Sensitivity Improvement of Spin-Exchange Relaxation Free Atomic Magnetometers by Hybrid Optical Pumping of Potassium and Rubidium

An optically pumped atomic magnetometer using a hybrid cell of potassium and rubidium atoms was demonstrated to yield high sensitivity to magnetic fields. We operated the magnetometer with the four possible combinations of optically pumped and optically probed atoms and found that the combination of optically pumped potassium and optically probed rubidium showed the highest sensitivity among the four combinations because the rubidium atoms were denser than those of potassium. Furthermore, we investigated the dependence of the sensitivity on the power densities of the pump and probe beams and the wavelength of the probe beam. The magnetometer using the hybrid cell required higher pump-beam power and had narrower magnetic linewidth than those of the single alkali-metal cell. However, the magnetic linewidth was larger than the theoretical value, ignoring the spin relaxation caused by the spin-exchange collisions. By adjusting the laser conditions, the highest sensitivity approached 30 fT_rms/Hz^1/2.

I. Introduction

Biomagnetic field measurement methods such as magnetoencephalography (MEG) and magnetic resonance imaging (MRI) are of considerable practical concern from the standpoint of non-invasive analysis of human brain function. Superconducting quantum interference devices (SQUIDs), which have ultra-high sensitivity to magnetic fields in the femtotesla range, have been recently used to detect biomagnetic fields. However, these tools are extremely expensive to maintain because they require cryogenic cooling to sustain the superconductivity state regardless of whether they are in operation. Alternative sensors are therefore desirable for the development and spread of biomagnetic field measurements. The most remarkable of these are optically pumped atomic magnetometers, which were chiefly developed by the Romalis group at Princeton University [1]–[3]. These magnetometers have high sensitivity, which has been theoretically estimated to be 0.01 under spin-relaxation free (SERF) conditions [2]. Several research groups have empirically applied optically pumped atomic magnetometers to biomagnetic field measurements [3]–[6]. Therefore, we have investigated their properties and reported the suitability of the potassium atomic magnetometer for biomagnetic field measurements by use of a phantom model of the human head [7].