Source: Science and Technology Daily
In classical mechanics, the state of an object can be measured precisely, and the interference of observation and measurement on the object of observation is negligible, but in the microscopic world, the interference cannot be ignored in any way. When you make measurements on the quanta, you will find that the results of the measurements are completely random, and the results obtained are always different.
Quantum Technology Series (2)
◎ Reporter Wu Changfeng
In quantum physics, something is strictly unknowable. For example, you can never know the position and momentum of an electron at the same time, and you don't know which side will face up until the coin falls. Before the measurement, the position of the electron, the momentum, etc., is the superposition of various possible states; before the coin lands at rest, its state is the superposition of two states of "face up" and "back up", and only when measured, it will choose a definite state to present.
The instantaneous random mutation in the process of measurement is a great magic in quantum mechanics, which also means that measurement is much more important in quantum mechanics than in classical mechanics.
The world's most sophisticated measuring instrument is the Laser Interferometer Gravitational Wave Observatory (LIGO), which humans used to observe gravitational wave events for the first time, representing the highest level of measurements currently available to humans. In order to further improve the accuracy of measurements, scientists have invariably focused on quantum precision measurement technology based on quantum mechanics. What kind of technology is this?
Classical measurement - you measure or not, I do not increase or decrease
After the emergence of the new crown epidemic, a human body indicator has received unprecedented attention, that is, body temperature, and the measurement of human body temperature is a physical quantity measurement.
There is no science without measurement. Modern science is established and developed in the cycle of "hypothesis-test-model-theory". Increasing measurement accuracy by an order of magnitude often leads to new physical discoveries. The definition of the unit of physical quantity, the accuracy of the measured value, the size of the physical constant, and whether the constraint relationship is valid have become the key to testing the laws of physics.
In classical mechanics, the state of an object can be measured precisely, and the interference of observation and measurement on the object of observation is negligible, but in the microscopic world, the interference cannot be ignored in any way.
In fact, the measurement of any physical quantity is accompanied by noise, which interferes with our precise control of the system. It is generally believed that classical noise mainly comes from technical defects, unsatisfactory instruments and other factors, with the development of science and technology, the classical noise of the system is greatly reduced, often negligible.
According to the mathematical central limit theorem, repeating an independent measurement N times (N is much greater than 1) satisfies the normal distribution, and the error of its measurement can reach 1/formula of a single measurement. As a result, the measurement accuracy is increased to the formula multiple of a single measurement. This is the measurement limit under the framework of classical mechanics – the shot noise limit.
The minimum noise that can be achieved by classical measurements is the shot noise, which corresponds to the standard quantum limit of the measurement. In 1927, Heisenberg proposed the famous uncertainty principle in quantum mechanics, arguing that the position of a particle and momentum cannot be determined at the same time, and the more accurately the position is determined, the less accurate the determination of momentum, and vice versa.
Heisenberg's uncertainty principle appears to be a veil obscuring these observable true values. In fact, this is the precision allowed to indicate that these variables can only be defined to the Heisenberg limit. The difference between quantum noise and classical noise is that such as thermal noise, shot noise, etc. are temperature-dependent — the lower the temperature, the lower the noise. When the temperature reaches absolute zero, the classical noise will disappear completely. But you can't eliminate quantum noise — because according to quantum mechanics, space is always filled with fluctuating energy, and quantum noise is active throughout the universe.
Quantum measurements - neither 1 nor 2, both 1 and 2
Quantum theory has had great success in revealing and applying the laws of the microscopic world, which is also known as the first quantum revolution, and many of the major inventions derived from it are mainly based on the application level of macroscopic embodiment of quantum laws.
As scientists conduct in-depth research on properties such as quantum superposition and quantum entanglement, humans have been able to directly actively prepare, precisely manipulate and measure the state of individual quantum objects (photons, atoms, molecules, electrons, etc.), thus being able to use quantum laws to understand and transform the world in a new "bottom-up" way. The rapid development of quantum regulation and quantum information technology marks the rise of the second quantum revolution.
If we want to know and understand the quantum, we must know the physical state of the quantum, such as how it moves, how much energy it has, and so on. If you make measurements on quanta, you will find that the results of the measurements are completely random. This is because quanta have many wonderful phenomena and properties that differ from the macrophysical world, such as quantum superposition.
"In the macroscopic world we live in, quantum superposition cannot exist or be sustained. In the macro classic world
in, 1 is 1 and 2 is 2. In the microscopic quantum world, a state can exist between 1 and 2, which is neither 1 nor 2, but it is both 1 and 2. Zhang Wenzhuo, an associate researcher at the Shanghai Research Institute of the University of Science and Technology of China, said.
"It's like Sun Wukong's fen-body technique." A Monkey King can appear in multiple places at the same time, and the various incarnations of Monkey King are like its superposition. Pan Jianwei, an academician of the Chinese Academy of Sciences and a professor at the University of Science and Technology of China, explained, "In daily life, it is impossible for a person to appear in two places at the same time. But in the quantum world, as a microscopic object, it can appear in many places at the same time. ”
The macroscopic classical world follows the laws of classical mechanics, while in the quantum world, it follows the laws of quantum mechanics. In quantum mechanics, photons (a type of quantum) can vibrate in a certain direction, called polarization. Because of quantum superposition, a photon can be in the superposition of two quantum states of horizontal and vertical polarization at the same time. Scientific experiments have proved that because of the existence of quantum superposition effects, once measured, the state of the quantum will be destroyed or changed. Therefore, if you take an instrument to measure the quantum, you will find that the result of the measurement is completely random, and for the same state, no matter how carefully observed, the result will always be different.
Three "rulers" - quantum characteristics make the measurement accuracy continue to improve
Due to the limitations of the uncertainty principle of quantum mechanics, the measurement accuracy cannot be improved indefinitely, and this ultimate limit is called the Heisenberg limit.
However, there are two ways to improve measurement accuracy: the first is to prepare and utilize a "ruler" with higher resolution; the second way is to reduce measurement errors and improve measurement accuracy by repeating measurements multiple times. In recent years, it has been found that the use of the basic properties of quantum mechanics, such as quantum coherence, quantum entanglement, quantum statistics and other characteristics, can achieve high-precision measurements that break through the limit limit of classical bulk noise, which is equivalent to finding a highly sensitive quantum "ruler".
According to the application of quantum properties, quantum measurement also has three "rulers", the first "ruler" is based on microscopic particle energy levels measurement; the second "ruler" is based on quantum coherence measurement; the third "ruler" is based on quantum entanglement measurement.
The first "ruler" has gradually been used in atomic clocks and other fields since the 1950s. According to Bohr's theory of the atom, atoms emit electromagnetic waves when they jump from an "energy state" to a low "energy state." The characteristic frequency of this electromagnetic wave is discontinuous, which is what people call the resonance frequency.
In 1967, the International Conference of Weights and Measures redefined the second based on the vibration of the cesium atom, that is, the transition between the two ultrafine energy steps of the ground state of the cesium 133 atom corresponds to the duration of 9192631770 cycles of radiation. This was the first major contribution of quantum theory to the measurement problem.
The second "ruler" of quantum measurement is a measurement technique based on quantum coherence, using the quantum baud nature of matter to measure external physical quantities by interferometry. Now it has been widely used in gyroscopes, gravimeters, gravity gradient meters and other fields. For example, cold atom interference quantum gyroscopes can be applied to highly sensitive navigation systems due to their excellent characteristics of ultra-high precision and ultra-high resolution.
The last "ruler" of quantum measurement - measurement technology based on quantum entanglement. Theoretically, if the quantum state of the N quantum "ruler" is in an entangled state, the effect of the external environment on these N quantum "rulers" will be coherent and superimposed, so that the final measurement accuracy reaches 1/N of a single quantum "ruler". This accuracy breaks through the bulk noise limit of classical mechanics and is the highest precision that can be achieved in the scope of quantum mechanical theory - the Heisenberg limit.
In 2018, the research team led by Academician Guo Guangcan of the University of Science and Technology of China approached the optimal Heisenberg limit for the first time internationally. In January 2021, the research team led by Academician Guo Guangcan simultaneously achieved the measurement of three parameters to reach the extreme accuracy of Heisenberg. At present, scientists have achieved experimental demonstrations of physical quantity measurements such as phase measurements in physical systems such as photons, ion traps and superconductivity, breaking through the classical measurement limit and approaching or reaching the Heisenberg limit.