Keigo Kamada

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 fTrms/Hz1/2.

Human magnetoencephalogram measurements using newly developed compact module of high-sensitivity atomic magnetometer

In the field of biomagnetic measurements, optically pumped atomic magnetometers (OPAMs) are expected to be alternative sensors to magnetometers based on superconducting quantum interference devices (SQUIDs). In addition, miniaturized OPAMs are required for practical use. To address this issue, we developed a compact module of a high-sensitivity OPAM with a pump–probe arrangement and potassium used as the sensing atom. Because the noise spectrum density of the OPAM reached 21 fTrms/Hz1/2 at 10 Hz, we attempted to use it to measure human magnetocardiograms (MEGs). Compared with the results obtained with SQUID-based magnetometers, we could successfully observe distinct features of event-related desynchronization in the 8–13 Hz band associated with eye opening. The results demonstrate the feasibility of using the OPAM module for neuromagnetic field measurements.

Human MCG measurements with a high-sensitivity potassium atomic magnetometer.

Measuring biomagnetic fields, such as magnetocardiograms (MCGs), is important for investigating biological functions. To address to this need, we developed an optically pumped atomic magnetometer. In this study, human MCGs were acquired using a potassium atomic magnetometer without any modulating systems. The sensitivity of the magnetometer is comparable to that of high-T(c) superconducting quantum interference devices (SQUIDs) and is sufficient for acquiring human MCGs. The activity of a human heart estimated from the MCG maps agrees well with that measured with SQUID magnetometers. Thus, our magnetometer produces reliable results, which demonstrate the potential of our atomic magnetometer for biomagnetic measurements.

Development of an optically pumped atomic magnetometer using a K-Rb hybrid cell and its application to magnetocardiography

We have developed an optically pumped atomic magnetometer using a hybrid cell of K and Rb. The hybrid optical pumping technique can apply dense alkali-metal vapor to the sensor head and leads to high signal intensity. We use dense Rb vapor as probed atoms, and achieve a sensitivity of approximately 100 fTrms/Hz1/2 around 10 Hz. In this case, the sensitivity is limited by the system noise, and the magnetic linewidth is narrower than that for direct Rb optical pumping. We demonstrated magnetocardiography using the magnetometer and obtained clear human magnetocardiograms.

Optimization of Bandwidth and Signal Responses of Optically Pumped Atomic Magnetometers for Biomagnetic Applications

Recently developed ultrasensitive optically pumped atomic magnetometers are promising for biomagnetic measurements such as magnetoencephalograms and magnetocardiograms. The magnetometers bandwidth and amplitude of signal response are important factors for biomagnetic measurements. These factors depend on various operating parameters such as power density and wavelengths of laser beams. By focusing on the transverse spin relaxation time, we theoretically and experimentally studied the change in the bandwidth and amplitude of the signal response. By considering the effect of the attenuation of a pump beam inside a cell, we showed good agreement between theoretical and experimental results. Furthermore, the magnetometers integrated performances for the factors were evaluated by changing the power density of the pump beam. Measured data indicated that the bandwidth of signal response depended on the power density of the pump beam and that the bandwidth could be tuned to a desirable frequency range.

Effect of Spatial Homogeneity of Spin Polarization on Magnetic Field Response of an Optically Pumped Atomic Magnetometer Using a Hybrid Cell of K and Rb Atoms

We measured the spatial homogeneity of spin polarization, which is directly related to the sensitivity and magnetic linewidth of an optically pumped atomic magnetometer with a hybrid cell of K and Rb atoms. This was done by changing the position of the probe beam. Furthermore, for comparison, we also measured the sensitivity and magnetic linewidth of atomic magnetometers with single K and single Rb cells, and found that optically thick atoms were spin polarized homogeneously with the hybrid cell. Optically thin alkali metal vapor can be spin polarized homogeneously, and the spin polarization transfers to an optically thick alkali metal vapor. An atomic magnetometer with the hybrid cell was found to be more effective in realizing simultaneous signal measurements at different locations with the uniform sensitivity and magnetic linewidth.

Noise reduction and signal-to-noise ratio improvement of atomic magnetometers with optical gradiometer configurations.

In the field of biomagnetic measurement, optically-pumped atomic magnetometers (OPAMs) have attracted significant attention. With the improvement of signal response and the reduction of sensor noise, the sensitivity of OPAMs is limited mainly by environmental magnetic noise. To reduce this magnetic noise, we developed the optical gradiometer, in which the differential output of two distinct measurement areas inside a glass cell was obtained directly via the magneto-optical rotation of one probe beam. When operating in appropriate conditions, the sensitivity was improved by the differential measurement of the optical gradiometer. In addition, measurements of the pseudo-magnetic noise and signal showed the improvement of the signal-to-noise ratio. These results demonstrate the feasibility of our optical gradiometer as an efficient method for reducing the magnetic noise.

Detecting rotating magnetic fields using optically pumped atomic magnetometers for measuring ultra-low-field magnetic resonance signals

In this paper, we describe the detection of rotating magnetic fields using optically pumped atomic magnetometers (OPAMs) for measuring magnetic resonance (MR) signals. From the results of rotating- and alternating-magnetic-field measurements, we found that to detect a rotating magnetic field with high sensitivity, the rotation direction of the magnetic field to be measured must select the bias-magnetic-field direction of OPAM. In addition, the OPAM sensitivity for rotating magnetic fields should be twice that for alternating magnetic fields. These results indicate that for measuring MR signals, magnetic fields caused by rotating magnetizations can be detected with the sensitivity of 10 fT(rms)/Hz order at 1 kHz using OPAMs.

Magnetic Field Mapping and Biaxial Vector Operation for Biomagnetic Applications Using High-Sensitivity Optically Pumped Atomic Magnetometers

Optically pumped alkali-metal atomic magnetometers are expected to be used not only for biomagnetic field measurements but also for magnetic resonance imaging because of their potential ultrahigh sensitivity. Here, we studied magnetic field mapping and biaxial vector operation using atomic magnetometers. A potassium atomic magnetometer was used in these measurements. First, we obtained sensor output signals by solving the Bloch equation. Next, we measured magnetic field distributions generated by a current dipole electrode that was placed in a spherical phantom, which simulated a group of simultaneously activated neurons in the human brain. We obtained vector contour maps of the magnetic field distributions from the dipoles oriented parallel and orthogonal to the pump laser beam and have found good agreement with theoretical magnetic field distributions. These results demonstrate practical applications of magnetic field mapping and biaxial vector operation using optically pumped atomic magnetometers.

Measurements of Magnetic Field Distributions With an Optically Pumped K-Rb Hybrid Atomic Magnetometer

We have developed a K-Rb hybrid optically pumped atomic magnetometer (OPAM) for biomagnetic measurements. With this hybrid OPAM, we used a linear photodiode array and a charge-coupled device (CCD) sensor to instantaneously measure 1-D and 2-D magnetic field distributions generated by a test coil, respectively. The measured distributions were compared with those calculated using the Biot-Savart law and showed good agreement for the linear photodiode array; however, in the case of the CCD sensor in the area away from the test coil, the intensity of the detected magnetic field was slightly different from the calculated one, possibly because of the diffusion of the spin-polarized atoms as well as smear and blooming in the CCD sensor. The sensitivity of the OPAM was 5-6 pT/Hz1/2 using the linear photodiode array and ~10 pT/Hz1/2 using the CCD sensor. In future experiments, we plan to fabricate a sensor cell with a high density of probe atoms and reduce the noise generated in the electronic circuitry of the detectors in order to increase sensitivity.