In the previous two articles, we learned about the basic principles of quantum precision measurement and became familiar with a key material—the NV center, which can sense magnetic fields and temperature, and can read quantum state information with extremely high precision.

Recently, the Quantum Information Key Laboratory of the University of Science and Technology of China, in collaboration with multiple enterprises, formed an "industry-academia-research-application" joint research team. Using quantum precision measurement technology, they developed the world's first quantum current transformer for high-voltage alternating current monitoring. It has been stably operating in a 110kV substation for over a year. The core component of this device is the NV center we mentioned earlier. It turns a "diamond defect" into a quantum sensor capable of measuring kiloampere-level large currents.
The "Neuron" in the Power Grid: Current Transformer

In modern power systems, current transformers are very fundamental and important devices. Their function is to convert the large current in high-voltage lines into a small current that instruments can read, helping us monitor, protect, and control the operation of the entire power grid. It's like the neurons in our body, which can sense various external or internal information at any time and unify it to the brain for further processing.

Traditional current transformers mainly rely on the magnetic effect of current and the principle of electromagnetic induction.

The magnetic effect of current can be simply summarized as "electricity generates magnetism," meaning that when current passes through a conductor, a circular magnetic field is generated around the conductor. The direction follows the right-hand screw rule: if you make a fist with your right hand, the thumb points in the direction of the current, and the other four fingers point in the direction of the magnetic field. A constant current, such as direct current, produces a constant magnetic field around it; a changing current, such as alternating current, produces a varying magnetic field.

Magnetic effect of current and the right-hand screw rule

The principle of electromagnetic induction can be summarized as "magnetism generates electricity," meaning that when the magnetic flux passing through a closed circuit changes, an induced electromotive force is generated in the circuit, which in turn produces an induced current. Traditional current transformers usually have two sets of coils and an iron core inside. The two mechanisms of "electricity generating magnetism" and "magnetism generating electricity" work together to form the core working principle.

Structure of traditional alternating current transformer

For traditional alternating current transformers, when a large current on the primary side passes through the primary winding (a coil group made by winding wire) of the current transformer, an alternating magnetic field (electricity generating magnetism) is generated in the iron core. This alternating magnetic field is conducted through the iron core to the secondary winding, inducing a current (magnetism generating electricity) in the secondary winding. The induced current on the secondary side is proportional to the current on the primary side, with the proportionality coefficient related to the number of turns of the two coil groups. In this way, the large current that is difficult to measure directly can be converted into a small current measurable by general instruments, greatly reducing the cost and difficulty of current measurement. Although this technology has been used for over a hundred years, it is gradually showing obvious shortcomings in some key aspects:

1. Prone to magnetic saturation: When the current is too large, the magnetic flux conducted by the iron core will "saturate," causing measurement distortion;

2. Residual magnetism problem: After the current disappears, some magnetism may remain in the iron core, affecting the next measurement;

3. Poor versatility: This device is suitable for measuring power frequency alternating current around 50Hz, but is somewhat powerless for direct current and low-frequency alternating current;

4. Poor anti-interference ability: In outdoor or high-voltage environments, measurement results are easily affected by temperature changes, electromagnetic interference, and other factors;

5. High installation cost: Due to the presence of the iron core, the device is large and heavy, weighing dozens of kilograms, usually requiring hoisting operations; and since the primary side needs to connect to the high-voltage circuit, additional electrical isolation is required, which also brings extra installation and maintenance costs.

6. Requires offline calibration: Usually requires manual periodic inspection, power outage maintenance, resulting in low efficiency.

To solve these problems, scientists have been searching for an alternative that does not rely on iron cores, is anti-interference, compact, and more accurate. Quantum precision measurement, especially magnetic field detection technology based on NV centers, exactly has these potentials. It does not require an iron core, remains stable under environmental changes, is small in size, responds quickly, and the measurement results are based on physical constants, requiring no manual on-site calibration, and can even measure both alternating and direct currents simultaneously.

The Quantum "Insight" in Diamonds

We introduced the principle of the NV center in the previous article, but it cannot directly measure current; it measures magnetic fields. So how do we get current from magnetic fields? According to the principle of electromagnetic induction, current generates a circular magnetic field around it. Using the current magnitude, we can calculate the magnetic field strength at a certain point in space. Conversely, according to Ampere's circuital law, we can calculate the current magnitude from the magnetic field strength around the current. Therefore, we can indirectly calculate the current magnitude by measuring the magnetic field strength.

The core of the quantum current transformer is to arrange multiple NV center probes evenly around the conductor in a ring structure. When current flows through the conductor, it generates a magnetic field around it. The NV centers interact with the magnetic field and, through the continuous wave optically detected magnetic resonance (CW-ODMR) technology we introduced earlier, convert the magnetic field strength into optical signals. These optical signals are transmitted via optical fibers to the control system, where a computer analyzes the magnetic field strength and then calculates the current value using formulas.

To improve measurement stability and accuracy, the "industry-academia-research-application" joint research team from the University of Science and Technology of China used four NV center sensors evenly distributed on a magnetic shielding ring around the conductor. The magnetic shielding ring suppresses interference from the Earth's magnetic field or other electromagnetic devices. Multiple probes working simultaneously act like "repeated measurements," which not only improves the signal-to-noise ratio but also reduces errors caused by the conductor deviating from the center. The device's control system can also perform real-time feedback based on measurement results, taking actions such as temperature compensation and laser adjustment to ensure stable and reliable measurements.

Schematic diagram of the diamond NV center quantum current transformer experimental device and measurement principle
(Image source: Reference [1])

Hardcore Performance of the New Device

One of the biggest features of this device is that its measurement data can ultimately be traced back to physical constants and time measurement. According to our formula for calculating magnetic field strength: magnetic field strength equals half the frequency difference divided by the electron gyromagnetic ratio of the NV center, where the electron gyromagnetic ratio is a fixed constant.

In other words, throughout the entire current measurement process, the physical quantity that truly affects the accuracy of the measurement results and needs to be measured is "frequency," which is the reciprocal of the time period. Measuring frequency is essentially measuring time. This means that in the future, devices can be calibrated online in real-time via satellite remote timing, improving the calibration frequency, efficiency, and single calibration ratio of a large number of current transformers, reducing economic losses and time costs caused by power outages for maintenance and manual calibration. Additionally, different current transformers can be synchronized globally with unified standards, fundamentally revolutionizing the metering, protection, and monitoring systems of the power grid.

The quantum current transformer performs excellently in the laboratory. First, it has high measurement accuracy, with an error as low as 0.05% within the current range of 0–1000A (amperes). Second, it achieves a linearity of 0.16%, meaning it maintains good measurement accuracy whether measuring large or small currents. Third, it has high sensitivity, reaching 6nT/√Hz (nanotesla per root hertz), capable of detecting very weak magnetic field changes. Fourth, it covers a wide bandwidth, monitoring AC currents from low frequency 1Hz to mid-frequency 1.4kHz. At the same time, it has excellent temperature drift control, basically eliminating the impact of temperature changes on current measurement accuracy. Moreover, it features low insulation requirements, small device size, and compatibility with both AC and DC.

More importantly, this device has completed field operation testing. It was officially connected to the State Grid for commissioning in 2022 and successfully operated for 240 hours after being connected at the 110kV Qianshui Road Substation in Hefei City, Anhui Province. This is the first time quantum precision measurement technology has been "connected to the grid" in a power system, meaning it can work long-term in a real industrial environment without relying on laboratory conditions.

The world's first quantum current transformer grid-connected demonstration application at the Hefei 110kV substation.
(Image source: Reference [1])

This achievement was also recognized by the quantum technology service platform "Photon Box" as one of the "Top Ten Social Impact Events of Chinese Quantum Companies in 2022" and has been permanently collected by the Anhui Innovation Museum as a landmark device for public display.

The era of quantum measurement has arrived.

The emergence of the quantum current transformer is not just a new instrument but represents the beginning of the "quantum era" in the power industry. Of course, as the first product of quantum precision measurement technology in the power sector, this quantum current transformer still has areas for improvement. The research team is attempting to integrate the laser, photodetector, and diamond probe together, eliminating the optical fiber used for intermediate connections to reduce interference caused by fiber vibrations and improve measurement stability.

At the same time, they are developing a closed-loop feedback control system to keep the sensor operating near a "zero magnetic field" state to further enhance measurement accuracy. In the future, this system will be optimized for more complex ultra-high voltage DC scenarios and support seamless integration with remote monitoring systems and industrial data platforms, becoming a true "quantum-level fundamental unit" in the smart grid.

In the "Metrology Development Plan (2021–2035)," the State Council of China clearly proposed implementing the "Quantum Metrology" plan to build a national modern advanced measurement system centered on quantum metrology, with first-class scientific and technological levels, meeting the needs of the times and international development trends. The research and application of the quantum current transformer is a typical achievement of quantum metrology landing in the power industry.

In the future, similar quantum measurement devices may also appear in various industries such as mineral exploration, electromagnetic detection, navigation and positioning, and environmental monitoring. Quantum precision measurement technology will redefine the way humans perceive the world with disruptive high performance and lay a new foundation for modern scientific and technological development and industrial applications on a brand-new metrological standard!

References:

[1] Shao-Chun Zhang, Long Zhao, et al. High-voltage current sensing with nitrogen-vacancy centers in diamond. APL Photonics 2025; 10 (3): 036117