Improvement of Frequency Standards with Quantum Entanglement

The optimal precision of frequency measurements in the presence of decoherence is discussed. We analyze different preparations of $n$ two-level systems as well as different measurement procedures. We show that standard Ramsey spectroscopy on uncorrelated atoms and optimal measurements on maximally entangled states provide the same resolution. The best resolution is achieved using partially entangled preparations with a high degree of symmetry.