We demonstrate the ability to coherently control ultracold atomic Rb collisions using frequencychirped light on the nanosecond time scale. For certain center frequencies of the chirp, the rate of inelastic trap-loss collisions induced by negatively chirped light is dramatically suppressed compared to the case of a positive chirp. We attribute this to a fundamental asymmetry in the system: an excited wavepacket always moves inward on the attractive molecular potential. For a positive chirp, the resonance condition moves outward in time, while for a negative chirp, it moves inward, in the same direction as the excited wavepacket; this allows multiple interactions between the wavepacket and the light, enabling the wavepacket to be returned coherently to the ground state. Classical and quantum calculations support this interpretation.PACS numbers: 32.80. Qk, 32.80.Pj, 34.50.Rk Traditionally, the fields of ultracold physics [1] and short-pulse coherent control [2, 3] have followed independent paths. Besides the obvious incompatibility of sub-picosecond timescales with the motion of slow atoms, cooling typically deals with translational degrees of freedom while coherent control involves internal degrees of freedom. A potentially fruitful collaborative venture is the production of ultracold molecules by coherently-controlled photoassociation of ultracold atoms. Despite many theoretical proposals on this topic [4,5,6,7,8,9,10,11], experiments to date [12,13] have demonstrated coherent control of only the photodestruction of ultracold molecules, not of their collisional formation. Here we describe our use of frequency-chirped light on the nanosecond timescale [14] to coherently control ultracold collisions in Rb. We observe significant differences in the collision rates for the two chirp directions. For the negative chirp, a wavepacket excited to the attractive molecular potential can subsequently be returned coherently to the ground state, thereby reducing the collision rate.Ultracold molecules [15] are of significant current interest due to potential applications in a variety of fields: ultracold chemistry, quantum computing, novel quantum degenerate systems, and tests of fundamental symmetries. One method of ultracold molecule production is photoassociation [16], where two ultracold atoms collide in the presence of laser light and undergo a free-to-bound transition to form an excited molecule. Spontaneous emission can subsequently produce ultracold molecules in the electronic ground state, but typically in a distribution of high vibrational levels. If the entire process could be done coherently, it might be possible to populate a * Present address:

We demonstrate that judicious shaping of a nanosecond-time-scale frequency chirp can dramatically enhance the formation rate of ultracold (87)Rb(2) molecules. Starting with ultracold (87)Rb atoms, we apply pulses of frequency-chirped light to first photoassociate the atoms into excited molecules and then, later in the chirp, deexcite these molecules into a high vibrational level of the lowest triplet state a (3)Σ(u)(+). The enhancing chirp shape passes through the absorption and stimulated emission transitions relatively slowly, thus increasing their adiabaticity, but jumps quickly between them to minimize the effects of spontaneous emission. Comparisons with quantum simulations for various chirp shapes support this enhancement mechanism.

We use frequency-chirped light on the nanosecond time scale to produce ultracold 87 Rb2 molecules in the lowest triplet state via the process of photoassociation. Comparing to quantum simulations of the molecular formation, we conclude that coherent stimulated emission plays an important role and is primarily responsible for the significant difference observed between positive and negative chirps.

We present a novel technique for producing pulses of laser light whose frequency is arbitrarily chirped. The output from a diode laser is sent through a fiber-optical delay line containing a fiberbased electro-optical phase modulator. Upon emerging from the fiber, the phase-modulated pulse is used to injection-lock the laser and the process is repeated. Large phase modulations are realized by multiple passes through the loop while the high optical power is maintained by self-injection-locking after each pass. Arbitrary chirps are produced by driving the modulator with an arbitrary waveform generator.

We utilize various techniques to characterize the residual phase modulation of a waveguide-based Mach-Zehnder electro-optical intensity modulator. A heterodyne technique is used to directly measure the phase change due to a given change in intensity, thereby determining the chirp parameter of the device. This chirp parameter is also measured by examining the ratio of sidebands for sinusoidal amplitude modulation. Finally, the frequency chirp caused by an intensity pulse on the nanosecond time scale is measured via the heterodyne signal. We show that this chirp can be largely compensated with a separate phase modulator. The various measurements of the chirp parameter are in reasonable agreement.