1000th issue of Uspekhi Fizicheskikh Nauk journal. Instruments and methods of investigation
Accurate frequency and time dissemination in the optical domain
N.N. Kolachevskya,c,b aLebedev Physical Institute, Russian Academy of Sciences, Leninsky prosp. 53, Moscow, 119991, Russian Federation bInternational Center for Quantum Optics and Quantum Technologies (the Russian Quantum Center), ul. Novaya 100, Skolkovo, Moscow Region, 143025, Russian Federation cNational Research Nuclear University ‘MEPhI’, Kashirskoe shosse 31, Moscow, 115409, Russian Federation
The development of the optical frequency comb technique has enabled a wide use of atomic optical clocks by allowing frequency conversion from the optical to the RF range. Today, the fractional instability of such clocks has reached the record eighteenth digit level, two orders of magnitude better than for cesium fountains representing the primary frequency standard. This is paralleled by the development of techniques for transferring accurate time and optical frequency signals, including fiber links. With this technology, the fractional instability of transferred frequency can be lowered to below 10−18 with an averaging time of 1000 s for a 1000 km optical link. At a distance of 500 km, a time signal uncertainty of 250 ps has been achieved. Optical links provide the opportunity for comparing optical clocks and creating a synchronized time and frequency standard network at a new level of precision. Prospects for solving new problems arise, including the determination of the gravitational potential, the measurement of the continental Sagnac effect and the precise testing of fundamental theories.