Experimental demonstration of entanglement-enhanced rotation angle estimation using trapped ions

We experimentally investigate three methods, utilizing different atomic obs ervables and entangled states,to increase the sensitivity of rotation angle measurements beyond the "standard quantum limit" for nonentangled states. All methods use a form of quantum mechanical "squeezing." In a system of tw o entangled trapped Be-9(+) ions we observe a reduction in uncertainty of r otation angle below the standard quantum limit for all three methods includ ing all sources of noise. As an application, we demonstrate an increase in precision of frequency measurement in a Ramsey spectroscopy experiment.