Context :

Aims : We aim at constraining the stellar population properties of quiescent galaxies .
These properties reveal how these galaxies evolved and assembled since z \sim 1 up to the present time .

Methods : Combining the ALHAMBRA multi-filter photo-spectra with the SED-fitting code MUFFIT , we build a complete catalogue of quiescent galaxies via the dust-corrected stellar mass vs colour diagram .
This catalogue includes stellar population properties , such as age , metallicity , extinction , stellar mass and photometric redshift , retrieved from the analysis of composited populations based on two independent sets of SSP models .
We develop and apply a novel methodology to provide , for the first time , the analytic probability distribution functions ( PDFs ) of mass-weighted age , metallicity , and extinction of quiescent galaxies as a function of redshift and stellar mass .
We adopt different star formation histories to discard potential systematics in the analysis .

Results : The number density of quiescent galaxies is found to increase since z \sim 1 , with a more substantial variation at lower mass .
Quiescent galaxies feature extinction A _ { V } < 0.6 , with median values in the range A _ { V } = 0.15 – 0.3 .
At increasing stellar mass , quiescent galaxies are older and more metal rich since z \sim 1 .
A detailed analysis of the PDFs reveals that the evolution of quiescent galaxies is not compatible with passive evolution and a slight decrease is hinted at median metallicity 0.1 – 0.2 dex .
The intrinsic dispersion of the age and metallicity PDFs show a dependence with stellar mass and/or redshift .
These results are consistent with both sets of SSP models and the alternative SFH assumptions explored .
Consequently , the quiescent population must undergo an evolutive pathway including mergers and/or remnants of star formation to reconcile the observed trends , where the “ progenitor ” bias should also be taken into account .

Conclusions :