We have observed Bragg diffraction of a Bose-Einstein condensate of sodium atoms by a moving, periodic, optical potential. The coherent process of Bragg diffraction produced a splitting of the condensate with unidirectional momentum transfer. Using the momentum selectivity of the Bragg process, we separated a condensate component with a momentum width narrower than that of the original condensate. By repeatedly pulsing the optical potential while the atoms were trapped, we observed the trajectory of the split atomic wave packets in the confining magnetic potential.

letters to nature 218 NATURE | VOL 398 | 18 MARCH 1999 | www.nature.com optimizing the performance of the interferometers at low frequencies, where both equations (4) and (6) become more signi®cant. It appears natural to perform such studies in the quiet environment of space, perhaps through future re®nements of LISA-type set-ups 27 .The above discussion of gravity-wave interferometers shows that the smallness of the Planck length does not preclude the possibility of direct investigations of space-time fuzziness. This complements the results of studies 28,29 which have shown that indirect evidence of quantum space-time¯uctuations could be obtained by testing the predictions of theories consistent with a given picture of thesē uctuations. Additional encouragement for experiment-driven progress in understanding the interplay between gravity and quantum mechanics comes from recent studies 30,31 in the area of gravitationally induced phases, the signi®cance of which has been emphasized in refs 32 and 33. M

Extraction of sodium atoms from a trapped Bose-Einstein condensate (BEC) by a coherent, stimulated Raman process is demonstrated. Optical Raman pulses drive transitions between trapped and untrapped magnetic sublevels, giving the output-coupled BEC fraction a well-defined momentum. The pulsed output coupling can be run at such a rate that the extracted atomic wave packets strongly overlap, forming a highly directional, quasi-continuous matter wave.

We have extended two-photon Doppler-free spectroscopy to the vacuum ultraviolet spectral region, to accurately measure the He 1 1 S-2 1 S transition at 120 nm. Our result is 4 984 872 315 (48) MHz. This yields a ground state Lamb shift of 41 104(48) MHz, in fair agreement with theory and other experiments. This approach has the potential for significantly better accuracy once improvements in the laser and the wavelength metrology are implemented. [S0031-9007(98)

We have demonstrated the exchange of sifted quantum cryptographic key over a 730 meter free-space link at rates of up to 1.0 Mbps, two orders of magnitude faster than previously reported results. A classical channel at 1550 nm operates in parallel with a quantum channel at 845 nm. Clock recovery techniques on the classical channel at 1.25 Gbps enable quantum transmission at up to the clock rate. System performance is currently limited by the timing resolution of our silicon avalanche photodiode detectors. With improved detector resolution, our technique will yield another order of magnitude increase in performance, with existing technology.