[scientizscht (OP)]

Hello

What is the smallest electro/magnetic field that can be detected with precision?

Thanks!

[alancalverd]

The human eye can detect about 5 visible photons. X-ray detectors respond to single photons. At low frequencies (low photon energies) the detection limit is set by the integration period required to distinguish the target signal from thermal noise, so the question is one of practical rather than theoretical limitation.

[Bored chemist]

A SQUID can measure magnetic fields smaller than those produced by the beating of your heat.
http://hyperphysics.phy-astr.gsu.edu/hbase/Solids/Squid.html

https://en.wikipedia.org/wiki/SQUID

[evan_au]

Cryogenically cooled detectors on large telescopes can detect single photons that strike a CCD cell.

Unfortunately, the wiring and insulation between the CCD cells takes up some space on the surface of the chip, so they can't quite reach 100% photon efficiency.

See: https://en.wikipedia.org/wiki/Charge-coupled_device#Use_in_astronomy