Rydberg atoms are unusual in that they contain an electron that has been excited to such a high energy level that it orbits a very long way from the nucleus. Since the outer electron is so loosely bound, Rydberg atoms are highly sensitive to external perturbations, such as electric fields. In the present study, for example, hydrogen atoms were used in which the electron had been excited by a laser beam so that its principal quantum number, n, was 27. The electron in one of these atoms can be as far as 37 nm from the nucleus.

Vliegen and Merkt began by using a laser to split up ammonia (NH₃) in a quartz capillary tube. As the gas left the tube, it underwent a supersonic expansion so that the hydrogen atoms were travelling at a speed of 720 ms^-1. The atoms then entered the gap between four metallic electrodes, where there is a rapidly changing electric field. As they did so, the atoms were excited by two ultraviolet laser beams to create Rydberg states.

By applying a sequence of voltages to the four electrodes, Vliegen and Merkt found that they could stop the Rydberg atoms in a time of 4.8 microseconds just 1.9 mm away from the position where the atoms were excited by the laser beams. They were then able to reflect the atoms back from the middle of the plates to their original positions with accelerations of 2 x 10⁸ ms^-2. And since the atoms are focused about six microseconds after being excited, Vliegen claims that their mirror also works as a cylindrical lens.

There could be some interesting applications of the new work. According to Vliegen, the new mirror could be used to perform interferometry experiments with Rydberg atoms. He even thinks the mirror could help prevent antihydrogen Rydberg atoms generated at the CERN “antimatter factory” from colliding with the walls of the experiment chamber and annihilating there.