An atom Michelson interferometer is implemented on an "atom chip." The chip uses lithographically patterned conductors and external magnetic fields to produce and guide a Bose-Einstein condensate. Splitting, reflecting, and recombining of condensate atoms are achieved by a standing-wave light field having a wave vector aligned along the atom waveguide. A differential phase shift between the two arms of the interferometer is introduced by either a magnetic-field gradient or with an initial condensate velocity. Interference contrast is still observable at 20% with an atom propagation time of 10 ms.

We report on the direct conversion of laser-cooled 41K and 87Rb atoms into ultracold 41K87Rb molecules in the rovibrational ground state via photoassociation followed by stimulated Raman adiabatic passage. High-resolution spectroscopy based on the coherent transfer revealed the hyperfine structure of weakly bound molecules in an unexplored region. Our results show that a rovibrationally pure sample of ultracold ground-state molecules is achieved via the all-optical association of laser-cooled atoms, opening possibilities to coherently manipulate a wide variety of molecules.

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We have investigated photoassociation ͑PA͒ spectra of ultracold 88 Sr atoms near the 5s 2 1 S 0 +5s5p 1 P 1 atomic asymptote. The intensity modulation of the PA lines was used to reconstruct the ground-state scattering wave function, whose last nodal point was determined to be r 0 = 3.78͑18͒ nm. The PA lines also determined a precise lifetime of the 1 P 1 state to be 5.263͑4͒ ns.Precise knowledge of atom-atom interactions is of significant importance in the creation and manipulation of ultracold atoms or molecules as well as in their application to precision measurements. Investigations on light-induced atom losses in magneto-optical traps ͑MOTs͒, where collision processes are altered by the presence of near resonant photons ͓1͔, have triggered an emerging field of cold collisions with laser cooled atoms ͓2͔. The stability and dynamics of the Bose-Einstein condensates ͑BECs͒ are governed by the ultracold collisional properties ͓3-6͔. Their tunability via magnetic or optical method is utilized to produce ultracold molecules as well as molecular BECs ͓7-10͔. From a metrological point of view, atomic collisions introduce unwanted frequency shifts that critically limit the performance of the state-of-the-art atomic fountain clocks ͓11͔ and optical clocks ͓12͔.A two-body collision problem is most simplified for ultracold atoms. The ground-state wave function for the angular momentum l =0 ͑s-wave͒ state can be written as R͑r͒ = u g ͑r͒ / r with u g ͑r͒ the solution of an ordinary onedimensional ͑1D͒ Schrödinger equation and r the interatomic separation. The interaction is characterized by a single parameter, the s-wave scattering length a, which is associated with the last node of the scattering wave function u g ͑r͒ ͓13͔.One of the most versatile experimental techniques for probing the wave function, especially its last node, is the photoassociation ͑PA͒ spectroscopy ͓14͔ that measures the intensity modulation of the PA lines ͓15-17͔. Since the demonstration of the technique in laser-cooled Na ͓18͔ and Rb ͓19͔ atoms, there have been extensive theoretical as well as experimental studies in alkali-metal systems ͓2͔. However, analysis of these PA lines are not easy because the hyperfine structure in their ground state as well as the excited state gives rise to complicated molecular potentials. In contrast, alkaline-earth-metal system offers the simplest energy structures, i.e., the 1 S 0 ground state and the 1 P 1 excited state, which allows straightforward interpretation of PA spectra. Recently a PA line profile was obtained for Ca, which determined the possible range of the scattering length ͓20͔. A predissociation process was revealed through the broadening of the PA line profiles in Yb atoms ͓21͔. PA spectra near the dissociation limit was investigated for Sr, which determined the 1 P 1 excited state lifetime ͓22͔.In this Rapid Communication we report on the determination of the last node of the scattering wave function for the 5s 2 1 S 0 state of 88 Sr atoms by reconstructing the wave function through PA spectroscop...