\label{fig:polarization} Proof of principle for polarization in collisions around the Z pole energy at LEP ($E_{\rm beam}=$ 44.71 GeV). The measured transverse polarization of the two electron bunches is plotted as function of time. One of the two electron bunches was brought into collisions with a positron bunch at 4:10 am, and remained polarized at the same level as the non-colliding bunch for more than five hours afterwards.