We report on the sub-kilohertz optical spectroscopy on the 1 S 0 -3 P 1 intercombination transition in magnesium at 457 nm. The spectroscopic signal is probed by a time domain atom interferometer. The realization of this time domain atom interferometer with laser cooled and trapped atoms allows extremely long interaction times and leads to resolutions down to 491 Hz (FWHM). This corresponds to a high line Q factor of 1.3 3 10 12 . Because of the high accuracy in the determination of the line center, applications with respect to an optical frequency standard are possible. [S0031-9007(98) PACS numbers: 03.75. Dg, 39.20. + q, 39.30. + w, In recent years the field of atom interferometry [1] has opened up new areas in fundamental and applied atomic physics. This is based mainly on the small wavelength of atomic de Broglie waves leading to the high sensitivity of atomic interferometers. In parallel the operation of atom interferometers has shown that due to the small de Broglie wavelength the proposed high sensitivity demands a setup of highest mechanical stability. Very recently several atom interferometric measurements have reached or even surpassed the precision of conventional "state of the art methods." Prominent examples are the realization of a highly sensitive atom-based Sagnac gyroscope [2], the measurement of the Earth's gravitational acceleration [3], and the precise determination ofh͞m atom [4].The Ramsey-Bordé atom interferometer, in particular, has proven to have excellent perspectives for applications in high resolution spectroscopy [1]. In this type of atom interferometer, the atomic wave is split and the partial waves are reflected and recombined by nearly resonant laser light. The phase shift between the two interfering matter waves depends strongly on the frequency of the splitting laser beam producing the spectroscopic signal. The transfer of the Ramsey-Bordé scheme from the originally proposed beam experiment [5] to a time domain interferometer [6] increases the sensitivity and reduces systematic line shifts by several orders of magnitude.Different methods for high resolution optical spectroscopy on neutral atoms as well as ions are currently being discussed and evaluated in the context of the next generation of frequency standards and atomic clocks. Especially, cold neutral atoms are also being used in the development of new microwave clocks [7,8]. In this Letter we report for the first time on atom interferometric high resolution measurements of an optical transition with a line Q factor above 10 12 corresponding to a resolved linewidth well below 1 kHz. The results presented here should be compared to recent results from the methods of the two-photon spectroscopy [9] and spectroscopy on single trapped ions [10]. In two-photon spectroscopy the ac Stark shift affects the obtainable accuracy, whereas in single-ion spectroscopy the stability is limited by the small particle numbers. The atom interferometric measurement of optical transitions discussed here offers the combination of a high accuracy...

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Single neutral atoms have been stored in a magneto-optical trap and imaged with the help of a CCD camera. The trapping of individual atoms is marked by steps in the fluorescence signal corresponding to the capture or loss of a single atom. The statistics of these capture and loss processes is explained by means of a birth-death model. The spatial distribution and the trap losses are investigated for the single-atom case and compared with the behaviour of a cloud of up to 2 • 10 5 atoms.

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