Recently, the official WeChat public account of Tianjin Hi-Tech Industrial Development Zone announced that the first domestic chip atomic clock production line built by Tianjin Huaxintai Technology Co., Ltd. was completed and put into operation in Binhai Hi-Tech Industrial Development Zone. This production line has an annual production capacity of 30,000 units. Its completion and operation indicate that China has broken the foreign monopoly in the field of chip atomic clocks, overcome the "bottleneck" problem of key components, and met the urgent needs of domestic technical products in related fields. Chip atomic clocks are core basic devices in the field of time and frequency technology within electronic information technology. They are compact, low-power atomic clocks manufactured using micro-electro-mechanical systems (MEMS) technology. With the characteristics of high timing accuracy, low power consumption, and small size, they are suitable for application fields such as satellite navigation and timing, communication synchronization, and underwater detection, and have a wide range of application prospects. Atomic clocks, which sound esoteric, have actually integrated into our lives. For example, the well-known Beijing Time is the result of the weighted average of more than 150 atomic clocks around the world that jointly keep time. The measurement of various physical constants, as well as power systems and communication systems, are all inseparable from high-precision atomic clocks. What kind of clock is an atomic clock exactly? Why can it provide such precise time? Next, let's learn about the principles and applications of atomic clocks! 01. The Great Role of Atomic Clocks With the progress of science, people found that the accuracy of quartz clocks (the most accurate timers before the invention of atomic clocks) was insufficient to support scientific research. The pillars of modern physics are relativity and quantum mechanics, and on a macroscopic scale, their conclusions are almost the same as those of Newtonian mechanics. That is to say, we cannot perceive the difference between Newtonian mechanics, relativity, and quantum mechanics in the macroscopic world. However, the difference between relativity and Newtonian mechanics appears at the 15th decimal place, which is impossible to measure with a quartz clock. According to Einstein's general relativity, gravity can distort space and time and cause time dilation. According to this theory, time at the top of Mount Everest is 1 second faster than time at sea level on average over 80,000 years. This is far beyond the measurement accuracy of quartz clocks. As we all know, China has independently developed the "Beidou" satellite navigation system, which provides global users with all-weather, all-time, high-precision positioning, navigation, and timing services. The positioning of satellite navigation relies on electromagnetic waves to transmit signals. The speed of electromagnetic waves propagating in a vacuum is about 3×10⁸ meters per second. Multiplying this speed by the signal propagation time gives the distance the signal travels. Since this speed is extremely fast, a 1-second error in the timing clock will lead to a 300,000-kilometer deviation in ground positioning! Therefore, for global satellite positioning systems, to provide sufficiently accurate navigation services, there must be sufficiently accurate clocks. The atomic clock we are going to introduce can achieve this high-precision timing, so it has become a commanding height technology in global satellite positioning. 02. The Definition of "Second" To understand the timing principle of atomic clocks, we must first understand the definition of the time unit "second". The current International System of Units uses the definition of "second" adopted at the 13th General Conference on Weights and Measures held in 1967, which is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom. We know that the basic unit of matter is the atom, which is composed of a nucleus and electrons. According to the Bohr model, electrons in an atom move around the nucleus and can only move in specific, discrete orbits. Electrons in each orbit have discrete energy, which is called "energy level". Electrons can transition between different orbits. When an electron absorbs electromagnetic waves of a specific frequency, it can transition from a lower energy level to a higher one; when an electron transitions from a higher energy level to a lower one, it can also radiate electromagnetic waves of a specific frequency. The frequency of the electromagnetic waves absorbed or radiated by electrons during the transition corresponds to the energy difference between the transition energy levels one by one. Therefore, the frequency and period of the electromagnetic waves radiated during the transition between the two hyperfine levels of the ground state of the cesium-133 atom are also fixed. Physicists stipulate that the time equal to 9,192,631,770 times this fixed period is a "second". 03. The Principle of Atomic Clocks After knowing the definition of "second", the principle of atomic clocks is easier to understand. Everyone has used a radio. The working principle of an atomic clock is actually similar to the process of using a radio. Suppose we want to listen to a radio station with a frequency of 100MHz. We first need to tune the frequency to around 100MHz. When the frequency is close to 100MHz, some vague sounds will come from the radio. If the previous frequency is less than 100MHz, we will find that the received sound becomes clearer and clearer as we increase the frequency; if the previous frequency is higher than 100MHz, we will find that the received sound becomes clearer and clearer as we decrease the frequency. In the process of increasing or decreasing the frequency, the modulated frequency gets closer and closer to 100MHz, and when the received sound is the clearest, that frequency is exactly the 100MHz we want. In an atomic clock, there is a similar frequency modulation process. First, we use the first state-selecting magnet to screen out the cesium atoms in the lower energy level from a large number of cesium atoms in two hyperfine energy levels. Then, under the action of a collimator, these atoms form an atomic beam and pass through the microwave cavity without colliding with each other. The microwave cavity radiates electromagnetic waves with a frequency close to 9,192,631,770Hz. Under the action of the electromagnetic waves, some of the atoms passing through the microwave cavity will transition from the lower energy level to the higher one. After the atoms pass through the microwave cavity, the second state-selecting magnet will separate the atoms in the higher energy level from those in the lower one, and the detector will detect the number of atoms in the higher energy level. The closer the frequency of the electromagnetic waves in the microwave cavity is to 9,192,631,770Hz, the more atoms transition to the higher energy level, and the more atoms in the higher energy level detected by the detector. When the number of atoms in the higher energy level detected reaches the maximum, the frequency of the electromagnetic waves in the microwave cavity is exactly 9,192,631,770Hz, and we can use the time-frequency signal that controls the microwave cavity as the standard for our timing. An atomic clock is like a radio in the hands of physicists. If you can understand the principle of a radio, you can definitely understand the principle of an atomic clock. 04. Why It Is the Most Precise Timing Device Why can an atomic clock achieve an error of only 1 second in about 5 billion years? This has to start from the principle of timing. Timing is to measure the number of occurrences of a certain periodic motion, and the unit of time is a specification for the number of occurrences of a specific periodic motion. The ancients stuck a stick in the ground, observed the periodic change of the position of the stick's shadow from sunrise to sunset, and then marked the time of the day according to the different positions of the shadow during the day. This kind of clock is called a "sundial". In the development of timing technology, people also used flowing water and quicksand to measure the passing time, creating water clocks and hourglasses. The motion cycles of sundials, water clocks, and hourglasses are all unstable and are greatly affected by factors such as seasons, temperature, and friction. This leads to uncertainty in the period and frequency of periodic motion, so timing is naturally inaccurate. Later, people invented more stable mechanical clocks, which are less affected by factors such as friction, and because their motion frequency is high, this error is greatly reduced. A mechanical clock that moves 10,000 times in 1 hour counts the time of 10,000 periodic motions of the mechanical clock as 1 hour. That is to say, if one count is missed during counting, the timing will only be slow by one ten-thousandth of an hour. Now, a good mechanical clock has an error of only about ten seconds in a day. Later, people made more accurate quartz clocks. Quartz clocks rely on the periodic oscillation of quartz crystals under the action of electric current, which can occur 32,768 times in 1 second. Compared with mechanical clocks, the period of quartz clocks is more stable, and the timing is more accurate. An ordinary quartz clock will not have an error of more than 2 seconds in a day. After quartz clocks, atomic clocks were invented. Atomic clocks rely on the periodic motion of electromagnetic waves emitted by atoms during transitions between energy levels. The frequency of this periodic motion is very stable and will not change due to external influences, making it the most suitable periodic motion for timing. In 1 second, a cesium atomic clock can move 9.1 billion times, a strontium atomic clock can move 430 trillion times, and a ytterbium atomic clock can move 518 trillion times. Therefore, the accuracy of the space cold atomic clock group in the "Mengtian" experimental module can reach an error of about 1 second in 5 billion years! To sum up, the reasons why atomic clocks are so accurate can be roughly divided into two points: first, the stability of periodic motion. The periodic motions other than those in atomic clocks are easily affected by external factors; second, the high frequency of periodic motion. The periodic motions that can occur in an atomic clock within 1 second are hundreds of millions of times, far exceeding those of other timers. 05. How to Discover "New Physics" with Atomic Clocks According to established physical laws, clocks should tick at a constant rate, but physical phenomena beyond the scope of the standard model will cause tiny charges to appear in atomic energy levels. This should affect the speed at which clocks run, but this change is so small that it can only be detected with extremely accurate clocks—and this is where atomic clocks come into play. "Atomic clocks bring cosmology and astrophysics to Earth, enabling people to search for ultralight particles that can explain dark matter in laboratories," Calmet said. Atomic clocks use atoms with two potential energy states to measure time. When atoms absorb energy, they enter a higher energy state. Then, they eventually release this energy and fall back to a lower ground state. In an atomic clock, a group of atoms is prepared by using microwave energy to put them in a higher energy state, and their characteristic and consistent rate of oscillation between states (their resonant frequency) is used to measure time precisely. For example, all cesium atoms resonate at the same frequency, which means that the standard measurement of one second can be defined as 9,192,631,770 cycles of cesium. Since the variation in this cycle per second is much smaller than the swing of a pendulum, this makes atomic clocks extremely accurate. Calmet explained: "It has recently been realized that dark matter may consist of ultralight particles that interact extremely weakly with ordinary matter. If so, dark matter would essentially behave as a classical wave that interacts with electrons and protons. This dark matter wave would give these particles some small kicks." Calmet added that these ultralight dark matter particles hitting the components of atoms would cause time variations in the fundamental constants of the universe, such as the fine-structure constant or "alpha"—a measure of the strength with which particles couple via the electromagnetic force—and the mass of the proton. "Because atomic clocks are extremely precise devices, they will be able to detect these kicks, thereby discovering ultralight dark matter," he continued. "By comparing two clocks, one sensitive to changes in alpha and one less so, we can obtain limits on the time variation of this fundamental constant, thereby placing limits on ultralight particles." Calmet believes that this technology can also be used to study another problematic aspect of the universe for physicists: dark energy, an unknown force that drives the accelerated expansion of space. Although Calmet admits that dark energy is more likely to be explained by the cosmological constant, a form of energy almost opposite to gravity that can stretch the structure of space and push galaxies apart, there is a small possibility that it is related to ultralight particles. In this case, future clocks may also be sensitive to this particle and its associated waves. "While clocks have not yet discovered new physics at this stage, we have been able to develop a new theoretical framework to probe generic new physics with clocks and have been able to derive the first model-independent limits on physics beyond the standard model within this approach," Calmet concluded. "We are creating a new field at the interface of atomic, molecular, and optical physics with traditional particle physics." These results are exciting! Sources: Sensor Expert Network, Xingmi Space, Popular Science Jiangsu