(Top left) A stationary bucket is not an effective decelerator, since water tends to enter too fast and splash out. (Top right) A better option is a backward-moving bucket that can catch and smoothly decelerate the water. However, this system only works for a single squirt of water, not a continuous stream. The same is true for current molecular deceleration methods, which can only be applied to pulses but not continuous beams. (Bottom left) The present device acts as a continuous decelerator by using a rotating guide, or “tube,” in the shape of a spiral (green). In the experiment, a molecular beam (blue) is directed through a circular-shaped guide (pink). (Bottom right) Those molecules moving fast enough enter the spiral guide, where they are decelerated before exiting through the opening in the center.