Electric vehicles (EVs) are leading the transition to sustainable transportation. However, effective battery management remains a significant challenge, as traditional monitoring techniques often lack adequate precision and noise resistance. Diamond quantum sensors provide a cutting-edge solution by using nitrogen-vacancy (NV) centers, which are the key elements within the diamond sensors that enable them to detect even the smallest changes in magnetic fields, making them ideal for accurately monitoring battery systems. While these advancements show promise, large-scale industrial adoption remains a challenge, requiring further optimization and integration into manufacturing processes.
Diamond quantum sensors are emerging as versatile, high-sensitivity tools for measuring magnetic and electric fields, temperature, and pressure. Additionally, these sensors offer biocompatibility, making them suitable for diverse applications beyond just energy systems. However, the use of diamond crystals for quantum sensors is often limited by the small size of the available substrates—typically a few millimeters in diameter—due to manufacturing constraints.
In a recent study, a team of researchers led by Professor Mutsuko Hatano and Professor Takayuki Iwasaki from the Department of Electrical and Electronic Engineering, School of Engineering, Institute of Science Tokyo (Science Tokyo), Japan, utilized heteroepitaxial growth technology to address size limitations in diamond substrates. They have developed a platform for heteroepitaxial (111) diamond quantum sensors, featuring preferentially aligned NV centers on large substrates. This breakthrough could pave the way for their use in monitoring EV batteries. The diamond crystal substrates were fabricated in collaboration with Shin-Etsu Chemical Co., Ltd. and the National Institute of Advanced Industrial Science and Technology (AIST). This technique enables diamond growth on non-diamond substrates and enhances material quality and sensor performance. Their findings were published in Advanced Quantum Technologies on January 18, 2025.
The team successfully grew a self-standing heteroepitaxial chemical vapor deposition (CVD) diamond film with a (111) orientation and a thickness of 150 μm on a non-diamond substrate, which was then separated to ensure high uniformity and crystallinity, offering superior industrial productivity. A 150-μm thick NV-diamond layer was then deposited on the heteroepitaxial diamond, achieving a T₂ (spin coherence time) value of 20 μs, corresponding to a substitutional nitrogen defect concentration of 8 ppm. The tilt correction mechanism was introduced in the sensor head to compensate for the miscut angle (deviation in crystal orientation) inherent in CVD substrates, enabling sensor performance nearly equivalent to that of conventional substrates.
Using continuous wave optically detected magnetic resonance spectroscopy in a fiber-top sensor configuration, the team estimated the NV concentration and T₂* (decoherence time) to be 0.05 ppm and 0.05 μs, respectively. The sensor's gradiometer setup, with two sensors positioned on either side of the busbar, demonstrated a noise floor of less than 20 nT/Hz0.5 without magnetic shielding. Additionally, the Allan deviation of magnetic field noise remained below 0.3 μT, which enabled the detection of busbar currents as low as 10 mA over an accumulation time of 10 ms to 100 s.
"The ability to measure currents accurately while minimizing interference makes this sensor a promising candidate for monitoring battery systems in electric vehicles, where precision and reliability are paramount," says Hatano. To improve detection in noisy automotive environments, the team plans to increase NV center density using electron beam irradiation, enhancing sensitivity. They will also boost fluorescence collection efficiency, and extend coherence time with advanced quantum protocols for more accurate, long-lasting current detection.
This study demonstrates the industrial manufacturing potential of quantum-grade diamond substrates and their applications in quantum technologies, including electric vehicle battery monitoring, medical diagnostics, and energy devices. "This success contributes to the acceleration of quantum technologies, particularly in sectors related to sustainable development goals and well-being," concludes Hatano.
