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A magnetometer is a device used to measure the direction and strength of the magnetic field at a particular location. Magnetometers are employed across several sectors such as energy, healthcare, aerospace & defense, consumer electronics, surveys, and industrial. A magnetometer is an instrument with a sensor that measures magnetic flux density B (in units of Tesla or As/ m2) at the point in space where the sensor is located. A magnetic field drops in intensity with the cube of the distance from the object. Therefore, the maximum distance that a given magnetometer can detect the object is directly proportional to the cube root of the magnetometer’s sensitivity. The sensitivity is commonly measured in Tesla.
The Earth generates a weak magnetic field that produces flux densities (in air) of about 18 micro Tesla in some parts of South America to a high of over 60 microTesla in the Arctic Circle and Antarctica. Since magnetic flux density in air is directly proportional to magnetic field strength H [A/m], a magnetometer is capable of detecting fluctuations in the Earth’s field.
The magnetometer sensor is even present in the tablet or smartphone that utilizes the modern solid state technology to create a miniature Hall-effect sensor that detects the Earth’s magnetic field along three perpendicular axes X, Y and Z. The Hall-effect sensor produces voltage which is proportional to the strength and polarity of the magnetic field along the axis each sensor is directed. The sensed voltage is converted to digital signal representing the magnetic field intensity. Other technologies used for magnetometer may include magneto resistive devices which change the measured resistance based on changes in the magnetic field.
Magnetometry has two general aims: measuring magnetic fields highly precisely and on the smallest scale. Magnetometers have been used intensively for a long time – as compasses to measure the earth’s magnetic field, for geological studies or to analyze nanostructured magnetic layers in hard drives for data storage. There have been numerous breakthroughs in the scientific and technological use of magnetic fields during the past decades, yet the detection of smallest magnetic fields with highest spatial resolution at room temperature has proven to be a great scientific challenge.
Magnetometers are classified into two categories:Vector magnetometers, that measure the flux density value in a specific direction in 3 dimensional space. An example is a fluxgate magnetometer that can measure the strength of any component of the Earth’s field by orienting the sensor in the direction of the desired component.A SQUID sensor is basically a magnetic flux-voltage converter having an extremely low magnetic flux noise. The physical quantities (magnetic field, current, voltage, displacements, etc.) to be detected are converted in a magnetic flux by using suitable flux transformer circuits.Scalar magnetometers that measure only the magnitude of the vector passing through the sensor regardless of the direction. Quantum magnetometers are an example of this type of magnetometer.
Quantum Magnetometers
Quantum magnetometers take advantage of the spin of subatomic particles (nuclei and unpaired valence electrons). Through a process of polarization, particles are caused to precess in the earth’s ambient magnetic field. The resulting frequency of precession can be translated directly into magnetic field units. Quantum results are scalar (total field intensity) as opposed to vector (i.e. from fluxgate geophysical instruments or GEM’s Suspended dIdD technology).
The spin of nuclei and unpaired valence electrons is associated with the magnetic moment and is characteristic for each particular particle. Coupling of each particle’s magnetic moment with the applied field is quantized or limited to a discrete set of values as determined by quantum mechanical rules. In the ambient magnetic field, there are 2I + 1 orientations for electrons and for nuclei (i.e. protons and Helium 3). For each of these, I = ½. There are therefore, only 2 orientations allowed (parallel and anti-parallel to magnetic field). Since the populations of each of the orientations are different, an assembly of magnetic moments will produce a tiny net macroscopic magnetization that is aligned with the magnetic field.
Macroscopic nuclear or electron spin magnetization is static. If elementary magnetic moments are forced out of alignment with the direction of the ambient magnetic field, the corresponding particles precess (i.e. rotate) around the field in a plane of precession perpendicular to the field direction. They precess with an associated angular frequency, called the Larmor frequency which is in general proportional to the magnetic field value.
However, in weak magnetic fields, such as the Earth’s, the signals of all scalar magnetometers are just too weak for simple measurement of Larmor frequency. They must be boosted in intensity or “polarized” to ensure sufficient sensitivity of measurement. Due to the distribution of local magnetic fields, all particles in the sensor precess with naturally different frequencies and lose synchronism over time. The signal associated with the precession decays exponentially and the characteristic time of decay is called “transversal” relaxation time T2.
Similarly, if we apply a magnetic field to an assembly of spins, it takes time to establish macroscopic magnetization. The increase is again exponential with the time constant, T1, called “longitudinal” relaxation time. The intensity of magnetization is proportional to the strength of the applied magnetic field. The strength of the magnetization and therefore, of the detectable precession signal, depends on the difference in populations of the two orientations of magnetic moments.
Increasing that difference is called polarization and can be achieved in three ways in quantum magnetometers:
- Application of strong auxiliary magnetic field (actually flux density) to polarize nuclear, usually protons.
- Transfer of natural polarization of auxiliary electrons to protons (Overhauser effect).
- Optical manipulation or “pumping” of electrons by elevating them to a higher state selectively.
Note: In practice, T2 is very short in solid samples. All quantum magnetometers therefore use liquid or gaseous sensors. In liquids and gases T1 and T2 assume values between a fraction of a second to several seconds. An exception is Helium 3, which has a T2 value of several hours or even days.
Ultimate precision for sensor technology using qubits and machine learning
There are currently limits to how we can measure things. A well-established rule is the so-called standard quantum limit: the precision of the measurement scales inversely with the square root of available resources. The more resources – time, radiation power, number of images, etc. – you have access to, the more accurate your measurement will be. This will, however, only get you so far: extreme precision also means using excessive resources.
Researchers from Aalto University, Finland, ETH Zurich, Switzerland, and MIPT and Landau Institute, Moscow, have developed a way to measure magnetic fields using quantum technologies. They have demonstrated a new method that combines quantum phenomena and machine learning to realise a magnetometer with accuracy beyond the standard quantum limit.
Their paper, published in the prestigious journal npj Quantum Information shows how to improve the accuracy of magnetic field measurements by exploiting the coherence of a superconducting artificial atom, a qubit. It is a tiny device made of overlapping strips of aluminium evaporated on a silicon chip – a technology similar to the one used to fabricate the processors of mobile phones and computers.
When the device is cooled to a very low temperature, magic happens: the electrical current flows in it without any resistance and starts to display quantum mechanical properties similar to those of real atoms. When irradiated with a microwave pulse the state of the artificial atom changes. It turns out that this change depends on the external magnetic field applied: measure the atom and you will figure out the magnetic field. But to surpass the standard quantum limit, yet another trick had to be performed using a technique similar to a widely-applied branch of machine learning, pattern recognition.
‘We use an adaptive technique: first, we perform a measurement, and then, depending on the result, we let our pattern recognition algorithm decide how to change a control parameter in the next step in order to achieve the fastest estimation of the magnetic field,’ explains Andrey Lebedev, corresponding author from ETH Zurich, now at MIPT in Moscow. ‘This is a nice example of quantum technology at work: by combining a quantum phenomenon with a measurement technique based on supervised machine learning, we can enhance the sensitivity of magnetometers.
On April 1 2019, the Fraunhofer-Gesellschaft launches the lighthouse project “Quantum Magnetometry” : Freiburg’s Fraunhofer institutes IAF, IPM and IWM want to transfer quantum magentometry from the field of university research to industrial applications. In close cooperation with three further Fraunhofer institutes (IMM, IISB and CAP), the research team develops highly integrated imaging quantum magnetometers with highest spatial resolution and sensitivity. The project QMag runs until 2024 and is founded with a total of € 10 million euros in equal parts by the Fraunhofer-Gesellschaft and the federal state of Baden-Württemberg.
The lighthouse project QMag enables the use of single electrons to detect smallest magnetic fields. This allows to use magnetometers in industry, for example for defect analysis of nanoelectronic circuits, for the detection of hidden material fissures or to realize especially compact magnetic resonance imaging scanners (MRI). “Our lighthouse projects set important strategic priorities to develop concrete technological solutions for Germany as an economic location. QMag paves the way for a Fraunhofer lighthouse in the field of quantum technology. The ambition of the excellent scientists who take part in the project is to significantly improve the technology and to define it internationally. In this way a long-term transfer of the revolutionary innovations of quantum magnetometry to industrial applications can be achieved”, explains Fraunhofer President Prof. Reimund Neugebauer.
The QMag consortium has set itself the goal of bringing quantum magnetometry from laboratory to application and making it usable in industry. In order to do so, the Fraunhofer Institutes will develop two complementary magentometers which are able to measure smallest magnetic fields and currents with highest spatial resolution, respectively highest magnetic sensitivity, at room temperature.
More specifically, the project partners aim to demonstrate and test two systems, which are based on the same physical measuring principle and method but which target different applications: On the one hand, a scanning probe magnetometer based on NV centers in diamond will allow highest precision measurements of nanoelectronic circuits. On the other hand, measuring systems based on highly sensitive optically pumped magnetometers (“OPMs”) for applications in material probing and process analysis will be realized.
Nanoscaled magnetometry based on NV centers
A scanning probe magnetometer is able to measure magnetic fields with highest spatial resolution at room temperature. The magnetometer consists of single atomic vacancy complexes in diamond crystals which function as the smallest possible magnet. A nitrogen vacancy center (“NV center”) in diamond plays the central part. An NV center develops when two neighboring carbon atoms are removed and one is replaced with a nitrogen atom. The resulting vacancy is then occupied by the spare electron of the nitrogen atom. This electron possesses a magnetic momentum, which, after being oriented, can be used as a magnet for the magnetic field that is to be measured. Within Qmag, an NV center will be placed in the nanoscaled tip of a diamond measuring head. When this sensor tip is being moved across a sample within a scanning probe microscope, local magnetic fields can be measured with extremely high spatial resolution. In this manner the electricity distribution in nanoelectronic circuits can be made visible, considering that even the smallest electronic current produces a magnetic field that can be visualized using the quantum magnetometer.
“Our aim is to develop quantum magnetometers with exceptional sensory characteristics, compactness and mode of operation, which allow innovative industrial applications, and furthermore simplify the evolution of complex electronic systems in the future”, says Prof. Dr. Oliver Ambacher, project manager and director of Fraunhofer IAF.
OPMs for chemical analytics and material testing
The second sensor system of QMag uses the magnetic field dependency of electronic transitions in alkali atoms: optically pumped magnetometers (“OPMs”) are a category of sensors that are being used to measure extremely weak magnetic fields. Just as NV centers, OPMs do not require extreme cooling and are therefore qualified for industrial use. The focus of the scientific work of QMag lies on the development of complete measuring systems based on existing magnetometer prototypes.
In OPMs alkali atoms in the gas phase are prepared with the help of a circular polarized laser beam so that all their magnetic moments have the same orientation. Inside the measured magnetic fields the magnetic moments experience a synchronous precession that can be measured via the absorption of a laser beam of adequate wavelength. The measurement can be done with such a high precision that even magnetic fields of femto-Tesla range are detectable – which is approximately the size of magnetic fields that our brain waves produces while we think. Due to their sensitivity, OPMs can be used as detectors for nuclear magnetic resonance signals (“NMR”). “In QMag, we develop complete measuring systems based on existing single sensor prototypes, which opens up innovative application scenarios, especially in the field of low-field NMR for chemical analytics and material testing”, explains Prof. Dr. Karsten Buse, director of Fraunhofer IPM.
Furthermore, the consortium will realize demonstrators for key applications for the mechanics of materials. The magnetic detection of mechanical micro-fissures is a highly sensitive tool for material characterization and component testing and therefore a very relevant field of application. “The high sensitivity of OPM sensors at low frequencies and room temperature opens up completely new application possibilities for material testing. Microscopic material defects can be measured non-destructively on the basis of their magnetic stray field signals”, highlights Prof. Dr. Peter Gumbsch, director of Fraunhofer IWM.
Alongside the core team, three additional Fraunhofer institutes contribute their scientific and technological competencies for the development of quantum technological key components. The consortium is completed by the academic expertise of Prof. Dr. Jörg Wrachtrup (University of Stuttgart) in the field of diamond-based quantum technology and by Prof. Dr. Svenja Knappe (University of Freiburg in cooperation with the University of Colorado Boulder) in the field of atom gas magnetometry.
Quantum Magnetometer Market
According to “Quantum Magnetometer Markets: 2020 to 2029,” a new report from Inside Quantum Technology, the market for atomic clocks will exceed $700 million by 2025. The main drivers will be the need for more sensitive magnetometers in medical imaging, defense applications and geophysical applications such as oil and gas applications. While all quantum magnetometers offer users enhanced sensitivity compared with classical magnetometers, there is growing competition in the field between optically pumped magnetometers, proton magnetometers, Overhauser magnetometers, SERFs, NV-diamond vapor cell magnetometers and SQUIDs.
Geophysical studies and exploration is by far the biggest market for magnetometers — this area has used classical magnetometers for half a century. The use of quantum magnetometers will expand the scope of, for example, mineral explorations, magnetic survey and hazard detection. The compelling value proposition of quantum magnetometers will drive revenues from sales of these devices to approximately $315 million by 2025.
New applications for quantum magnetometers are also beginning to appear. For example, beyond medical imaging, the extreme sensitivity of SQUIDs makes them ideal for biological investigations of various kinds. While, quantum magnetometers have been shown to add value to some established markets, interesting – and potentially profitable – new applications for quantum magnetometers are also beginning to appear. And NV-diamond center magnetometers are being used in navigation systems where conventional GPS won’t work.
Quantum magnetometers have a large number of applications in the military. Both researchers in the US and China are working on such applications and this report discusses the impact that an era of Sino-American tensions may have on the quantum magnetometer business. In total the military/aerospace applications for quantum magnetometers will generate $120 million.
