Optical clock "Ascend to heaven" helps precise satellite navigation
Optical clock "Ascend to heaven" helps precise satellite navigation
Scientists have never stopped pursuing precise time. At present, the most accurate clock in the world is undoubtedly the optical clock. Although researchers have long proposed applying optical clocks to satellites to improve the accuracy of satellite positioning, how to maintain optical clocks in space as stable as on Earth has always been the focus of debate.
One hour is made up of 60 minutes, and one minute is made up of 60 seconds. How long is one second? It is a grid of the second hand on a clock and a jump of numbers on an electronic clock, but not many people know how long a second is, and not many people care about it.
But scientists' pursuit of precise time has never stopped. At present, the most accurate clock in the world is undoubtedly an optical clock. Although researchers have long proposed applying optical clocks to satellites to improve the accuracy of satellite positioning, how to maintain optical clocks in space as stable as on Earth has always been the focus of debate.
Key components or send optical clocks into space
Recently, in Optica, an influential academic research journal of the Optical Society of America, researchers announced a compact automatic laser frequency comb system. It is an important part of optical clocks because they are like gears that break the optical clock's oscillations into lower frequencies faster and connect to a microwave-based reference atomic clock. In other words, the frequency comb can accurately measure the optical oscillation and be used to obtain time.
However, to better understand the importance of this research, we must first understand the principles of optical clocks. At present, optical clocks are divided into two categories: single ion trapped optical clocks and cold atomic optical lattice clocks. Regardless of the optical clock, it is necessary to use laser cooling technology to decelerate and cool the ions or atoms in an ultra-high vacuum environment, and finally achieve the "trap" of the magneto-optical trap, and then use ion trap or optical lattice technology to achieve ion Or atoms are "long-term imprisoned" and locked with a laser. Finally, femtosecond optical comb technology is used to achieve a coherent link between optical and microwave frequencies, which is the new technology mentioned above.
Simply put, in terms of shape, the femtosecond optical comb is very similar to a "comb". When it is locked by a mode-locked laser, it becomes a ruler that can measure the optical frequency. Each comb tooth is this comb. The scale of the light ruler. The optical frequency comb converts the optical frequency measurement into a series of radio frequency measurement, which is a revolution in the realization of optical clock and optical frequency precision measurement.
Fang Zhanjun, director of the Institute of Time-Frequency Metrology, Chinese Academy of Metrology, explained in an interview with a reporter from China Science News: “The femtosecond laser optical frequency comb is one of the key technologies in optical clock research. It realizes optical frequency and microwave The frequency and the coherent link between different optical frequencies make the original extremely complex and arduous absolute optical frequency measurement relatively simple."
Because of the complexity and importance of the femtosecond beam, the volume of the optical clock components is relatively large. However, the compact automatic laser frequency comb system announced this time is only 22 cm x 14.2 cm and weighs about 22 kg. It is made on the principle of optical fiber, so it can be installed into satellites and can withstand extreme acceleration when leaving the earth. The resulting force and the effect of temperature changes. More importantly, its power consumption is less than 70 watts, enough to meet the requirements of satellite equipment.
Time and satellite positioning are more accurate
What is the relationship between improving time accuracy and satellite positioning? Fang Zhanjun explained: "The principle of satellite positioning and navigation is the same as that of radio positioning and navigation, that is, geometric distance measurement is achieved through time measurement, and the accuracy of time measurement determines the accuracy of positioning and navigation."
Generally speaking, clocks are installed in current satellite navigation and positioning systems. The stability of these clocks determines the time measurement accuracy of the positioning and navigation system, and thus determines the accuracy of the positioning and navigation. At present, the rubidium atomic clock and the cesium atomic clock are used on navigation satellites.
"Both rubidium clocks and cesium clocks are microwave atomic clocks, and their day stability is between 10-14 and 10-15, which limits the accuracy of satellite positioning to the order of meters. If in the future a higher-stability optical clock is used to replace the present The rubidium and cesium clocks used may increase the accuracy of satellite positioning and navigation to the centimeter level.” Fang Zhanjun said, “It must be noted that to achieve centimeter-level positioning and navigation, only high-stability satellites are placed on the satellite. Spaceborne optical clocks are not enough. They also need more accurate ephemeris parameters, more accurate ionospheric and tropospheric time delay correction models, and more accurate spaceborne atomic clock calibration and synchronization technology."
At present, according to incomplete statistics, there are 8 units in my country that have developed 10 optical clocks of 7 different types. Among them, in the direction of single-ion trapped optical clocks, the Wuhan Institute of Physics and Mathematics of the Chinese Academy of Sciences is conducting research on calcium (Ca+) ion optical clocks and aluminum (Al+) ion optical clocks, and Huazhong University of Science and Technology is conducting research on aluminum (Al+) ion optical clocks; In the direction of cold atomic optical lattice clocks, the Chinese Academy of Metrology and the National Time Service Center of the Chinese Academy of Sciences are conducting research on strontium (Sr) atomic optical lattice clocks, and East China Normal University and the Wuhan Institute of Physics of the Chinese Academy of Sciences are developing Ytterbium (Yb) atomic light. Lattice clock research.
In 2012, the Wuhan Institute of Physics, Chinese Academy of Sciences completed the development of the first domestic calcium (Ca+) ion optical clock. The absolute frequency measurement data was received by CCTF. The current frequency uncertainty is 7×10-17. In 2015, the Chinese Academy of Metrology The first domestic strontium (Sr) atomic optical lattice clock was developed. The absolute frequency measurement uncertainty was received by CCTF, and the frequency uncertainty was 2.3×10-16.
"However, there is still a big gap between my country’s optical clock research and the world’s advanced level. Starting in 2005, optical clock research has successively received the Ministry of Science and Technology’s 973 Program, Science and Technology Support Program, Natural Science Foundation of China’s Major Research Program and 2016 The initial national key R&D project support." Fang Zhanjun said.
1 second change
Although in daily life, people don’t have a strong sense of one more or one second, one minute earlier or one minute late, but accurate time can not only achieve higher-accuracy universal time (UTC), but also because The optical clock is currently the most accurate physics experimental device. It can also be used to test the correctness of basic physical theories such as gravitational redshift and whether the basic physical constants change with time. In addition, in the field of geodesy in the theory of relativity, scientists measure the frequency difference between two high-precision optical clocks placed in different places and calculate the difference in gravitational potential and altitude between the two places, and realize the gravitational potential on the earth in the future. High-precision monitoring of changes.
However, in the research process of optical clocks, researchers not only need to solve the technical problems of femtosecond optical combs, but also need to solve key technologies such as atomic manipulation, precision laser frequency control, ultra-high vacuum, and precision constant temperature vibration isolation. Fang Zhanjun said: "These are all system devices, and they are relatively complex and technically difficult."
In fact, before optical clocks, humans have experienced several improvements in time cognition. In the 1820s, French scientists defined the length of a second as a mean solar second based on the Earth's rotation period. 1 second is 1/86400 of a mean solar day. This definition of 1 second has been used until 1960. "At that time, astronomical observations found that the earth's orbital motion around the sun was more stable than that of the earth. The average solar second was replaced by the ephemeris second derived from the earth's orbital period. One second is 1/31556925.9747 of a tropical year." Fang Zhanjun said.
In 1967, the definition of time was refreshed again by the atomic clock. The more precise atomic second is defined based on the microwave transition period between the electronic energy levels in the cesium atom, that is, 1 second is equal to the superfine transition between the electronic ground state of the cesium 133 atom. The duration of 9192631770 cycles.
"From the mean solar second to the astronomical second to the atomic second, the frequency of the periodic motion on which the second definition depends has been increased from 10-5 Hz and 10-8 Hz to 10-10 Hz, and the resolution of time measurement has been greatly improved. The recurrence accuracy of the long definition is also improved accordingly. The future optical clock uses the light frequency transition period between the electron energy levels in the atom to define the second. The frequency of its periodic motion is generally on the order of 1014~1015 Hz. Now the accuracy will be further improved." Fang Zhanjun said.