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Quantum magnetic-field sensors

In twos: Pairs of electrons tunnel through a barrier in a SQUID.
In twos: Pairs of electrons tunnel through a barrier in a SQUID.

While homo sapiens may not be able to detect magnetic fields, as “homo technicus”, human beings make use of a wide variety of technical sensors. Quantum effects are increasingly being exploited to collect information unavailable by conventional means – for example, in order to detect magnetic fields in living organisms or to use such fields for medical imaging purposes.

 

 

 

  • Medical imaging: In order to peer inside human beings (without having to cut them open), medicine has developed an arsenal of methods. One way to obtain an image from within is to measure the very weak magnetic fields produced by our brain when thinking – or by our beating heart. Sensors that are highly sensitive to such weak magnetic fields exploit quantum effects such as superconductivity.
  • Medical biomarkers: In some cases, medicine must send in “spies” to find out what is happening inside. For example, the path of nanoparticles introduced into a patient’s body can be followed by means of certain particle properties such as magnetism.
  • Quantum communication: Highly sensitive magnetic-field sensors can also be used outside of medicine. For example, single photons can be detected by means of these sensors – an important prerequisite for fundamental research as well as for applications of quantum communication.

To detect very weak magnetic fields, superconducting quantum interference devices (SQUIDs) are especially well suited for use as highly sensitive sensors. This special form of quantum technology, together with its requisite cryotechnology, has enjoyed top-caliber development and manufacturing and been used for measurement at PTB for some time now. For example, SQUID magnetometers have been used for several years to measure the tiny magnetic fields generated by the neural activity of the human brain. PTB is an international leader in both the manufacture of such SQUIDs (via superconductor thin-film technology) and the measurement technology based on them. At PTB, SQUID technology is complemented by so-called optically pumped magnetometers (OPMs) for which nuclear spins are “read out” by means of laser light and which – in contrast to SQUIDs – do not require cooling to low temperatures (approximately that of liquid helium).

Magnetic fields in living organisms are so small that the Earth’s magnetic field and the magnetic fields of our electrified world are gigantic by comparison. Therefore, PTB conducts its biomagnetic reference measurements, which are performed before real-life medical applications take place, in a specially shielded room, which is the “magnetically quietest place in the world” (Berlin Magnetically Shielded Room, BMSR). This facility and its associated equipment are also open to external partner institutions from industry and research.

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Cryogenic sensors are based on physical phenomena such as superconductivity at low temperatures. Superconducting quantum interferometers (SQUIDs) allow physical quantities that can be converted into a magnetic flux to be measured. These quantities are primarily magnetic fields and magnetic material parameters, but also electric currents and temperatures. Cryogenic sensors and SQUIDs enable precision measurements in metrology and fundamental research, but they are also used in commercial measuring systems for materials research or applied geophysics.

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One of the main applications of SQUIDs are biomagnetic measurements such as magnetoencephalography or low-field MRI. These applications require the operation of SQUID systems in low-noise cryogenic vessels to be able to detect the magnetic signals of the human body sensitively. Due to the special design of the cryogenic vessel, its noise contribution has been minimized to a negligible level, and record noise values of less than 200 aT Hz-1/2 have been attained.

Further innovations in this field can be attained by improving the manufacturing possibilities. To this end, the Josephson junctions have been miniaturized down to nanometer size, which has led to yet another noise reduction. This development has opened up new possibilities: novel SQUID systems can be set up and thus, new applications, both in biomagnetism and in fundamental research, are within reach.

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Optisch gepumpte Magnetometer zur Messung kleinster Magnetfelder

In optisch gepumpten Magnetometern werden gasförmige Atome als empfindliche Magnetfeldsonden eingesetzt. Dazu wird der quantenmechanische Zustand der Atome mit Laserlicht präpariert und die Wirkung eines Magnetfelds auf diesen mit Laserlicht ausgelesen. Bei der Präparation werden die Spins, der in einer Gaszelle befindlichen Atome zu einer kohärenten Rotation angeregt, sie werden dabei in einen bestimmten Spinzustand „gepumpt“. In einem Magnetfeld präzedieren die Spins dann kollektiv mit der Lamorfrequenz, die proportional zur magnetischen Flussdichte ist. Diese Wirkung auf den quantenmechanischen Zustand der Atome wird dann mittels laserspektroskopischer Methoden ausgelesen.

OPMs haben sich in den letzten beiden Dekaden rasant entwickelt und erreichen bereits ähnliche Empfindlichkeiten wie SQUIDs, ohne dabei kryogene Temperaturen zu erfordern. Als gasförmige Atome dienen z.B. verdampfte Alkalimetalle, wie Kalium, Rubidium oder Cäsium. Zudem können die Sensoren letztendlich klein und flexibel sein, sodass sie vorher nicht realisierbare Anwendungen – z. B. in der Medizin – ermöglichen.

Wir arbeiten daran OPMs weiterzuentwickeln um sie für neue Anwendungsgebiete in der Medizinphysik (z. B. Detektion und Abbildung magnetischer Nanopartikel) und in der Grundlagenphysik (z.B. genaue Messung kleinster Magnetfelder) einzusetzen. 

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