  Optically pumped magnetometers are becoming a promising alternative to
cryogenically-cooled superconducting magnetometers for detecting and imaging
biomagnetic fields. Magnetic field detection is a completely non-invasive
method, which allows one to study the function of excitable human organs with a
sensor placed outside the human body. For instance, magnetometers can be used
to detect brain activity or to study the activity of the heart. We have
developed a highly sensitive miniature optically pumped magnetometer based on
cesium atomic vapor kept in a paraffin-coated glass container. The magnetometer
is optimized for detection of biological signals and has high temporal and
spatial resolution. It is operated at room- or human body temperature and can
be placed in contact with or at a mm-distance from a biological object. With
this magnetometer, we detected the heartbeat of an isolated guinea-pig heart,
which is an animal widely used in biomedical studies. In our recordings of the
magnetocardiogram, we can in real-time observe the P-wave, QRS-complex and
T-wave associated with the cardiac cycle. We also demonstrate that our device
is capable of measuring the cardiac electrographic intervals, such as the RR-
and QT-interval, and detecting drug-induced prolongation of the QT-interval,
which is important for medical diagnostics.
