We describe a high-resolution spectroscopy method, in which the detection of single excitation events is enhanced by a complete loss of coherence of a superposition of two ground states. Thereby, transitions of a single isolated atom nearly at rest are recorded efficiently with high signal-to-noise ratios. Spectra display symmetric line shapes without stray-light background from spectroscopy probes. We employ this method on a 25 Mg + ion to measure one, two, and three-photon transition frequencies from the 3S ground state to the 3P, 3D, and 4P excited states, respectively. Our results are relevant for astrophysics and searches for drifts of fundamental constants. Furthermore, the method can be extended to other transitions, isotopes, and species. The currently achieved fractional frequency uncertainty of 5 × 10 −9 is not limited by the method.Quantum systems that are well isolated from their environments, e.g., tailored solid-state systems, photons, and trapped atoms, offer a high level of control [1]. Over the past decades, several experimental methods have been devised for quantum control of single trapped ions [2][3][4]. Developments are driven by the urge to make more accurate and precise clocks [5,6] as well as to address questions in different fields of research, e.g., properties of highly charged ions [7,8], ion-neutral collisions [9][10][11][12], molecular physics [13][14][15], and tests of fundamental physics [16][17][18][19][20]. High-resolution spectroscopy measurements [21][22][23][24][25] are of particular interest for studying spatial and temporal fine structure variations of the universe [26][27][28]. In such experiments, complex atomic and molecular structures need to be probed by singleor multi-photon transitions in isotopically pure samples revealing undisturbed transition line shapes. Weak transitions in trapped ions can be measured with various methods [3,6], and techniques based on the detection of momentum kicks altering the occupation of motional states from few absorbed photons have been developed that are applicable to strong electric dipole transitions as well [25,29]. In this Letter, we experimentally study single-and multi-photon transitions in a single, laser cooled 25 Mg + ion that can be near-perfectly isolated from its environment. We detect the decoherence of a superposition of two electronic ground states due to single scattering events and determine transition frequencies which are relevant for astrophysics and searches for variations of fundamental constants [30,31] with a fractional uncertainty of 5 × 10 −9 .For a simplified description of the method, consider an atom with three states. Two of these states, labeled |↑ and |↓ , which are long-lived and allow for coherent control, are used to study transitions to a third, excited state |e . After preparation in |↑ , a π/2 pulse on the |↑ → |↓ transition creates a superposition state |ψ ≡ 1 √ 2 (|↑ + |↓ ). A spectroscopy pulse probes the couplings |↑ → |e and |↓ → |e during a delay period τ . To decouple the superposition state ...