A magnetometer that can measure extremely small magnetic fields using superconductors with Josephson junctions (see Josephson effects). SQUIDs were invented in the mid-1960s soon after Josephson junctions. Because they can detect very small magnetic fields, including neural activity inside brains, SQUIDs are very useful in biology. They have also been used to investigate various aspects of quantum theory. The use of SQUIDs to determine certain physical quantities very accurately is an example of quantum metrology. If a SQUID is at the nano-scale, it is called a nano-SQUID.