For MEG measurements made thus far, only zero-field magnetometers have been used. When operated in a regime of frequent atomic collisions and in low magnetic fields, the decoherence mechanism through spin-exchange collisions can be suppressed (Happer and Tang 1973). This so-called spin-exchange relaxation-free (SERF) regime (Allred et al. 2002) allows for very high magnetometer sensitivities, but limits the dynamic range of the magnetometer. In very small magnetic fields, the spins are tilted by the magnetic field and a static reorientation results from the balance between precession and continuous pumping. Again, the orientation of the spins is measured with near-resonant light by detecting the transmission of resonant light. Often it is more advantageous to monitor the polarization rotation of a slightly detuned light beam, which is usually done with a balanced polarimeter. This method can cancel intensity noise of the laser light and also tolerate much higher optical thickness of the vapor, since the light is detuned from resonance. In order to increase the signal-to-noise ratio, phase-sensitive detection can be implemented. For the zero-field resonances, one parameter, such as the magnetic field or the probe light polarization, is modulated and the same frequency component of the light is detected at a fixed phase with the modulation. Typically, OPMs measure the magnitude of the magnetic field, but the zero-field OPMs used for MEG are operated to measure a magnetic-field component in a certain direction, e.g., through external field modulation with an additional Helmholtz coil pair.

All of the devices used for MEG demonstrations have been operated in an open-loop configuration, where the dynamic range was limited by the linewidth of the resonance to below 100 nT. While feedback systems have been demonstrated in principle, they have not yet reached the same sensitivities (Seltzer and Romalis 2004). Furthermore, the response of the magnetometer shows the behavior of a first-order low-pass filter with a width corresponding to the linewidth of the atomic resonance. This usually limits the intrinsic bandwidth to below 1 kHz.

Figure 2a shows a chip-scale high-sensitivity OPM, which was manufactured by use of a micro-electro-mechanical-system (MEMS) process (Mhaskar et al. 2012). In the center of the cube is an alkali-vapor cell, heated to generate a sufficient atomic density of the vapor (Knappe et al. 2005). A laser, on resonance with a transition of the atoms, is circularly polarized and optically pumps the atoms. The same laser is used to monitor the atomic polarization by detecting the transmitted light with a photodiode (Dupont-Roc et al. 1969; Shah et al. 2007). This is only one of many possible configurations with respect to polarization, sensor shape, and beam.