Dispersive magnetometry with a quantum limited SQUID parametric amplifier

There is currently fundamental and technological interest in measuring and manipulating nanoscale magnets, particularly in the quantum coherent regime. To observe the dynamics of such systems one requires a magnetometer with not only exceptional sensitivity but also high gain, wide bandwidth, and low backaction. We demonstrate a dispersive magnetometer consisting of a two-junction superconducting quantum interference device (SQUID) in parallel with an integrated, lumped-element capacitor. Input flux signals are encoded as a phase modulation of the microwave drive tone applied to the magnetometer, resulting in a single quadrature voltage signal. For strong drive power, the nonlinearity of the resonator results in quantum limited, phase sensitive parametric amplification of this signal, which improves flux sensitivity at the expense of bandwidth. Depending on the drive parameters, the device performance ranges from an effective flux noise of 0.29 $\mu {\Phi }_0$Hz${}^{-1/2}$ and 20 MHz of signal bandwidth to a noise of 0.14 $\mu {\Phi }_0$Hz${}^{-1/2}$ and a bandwidth of 0.6 MHz. These results are in excellent agreement with our theoretical model.