  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 magnetic-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.
