Clock transition

The black body shifts are not confined to Rydberg atoms, but also alter the frequency of atomic clock transitions. Itano et al. have shown that the Cs 9 GHz ground state hyperfine interval, the definition of the second, is increased by one part in 1014 when the temperature is raised from 0 K to 300 K.29. 

High-Resolution Spectroscopy and Absolute Frequency Measurement of the Clock Transition... 

A high-resolution spectrum of the clock transition is shown in Fig. 2. The clock-laser power was reduced to 30 nW to avoid saturation broadening. The fit with a lorentzian curve results in a linewidth of 170 Hz (FWHM), corresponding to a fractional resolution bv/v of 1.3 10-13. A spectral window of 200 Hz width contains 50% of all excitations. According to our present experimental control of the ion temperature, electromagnetic fields and vacuum conditions, no significant Doppler, Zeeman, Stark or collisional broadening of the absorption spectrum of the ion is expected beyond the level of 1 Hz. The linewidth is determined by the frequency instability of the laser and the lineshape is not exactly lorentzian... 

This part of the chain was simultaneously used for an absolute frequency measurement of the In+ 1Sq - 3P[Pg.581]

Fig. 1. On the left is a simplified energy-level diagram for l Hg+. The 281.5 nm quadrupole "clock" transition can be observed by monitoring the 194 nm fluorescence. If the ion has made a transition from the Si to the 5/2 level the 194 nm flourescence disappears. For the figure on the right, on the horizontal axis is plotted the relative detuning from line center in frequency units at 281.5 nm. On the vertical axis is plotted the probability that the fluorescence from the 6s Si - 6p pi first resonance transition, excited by laser radiation at 194 nm, is on immediately after the 281.5 nm pulse. The electric-quadrupole-allowed S-D transition and the first-resonance S-P transition are probed sequentially in order to avoid light shifts and broadening of the narrow S-D transition. The recoilless absorption resonance or carrier (central feature) can provide a reference for an optical frequency standard. (From ref. 11)...

If we make the simplifying assumption that the time-domain Ramsey method is used to interrogate the clock transition, then the frequency stability (two-sample Allan variance- - ) is given as... 

Two kinds of ion species are involved depending on their atomic level properties. One has two optical/peripheral electrons, such as A1+, In+, where the clock transition is based on a dipolar electric transition, and the other has only one optical electron, such as Ca+, Hg +, Sr+, and Yb+, for which the clock transition is based on either a quadrupolar or an octopolar dipole electric transition. With the first kind of ion, the cooling transition is cycling wherein 100% of the atoms relax to the lower level, while the cooling transition (nS to nP) of the second kind relaxes to two different-orbital lower levels the fundamental ( 5) and one metastable level ((n-1) D). The value of the relaxation branching ratio between the nS and metastable (n-1) D levels is such that a significant fraction of ions will populate the metastable (n-l)D level. Thus, another laser is required to pump the atomic ions from the (n-l)D level back to the optically excited state nP. 

At NPL, two projects exist. The first project concerns the strontium ion, Sr [58], which is studied also at the National Research Council (NRC) [59], in Ottawa. The clock transition is at 674 nm with a line width of 0.4 mHz. The line has been explored already at a resolution of 5 Hz and the frequency has been measured to within 1.7 Hz. The second project is based on an octopole electric transition S F),... 

The 237 nm " Indium line (Max Planck Institute at Garching and Erlangen University in Germany) [60,61] of 0.8 Hz natural width is expected to have an accuracy of 10 , see Figure 11.21. The clock transition measured with an experimental line width of 170 Hz (1.3 x 10 ), has an accuracy of 230 Hz. 

FIGURE 11.21 An example of a clock atomic transition. The excitation probability of the clock transition (the atomic oscillator) is measured through the quantum jump number vs. the laser tuning of the local oscillator. Each probe pulse is of 90 ms duration, and twenty probe cycles were performed for each value of the detuning. (Reproduced with the permission of the Physikalisch-Technisehe Bundesanstalt.)... 

Matsubara, K. Hayasaka, K. Li, Y Hiroyuki, L Nagano, S. Kajita, M. Hosokawa, M. Frequency measurement of the optical clock transition of Ca-I- ions with an uncertainty at 10 level. Appl. Phys. Express 2008, 7, 067011-3. 

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