Ramsey interferometry is routinely used in quantum metrology for the most
sensitive measurements of optical clock frequencies. Spontaneous decay to the
electromagnetic vacuum ultimately limits the interrogation time and thus sets a
lower bound to the optimal frequency sensitivity. In dense ensembles of
two-level systems the presence of collective effects such as superradiance and
dipole-dipole interaction tends to decrease the sensitivity even further. We
show that by a redesign of the Ramsey-pulse sequence to include different
rotations of individual spins that effectively fold the collective state onto a
state close to the center of the Bloch sphere, partial protection from
collective decoherence and dephasing is possible. This allows a significant
improvement in the sensitivity limit of a clock transition detection scheme
over the conventional Ramsey method for interacting systems and even for
non-interacting decaying atoms
