While superconductivity is a collective phenomenon of all electrons and ions in a superconductor’s crystal lattice, the charge carriers flowing through the material, and carrying the so-called super-current can be approximately understood as a compound particle formed by a pair of electrons, known as a Cooper pair, which is able to flow through the crystal lattice without interactions with the static ions, thus flowing without resistance, unlike unpaired electrons.
However, this behaviour means that the momentum of Cooper pairs is not frequently reset as happens with single electrons carrying a current. Therefore, when the electric field inducing the flow of the super-current changes, the movement of the Cooper pairs is not immediately affected, due to the inertia of the pairs.
The change in super-current then lags behind the change in voltage, opposing it as an inductance. In a traditional conductor, inductance happens as energy is stored in a magnetic field, which then sustains the flow of charges. As in this case the energy is instead stored as kinetic energy of the Cooper pairs, this kind of inductance is known as kinetic inductance.
Another important property of BCS theory, the underlying microscopic theory of superconductivity, is that Cooper pairs have a small binding energy. Typically this energy gap is in the order of 1 to 100 meV for Type I superconductors, which is sufficiently low to be sensitive to photons down to the THz range, or to [[Phonon-mediated sensing of substrate|phonons in a crystal substrate]] over which the superconductor may be deposited. When these break, the electrons that compose them are then free to conduct electricity in the classical manner, though as they behave as “quasi-” Cooper pairs, they are often known as quasiparticles.
This provides electricity an alternate channel to flow through without inductance, but with resistivity. The total flowing current is then divided between the two channels, lowering the overall inductance experienced by the flowing current.
As this effect is linearly proportional on the changes in density of Cooper pairs and quasiparticles, measuring it allows for a linear measurement of the energy being deposited within the volume of superconductor.