Advances in Biomagnetism pp 67-72 | Cite as

# Evolution of the *SQUID* and its Use in Biomagnetic Research

## Abstract

With benefit of several decades of hindsight, I think London’s hypothesis, that superconductivity is a state characterized by long-range phase coherence, must have required great intellectual courage. He suggested that the theory of wave-particle duality known as quantum theory, hitherto applied only to submicroscopic systems such as individual atoms and molecules, should now be applied to vastly larger systems, such as a one-kilometer loop of superconducting wire. He proposed this startling hypothesis as a trivially simple way of understanding the Meissner effect, the exclusion of magnetic field from a bulk material when it is cooled through the superconducting transition temperature. A consequence of the hypothesis was that if magnetic field is trapped inside a hole or imperfection in a piece of bulk superconductor, the total flux contained within the hole must be quantized. The flux within the hole must be an integral multiple of a basic value, denoted Ф_{o}. He suggested Ф_{o} = *h/e* (Planck’s constant divided by the electronic charge) as an obvious possibility for the magnitude of the quantum of flux. The quantity *h/e* had long been known as a factor in the magnetic effect in atomic spectra known as Zeeman splitting. In 1961, several years after London’s death, it was shown experimentally (and immediately confirmed theoretically, as Prof. Bill Little once put it) that the flux quantum is Ф_{o} = *h/e* (or 2.07 × 10^{−15} Wb), rather than *h/e* (Doll and Nabauer, 1961; Deaver and Fairbank, 1961). It was already known that the superconducting state involved electron pairing (with charge 2*e*), but it seems no one had thought to relate this to flux quantization.

### Keywords

Microwave Coherence Boulder## Preview

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