Abstract

Selective spectroscopic detection of an individual ion and control of its centre-of-mass motion are prerequisite for the ion’s application as a frequency standard [1] or a quantum computer [2]. A single dynamically trapped Ba⁺-ion has been irradiated by monochromatic 493-nm and 650-nm laser light close to resonance with the S_1/2 - P_1/2 resonance line and the D_3/2 - P_1/2 line, respectively. Its fluorescence was detected, and it was 3D-cooled down into the Lamb-Dicke regime far below the Doppler limit and with the help of electronic Raman excitation. When irradiation by tunable 1.76µm- light on resonance of the E2 line S_1/2 - D_5/2 (Fig. 1) alternates with the visible light, successfully exciting the ion is signalled by missing resonance scattering in the next half cycle ("off"). Recording the "on-off rate VS tuning the IR light shows an absorption spectrum of the ion’s E2 line including sidebands from residual motion in the trapping potential (Fig. 2a). Since lower and upper sideband scale like <n> and <n + 1>, respectively, the mean vibrational quantum number <n> is derived. When in the D_5/2 state, the ion decouples from the visible light, and its distribution over the vibrational states is non-thermal and non-classic, since sideband excitation and "off detection (missing fluorescence) both redistribute the ion over n. An excitation spectrum of "on-off-on" events represents Stimulated emission, so far not detectable on dipole-forbidden lines of a single ion, and both vibrational sidebands scale as 2<n> + 1 [3] (Fig. 2b). More than one subsequent "off" detection before the next "on" detection of fluorescence reduce <n>, i.e. impose stochastic cooling to the ion in the metastable "dark" state [4]. The probability for sideband excitation oscillating upon increasing number q of subsequent "off detections (Fig. 3), indicating preservation of some coherence during the q-1 intermissions of the interaction with the IR-light when scattering the monitor light is unsuccessfully attempted.

© 1996 IEEE