Precision spectroscopy with two correlated atoms

Understand

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 3d D5/2 state of 40Ca+ and to a measurement of the linewidth of an ultrastable laser exciting a pair of 40Ca+ ions.