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

摘要

The magnetic field noise in superconducting quantum interference devices(SQUIDs) used for biomagnetic research such as magnetoencephalography orultra-low-field nuclear magnetic resonance is usually limited by instrumentaldewar noise. We constructed a wideband, ultra-low noise system with a 45 mmdiameter superconducting pick-up coil inductively coupled to a current sensorSQUID. Thermal noise in the liquid helium dewar is minimized by usingaluminized polyester fabric as superinsulation and aluminum oxide strips asheat shields, respectively. With a magnetometer pick-up coil in the center ofthe Berlin magneti- cally shielded room 2 (BMSR2) a noise level of around 150aT Hz$^{-1/2}$ is achieved in the white noise regime between about 20 kHz andthe system bandwidth of about 2.5 MHz. At lower frequencies, the resolution islimited 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 canquantitatively reproduce its measured field noise.