Measuring tasks in factory and process automation, as well as in automotive and medical technology, are constantly presenting sensors with new challenges. There is a demand for precise, reproducible and scalable methods that can be integrated advantageously into applications under adverse operating conditions - at extreme temperatures, in electromagnetic fields or with limited installation space. In the field of magnetometry, or magnetic field measurement, quantum sensors, which achieve a high level of sensitivity and accuracy compared to conventional sensors, have been the focus of attention for several years. Unlike Hall effect or magnetoresistive sensors, for example, they react robustly to temperature fluctuations and measure without hysteresis. This is based on quantum physical interactions on the subatomic scale, which can be generated and controlled in diamond crystals.

Using diamonds and lasers to measure magnetic fields

Quantum Technologies, a spin-off from Leipzig University, has been developing in the field of quantum magnetometers since 2020. The target applications include highly sensitive angle encoders, robust sensors for current sensors and the monitoring of electric motors - all of which are problematic areas of application, even for quantum sensors. This is because either the necessary cooling is disadvantageous when integrating the measuring methods or microwave radiation is required, which in turn can generate heat. It is often difficult to galvanically isolate the sensor and the measuring environment.

Dr. Robert Staacke, co-founder and CEO of Quantum Technologies, was already researching magnetic field sensors that use quantum effects as a doctoral student. The scientists typically focus on so-called color centers in diamond crystals, in particular nitrogen-vacancy centers (NV). They are suitable for optically measuring magnetic fields and their changes. NV centers are formed when one of the carbon atoms in the crystal lattice of the diamond - a modification of the element carbon - is replaced by nitrogen, while an adjacent carbon atom is missing. This defect pair of nitrogen atom (N) and lattice vacancy (V) is characterized by the fact that it has a spin of 1. External magnetic fields influence the energy levels of the spin states of the center, which in turn results in a change in fluorescence. Light can be used to excite the NV center energetically. If the resulting fluorescence is monitored, this allows conclusions to be drawn about the strength of the magnetic field in question.

Microwaves banned from the sensor

The measuring principle of the magnetic field sensors from Quantum Technologies.

© Quantum technology

In contrast to squids (Superconducting Quantum Interference Devices), NV magnetometers can record further measuring ranges and work up to room temperature and beyond. However, it is typically necessary to align the NV centers spatially to the magnetic field and to use microwave radiation. Although NV sensors do not require the complex cooling equipment needed for squids, microwaves also require special setups - for example antennas close to the diamond or shielding enclosures to increase the sensitivity of the measurement.

Quantum Technologies is therefore pursuing a technology approach that works independently of the direction of the magnetic field and without microwave radiation: Instead of an extended and homogeneous diamond crystal, diamond nanoparticles are used on the tip of an optical fiber. The statistically distributed orientation of the particles eliminates the unavoidable directional dependence (anisotropy) of the material properties in crystals. The magnetic field measurement becomes isotropic.

This technology has two advantages: Firstly, the sensors work purely optically. They can be easily positioned at the end of an optical fiber without any electrically conductive or magnetic material. This allows the measurement to be galvanically decoupled and unaffected by electromagnetic interference sources. Secondly, the measurement setup is very small, simple and can also be integrated into applications that are difficult to access, such as electric motors.

This was not always the case. The typical picture of a laboratory for research at NV centers shows complicated test setups, huge optical tables, littered with components and devices. The Quantum Technologies team's approach was rather atypical for science, because the aim was not to measure even more precisely with even more complexity in order to uncover one more physical effect, but rather to ask how the complexity could be reduced and the system simplified?

What emerged as the minimum equipment looks like this: Diamond nanoparticles, a light source and a detector are required. The red fluorescence is then evaluated. Although the result is not the most sensitive sensor that can be achieved with NV centers, galvanic isolation is achieved. The diamond nanoparticles create an isotropic measuring possibility and a tiny measuring point, which opens up new application possibilities for the sensors.

Measuring without being a quantum physicist

The directional and interference-decoupled measurement as well as the small footprint of the sensor are key to the applications that Quantum Technologies wants to develop: directly in the air gap of an electric motor, for example, or within an electric vehicle battery, in high-voltage energy systems, in invasive medical technology and radiology or in non-destructive inline material testing for the metal industry.

According to Dr. Staacke, it is important in practice that the red fluorescence of varying intensity provides an output signal that is easy to interpret: Although the physical relationships behind the measurement are really complicated, changes in brightness can be easily understood by anyone, even without being a quantum physicist.

A single measuring point is one of the simpler application scenarios. Scanning processes, for example with a moving sensor in material characterization, are also relatively easy to implement. The idea of a quantum magnetic field camera is somewhat more complex, but demonstrators have already been set up for this too. For this, a large surface was covered with diamond nanoparticles, these were excited using an LED and the red fluorescence was then observed using a CCD camera.

The author: Tobias Wölk is Product Manager Automation Technology & Active Components at Reichelt Elektronik.

© Reichelt Electronics

Demonstrators for position and frequency measurement on rotating magnets, electric motor monitoring and the megapixel magnetic field camera are intended to show Quantum Technologies' customers the practical benefits and variety of potential applications. To accelerate the market entry process, the company entered into a partnership with Reichelt Elektronik in 2023.