The phase shift of the hyperfine Larmor precession of an individual ground-state 171 Yb 1 ion upon pulsed variation of the ambient magnetic field has been measured by microwave-optical double resonance interpreted in terms of Mach-Zehnder interferometry. Averaging over an ensemble of individual measurements, compared with measurements on an ensemble of ions, demonstrates quantum ergodicity. Even a single measurement yields (incomplete) phase information. Outside the peaks and dips of the interferogram, where ion probing is incompatible with ion preparation, the results of measurements are stochastic. This is demonstrated by laser exciting the ion on an E2 line. [S0031-9007(99)08549-X] PACS numbers: 03.75.Dg, 32.80.Pj, 39.20. + qOne of the most sensitive and precise spectroscopic techniques is microwave-optical double resonance (MODR) [1,2], in particular when the atoms interact with spatially separated microwave fields [3]. Its variant that features instead separated optical signal fields has been shown to represent an atom interferometer: Only that component of the de Broglie wave which has, in fact, interacted with the light and is now ascribed to the excited state of the signal transition is affected by the light recoil and spatially separates from the ground-state component [4][5][6][7]. It has been pointed out that the states of a spin-type degree of freedom may replace spatial separation and be sufficient in order to make this scheme of atom-light interaction qualify as a Mach-Zehnder-type atom interferometer, however, in configuration space [8-10]. We applied microwave-optical double-resonance spectrometry to 10 6 laser-cooled 171 Yb 1 ions [11], confined in an electrodynamic ion trap [12,13], and to a single ion of this species [14].Such interferometry has been extended to some 30 trapped 171 Yb 1 ions and, moreover, to an individual ion. Here, each measurement may yield the same result, or a random result, depending on whether the quantum evolution, after identical ion preparation, results in an ion state compatible with the detection, or not, respectively. Ramsey fringes appear in the mean rate of fluorescence detections after averaging over many observations. Quantum projection noise [15] of periodically varying degrees is found superimposed upon the fringes. Opposite Larmor phase shifts applied to the upper and lower hyperfine (hf) level contributions to the ion's ground-state wave function between the two signal pulses result in a phase shift of the fringes. Restricting the averaging to smaller numbers of observation makes the projection noise increase. Individual, nonaveraged observations identify the complete microscopic state of the system: They are selective measurements unlike measurements on an ensemble [16]. Sequences of such measurements recorded upon certain rf detunings yield invariably "on" or "off" results, but random variations of on and off at intermediate rf frequencies. This interchange of deterministic and stochastic results is demonstrated with the rf replaced by light, i.e., by opt...