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Register a new userSuperconducting microwave magnetometer for absolute flux detection
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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 $Phi_0 approx 2.068times 10^{-15},mathrm{Wb}$, 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.
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We describe a dispersive nanoSQUID magnetometer comprised of two variable thickness aluminum weak-link Josephson junctions shunted in parallel with an on-chip capacitor. This arrangement forms a nonlinear oscillator with a tunable 4-8 GHz resonant frequency with a quality factor Q = 30 when coupled directly to a 50 $Omega$ transmission line. In the presence of a near-resonant microwave carrier signal, a low frequency flux input generates sidebands that are readily detected using microwave reflectometry. If the carrier excitation is sufficiently strong then the magnetometer also exhibits parametric gain, resulting in a minimum effective flux noise of 30 n$Phi_0$/Hz$^{1/2}$ with 20 MHz of instantaneous bandwidth. If the magnetometer is followed with a near quantum-noise-limited Josephson parametric amplifier, we can increase the bandwidth to 60 MHz without compromising sensitivity. This combination of high sensitivity and wide bandwidth with no on-chip dissipation makes this device ideal for local sensing of spin dynamics, both classical and quantum.
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