The weakest magnetic field measured is 0.5 femtotesla/√Hz, and was achieved by Dr. Sheraz Khan (Pakistan) and Dr. David Cohen (USA), at the Martinos Center, Massachusetts General Hospital, Charlestown, Massachusetts, USA, and was published in the Review of Scientific Instruments on 1 May 2013.
The measurement was to determine the magnetic "noise" produced by the wall in a magnetically shielded room. The magnetically shielded room is used for measurements of unimaginably weak magnetic fields, such as those produced by a brain or heart. Studies at this level allow a better understanding of the complex processes at work in the human body. To measure these magnetic fields accurately, the influence of other magnetic sources (such as the wall) must be measured and accounted for. In doing so, Drs Khan and Cohen recorded the weakest magnetic field ever measured.
Comments below may relate to previous holders of this record.