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Quantum sensors for magnetic fields

In optically pumped magnetometers, atomic vapors are used as sensitive magnetic field probes. For this purpose, the quantum-mechanical state of the atoms is prepared by using laser light and the effect of a magnetic field on this state is read out by laser light. During the preparation, the spins of the atoms in a gas cell are excited to a coherent rotation by "pumping" them into a certain spin state. In a magnetic field, the spins then precess collectively with the Larmor frequency which is proportional to the magnetic flux density. This effect on the quantum-mechanical state of the atoms is subsequently read out by means of laser-spectroscopic methods.

OPMs have rapidly developed in the past two decades and now reach sensitivities similar to those of SQUIDs without requiring cryogenic temperatures. Evaporated alkaline metals such as potassium, rubidium or cesium serve as atomic vapors. In addition, it is possible to design small and flexible sensors that can now be used in applications which were previously not feasible (e.g. in medicine).

We are working on further developing OPMs to use them in new fields of application in medical physics (e.g. detecting and imaging magnetic nanoparticles) and in fundamental physics (e.g. the accurate measurement of extremely small magnetic fields).

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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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The core facility "metrology for ultralow magnetic fields" enables external scientists to get access to know-how and infradtructure for measurements of extremely small magnetic fields. As sensors we employ SQUID magnetometer developed at PTB capable of measuring down to a few femto Tesla, with a noise down to 150 aT/sqrt(Hz), which is among the most sensitive SQUID-systems worldwide. 

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