The atomic clock is a timing device with an accuracy of 1 second every 20 million years. It was originally created by physicists to explore the essence of the universe;They never thought that this technology could one day be applied to the global navigation system.
The atomic clock is composed ofIsidor Isaac Rabi And his students found out.
Chinese name
atomic clock
Purpose
Explore the essence of the universe
Application
Global navigation system
Precision
It can reach an error of 1 second every 20 million years
Time of occurrence
1950s
Principle
According to the basic principles of atomic physics
What people usually useclocks and watchesFor high accuracy, there will be an error of about one minute every year, which has no impact on daily life. However, more accurate timing tools are needed in highly demanding production and scientific research.At present, the most accurate timing tool in the world is the atomic clock, which appeared in the 1950s.Atomic clocks use electromagnetic waves emitted by atoms when they absorb or release energy to time.Because this electromagnetic wave is very stable, coupled with the use of a series of precision instruments for control, the atomic clock timing can be very accurate.The elements currently used in atomic clocks are hydrogencesium（Cesium）、rubidium(Rubidium), etc.The accuracy of atomic clocks can reach an error of 1 second every 20 million years.This provides a strong guarantee for astronomy, navigation and space navigation.
Fundamentals
according toAtomic physicsThe basic principle ofEnergy difference, that is, aroundNucleusDifferent aroundElectronic layerTo absorb or release electromagnetic energy.The electromagnetic energy here is discontinuous.When an atom moves from a“Energy state”When it transitions to a low "energy state", it will be releasedelectromagnetic wave。The characteristic frequency of this electromagnetic wave is discontinuous, which is called resonance frequency.Of the same atomresonance frequencyYes, for exampleCesium 133Its resonance frequency is 9 192 631 770Hz.thereforeCesium atomIt is used as a metronome to maintain highly accurate time.[1]
Discoverer
Rabi
In the 1930s,Isidor Isaac Rabi [6]And his studentsColumbia UniversityIn our lab to study atoms andNucleusBasic characteristics of.It is here that they rely on thisAtomic timerThis is a valuable first step in making clocks.During his research, Rabbi invented amagnetic resonanceWith this technology, he can measure the atomicNatural resonance frequency。For this reason, he also obtained 1944Nobel Prize。In the same year, he also first proposed "to discuss such an idea" (his students said so), that is, the accuracy of these resonance frequencies is so high that they can be used to make high-precision clocks.He also specifically proposed to use the so-called atomic“Hyperfine transition”Frequency of.This hyperfine transition refers to the transition between two states with subtle energy differences caused by different magnetic interactions between atomic nuclei and electronstransition。
In this kind of clock, a bunch ofAtomic beamThrough aOscillating electromagnetic field。When the hyperfine transition frequency of the atom is closer to the oscillation frequency of the magnetic field, the more energy the atom absorbs from the magnetic field, thus producing a transition from the original hyperfine state to another state.Through a feedback loop, one can adjust the frequency of the oscillating field until all atoms have completed the transition.The atomic clock is generated by using the frequency of the oscillating field, that is, the frequency that is exactly the same as the resonant frequency of the atomTime pulseOfMetronome。
Discovery history
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Until the 1920s, the most accurate clock still depended onpendulumIt swings regularly.The more accurate clocks that replace them are based onquartz crystalIt is made by regular vibration, and the error of this kind of clock is not more than one thousandth of a second every day.Even though it is so precise, it still cannot satisfy scientists' researchEinstein's theory of gravityNeeds.According to Einstein's theorygravitational fieldInside,Space and timeWill bend.Therefore, in theMount QomolangmaThe top clock is 30 million times faster than the same clock at sea level on average every dayOne second。So the only way to measure time accurately is to control the clock through the tiny vibration of the atom itself.
NIST F-1 Atomic ClockIt consists of 170components and partsComposition, includinglens，reflectorandLaser。The tube in the middle is 1.70 meters high, and cesium atoms move up and down in it, sending out extremely regular "signals".
In the 1930s,Columbia UniversityLaboratoryIsidor Isaac Rabi [6]And his students' achievements in studying the basic properties of atoms and their nuclei have made substantial progress in the development of clocks based on the atomic timers mentioned above.In the clock envisioned by Rabbi, a beam of atoms in a specific hyperfine state passes through a vibrationelectromagnetic field, fieldVibration frequencyThe closer to the hyperfine transition frequency of the atom, the more energy the atom will absorb from the electromagnetic field, and therefore experience the transition from the original hyperfine state to another state.The feedback loop can adjust the frequency of the vibration field until all atoms can transition.The atomic clock uses the frequency of the vibration field as the metronome to generate time pulse. At present, the frequency of the vibration field and the atomic resonance frequency have reached the level of complete synchronization.In 1949,Isidor Isaac Rabi [6]Of studentsRamsayIt is proposed that making atoms pass through the vibrating electromagnetic field twice can make the clock more accurate.In 1989, Ramsey won the Nobel Prize for this.
One of the earliest atomic clocks
After World War II,National Bureau of StandardsandNational Physical LaboratoryIt was announced thatAtomic resonance studyBased onatomic timeStandards.The first atomic clock in the world was set up by Essen andParryIt was built in cooperation, but this clock needs equipment in one room, so its practicability is not strong.Zakalias, another scientist, made the atomic clock a more practical instrument.Zakalayas plans to build what he callsatomic fountain This kind of atomic clock is very accurate, and it is enough to study the effect of gravity on time predicted by Einstein.During the development process, Zakalias introduced a small atomic clock that can be easily transferred from one laboratory to another.In 1954, he worked with MassachusettsmoleDeng worked together to build a commercial atomic clock based on his portable instrument.Two years later, the company produced the first atomic clock and sold 50 in four yearsCesium atomic clockAre descendants of this atomic clock.
In 1967, the research on atomic clocks was so fruitful that people redefined the second according to the vibration of cesium atoms.Today's atomic clocks are extremely accurate, with an error of no more than one second in 100000 years.After years of efforts, three atomic clocks -Cesium atomic clock, HydrogenMaserandRubidium atomic clock(Their basic principles are the same, but the difference lies in the use of elements and the observation means of energy changes), which have been successfully applied to space, satellite and ground control.So far, the most accurate atomic clock of these three categories isCesium atomic clock，GPSThe cesium atomic clock is finally adopted by the satellite system.
In February 2010, the aluminum ion optical clock developed by the National Bureau of Standards of the United States has reached an astonishing level of 3.7 billion years with an error of less than 1 second, becoming the most accurate atomic clock in the world.[2]
type
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Cesium atomic clock
Cesium atomic clock NIST 7
The cesium atomic clock uses the electrons inside the cesium atomenergy levelThe electromagnetic wave radiated from the inter jump is used as the standard to control the calibration of the electronic oscillator, and then control the movement of the clock.This kind of clock has a high degree of stability. At present, the best cesium atomic clock has reached 20 million years with a difference of 1 second.At present, the transition frequency of cesium atomic clocks is widely used as the time frequency standard internationally, and is widely used in astronomy, geodesy, national defense construction and other fields.[3]
Hydrogen atomic clock
Hydrogen atomic clock
Hydrogen atomic clockA precise timing device.hydrogenatomClock is a kind of precision clock widely used in many modern scientific laboratories and production departmentsAtomic energy levelThe electromagnetic wave radiated when jumping controls and calibrates the quartz clock, but it uses hydrogen atoms.The stability of this kind of clock is quite high, with only one billion changes every dayOne second。Hydrogen atomic clock is also commonly usedTime frequency standardIt is widely used in radio astronomy observation, high-precision time measurement, rocket and missile launch, nuclear submarine navigation and other aspects.The hydrogen atomic clock was first developed for American scientist Ramsey in 1960.Hydrogen atomic clock is a high-precision time and frequency standard, which has important applications in national defense, space technology and modern scientific experiments.
Rubidium atomic clock
The smallest rubidium atomic clock
It is the simplest and most compact of all atomic clocks.This clock uses rubidium gas in a glass chamber. When the surrounding microwave frequency is just right, it will be changed according to the optical rubidium frequencylight absorptionRate.Three kinds of atomic clocks cesium atomic clock, hydrogen maser and rubidium atomic clock have been successfully used in space, satellite and ground control.So far, the most accurate atomic clock among these three categories is the cesium atomic clock, and the cesium atomic clock is finally adopted by the GPS satellite system.
In addition, laser beams can also be used to prevent cesium atoms from moving forward and backward at a high speed, thus reducing the slight frequency change caused by Doppler effect.
CPT atomic clock
CPT atomic clockIt is a new type of atomic clock based on the coherent layout trapping principle of atoms. It is also the only atomic clock that can be miniaturized in principle at present. Its size and power consumption are much smaller than the rubidium atomic clock with the smallest size and power consumption.The smallest CPT atomic clock can be the size of a watch and powered by a button battery.Because of these characteristics, CPT atomic clock has good application prospects in military and civilian applications such as the timing of remote communication systems, synchronization of large-scale communication networks, and portability of weapons and equipment.For example, the CPT frequency standard is applied toGPS receiver, can significantly improve the accuracy of navigation and positioning.Western countries such as Europe and the United States have included the research and development of portable and miniaturized CPT frequency standards in their national strategic development goals.There are already two kinds of commodity CPT frequency standards on the market in the United States.[4]
Each atom has its own characteristic vibration frequency.The most familiar vibration frequency phenomenon is the orange light emitted by the element sodium in the salt when the salt is sprayed on the flame.An atom has multiple vibration frequencies, some of which are located in radio wavesband, some invisible lightBand, while others are in between.Cesium 133 is widely used as an atomic clock.Put cesium atomsResonantonWhen placed in an atomic clock, the transition frequency of one of them needs to be measured.Usually lockedcrystal oscillator To the main microwave resonance of cesium atoms.This signal is in the microwave of the radiospectrumIt happens to be similar to the transmission frequency of broadcast satellites, so engineers are very skilled at making instruments for this spectrum.
To make atomic clocks, cesium atoms are heated to vaporize and pass through a vacuum tube.In this process, cesium atomic gas first passes through a magnetic field used to select the appropriate energy state atoms, and then passes through a strong microwave field.The frequency of microwave energy oscillates in a very narrow frequency range, so that some frequency points can reach 9192631770Hz in each cycle.The frequency range of microwave generated by an accurate crystal oscillator is close to this precise frequency.When a cesium atom receives microwave energy of the correct frequency, the energy state will change accordingly.
Internal structure of cesium atomic clock
At the far end of the vacuum tube, another magnetic field separates cesium atoms that have changed their energy state because the microwave field is at the correct frequency.The detector at the end of the vacuum tube will display the cesium atoms hit on it in proportion, and show the peak at the microwave field at the correct frequency.This peak value is used to make minor corrections to the crystal oscillator, and make the microwave field at the correct frequency.This locked frequency is divided by 9192631770 to get one per second required by the common real worldpulse。[5]
working process
Cesium atomic clock is also known as“Fountain clock”Because the working process of cesium atomic clock is that cesium atoms rise and fall like fountains.This motion makes the calculation of frequency more accurate.The figure "Working Process of Cesium Atomic Clock" depicts the whole working process of cesium atomic clock in detail.This process can be divided into four stages:
Phase I
Working process of cesium atomic clock
The gas composed of cesium atoms is introduced into the vacuum chamber of the clock, and six mutually perpendicular infrared lasers (yellow lines) are used to illuminate the cesium atomic gas, making it close to each other and spherical, while the laser slows down the atomicMovement speedAnd cool it to nearAbsolute zero。
Phase II
Two vertical lasers gently lift the cesium atom balloon upward to form a "fountain" movement, and then turn off all lasers.This small thrust will make the cesium atom balloon lift up about 1m high and pass through a microwave cavity full of microwave. At this time, the cesium atom absorbs enough energy from the microwave.
Phase III
stayGravityUnder the action of, the cesium atom balloon starts to fall, passes through the microwave cavity again, and releases all the absorbed energy.When the cesium atoms whose state changes in the microwave cavity interact with the laser beam again, they will emit light energy.
Phase IV
At the exit of the microwave cavity, another laser beam shoots at the cesium atomic gas, and the detector will measure the intensity of the emitted fluorescence.
The above process will be repeated many times, and the frequency in the microwave cavity is different each time.From this, we can get a microwave with a certain frequency, which can change the energy state of most cesium atoms accordingly.This frequency is the natural resonance frequency of cesium atom, or the frequency of determining the length of seconds.[1]
Latest achievements
Announce
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A research result published by Science magazine on July 12, 2001 shows that American government scientists have integrated advanced laser technology with a singleAmalgam atomThe most accurate clock in the world has been developed.Scientists from the National Institute of Standards and Technology in Boulder, Colorado, developed this new atomic clock based on high-frequency invisible light waves and non microwave radiation.Because the development of this clock mainly depends onLaser technology, so it is named“All optical atomic clock”。
We knowAtomic clock"Tick" comes from the transformation of atoms. In current atomic clocks, cesium atoms are transformed in the microwave frequency range, while optical transformation occurs in a much higher frequency range than microwave transformation, so it can provide a more refinedtime scale , you can time more accurately.The newly developed all optical atomic clock emits a "ticking" sound of 1000 to the fifth power (the number obtained by adding 15 zeros after 1) when the pointer moves within 1 second, which is the most advanced clock now microwaveCesium atomic clock100000 times.Therefore, using it to measure time will be much more accurate.
The structure of all clocks includes two parts: devices that can move according to a fixed cycle, such as a pendulum;There are also some devices to calculate, accumulate and display the time loss, such as the gear driving the clock pointer.The atomic clock first developed about 50 years ago added a third part, that is, atoms that react to light and electromagnetic radiation at a specific frequency. These atoms are used to control the "pendulum".At present, the most advanced atomic clock uses 1 million atomic clocksliquid metalCesium atomic pairmicrowave radiationRespond to control the movement of the clock hands.Such a clock pointer moves about 10 billion times per second. The faster the clock pointer moves, the more accurate the clock calculates the time.howeverCesium atomic clockThe high-speed electronics technology used cannot calculate more times of clock pointer movement.Therefore, American scientists use a single cooled liquid mercury ion (that is, a mercury atom that has lost an electron) instead of cesium when they study a new type of all optical atomic clock, and compare it with a mercury atom that has the function equivalent to a pendulumfemtosecond (10 billion partsOne second）The laser oscillator is connected, and the clock is equipped with an optical fiber, which can decompose the optical frequency into microwave frequency pulses that can be recorded by the counter.
To make such an atomic clock, it is necessary to have a technology that can capture the corresponding ions and keep the captured ions still enough to ensure accurate data reading, and at the same time, it is necessary to ensure that the number of "ticks" can be accurately calculated at such a high frequency.The quality of this clock depends on its stability and accuracy, that is, the clock should provide a constant output frequency and make its measurement frequency consistent with the resonant frequency of the atom.
S. A. Diddams, the American physicist who led the research, said: "We have demonstrated the principle of this new generation of atomic clock for the first time. This clock may be more powerful than the current microwaveCesium atomic clock100 to 1000 times more accurate. "It can calculate the shortest time interval ever.Scientists predict that this clock can improve the application level of aviation technology and communication technology, such as mobile phone and optical fiber communication technology, and can also be used to adjust the precise orbit of satellites, aerospace in outer space, and link spacecraft.
A Strontium Lattice Atomic Clock with 5 Billion Years of No Deviation
Developed by the joint laboratory of the Institute of Astrophysics of the University of Colorado in the United States, the strontium lattice atomic clock maintains the previous record of the quantum logic clock developed for the National Institute of Standards and Technology in the United States, but the strontium lattice atomic clock is more accurate than the latter, and its accuracy can be increased by 50%.
Dr. Jun Ye of the Institute of Astrophysics believes that the strontium lattice atomic clock is controlled by the atomic oscillation between two energy levels, and the conversion between energy levels is precisely controlled by the laser device, which realizes the manufacture of the most accurate clock.In fact, the goal of scientists is to create a clock that will never deviate. The goal is not 5 billion years, but the age of the entire universe. That is to say, the clock will not slow down or go fast in the possible life cycle of the universe.
On October 18, 2022, the Russian Free Media Network reported that Chinese scientists had created the most accurate atomic clock on the earth.They can become the standard for calibrating all technical phenomena, and can more accurately "adjust" China's Beidou satellite navigation system.[7]
