© European Physical Society, EDP Sciences, 2025

Sensors have grown into an extremely large market, serving as the eyes and ears of any automation. As artificial intelligent (AI) systems continue to advance, the role of sensors will expand further. Applications in the SMART home sector and up to self-driving cars are unthinkable without a large number of sensors. However, the sensor market is vast and the path from an idea to a finished product is lengthy. Just because a sensor works well under stable laboratory conditions with very expensive measurement apparatus does not mean it is suitable or even competitive for mass market requirements [1].

In this article, we focus on quantum sensors based on Nitrogen-Vacancy-Centers (NV-centers) in diamond. NV-centers are not only very easy to manufacture and to control but exhibit also quantum properties at room temperature [2]. In addition, a significant portion of the intellectual property rights is held by companies and institutions within the European Union.

Distinction between quantum and conventional sensors

The distinction between quantum sensors and conventional sensors, which also obey quantum mechanics, lies in the fact that quantum properties are observable at the level of individual quantum objects. In contrast, conventional sensors rely on quantum mechanical collective effects that are only evident with many quantum systems combined [3]. Magnetic sensors based on the giant magnetoresistance effect (GMR), for example, are such collective quantum phenomena, like how the conduction properties of semiconductors are based on collective properties of the electrons in a crystal.

In contrast to other sensors, quantum sensors can be greatly reduced in size, as their properties are based on the effects of individual electrons, nuclei or atoms [4]. Since these electrons, nuclei and atoms have the same properties as elementary building blocks of nature, the signals are in most cases only dependent on natural constants. This allows easy self-calibration, extreme long service-free lifetime, independence from environmental influences and measurement accuracy up to the Heisenberg limit [5].. Such quantum sensors have discrete states and are ideally in a well-defined initial state. These properties make these sensors of extremely high quality and high spatial resolution.

Quantum Sensors based on Color Centers and Defects

In general, quantum states of atoms or molecules have extremely short lifetimes at room temperature and are so strongly influenced by the environment that the determination of a far field such as a magnetic field is not possible [5]. To make a quantum system usable for sensors, it must be well shielded from direct external influences. Diamond is particularly appropriate as a host material. Indeed it is a wide band gap material, interaction with free electrons is very low, and it has a high Debye temperature that suppresses the interaction with phonons. Other wide band gap materials such as GaN, AlN, SiC, BN are also known hosts expected to have usable centers with similar properties as those in diamond. Another advantage of these defects in such materials is that they are easy to detect: the excited states have a short lifetime and can be produced by photons in the visible range, making them easier to observe. The lifetime of excited states is typically only a few ns, so that a center at saturation can emit 109-1011 photons per second. Even with simple optical setup, typically 0.1% of these photons can be detected, thus achieving a rate of 106-108 photons per second for a single center [6]. But individual defects, atoms or molecules can only be used as quantum spin systems if they can also be initialized and their coherence time is long enough to manipulate them. The most prominent defect system is the nitrogen vacancy (NV) center in diamond. This center was already known as color center in the 1960s leading to a red coloration of the diamond. In the purest samples enriched with 12C atoms the spin coherence time reached a record of 2 milliseconds at room temperature. Despite intensive research, it has not yet been possible to find a center with better quantum properties at room temperature [7].

The sensor properties are then achieved by an interaction with the measured field e.g. the Zeeman effect for magnetic field [7] or Stark effect for electrical field strength [8]. In the case of the Zeeman effect, a magnetic field leads to a splitting of a spectral line proportional to the applied magnetic field and measuring the transitions between the sublevels allows to determine the field magnitude. A reduction of the fluorescence intensity can be observed at a certain microwave frequency. This corresponds to the transition between two states, where one state exhibits a lower fluorescence. This behavior is understood by looking at the lifetime of the states. In the case of the resonant transition, this is longer by a factor of 10, which leads to lower photon rates and a drop in fluorescence intensity. Sensors with a resolution of a few pico-Tesla have already been successfully tested [5]. Magnetic sensors based on individual color centers can also be installed on diamond AFM tips [9].

All-Optical Microwave-Free Sensors

Magnetic field sensors based on the Zeeman effect assume that the quantum numbers are conserved. In case of NV-centers exposed to magnetic fields > 10 mT, which are not aligned with the axis of the nitrogen and the vacancy, the quantum number is not conserved [10]. This leads to a lower fluorescence depending on the mismatch angle and the strength of the magnetic field due to spin mixing.

But also this quantum mechanical effect can be used to build a pure optical magnetic field sensor without using microwaves [7]. Diamond particles with random orientations to each other are used to eliminate the magnetic field angle dependency of the NVs, so that the effect depends only on the magnetic field strength. Fig. 1 shows the fluorescence vs. the absolute magnetic field for a diamond powder with a high NV content.

Fig. 1 Example of a commercial all optical magnetic sensor. The sensor head with a size of 125 µm is made of highly NV enriched diamond powder. The powder is excited with green light and the red fluorescent is used as a detection signal (picture from Quantum Technologies®)

This quantum mechanical effect can also be used to build a pure optical magnetic field sensor without using microwaves [7]. Diamond particles with random orientations to each other are used to eliminate the magnetic field angle dependency of the NVs, so that the effect depends only on the magnetic field strength. Fig. 1 shows the schematic experimental set-up where the sensor’s head made of diamond powder with a high NV content, is excited by green light and emits fluorescence that is proportional to the magnitude of the measured magnetic field. Both the green excitation light and the red fluorescence are transported through the same fiber and later separated by means of filter arrangements. This allows the construction of an all-optical magnetic field sensor with an extremely small footprint and perfect galvanic isolation without metallic material in the sensor head. The sensor shown in Fig. 2 is already commercially available, with a sensing volume of ~100 μm in diameter and ~50 µm thickness. An additional advantage of these sensors is the separation of all opto-electronics and the actual sensing element with the usage of an optical fiber. This makes the sensor insensitive to electromagnetic noise or ionizing radiation between the sensor head and measuring electronics.

Fig. 2 All optical quantum magnetic sensor used as the pickup head on an electric bass guitar and the corresponding signal spectrum with the different harmonics.

These specific properties distinguish them from GMR- or Hall effect-based sensors. Due to its galvanic isolation, such sensor is well suited for current measurements on high-voltage lines or for lightning detection. Thanks to its tiny dimension, it can be placed in hard-to-reach positions.

The output readout of this sensor is ultimately carried out by determining the fluorescence intensity or the fluorescence lifetime. Complex control using microwave technology is not necessary here, what simplifies the measuring setup extremely. In addition to the point-shaped sensors, the diamond particles can also be distributed over a large area and read out with a video camera. This allows to map a magnetic field area with very high resolution. This type of microscope is used in material analysis or in front end quality control.

Another interesting property is that the response to changing magnetic fields is extremely fast. The sensor can scan magnetic fields with a frequency of several MHz. In addition to the applications described above, we were able to develop an electric guitar pickup based on such sensors. Fig. 3 shows such a guitar with a corresponding sensing head below one of the strings. The sensor has a sampling rate of 20 kHz with a signal strength similar to conventional pickup coils, but with a much larger dynamic range. Like conventional pickups based on induction, this optical quantum sensor requires also a magnetic string to generate the signal. This magnetization can also be achieved with a permanent magnet.

Summary

The main distinguishing feature of quantum sensors from classical ones is that the transition between individual quantum states is measured for such sensors. The excitation can be produced by photons or electrons. Since the quantum state transitions are mainly determined by a few fundamental constants, the sensors do not need calibration, but their miniaturization to atomic size level requires a high degree of technological know-how and mastering. Unfortunately, because of manufacturing costs, the market share has been very low so far. Nevertheless, the microwave-free all-optical sensors based on NV-centers in diamond, described here, could be the first ones suitable for mass production. Their properties and availability could complement and ultimately replace conventional sensors based on Hall effect or GMR.

References

All Figures

	Fig. 1 Example of a commercial all optical magnetic sensor. The sensor head with a size of 125 µm is made of highly NV enriched diamond powder. The powder is excited with green light and the red fluorescent is used as a detection signal (picture from Quantum Technologies®)
In the text

	Fig. 2 All optical quantum magnetic sensor used as the pickup head on an electric bass guitar and the corresponding signal spectrum with the different harmonics.
In the text