We report on a two-channel magnetometer based on nonlinear magneto-optical rotation in a Cs glass cell with buffer gas. The Cs atoms are optically pumped and probed by free running diode lasers tuned to the D$_2$ line. A wide frequency modulation of
the pump laser is used to produce both synchronous Zeeman optical pumping and hyperfine repumping. The magnetometer works in an unshielded environment and spurious signal from distant magnetic sources is rejected by means of differential measurement. In this regime the magnetometer simultaneously gives the magnetic field modulus and the field difference. Rejection of the common-mode noise allows for high-resolution magnetometry with a sensitivity of pthz{2}. This sensitivity, in conjunction with long-term stability and a large bandwidth, makes possible to detect water proton magnetization and its free induction decay in a measurement volume of 5 cm$^3$
We present first, encouraging results obtained with an experimental apparatus based on Coherent Population Trapping and aimed at detecting biological (cardiac) magnetic field in magnetically compensated, but unshielded volume. The work includes magne
tic-field and magnetic-field-gradient compensation and uses differential detection for cancellation of (common mode) magnetic noise. Synchronous data acquisition with a reference (electro-cardiographic or pulse-oximetric) signal allows for improving the S/N in an off-line averaging. The set-up has the relevant advantages of working at room temperature with a small-size head, and of allowing for fast adjustments of the dc bias magnetic field, which results in making the sensor suitable for detecting the bio-magnetic signal at any orientation with respect to the heart axis and in any position around the patient chest, which is not the case with other kinds of magnetometers.
