The NIST team measured frequency ratios, the quantitative relationships between the frequencies of the atoms as measured in three pairs (ytterbium-strontium, ytterbium-aluminum, aluminum-strontium). Frequency is the most accurately measured single quantity in science. The new NIST results reported in Nature also set other important records. Comparisons are crucial to the international community’s selection of one or more atoms as the next time standard. The atomic clocks used in the new comparisons tick at much higher optical frequencies, which divide time into smaller units and thus offer greater precision. Since 1967, the second has been defined based on the cesium atom, which ticks at a microwave frequency.
The comb-based signal transfer technique had been demonstrated previously, but the latest work was the first to compare state-of-the-art atomic clocks. NIST researchers developed two-way transfer methods to precisely compare optical clocks over the air, even in conditions of atmospheric turbulence and laboratory vibrations. The key to the air link was the use of optical frequency combs, which enable accurate comparisons of widely different frequencies. This work shows how the best atomic clocks might be synchronized across remote sites on Earth and, as time signals are transferred over longer distances, even between spacecraft. NIST researchers previously described in detail how they transferred time signals over the air link between two of the clocks, the NIST ytterbium and JILA strontium clocks, and found the process worked as well as the fiber-based method and 1,000 times more precisely than conventional wireless transfer schemes. The team’s measurements were so accurate that uncertainties were only 6 to 8 parts in 10 18 - that is, errors never exceeded 0.000000000000000008 - for both fiber and wireless links. The study compared the aluminum-ion clock and ytterbium lattice clock, located in different laboratories at NIST Boulder, with the strontium lattice clock located 1.5 kilometers away at JILA, a joint institute of NIST and the University of Colorado Boulder. The new measurements were challenging because the three types of atoms involved “tick” at vastly different frequencies, because all the many network components had to operate with extreme accuracy, and because the wireless link required cutting-edge laser technology and design. “These comparisons are really defining the state of the art for both fiber-based and free-space measurements - they are all close to 10 times more accurate than any clock comparisons using different atoms performed so far,” NIST physicist David Hume said.
These atomic clock comparisons place the scientific community one step closer to meeting the guidelines for redefinition of the second. In a significant advance toward the future redefinition of the international unit of time, the second, a research team led by the National Institute of Standards and Technology (NIST) has compared three of the world’s leading atomic clocks with record accuracy over both air and optical fiber links.ĭescribed in the March 25 issue of Nature, the NIST-led work is the first to compare three clocks based on different atoms, and the first to link the most advanced atomic clocks in different locations over the air.
0 kommentar(er)
