Spin Damping in an RF Atomic Magnetometer

Under negative feedback, the quality factor Q of a radio-frequency magnetometer can be decreased by more than two orders of magnitude, so that any initial perturbation of the polarized spin system can be rapidly damped, preparing the magnetometer for detection of the desired signal. We find that noise is also suppressed under such spin-damping, with a characteristic spectral response corresponding to the type of noise; therefore magnetic, photon-shot, and spin-projection noise can be measured distinctly. While the suppression of resonant photon-shot noise implies the closed-loop production of polarization-squeezed light, the suppression of resonant spin-projection noise does not imply spin-squeezing, rather simply the broadening of the noise spectrum with Q. Furthermore, the application of spin-damping during phase-sensitive detection suppresses both signal and noise in such a way as to increase the sensitivity bandwidth. We demonstrate a three-fold increase in the magnetometer's bandwidth while maintaining 0.3 fT/\surdHz sensitivity.Comment: 24 pages, 7 figure

Magnetic field imaging with microfabricated optically-pumped magnetometers.

A multichannel imaging system is presented, consisting of 25 microfabricated optically-pumped magnetometers. The sensor probes have a footprint of less than 1 cm2 and a sensitive volume of 1.5 mm &times; 1.5 mm &times; 1.5 mm and connect to a control unit through optical fibers of length 5 m. Operating at very low ambient magnetic fields, the sensor array has an average magnetic sensitivity of 24 fT/Hz1/2, with a standard deviation of 5 fT/Hz1/2&nbsp;when the noise of each sensor is averaged between 10 and 50 Hz. Operating in Earth's magnetic field, the magnetometers have a field sensitivity around 5 pT/Hz1/2. The vacuum-packaged sensor heads are optically heated and consume on average 76 &plusmn; 7 mW of power each. The heating power is provided by an array of eight diode lasers. Magnetic field imaging of small probe coils was obtained with the sensor array and fits to the expected field pattern agree well with the measured data.</p

A new generation of magnetoencephalography: room temperature measurements using optically-pumped magnetometers

Advances in the field of quantum sensing mean that magnetic field sensors, operating at room temperature, are now able to achieve sensitivity similar to that of cryogenically cooled devices (SQUIDs). This means that room temperature magnetoencephalography (MEG), with a greatly increased flexibility of sensor placement can now be considered. Further, these new sensors can be placed directly on the scalp surface giving, theoretically, a large increase in the magnitude of the measured signal. Here, we present recordings made using a single optically-pumped magnetometer (OPM) in combination with a 3D-printed head-cast designed to accurately locate and orient the sensor relative to brain anatomy. Since our OPM is configured as a magnetometer it is highly sensitive to environmental interference. However, we show that this problem can be ameliorated via the use of simultaneous reference sensor recordings. Using median nerve stimulation, we show that the OPM can detect both evoked (phase-locked) and induced (non-phase-locked oscillatory) changes when placed over sensory cortex, with signals ~4 times larger than equivalent SQUID measurements. Using source modelling, we show that our system allows localisation of the evoked response to somatosensory cortex. Further, source-space modelling shows that, with 13 sequential OPM measurements, source-space signal-to-noise ratio (SNR) is comparable to that from a 271-channel SQUID system. Our results highlight the opportunity presented by OPMs to generate uncooled, potentially low-cost, high SNR MEG systems