Illustration: Courtesy of S. D. Hogan, Ch. Seiler, and F. Merkt from ETH Zurich

Figure 1: Simulations of phase-space distributions showing the velocity vs position of the molecular cloud of $H2$ in the $n=30$ Rydberg state at different junctures of the deceleration and trapping process. When created by photoexcitation at position and time zero, the Rydberg molecules have a mean longitudinal velocity of $500m/s$. In about $10μs$ the molecules are decelerated over a longitudinal distance of only $3mm$ to zero mean velocity. At this position the molecules are then held in an electrostatic trap for about $50μs$. At any juncture, the molecules can be field-ionized and detected. Apart from reflecting the inhomogeneity of the electric field applied, the distributions illustrate that the conservative Stark interaction does not increase phase-space density (the density of points in each of the subsequent distributions).