[kuhsaft]

> AMBIIENT’s goal was biological imaging.. sensors near a body for medical/neuroscience applications.

As with all DARPA projects, there is a civilian use-case and a military use-case.

That demo was at a conference, in a city, surrounded by electronics and RF noise. The fact that it worked at all in that environment is surprising. As the subject got closer to the apparatus, the signal became larger than the background noise. So, I think the distance is limited mostly by background noise. The press release did state that it was an optimal environment for locating their target, i.e., an isolated person with only geomagnetic noise and known signatures of the aircraft.

[quietsegfault]

If background noise was the factor, which it's not, but if it was, the background noise to combat would be the thermal noise floor of any physical conductor at any temperature above absolute zero. But that's not the factor. You're assuming the cardiac field is like a radio signal being transmitted rather than a local field effect. A magnetic dipole doesn't radiate energy outward the way a radio antenna does. It creates a static field that exists in the space around it, and that field geometrically collapses with distance. I ran the numbers on Wolfram Alpha, and at 1 meter from your chest the field is around 100 femtotesla. At 10 meters it's around 100 attotesla. At 5 kilometers it's around 10^-27 tesla.

Oh, but you say that you simply cool the sensor to 0K. Cooling helps, but you're still many orders of magnitude short even at near 0K, and you're doing this in Iranian mountains, not a dilution refrigerator.

[kuhsaft]

You’re calculating based on classical mechanics. The sensors are optical-atomic magnetometers, using quantum mechanics to measure magnetic vector potentials (MVP), which behave differently and can produce seemingly non-local effects (Aharonov–Bohm effect).

> A magnetic dipole doesn't radiate energy outward the way a radio antenna does.

See Section 15-5 of https://www.feynmanlectures.caltech.edu/II_15.html#Ch15-S5

Veritasium has a video on the weirdness of MVP: https://m.youtube.com/watch?v=XKSjCOKDtpk

See https://pubs.aip.org/aip/adv/article/13/2/025127/2877320/Dif... for an experiment measuring MVP with an optical-atomic magnetometer.

> Oh, but you say that you simply cool the sensor to 0K.

These sensors do not require cryogenics and have been developed with sensitivities of 10s of fT/√Hz, so approaching the quantum noise limit.

Essentially, you can think of it as measuring the energy of a magnetic dipole in space instead of measuring a magnetic field.