A diamond defect small enough to exist at the atomic scale may help simplify one of the least visible but most important layers of the electric-power system: the calibration chain that tells engineers whether a current measurement can be trusted.
Researchers at Japan’s National Institute of Advanced Industrial Science and Technology (AIST), the Institute of Science Tokyo and the National Institutes for Quantum Science and Technology (QST) have developed a compact current comparator that uses a diamond quantum sensor to evaluate both alternating-current and direct-current ratios with the same basic instrument configuration.[1]
The practical advance is more important than the word “quantum.” AC and DC current-ratio calibration have traditionally required different measurement principles and separate systems. The new design brings both into one architecture while shrinking the comparator’s central section to less than one-third the size and less than one-tenth the mass of conventional equipment. In repeatability tests at a 1-ampere input, the team demonstrated performance of about 5×10−8 A/A for both AC and DC measurements.[1]
The peer-reviewed work appeared in Scientific Reports on August 11, 2026 under the title “Current comparator for both AC and DC ratio measurements with 10−8-level type-a uncertainty.” AIST published an English research summary on September 28.[2][3]
Why measure a current ratio instead of simply measuring current?
Power stations, transmission systems and industrial installations routinely handle currents reaching hundreds of amperes. Those currents are too large to feed directly into precision instruments, so current transformers reduce them to manageable values—often a few amperes—while preserving a known proportional relationship.
The accuracy of that relationship matters. A transformer intended to convert 100 amperes into 5 amperes is useful only if engineers know the ratio precisely. Small errors can propagate into energy metering, equipment diagnostics, protection systems and efficiency measurements. Current transformers therefore need calibration against standards that are substantially more accurate than the equipment being tested.[1]
A current comparator is one such standard instrument. It balances magnetic flux generated by two windings until the net flux approaches zero, then infers the current ratio from the winding ratio. The null method can deliver exceptional accuracy.
AC and DC have traditionally required different calibration systems
Conventional AC current comparators detect imbalance through electromagnetic induction. That works because an alternating magnetic field induces a measurable voltage in a detection coil. A steady DC magnetic field does not provide the same signal, so DC ratio measurement has relied on different approaches and equipment.[1]
The Japanese team replaced that conventional detection function with a diamond quantum sensor capable of responding directly to magnetic flux in both AC and DC conditions. That makes a common measurement architecture possible.
The useful part of the diamond is a defect
The sensor does not rely on a flawless gemstone. It uses nitrogen-vacancy, or NV, centers—atomic-scale defects in diamond in which a nitrogen atom sits beside a vacant lattice site. The electron spin associated with the defect can maintain a useful quantum state at room temperature.
In the AIST-led device, green laser light and microwaves are applied to the quantum diamond. The intensity of red fluorescence changes with the quantum state, which in turn depends on the local magnetic field. Reading that fluorescence lets the instrument detect the very small residual magnetic flux in a gap in the comparator’s magnetic core.[1]
The sensor can also separate magnetic-field and temperature information, helping reduce the effect of temperature drift. Eliminating the electromagnetic-induction detection coil used in conventional AC comparators allows the central magnetic structure to become much smaller.
One-third the size, one-tenth the mass
The research team reports that the central section is less than one-third the size and less than one-tenth the mass of the conventional configuration. In metrology, that is not merely an ergonomic improvement.
National-standard and precision-calibration systems often become large and mechanically demanding because stability is more important than portability. Shrinking the core measurement section creates a path toward simpler calibration laboratories and, eventually, more deployable precision systems.
AIST is careful not to present the prototype as a finished commercial instrument. Its next goals include widening the frequency range, improving sensitivity and building a more robust system—potentially with fiber-integrated optics—that could eliminate the large optical-table structures still associated with laboratory implementations.[1]
About 300 Hz for AC, with comparable repeatability in DC
For AC measurements, the researchers extracted the frequency component corresponding to the input current and measured the small deviation from the ideal ratio determined by the windings. Up to roughly 300 Hz, the measured behavior agreed with expectations based on the magnetic material’s frequency response.[1]
Using Allan deviation to study repeatability, the AC measurement noise fell to about 50 nA, corresponding to repeatability of roughly 5×10−8 A/A at a 1 A input. AIST says this exceeds the 1×10−6 A/A performance level required for the relevant national-standard work.[1]
DC measurement is harder in a different way. There is no frequency component to isolate and Earth’s magnetic field becomes a practical disturbance. The team therefore compared stable periods with the DC current on and off. The resulting repeatability was again about 5×10−8 A/A at 1 A.[1]
Three institutions supplied three different pieces of the system
The collaboration is central to the result. AIST brought precision electrical metrology and responsibility for maintaining current-comparator national standards. The Institute of Science Tokyo contributed diamond quantum-sensor technology. QST contributed techniques for forming high-quality nitrogen-vacancy defects in diamond.[1]
The announced research team includes AIST’s Yasutaka Amagai, Yuta Kainuma, Hidekazu Muramatsu and Hiromitsu Kato; Professor Takayuki Iwasaki of the Institute of Science Tokyo; and QST center director Takeshi Ohshima, among others.[1]
The current comparator itself has a history stretching back more than 60 years
The core idea is not new. High-accuracy calibration of AC current transformers drove development of the current comparator in the late 1950s and early 1960s at the Nikola Tesla Institute in Belgrade and Canada’s National Research Council. A landmark 1961 paper by N.L. Kusters and W.J.M. Moore described the use of the current comparator for absolute current-transformer calibration, and the approach was later extended to DC measurement.[4]
A 2024 CERN technical history traces two major development paths from that period: precision calibration of current transformers and the need for highly accurate DC measurement in particle accelerators. The zero-flux concept became a durable part of electrical metrology.[5]
The Japanese work does not throw that principle away. Instead, it changes how the near-zero magnetic flux is detected. A measurement concept born in the mid-20th century is being updated with a 21st-century quantum material.
The ampere itself entered a new era in 2019
Electrical metrology has already undergone a deeper conceptual change. Since the 2019 revision of the International System of Units, the ampere has been defined by fixing the elementary charge e at exactly 1.602 176 634×10−19 coulomb. In physical terms, the unit is tied to a precisely defined amount of charge flowing per second rather than to the older force-between-conductors definition.[6]
That does not mean engineers measure a 500-ampere feeder by literally counting electrons. Practical electrical metrology still depends on a traceability chain connecting the SI definition to national standards, calibration laboratories, instrument transformers and working instruments.
The new comparator sits inside that chain. Its value is in making the transfer of accuracy simpler and potentially more universal.
Renewable power makes the AC/DC divide less tidy
Traditional public grids evolved around AC. Modern energy systems increasingly contain large DC domains as well. Solar panels generate DC. Batteries store DC. Electric vehicles use DC battery systems. Power converters continually translate between AC and DC.
Data centers, advanced industrial drives and other power-electronics-heavy systems add further pressure for accurate measurement across both domains. Maintaining completely separate calibration architectures for AC and DC becomes increasingly awkward as the physical system itself mixes them.
AIST therefore describes the work as foundational technology for unifying and streamlining calibration systems and for standardizing current measurement in next-generation grids where AC and DC coexist.[2]
The right way to judge “quantum” technology is by what it replaces
Quantum sensing can be an easy field to overstate. The useful question is not whether a device contains a quantum effect—every semiconductor ultimately does—but whether that effect enables a measurement that is smaller, more accurate, more robust or otherwise difficult with conventional technology.
Here the claims are concrete: one setup for AC and DC ratios, removal of a conventional induction detection coil, a central section dramatically reduced in size and mass, and repeatability in the 10−8 range.[2]
The remaining challenge is engineering. Frequency coverage, long-term drift, ruggedness, alignment, calibration procedures, optical integration and cost all matter before a laboratory prototype becomes infrastructure.
But the direction is significant. As power systems become more electronically controlled and more dependent on accurate conversion between AC and DC, the instruments used to verify electrical measurements need to become more flexible too.
In this case, the enabling component is not a larger transformer or a heavier magnetic core. It is a deliberately engineered imperfection inside diamond.
