Penning-trap eigenfrequency measurements with optical radiofrequency detectors

We use an electric-dipole laser-driven transition to precisely measure the cyclotron-frequency ratios of the pairs 42Ca+ - 40Ca+, 44Ca+ - 40Ca+, and 48Ca+ - 40Ca+ in a 7-tesla Penning trap. A single laser-cooled (T approximate to 1 mK) ion serves, together with photon-counting and photon-imaging units, as a radiofrequency detector covering a broadband frequency spectrum, in the present case from kHz to a few MHz. Such detectors (40,42,44,48Ca+) allow measuring extremely small forces increasing the sensitivity in Penning-trap mass spectrometry. The direct determination of the ions' amplitudes makes a cyclotron-frequency measurement process more robust against inhomogeneities of the magnetic field and/or deviations of the electric quadrupole field due to mechanical imperfections of the trap.