The sensor head is made up of a series of coils that cancel out unwanted signals and amplifies the signals that are needed. So the tiny magnetic fields produced by a person's heart can be transmitted into the heavily shielded environment. What we've been able to do is combine existing technology from the areas of atomic physics and medical physics in a completely unique way.
Supposedly, this development came as a "by-product" of a research grant titled "Creating Long Chain Entanglement Using a Phase Sensitive Micromaser". This is another example of not judging a scientific work by its name. You just never know what direct benefits that you will get even when the subject matter being studied sound esoteric.