Ion optical clock

Bize, S. et al.. Testing the stability of fundamental constants with the Hg single-ion optical clock, Phys. Rev. Lett., 90, 150802, 2003. 

S. Bize, S.A. Diddams, U. Tanalca, C.E. Tanner, W.H. Oskay, R.E. Drul-linger, T.E. Parker, T.P. Heavner, S.R. Jefferts, L. Hollberg, W.H. Itano, J.C. Bergquist Testing the stability of fundamental constants with the Hg" single-ion optical clock. Phys. Rev. Lett. 90, 150 802 (2003)... 

Realisation of an optical clock requires first of all a narrow and stable resonance as is the case for a trapped indium ion - and, secondly, precise frequency determination in comparison with the cesium clock. We have performed two measurements of the absolute frequency of the n In+ —> 5s5p clock... 

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. 

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)...

In 1995, the first caesium fountain atomic clock was constructed at the Paris Observatory in France. A fountain clock, NIST-Fl, was introduced in 1999 in the US to function as the country s primary time and frequency standard NIST-Fl is accurate to within one second in 20 x 10 years. While earlier caesium clocks observed Cs atoms at ambient temperatures, caesium fountain clocks use lasers to slow down and cool the atoms to temperatures approaching 0 K. For an on-line demonstration of how NIST-Fl works, go to the website http //tf.nist.gov/cesium/fountain.htm. Current atomic clock research is focusing on instruments based on optical transitions of neutral atoms or of a single ion (e.g. Sr ). Progress in this area became viable after 1999 when optical counters based on femtosecond lasers (see Box 26.2) became available. 

To summarize the above paragraph, trapped ions can serve as exceptional tools for new and functioning atomic clocks, in both the microwave and optical frequency domains. 

In this Section, we will describe briefly the most recent projects of atomic clocks involving/based on ion traps as described above. The first part concerns micro-wave clocks, while the one following will be dedicated to optical frequency clocks. Performances of atomic standards can be evaluated only by comparison (frequency beatings) with another devices. When a new atomic standard can be presumed to out-perform the norm, it can be evaluated only from the comparison with a second system, which must be build in a similar way. It is worth noting that performances of each scheme depend on the local oscillator a quartz (eventually, cryogenic) oscillator for the microwave range, and a laser for the optical one. 

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. 

Roth, B., Koelemeij, J.C.J., Daerr, H., Ernsting, L, Jorgensen, S., Okhapkin, M., Wicht, A., Nevsky, A., and Schiller, S., Trapped ultracold molecular ions Candidates for an optical molecular clock for a fundamental physics mission in space, in Proc. of the 6th Intemat Conf. on Space Optics, ESTEC, Noordwijk, The Netherlands, ESA-SP 621,2006. 

The next stage in the development of atomic clocks that utilize the control of atoms will be characterized by the use of optical transitions in atoms, specifically laser-cooled trapped atoms or ions, and the laser synthesis of optical and microwave radiation (Chapter 6). 

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