In the past fifteen years, research on atomic frequency standards has focused on three main topics. First, the realization of the unit of time has reached a relative accuracy level of a few parts in 10 -16, 100 times better than previously, due to the introduction of laser cooling techniques. Then, the advent of femtosecond laser frequency combs has triggered an impressive acceleration in the development of accurate optical clocks, which is leading to a further improvement in the second realization. Third, great efforts have been made in the challenging generation of compact, high stable standards, both for laboratory and satellite environments.
Laser cooled atomic clocks are based on ultra-low temperature atomic samples obtained by laser manipulation. Slow atomic samples lead to precise spectroscopy, and then to highly accurate atomic clocks, as interaction times are longer and Doppler effects negligible.
In 1996 the first laser cooled Cs atomic fountain was demonstrated [1]: about 106 Cs atoms are cooled down to 1 µK (about 1 cm/s rms speed), then launched in a ballistic flight. During the flight a Ramsey spectroscopy technique is implemented allowing the comparison between an external local oscillator and the reference transition at 9192631770 Hz which defines the second. The accuracy of this standard was 3·10-15 in 1996, reaching a few parts in 10-16 today [2]. Since then, four laboratories have provided data for the calibration of the International Atomic Timescale (TAI) on a regular schedule with a combined uncertainty of about 1·10-15 [2, 3].
Laser Cooled Cs fountain IT-CsF2 optical bench.
At INRIM, the Cs fountain IT-F1 regularly provides data to TAI and a new generation Cs fountain is now under development, with an expected accuracy of 1·10-16. The new fountain structure will be cooled by liquid nitrogen and then operated at 80 K: this will reduce the blackbody radiation shift, one of the main limits of the present fountains, to a negligible level.
Nowadays, the advent of the frequency comb generators [4] based on supercontinuum coherent radiation by a femtosecond pulsed laser and photonic nonlinear fibers allows direct comparison between optical and microwave radiations with an uncertainty source due to the comparison itself of few parts in 10-18. This has thrown open the possibility of a new atomic clock generation, the so called optical clocks, that is also based on laser cooled atomic samples, but uses a transition in the optical domain as reference. These transitions have linewidths of a few tens of millihertz, that means quality factors of the transition Q of 1015-1016 , 105-106 times the present Q of atomic Cs fountains.
There are several atoms suitable for making an accurated optical clock, based on ions or neutral atoms, and in 2008 an Al+ and an Hg+ based clock demonstrated an accuracy of a few parts in 10-17, better than the present realization of the SI second based on Cs fountains [5].
At INRIM an optical clock based on laser cooled ytterbium is under development. Ytterbium offers a yellow 578 nm reference, the 1S0-3P0 intercombination transition, that is seriously forbidden but for a weak hyperfine mixing of intercombination pure states in fermionic isotopes. The natural linewidth is evaluated to be 10 mHz. Ytterbium has seven isotopes with reasonable natural abundance, among them the most interesting are the boson Yb174 and the fermion Yb171, with the lowest total angular momentum ½, that is considered the best candidate for an Yb based optical clock.
Laser cooled Yb clock experiment.
Ytterbium could be cooled down to a few tens of microkelvin by using two magneto optical traps, a first one at 399 nm that cools atoms down to 1 mK, and a 556 nm trap that allows the ultimate temperature. After this cooling process, the atoms are slow enough to charge a third optical trap at 759 nm, called the "magic wavelength", as this radiation does not shift Yb clock transition by the Stark effect because of a shift cancellation.
In this way, the Yb clock implements a spectroscopic scheme where the atoms are trapped in a lattice generated by 759 nm radiation and then they undergo a Rabi pulse of a 578 nm laser source. The great advantage of a lattice based clock is the longer interrogation time that implies the narrowing of the reference transition linewidth without compromising accuracy by the Stark shift, canceled out at the magic wavelength. The expected accuracy for the Yb clock is 10-17 in the medium term, and, so far, there is no theoretical evidence that sets limits to accuracy at the 10-18 level.
Laser cooled Yb clock: physical package detail.
As for the research on compact and highly stable clocks, it is mandatory as these secondary atomic frequency standards have an important role in synchronization and then in several applications both scientific and technological (such as telecommunications and satellite radionavigation). Optically pumped vapour cell atomic clocks represent a well-established type of secondary atomic frequency standard. In general, these clocks use a double resonance mechanism (microwave-optical radiations) to excite rubidium atoms in a vapour cell. Recently, pulsing the different operation phases of a laser pumped vapour cell clock has been recognized as one of the most effective techniques to reduce laser noise transferred to the clock transition (light shift) and then to improve the stability perspectives of these clocks. In more detail, at INRIM a laboratory prototype of Pulsed Optically Pumped (POP) vapour cell clock uses the free induction decay microwave signal and the Ramsey interaction scheme to detect the clock transition: the so called POP maser. The measured frequency stability of the POP maser is αy(τ)=1·10-12 τ-1/2 up to integration times of about 104 s reaching the level of 5·10-15 (drift removed).
The POP maser is based on the idea that the pumping phase, the microwave interrogation and the detection of clock transition are no longer simultaneous as in the RF-optical double resonance approach of the traditional vapor cell atomic frequency standards but occur at different times. In this way during the clock interrogation the atomic sample behaves as a pure 2-level system, minimizing the mutual influences of the different signals normally present in a gas cell and strongly reducing the transfer of pumping source instabilities to clock transition. The pulsed optical pumping technique in particular avoids the light shift effect and the noise conversions from the laser to the clock signal. Microwave detection makes it possible to reach the thermal noise limit without any laser background signal. A higher atomic quality factor Qa is then obtained, due to the direct detection of induced microwave coherence instead of population inversion.
[1] A. Clairon et al: Proc. 5th Symp. Frequency Standards and Metrology (Woods Hole, MA) ed J.C. Bergquist (Singapore: World Scientific), pp. 45-59, 2006.
[2] F. Levi, D. Calonico, L. Lorini, A. Godone: Metrologia, Vol. 43, pp. 545-555, 2006.
[3] S. Jefferts et al: Metrologia, Vol. 39, pp. 321-326, 2002; S. Weyers, U. Huebner, R. Schroeder, C. Tamm, A. Bauch: Metrologia, Vol. 38, pp. 343-352, 2001; C. Vian et al: IEEE Trans. Instrum. Meas., no. 54, pp. 833-836, 2005.
[4] S.T. Cundiff, J. Ye, J.L. Hall: Rev. Mod. Phys., no. 75, pp. 325-342, 2003.
[5] T. Rosenband et al: Science, no. 319, pp. 1808-1812, 2008.