Superconducting microwave magnetometer for absolute flux detection

Superconducting quantum interference devices (SQUIDs) are among the most sensitive detectors for out-of-plane magnetic field components. However, due to their periodic response with short modulation period $M = 1 \Phi_0$, determined by the magnetic flux quantum $\Phi_0 \approx 2.068\times 10^{-15}\,\mathrm{Wb}$, it is difficult to infer the value of the magnetic flux unambiguously, especially in case the magnetic flux enclosed in the SQUID loop changes by many flux quanta. Here, we demonstrate that by introducing a second degree of freedom in the form of a second SQUID, we substantially enhance the modulation period $M$ of our device without sacrificing sensitivity. As a proof of concept, we implement our idea by embedding two asymmetric direct current SQUIDs into a common tank circuit. By measuring the reflection coefficient of the device, we extract the two lowest energy eigenfrequencies as a function of the external magnetic flux created by a superconducting field coil, from which we experimentally deduce a modulation period $M \geq 15 \Phi_0$, as well as the magnetic offset-field $B_0 = 22\,\mathrm{nT}$ present in our experiment.