multipoles range $\Delta \ell = 2-47$ not including an instrumental systematic
uncertainty that the WMAP team estimates at $\pm 1.5^\circ$. This improves the
bound obtained by WMAP team (Komatsu et al., 2010). Moreover we show, for the
first time at low multipoles, the angular power spectrum $\alpha_\ell$ in
search of a possible scale dependence of the birefringence effect. Our findings
are compatible with no detection at all angular scales probed here. We finally
forecast the capabilities of Planck in tightening the present constraints.
