# An ultra-sensitive and wideband magnetometer based on a superconducting quantum interference device

## TL;DR

This paper presents a superconducting quantum interference device (SQUID)-based magnetometer with ultra-low noise and wide bandwidth, suitable for biomagnetic applications, achieved through innovative thermal noise mitigation techniques.

## Contribution

The authors developed a wideband, ultra-sensitive SQUID magnetometer with minimized thermal noise, achieving record low noise levels in the frequency range relevant for biomagnetic research.

## Key findings

- Achieved a noise level of around 150 aT/Hz^{1/2} between 20 kHz and 2.5 MHz.
- Successfully modeled the magnetic noise of the shielded room to match measurements.
- Demonstrated effective thermal insulation techniques to reduce dewar noise.

## Abstract

The magnetic field noise in superconducting quantum interference devices (SQUIDs) used for biomagnetic research such as magnetoencephalography or ultra-low-field nuclear magnetic resonance is usually limited by instrumental dewar noise. We constructed a wideband, ultra-low noise system with a 45 mm diameter superconducting pick-up coil inductively coupled to a current sensor SQUID. Thermal noise in the liquid helium dewar is minimized by using aluminized polyester fabric as superinsulation and aluminum oxide strips as heat shields, respectively. With a magnetometer pick-up coil in the center of the Berlin magneti- cally shielded room 2 (BMSR2) a noise level of around 150 aT Hz$^{-1/2}$ is achieved in the white noise regime between about 20 kHz and the system bandwidth of about 2.5 MHz. At lower frequencies, the resolution is limited by magnetic field noise arising from the walls of the shielded room. Modeling the BMSR2 as a closed cube with continuous \mu-metal walls we can quantitatively reproduce its measured field noise.