  We discuss a device capable of filtering out two-mode states of light with
mode populations differing by more than a certain threshold, while not
revealing which mode is more populated. It would allow engineering of
macroscopic quantum states of light in a way which is preserving specific
superpositions. As a result, it would enhance optical phase estimation with
these states as well as distinguishability of "macroscopic" qubits. We propose
an optical scheme, which is a relatively simple, albeit non-ideal, operational
implementation of such a filter. It uses tapping of the original polarization
two-mode field, with a polarization neutral beam splitter of low reflectivity.
Next, the reflected beams are suitably interfered on a polarizing beam
splitter. It is oriented such that it selects unbiased polarization modes with
respect to the original ones. The more an incoming two-mode Fock state is
unequally populated, the more the polarizing beam splitter output modes are
equally populated. This effect is especially pronounced for highly populated
states. Additionally, for such states we expect strong population correlations
between the original fields and the tapped one. Thus, after a photon-number
measurement of the polarizing beam splitter outputs, a feed-forward loop can be
used to let through a shutter the field, which was transmitted by the tapping
beam splitter. This happens only if the counts at the outputs are roughly
equal. In such a case, the transmitted field differs strongly in occupation
number of the two modes, while information on which mode is more populated is
non-existent (a necessary condition for preserving superpositions).
