Absolute magnetometer based on the high-frequency modulation of the kinetic inductance of a superconducting thin film

Abstract

The high-frequency thermal modulation of a superconducting closed loop just below its critical temperature gives rise to a spectacular divergence of its kinetic inductance. It is shown that this periodic divergence due to that of the London penetration length λ permits very fine magnetic measurements. A superconducting magnetometer has been designed that can detect the absolute intensity of weak magnetic fields, with a noise level of 10−12 T/√Hz. This absolute level was reached by the double thermal modulation of a superconducting disk around its critical temperature and of a neighboring superconducting closed loop. As a consequence of the Meissner effect, the superconducting disk gives an estimation of the applied perpendicular magnetic field and removes any incertitude about the number of flux quantums inside the loop. Then, the modulation of the temperature of the loop increases the absolute sensitivity. The detection is performed by a very low-loss superconducting LC resonator set at the input of a cryogenic preamplifier. The thermal modulation is performed by pigtailed laser diodes. Practical limitations and further improvements are discussed.