In the Quantum Measurement section (the Quantum Metrology div.), ion trap technique has been studied to realize a future frequency standard with higher performance than that of a current one. Recently, researchers in the section succeeded in observing a single ion at rest in vacuum. (See NRLM News, vol. 11, 1995.)In the case of laser-cooled trapped single ions in ultra-high vacuum condition, the perturbation to them can be extremely small. Therefore, it is predicted that a frequency standard using them would improve accuracy to hundreds times as high as that of a current one.

The observed single ion is a singly-ionized ytterbium (Yb⁺).Yb⁺ ions are confined in a small space surrounded by three electrodes.(This device is called an ion trap.)The electrodes, as shown in Picture 1, are placed in a chamber, made of glass, evacuated in the ultra-high vacuum condition.The ions just stored in ion traps move around in the traps with a speed of about seven times as high as that of the Japanese bullet train, Shinkansen.Therefore, it is impossible to observe them. To observe the trapped ions, they are irradiated by laser radiation to stop the motion of them by radiation pressure.(This method is called laser cooling.)

Picture 1: The ion trap for laser-cooling of Yb⁺.
The ion trap is placed in a chamber, made of glass, evacuated in ultra-high vacuum condition. The trap is composed of the three electrodes which look like disks of golden color at the center of the picture.The middle electrode of the three is a ring with an inner diameter of 5 mm.(Unfortunately, we cannot see the hallow in this picture.)The ions are confined around the center of the hallow.(The white parts in the picture are insulators.)

Picture 2: Image of laser-cooled trapped Yb+ ions. The bright oval near the center of the picture is the fluorescence of the large number of laser-cooled trapped Yb+ ions.The major axis of the ion cloud is 300 micro m.The bright part on the right of the ion cloud is reflection of a laser beam on the electrode surface.This picture was taken by a ultra-high sensitive camera with a photon counting technique.

Figure:Measurement of the number of trapped Yb+ ions. (Observation of the quantum jumps.)The time record of the fluorescence intensity when the fluorescence of Yb+ ions was extinguished one by one.The extinguishment was made by transfer Yb+ ions from the cycle of the fluorescence emission to the state of no fluorescence emission.One step of the fluorescence-intensity drop corresponds to the fluorescence of one ion.Four ions are trapped in this case.

When the number of trapped ions is small, the number is determined by extinguishing the fluorescence of each ion one by one.Figure shows the fluorescence intensity during this measurement.One drop of the fluorescence intensity corresponds to that of one ion.Four ions are trapped in the experiment of the figure.In this measurement, the fluorescence of the ions is extinguished by transferring them from the cycle of the fluorescence emission to the state of no fluorescenceemission.It should be noted that the drops of the fluorescence intensity appear to finish in a moment and look like steps or jumps.This means that change of the energy level of the ions finishes in a moment.These jumps prove that one of the principles in quantum mechanics that change of the energy levels is accomplished in a moment.(It is called the quantum jumps.)

Thus, it is possible to detect single ions with high sensitivity when we apply quantum-jumps signals for detection.In the Quantum Measurement section, day and night, researchers have been investigating single or a small number of laser-cooled trapped Yb⁺ ions for realization of a new frequency standard in an optical region.