Ultracold cesium atoms are prepared in the ground energy band of the potential induced by an optical standing wave. We observe Bloch oscillations of the atoms driven by a constant inertial force. We measure the momentum distribution of Bloch states and effective masses different from the mass of the free atom. [S0031-9007 (96)00366-3] PACS numbers: 32.80.Pj, 03.75.-b 45080031-9007͞96͞76(24)͞4508(4)$10.00

We review recent advances in the development of colloidal fluorescent semiconductor nanocrystals (a class of quantum dots) for biological labeling. Although some of the photophysical properties of nanocrystals are not fully understood and are still actively investigated, researchers have begun developing bioconjugation schemes and applying such probes to biological assays. Nanocrystals possess several qualities that make them very attractive for fluorescent tagging: broad excitation spectrum, narrow emission spectrum, precise tunability of their emission peak, longer fluorescence lifetime than organic fluorophores and negligible photobleaching. On the down side, their emission is strongly intermittent ("blinking") and their size is relatively large for many biological uses. We describe how to take advantage of nanocrystals' spectral properties to increase the resolution of fluorescence microscopy measurements down to the nanometer level. We also show how their long fluorescence lifetime can be used to observe molecules and organelles in living cells without interference from background autofluorescence, a pre-requisite for single molecule detectability. Finally, their availability in multicolor species and their single molecule sensitivity open up interesting possibilities for genomics applications.

The long (but not too long) fluorescence lifetime of CdSe semiconductor quantum dots was exploited to enhance fluorescence biological imaging contrast and sensitivity by time-gated detection. Significant and selective reduction of the autofluorescence contribution to the overall image was achieved, and enhancement of the signal-to-background ratio by more than an order of magnitude was demonstrated.