A frequency comb for atomic clock comparisons

Currently, the second, as defined by the International System of Units, is based on the ground-state hyperfine transition in the caesium atom, a primary microwave frequency standard with an accuracy of 1 part in 10¹⁵ (that is, on the order of the femtosecond). In comparison, optical clocks, based on atomic transitions in the UV-visible, operate at higher frequencies and correspondingly provide higher accuracies reaching levels of 1 part in 10¹⁸ (of the order of the attosecond).

Frequency combs provide a bridge between the microwave domain, where electronic frequency counters operate, and the optical domain of atomic resonances. In this way, in a single, direct and phase-coherent step, frequency combs offer an accurate means to count the optical oscillations of atomic clocks. Moreover, they enable the comparison of optical clocks based on similar or different atomic transitions, which has the potential to shed light on the time variations of fundamental constants for example.

In the scope of carrying out such accurate and relative measurements of optical atomic clocks, our laboratory is developing a highly stable ytterbium-doped fibre frequency comb.

Currently, the second, as defined by the International System of Units, is based on the ground-state hyperfine transition in the caesium atom, a primary microwave frequency standard with an accuracy of 1 part in 10¹⁵ (that is, on the order of the femtosecond). In comparison, optical clocks, based on atomic transitions in the UV-visible, operate at higher frequencies and correspondingly provide higher accuracies reaching levels of 1 part in 10¹⁸ (of the order of the attosecond).

Frequency combs provide a bridge between the microwave domain, where electronic frequency counters operate, and the optical domain of atomic resonances. In this way, in a single, direct and phase-coherent step, frequency combs offer an accurate means to count the optical oscillations of atomic clocks. Moreover, they enable the comparison of optical clocks based on similar or different atomic transitions, which has the potential to shed light on the time variations of fundamental constants for example.

In the scope of carrying out such accurate and relative measurements of optical atomic clocks, our laboratory is developing a highly stable ytterbium-doped fibre frequency comb.