We present a method for the effective preparation of a Bose-Einstein condensate (BEC) into the excited bands of an optical lattice via a standing-wave pulse sequence. With our method, the BEC can be prepared in either a single Bloch state in a excited-band, or a coherent superposition of states in different bands. Our scheme is experimentally demonstrated by preparing a 87 Rb BEC into the d-band and the superposition of s-and d-band states of a one-dimensional optical lattice, within a few tens of microseconds. We further measure the decay of the BEC in the d-band state, and carry an analytical calculation for the collisional decay of atoms in the excited-band states. Our theoretical and experimental results consist well.

Superradiance scattering from a Bose-Einstein condensate is studied with a two-frequency pumping beam. We demonstrate the possibility of fully tuning the backward mode population as a function of the locked initial relative phase between the two frequency components of the pumping beam. This result comes from an imprinting of this initial relative phase on two matter wave gratings, formed by the forward mode or backward mode condensate plus the condensate at rest, so that cooperative scattering is affected. A numerical simulation using a semiclassical model agrees with our observations. [10][11][12], and quantum states storage and retrieval [13,14].In a typical BEC superradiance experiment, an elongated condensate is illuminated by an off-resonant pumping laser pulse along its short axis. Due to the phasematching condition and mode competition, highly directional light is emitted along the long axis of the condensate, in the so-called end-fire modes. Consequently, the recoiled atoms acquire a well-defined momentum at ±45• angles with respect to the pumping laser direction. These atomic modes are referred to as forward modes. This forward scattering is interpreted as optical diffraction from a matter wave grating [1,5]. Meanwhile, atoms in the condensate may scatter photons in the end-fire modes back into the pumping mode and recoil at ±135• angles, forming the so-called backward modes [3,7], when the pumping pulse is short and intense. This pattern was interpreted as a result of diffraction of atoms off a light grating. A four wave mixing interpretation was proposed, involving two optical fields-the pumping laser field and an end-fire mode, and two matter wave modes-the condensate and a mode of momentum [7].There is an energy mismatch of four times the recoil frequency for this backward scattering, due to the increased kinetic energy of recoiled atoms [3,6]. Recently, a two-frequency-pumping scheme has been implemented [15][16][17], where the pumping beam consisted of two frequency components and the frequency difference was controlled to compensate for the energy mismatch * E-mail: xjzhou@pku.edu.cn † E-mail: xuzongchen@pku.edu.cn FIG. 1. (Color online) Schematic diagram of our experiment.The two-frequency pumping beam is incident along the short x direction, with a linear polarization along the y direction. The atomic side modes are denoted in momentum space, each labeled with a pair of integers which describe the order in the x and z directions, respectively. Within this notation, atoms of the condensate at rest are in mode (0, 0). A forward scattering event transfers an atom from mode (n, m) to mode (n + 1, m ± 1), and a backward event transfers one to mode (n − 1, m ± 1).The end fire mode e± is along the long axis of the condensate in z direction.and excite the backward scattering on a long time scale with a weak pump intensity. The presence of the backward mode in the spectroscopic response [16] and the enhancement of the diagonal sequential scattering [17] have been reported. In those experiments the rel...

We analyze the effects of sequences of standing wave pulses on a Bose-Einstein condensate (BEC). Experimental observations are in good agreement with a numerical simulation based on the band structure theory in the optical lattice. We also demonstrate that a coherent control method based on such sequences of pulses is very efficient for experimentally designing specific momentum states.

We develop a simple yet powerful technique to study Bogoliubov-Cherenkov radiation by producing a pulsed atom laser from a strongly confined Bose-Einstein condensate. Such radiation results when the atom laser pulse falls past a Bose-Einstein condensate at high-hypersonic speeds, modifying the spatial profile to display a characteristic twin jet structure and a complicated interference pattern. The experimental observations are in excellent agreement with mean-field numerical simulations and an analytic theory. Due to the highly hypersonic regime reached in our experiment, this system offers a highly controllable platform for future studies of condensed-matter analogs of quantum electrodynamics at ultrarelativistic speeds.

We study the diffraction phase of different orders via the Dyson expansion series, for ultracold atomic gases scattered by a standing-wave pulse. As these diffraction phases are not observable in a single-pulse scattering process, a temporal Talbot-Lau interferometer consisting of two standing-wave pulses is demonstrated experimentally with a Bose-Einstein condensate to explore this physical effect. The role of the diffraction phases is clearly shown by the second standing-wave pulse in the relative population of different momentum states. Our experiments demonstrate obvious effects beyond the Raman-Nath method, while they agree well with our theory by including the diffraction phases. In particular, the observed asymmetry in the dependence of the relative population on the interval between two standing-wave pulses reflects the diffraction phase differences. The role of interatomic interaction in the Talbot-Lau interferometer is also discussed.