Working Group 4.43
Single-ion optical frequency standard with ¹⁷¹Yb⁺

Electrically charged atoms (ions) can be stored in electric or magnetic traps over practically unlimited periods of time. In the Paul trap, the storage field is an alternating electric field which has the form of a quadrupole. Such a field can be generated by a simple electrode system. To load ions into the trap, neutral atoms are evaporated in the storage volume and are ionized with the aid of an electron source. Laser cooling allows stored ions to be located in the centre of the trap. The thermal motion can be suppressed to such an extent that stored ions arrange in a crystalline structure.

The amplitude of the motion of a single laser-cooled ion in a trap whose size lies in the millimetre range is typically not greater than 0.1 µm. Since the motional amplitude is smaller than the wavelength of light, the ion can be resonantly excited by a light field without linear Doppler frequency shifts. Also the shifts of the transition frequency caused by the trap field are extremely small since the field vanishes in the center of the trap. For the ¹⁷¹Yb⁺ ion investigated at PTB and for some other ions, one expects that the relative systematic uncertainty in the realization of atomic transition frequency can be reduced to 1×10^-18.

The reference transition of the investigated ¹⁷¹Yb⁺ single-ion frequency standard is the transition from the ground state to the metastable state ²D_3/2(F=2). The wavelength of this transition is at 436 nm, and its natural linewidth is 3.1 Hz. Presently the transition is resolved with a linewidth of 10 Hz. The absolute optical transition frequency was determined with a relative uncertainty of 3×10^-15. For this measurement, the Caesium fountain frequency standard CSF1 was used as the reference (Working Group 4.41). The output signals of the ¹⁷¹Yb⁺ standard and of CSF1 were compared using an optical frequency comb generator (Working Group 4.44). It is the objective of the present work to obtain more precise information about shifts of the atomic transition frequency due to the trap field. The necessary measurement accuracy can not be achieved by comparisons with conventional frequency standards. Therefore two independent ¹⁷¹Yb⁺ frequency standards are directly compared under different operating conditions.

More information on the ¹⁷¹Yb⁺ frequency standard

• Design of the employed ion trap
• Image of crystal of laser-cooled ions
• Scheme of an optical ion trap frequency standard
• Resonance signal of a trapped ¹⁷¹Yb⁺ ion