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Ramsey interferometry is routinely used in quantum metrology for the mostsensitive measurements of optical clock frequencies. Spontaneous decay to theelectromagnetic vacuum ultimately limits the interrogation time and thus sets alower bound to the optimal frequency sensitivity. In dense ensembles oftwo-level systems the presence of collective effects such as superradiance anddipole-dipole interaction tends to decrease the sensitivity even further. Weshow that by a redesign of the Ramsey-pulse sequence to include differentrotations of individual spins that effectively fold the collective state onto astate close to the center of the Bloch sphere, partial protection fromcollective decoherence and dephasing is possible. This allows a significantimprovement in the sensitivity limit of a clock transition detection schemeover the conventional Ramsey method for interacting systems and even fornon-interacting decaying atoms.

Protected state enhanced quantum metrology with interacting two-level ensembles