Superconducting quantum interferometers

Quantum interferometers for magnetic microscopy of microstructures and electric circuits at room temperature

The focus of our recent work has been the development of a scanning magnetic microscope based on high-Tc superconducting quantum interferometers (SQUIDs) with a ferromagnetic antenna. An extremely soft magnetic amorphous foil was used to guide the flux from room temperature samples to the liquid nitrogen-cooled SQUID-sensor and back again. The flux guide passes through the pick-up loop of the high-Tc SQUID providing an improved coupling of the object’s magnetic flux to the SQUID.

The device measures the z-component (direction perpendicular to the sample surface) of the stray field of the sample, which is rastered with submicron precision in x-y direction by a motorized computer controlled scanning stage. A lateral resolution of better than 10 µm was achieved. Fields of wires carrying currents of less than 1 µA could be detected with a time constant of 1 sec. An increase in the flux noise of the SQUID due to the presence of the soft magnetic foil was not essential.

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Copyright:
— M. Faley
  1. M. I. Faley et al., IEEE Transactions on Appl. Supercond., 27, No.4, 1600905 (2017).
    DOI: 10.1109/TASC.2016.2631419

High-Tc quantum interferometers for biomagnetic measurements

The expected increase in the price of helium is set to seriously impact on operation costs associated with magnetoencephalography (MEG) which currently relies on low-Tc SQUIDs that are cooled exclusively by liquid helium. High-Tc SQUIDs are cooled by cheap, easy-to-handle and readily available liquid nitrogen, and can be placed much nearer to magnetic sources than low-Tc SQUIDs. The problem lies in producing HTc SQUIDs with a magnetic field resolution better than 10 fT/sqrt(Hz) at 77 K, required for their application in MEG systems.

We have developed 16-mm high-Tc SQUID magnetometers possessing a magnetic field resolution ≈ 4 fT/sqrt(Hz) at 77 K and, together with INM-4 FZJ, have demonstrated MEG measurements with high-Tc SQUIDs compared to those obtained using a commercial low-Tc MEG system. The low intrinsic noise and closer positioning of the high-Tc SQUIDs to the cortex contributed to the high signal-to-noise ratio of the MEG data obtained using the high-Tc system. These results open up new ways to upgrade MEG systems using low-Tc SQUIDs, which would make these systems independent of helium, more user-friendly and would save on a large part of the operating costs leading, in turn, to the widespread utilization of MEG systems.

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Comparison of auditory evoked MEG averaged signals acquired at a similar location over the left hemisphere using the low-Tc (50 positions, blue, upper curve) and the high-Tc (16 positions, red, lower curves) SQUID systems. The time range highlighted in grey was used for source analysis.
Copyright:
— M. Faley
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Results from source analysis of auditory evoked neuromagnetic field responses, as recorded by high-Tc SQUID (red circle) and low-Tc SQUID (blue triangle) systems. The location of the dipolar source from the high-Tc SQUID system (red) is, as expected, in the region of the left primary auditory cortex and is in very good agreement with the source analysis using data recorded by the low-Tc SQUID system (blue).
Copyright:
— M. Faley
  1. M. I. Faley et al., IEEE Transactions on Applied Superconductivity, 23 Issue 3 part 1 p.1600705(5) (2013). DOI: 10.1109/TASC.2012.2229094.
  2. J. Dammers et al., Appl. Phys. Lett. 104, 213705 (2014); http://dx.doi.org/10.1063/1.4880097.
  3. M. I. Faley et al., Superconductor Science and Technology 30, 083001 (2017). https://doi.org/10.1088/1361-6668/aa73ad

High-Tc quantum interferometers for geomagnetic surveys

The most often used high-Tc flip-chip magnetometers have a pick-up loop size of about 8 mm x 8 mm, a magnetic field sensitivity 1 nT/Φ0 and magnetic field resolution down to about 15 fT/sqrt(Hz) at 77 K.

These magnetometers are able to operate outside magnetic shielding, which is essential for geomagnetic surveys.

Due to their relatively small size they can be integrated into a compact mobile vector magnetometer or tensor systems.

Cooling with liquid nitrogen has significant advantages compared to the low-Tc systems; the system’s signal-to-noise ratio and slew rates are significantly lower than those of room temperature systems.

Another advantage is that the supply of liquid helium is a serious problem, especially in remote areas, whereas liquid nitrogen can be readily obtained simply from the air.

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Nanofabrication and Quantum Sensing

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Prof. Dr. Michael Faley

Phone: +49 2461 61-4366
Fax: +49 2461 61-6444
E-Mail: m.faley@fz-juelich.de

 
Last Modified: 15.06.2022