You might not know that the most basic units in our daily life, such as "1 second," "1 meter," and "1 kilogram," actually have a rigorous and complete set of international standard definitions behind them. They form the foundation for the entire operation of science, industry, and national standards.

On November 16, 2018, at the 26th General Conference on Weights and Measures held in Versailles, France, scientists from around the world made an important decision: to redefine the four base units "kilogram," "ampere," "kelvin," and "mole" from their classical definitions based on physical artifacts to new definitions based on natural constants. Together with the previous definitions of "second," "meter," and "candela," all seven base units in the International System of Units (SI) were quantum-defined and no longer linked to physical objects. This reform officially took effect on May 20, 2019, marking the beginning of a new era in measurement: the quantum metrology era.

Quantum definitions of the International System of Units based on physical constants

This decision may seem a bit "technical geeky," but its significance is profound. This change means that humanity will fully define the world's basic units using "quantum physics" and "natural laws."

Why switch to natural constants?

Before this reform of the unit system, the international definitions of some basic units mainly relied on physical artifacts. For example, the unit of mass "kilogram" was defined in 1879 by scientists who made a cylinder from a platinum-iridium alloy and set its mass as "1 kilogram." This is the famous International Prototype Kilogram. It is kept at the International Bureau of Weights and Measures in France, and the mass measurements and derived quantities related to mass (such as acceleration) worldwide are based on it.

International Prototype Kilogram

However, this classical measurement method relying on physical references has significant problems.

First, the International Prototype Kilogram, as a physical standard, is unique. Any country wanting to calibrate its measurement equipment must refer to it and its replicas. If this prototype is contaminated, damaged, or lost, it would affect the global mass standard from the source.

Second, provincial and municipal metrology institutions in various countries need to replicate secondary standards and regularly send them for inspection. This hierarchical system of national standards - local standards - enterprise standards extends the chain of value transmission and calibration. The more levels in the value transmission, the more complex and large the verification system becomes, consuming more time, manpower, and resources, and accumulating greater precision loss.

Finally, the physical standard itself is not stable enough. Researchers found that although this alloy cylinder was carefully preserved, its mass changed slightly over decades, being on average about 50 micrograms lighter than its replicas. Although this number is very small, it is an error that cannot be ignored in high-precision scientific experiments and industrial manufacturing.

Not only the unit of mass, but other units such as the ampere for electric current and kelvin for temperature also have similar problems. Defining units based on physical objects or experimental devices is easily affected by environment, materials, time, and other factors, making it impossible to ensure global uniformity and long-term stability.

Therefore, scientists decided to redefine the basic units using physical constants in nature that are constant and unchanging (such as the speed of light in vacuum c, Planck constant h, elementary charge e, etc.) and the intrinsic quantum properties of microscopic particles (i.e., inherent characteristics that do not depend on external conditions or measurement methods, such as spin, angular momentum, and charge of microscopic particles). These physical constants do not change anywhere or anytime, and the intrinsic properties of microscopic particles are discrete, quantized, stable, reliable, and precisely reproducible. Thus, physical units can be bound to universal natural laws and natural constants.

Using them to define physical units ensures the long-term stability, objective universality, and future applicability of the measurement system. Moreover, replacing physical standards with natural benchmarks means that any laboratory worldwide, as long as it has suitable equipment, can independently reproduce the same standards at any time and place through universal definitions, without relying on any specific physical object or device. This shortens, speeds up, improves accuracy, and stabilizes the value transmission chain without the traditional "pyramid-style" hierarchical traceability or transmission. This is called "flattening of value transmission."

Pyramid-style value traceability system and flattened value traceability system

In short, the core idea of the "quantization" of the International System of Units is to define how we measure the world using eternal and unchanging physical laws of the universe, rather than man-made reference objects.

The "quantization" of the International System of Units

What is quantum precision measurement?

In the early 20th century, humans established the theory of quantum mechanics and began to understand how microscopic particles (such as electrons, atoms, photons) move and interact. This led to the first quantum revolution, giving rise to new technologies such as atomic energy, lasers, semiconductors, and nuclear magnetic resonance, fundamentally changing human society's way of life.

If a physical quantity has a smallest, indivisible basic unit, then this physical quantity is "quantized," and this basic unit is called a "quantum." In classical physics, physical quantities are considered infinitely divisible and continuously variable. For example, the energy of a macroscopic object can take any real number, including 1 joule, 1.5 joules, 1/3 joule, or even π joules.

But in quantum physics, quantized physical quantities can only exist in integer multiples of the basic unit. For example, the energy of electromagnetic waves can only be integer multiples of hν, and cannot be 0.5hν, 1.5hν, or other non-integer multiples. hν as the basic unit is called a "quantum of energy," where h is Planck's constant and ν is the frequency of the electromagnetic wave.

Continuous physical quantities and discrete physical quantities

Similarly, physical quantities of microscopic particles such as electric charge, magnetic flux, and angular momentum are quantized, corresponding to basic units like elementary charge, magnetic flux quantum, and reduced Planck constant. These quantized properties mean that electrons moving outside the atomic nucleus can only appear in certain specific, discrete orbits, and electrons in each orbit have specific energies called energy levels. This discontinuous "discreteness" is a fundamental law of the quantum world.

By the 1990s, scientists were no longer satisfied with passively "observing quantum phenomena"; they began actively manipulating microscopic particles, such as controlling atomic energy level changes and adjusting electron spin directions. This ability marked the arrival of the second quantum revolution and promoted the rise of quantum information technology. This revolution mainly includes three directions:

1. Quantum computing (using qubits for high-speed parallel computation);

2. Quantum communication (achieving long-distance, highly secure information transmission);

3. Quantum precision measurement (using the sensitivity of quantum states to the external environment to perform ultra-high precision measurements of physical quantities such as voltage, current, and magnetic fields).

"Quantum precision measurement" refers to using the quantum properties of these microscopic particles to perform high-precision measurements of various physical quantities. This method differs from classical measurement methods, which rely on macroscopic phenomena such as mechanical devices, circuits, or chemical reactions, whereas quantum measurement utilizes changes in quantum states, such as electron energy levels, atomic vibration frequencies, and particle spin directions.

Conceptual diagram of quantum precision measurement

A typical quantum measurement process can roughly be divided into several steps:

1. Initialize the quantum system: prepare the microscopic particles in a known initial state;

2. Interaction with the physical quantity to be measured: such as magnetic field, electric field, temperature, stress, current, etc.;

3. Quantum state changes: external influences cause identifiable changes in the particle state;

4. Read out the changes: use methods such as lasers or microwaves to "read" these changes;

5. Derive the physical quantity: through calculation, infer the measured magnetic field, current, or other physical quantities.

This method has many advantages: first, high precision, much more accurate than traditional methods, some even capable of detecting changes as small as one part in a billion; second, high sensitivity, able to detect very weak signals; third, strong stability, based on physical constants and microscopic particles, ensuring long-term measurement stability.

Currently, scientists have developed many quantum measurement technologies, such as ultra-cold atomic clocks, atomic interferometric absolute gravimeters, diamond NV center magnetic sensors, etc. These devices can be used to detect physical quantities like time, gravity, magnetic fields, and current.

The emergence and breakthroughs of quantum precision measurement technology have promoted the transition of the International System of Units to quantum standards. The innovation in metrology systems has in turn led to a global surge in the research and application of quantum precision measurement technology.

High-end technology has actually long entered daily life.

Quantum precision measurement sounds like high-end technology in laboratories, but in fact, it has been applied in many fields, some even closely related to our daily lives.

1. Atomic clock — setting the standard for the "second"

The "second" is the basic unit of time, and the current international definition of the "second" does not rely on mechanical clocks or Earth's rotation, but uses the electromagnetic radiation corresponding to the internal energy level transitions of the cesium-133 atom. The oscillation period of this radiation is very stable; the duration of 9,192,631,770 cycles is defined as "1 second." The cesium atomic clock has an error of no more than 1 second in 100 million years and is the core tool for time standards in modern metrology.

The world's first cesium atomic clock

Atomic clocks are one of the core devices in modern navigation systems (such as GPS and BeiDou). Navigation satellites must know very precisely when they send signals so that ground receivers can accurately calculate position errors. It can be said that without atomic clocks, there would be no precise navigation.

2. Atomic interferometer — able to "see" changes in Earth's gravity

The atomic interferometer is a highly sensitive gravimeter that can be used to detect crustal movements, groundwater changes, and even capture precursors to earthquakes. Its working principle is to measure changes in interference patterns of cold atoms during free fall to calculate tiny differences in gravitational acceleration.

3. Diamond NV center — measuring magnetic fields using "defects" in diamonds

There are also some quantum sensors that utilize a tiny defect inside diamonds called the NV center. This structure, composed of a nitrogen atom and a vacancy, has excellent quantum properties. Scientists can manipulate the NV center with lasers and microwaves to respond to physical quantities such as magnetic fields, current, and temperature for measurement.

This quantum sensor can be made very compact, even chip-sized or implantable into cells, for high-precision magnetic field imaging, industrial current monitoring, or biomedical detection.

4. A new revolution in the power industry: quantum current transformer

In China, scientists have developed the world's first current transformer based on quantum measurement technology. It uses NV center sensors to monitor high-voltage currents with high measurement accuracy, strong anti-interference ability, and remote calibration capability. This device has been successfully put into operation in substations of the State Grid.

Through this introduction, you should have a preliminary understanding of the revolutionary potential of quantum precision measurement. So, what kind of "toolkit" does such powerful measurement capability require? In the next article, we will focus on the diamond NV center to see how it stands out among many quantum systems and becomes the "handy tool" for scientists conducting quantum precision measurements!