The Cs Fountain

f2INRIM is one of the seven metrology laboratories that contribute to the calibration of the International Atomic Time (TAI) with primary frequency standards. The first Italian Cs fountain, ITCsF1, has been operating since 2003 with an accuracy of 5E-16. Since 2012 a cryogenic Cs fountain clock is under operation and is presently one of the most accurate realizations of the SI second, with a type B  accuracy of 1.7E-16.

The clock is operated according to a typical scheme: in the lower region of the vacuum chamber, Cs atoms are laser-cooled at a temperature of 1 microkelvin. Then they are launched upward and are probed by a microwave in the Ramsey interrogation scheme: on their flight, they first cross a resonant cavity and interact with a microwave tuned at the clock frequency; then, they evolve freely on a balistic flight, and interact again with the cavity while falling down. The number of atoms which perform the clock transition depends on the microwave frequency and the two-stage interrogation allows a much higher resolution on the detection.

The fountain is cooled at 89 K by liquid nitrogen, to minimize the effect of blackbody radiation on the clock transition frequency. Other effects which  generate a shift on the clock transition and are corrected for in ITCsF2 include the magnetic field (due to Zeeman effect), the collision between atoms, the spectral purity of the interrogation frequency, impairments in the microwave cavity feeding. Careful measurements of the height above the Geoid have been made in 2006 to determine the gravitational shift at better than the 1E-17 level, corresponding to a 10 cm accuracy.

About 20 days are needed to determine the clock frequency at the 2E-16 level. The measurement time depends mainly on the number of atoms which are loaded in the fountain.

Uncertainty budget of ITCsF2

Physical effect

Bias
(10-16)
Uncert. (10-16)
Zeeman effect 1166.5 0.8
Blackbody radiation -1.45 0.12
Gravitational redshift 260.4 0.1
Microwave leakage -1.2 1.4
Cavity Phase 0.2
2nd order cavity pulling 0.3
Background gas 0.5
Total Type B   1.7
Atomic density -3.6 2.2
Total 1420.7 2.8

Further readings

Contacts: giovanni.costanzo [at] polito.it, d.calonico [at] inrim.it, f.levi[at]inrim.it