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George W. Downs Laboratory of Physics California Institute of Technology, Pasadena, USA
In the two-fluid model introduced by Heinz London in 1934, the current in a superconductor includes contributions from both superconducting and normal electrons, corresponding to the familiar Cooper pairs and quasiparticles of the BCS theory. The pairs are lossless but have inertia, while the normal electrons are dissipative. Thus, in a circuit, the pairs contribute an induc- tance - the kinetic inductance - while the normal electrons contri- bute resistance. Absorption of energy - e.g. photons - capable of breaking the pairs into individual quasiparticles results in changes to both the inductance and resistance. These changes may be measured with high sensitivity through use of a resonant circuit, which also introduces the possibility of frequency multi- plexing. Over the past fifteen years, these simple ideas have been exploited to develop a variety of detector arrays for diverse applications in astrophysics across the spectrum, from millimeter to optical wavelengths. I will describe the basic physics, the development of the field, the current status, spin-offs, and provide examples of instruments currently being developed and deployed as well as some future applications.