Suppressed Spontaneous Emission for Coherent Momentum Transfer

Strong optical forces with minimal spontaneous emission are desired for molecular deceleration and atom interferometry applications. We report experimental benchmarking of such a stimulated optical force driven by ultrafast laser pulses. We apply this technique to accelerate atoms, demonstrating up to an average of $19\hslash k$ momentum transfers per spontaneous emission event. This represents more than an order of magnitude improvement in suppression of spontaneous emission compared to radiative scattering forces. For molecular beam slowing, this technique is capable of delivering a many-fold increase in the achievable time-averaged force to significantly reduce both the slowing distance and detrimental losses to dark vibrational states.

Figure 1

Pulse sequence of the stimulated force. As described below, the delay time $\tau$ between pump and dump pulses is chosen to be much smaller than the spontaneous emission lifetime ($\tau \ll 1/\gamma$).

Figure 2

Time domain (upper) and frequency domain (lower) illustration of single-beam processes in this work. The ML laser generates 30 ps pulses at 12.5 ns intervals (80 MHz, red). A Pockels cell increases this interpulse delay within each pair to 250 ns (4 MHz, blue) to ensure $>99.99\%$ decay probability between pulses. The excited state probability for an atom excited by the first pulse is represented by the yellow area in the time domain figure. The corresponding atomic spectrum is shown in the frequency domain figure.

Figure 3

Coherent Rabi flops on an optical frequency electric dipole transition. The oscillation period allows identification and matching of the $\pi$-pulse energy for both beams. Fluorescence is collected from the atom cloud transverse to the acceleration direction for single pulses from the pump beam (red), dump beam (blue), or a pump-then-dump sequence (black). The $\pi$-pulse condition is satisfied slightly above 1 nJ, and the probability of spontaneous emission (the vertical axis) is calibrated from the measurements using Eqs. (1) and (3), which are shown as dashed curves.

Figure 4

Effect of varying pulse energy on the arrival times of the atoms at the TOF detection position. Each vertical cross section is a TOF trace (see the Supplemental Material [53]). The dashed guide lines represent the theoretical arrival times if the indicated momentum had been transferred to the atoms. These diagnostic data were taken before optimizing the force, and the arrival times of the fastest 10% atoms correspond to $\mathrm{\Upsilon }\approx 6$.