Precision spectroscopy with two correlated atoms

We discuss techniques that allow for long coherence times in laser spectroscopy experiments with two trapped ions. We show that for this purpose not only entangled ions prepared in decoherence-free subspaces can be used but also a pair of ions that are not entangled but subject to the same kind of phase noise. We apply this technique to a measurement of the electric quadrupole moment of the 3d²D_5/2 state of ⁴⁰Ca⁺ and to a measurement of the line width of an ultra-stable laser exciting a pair of ⁴⁰Ca⁺ ions. PACS 03.67.-a; 06.30.Ft; 37.10.Ty