Observation of strong-field stabilization in a driven condensate

Very strong laser fields can profoundly alter or destroy atomic structure. While ionization rates typically increase with intensity, approximate theories counter-intuitively predict that above some very high threshold intensity, ionization rates should level off or even decrease with increasing intensity. This long-predicted phenomenon, known as strong-field stabilization, is expected to be accompanied by a spatial bifurcation of the electronic wavepacket. Direct observation of ground-state stabilization in pulsed-laser experiments has been hindered by the extreme laser intensities it requires. We report the first experimental observation of strong-field stabilization of a ground state in the low-frequency regime, using an analog quantum simulator of an atom driven by an ultrastrong and ultrafast light field. By shaking a focused optical dipole trap confining a Bose–Einstein condensate of strontium, we realize physics analogous to that of bound electrons in a strong pulsed laser field. We observe that atom loss (ionization) increases with shaking amplitude (laser pulse intensity) until a threshold, where the loss rate saturates and then decreases sharply. At the largest amplitudes, we observe the predicted bifurcation of the atomic density into two lobes by directly imaging the wavepacket. We experimentally map out the dependence of stabilization on the frequency, amplitude, shape, and duration of the emulated laser pulse, and compare the results to numerical calculations. These results showcase the cold atom platform as a powerful complementary approach for probing strong-field phenomena near or beyond the current frontiers of laser technology, with natural extensions to half-cycle pulses, multifrequency fields, circular or elliptical polarization, and high-harmonic generation.