Coherent manipulation of matter-waves on femtosecond timescales

Bridging the worlds of ultrafast physics and ultracold quantum matter opens new pathways to manipulate macroscopic wave functions on femtosecond timescales and to control the formation of ions and electrons in a quantum gas [1, 2] towards the creation of hybrid quantum systems.

The electric field of a single femtosecond laser pulse allows ultrafast phase imprinting in a Bose-Einstein condensate (BEC). The strong transient ac Stark shifts give rise to time-dependent attractive or repulsive potentials determined by the intensity profile and detuning of the laser pulse. Here we show that, even far detuned from any atomic resonances, peak accelerations exceeding 10⁹ m/s² can be used to engineer propagating ⁸⁷Rb matter-waves with unprecedented transversal beam temperatures below 18 nK. The combination of ultracold atoms and ultrashort laser pulses as instantaneous trigger allows a precise comparison of the experimental results and the semi-classical oscillator model of the optical dipole potential. Even though its validity becomes debatable at peak intensities up to 10¹³ W/cm² used in the experiment, we find that the transferred momentum is accurately described by this model and only limited by strong-field ionization. Additionally, we show that the interplay between an attractive potential and repulsive atomic interactions leads to a stable BEC with density enhancement. Simulations based on numerically solving the 3D Gross-Pitaevskii equation are in good agreement with the measurements confirming the coherent nature of the ultrafast phase imprinting.

Ultrashort laser pulses arise as a versatile tool for precise manipulation of ultracold gases regardless of the specific atomic level structure. Our experimental findings demonstrate a general scheme for engineering monoenergetic beams of atoms, particularly relevant for quantum chemistry, controlled collision experiments, and atom lithography.