We report on precise measurements of absolute nonlinear ionization probabilities obtained by exposing optically trapped ultracold rubidium atoms to the field of an ultrashort laser pulse in the intensity range of 1 × 10 11 to 4 × 10 13 W/cm 2 . The experimental data are in perfect agreement with ab-initio theory, based on solving the time-dependent Schrödinger equation without any free parameters. Ultracold targets allow to retrieve absolute probabilities since ionized atoms become apparent as a local vacancy imprinted into the target density, which is recorded simultaneously. We study the strong-field response of 87 Rb atoms at two different wavelengths representing non-resonant and resonant processes in the demanding regime where the Keldysh parameter is close to unity.

Plasma dynamics critically depends on density and temperature, thus well-controlled experimental realizations are essential benchmarks for theoretical models. The formation of an ultracold plasma can be triggered by ionizing a tunable number of atoms in a micrometer-sized volume of a 87Rb Bose-Einstein condensate (BEC) by a single femtosecond laser pulse. The large density combined with the low temperature of the BEC give rise to an initially strongly coupled plasma in a so far unexplored regime bridging ultracold neutral plasma and ionized nanoclusters. Here, we report on ultrafast cooling of electrons, trapped on orbital trajectories in the long-range Coulomb potential of the dense ionic core, with a cooling rate of 400 K ps−1. Furthermore, our experimental setup grants direct access to the electron temperature that relaxes from 5250 K to below 10 K in less than 500 ns.

Technical progress in both industry and science has enabled the preparation, manipulation, and probing of quantum systems with an unprecedented level of control. This provides the basis for a multitude of so‐called quantum technologies like quantum cryptography, quantum sensors and quantum computers, which are currently developing from laboratory experiments into commercial applications.

We report on ultrafast cooling of electrons in an initially strongly coupled ultracold plasma, created by ionization of a Bose-Einstein condensate using a femtosecond laser pulse. Our experiment grants direct access to the electron temperature.