Scanning probes

Scanning SQUID microscopy has long been used to probe mesoscopic systems by measuring local magnetometry and susceptibility in a non-invasive manner. Magnetic flux from nearby samples is coupled into the SQUID circuit through micron-scale pickup loops patterned near the junction, allowing the device to detect minute variations in local magnetic fields. In a DC scanning SQUID, the Josephson junction is biased near its critical current, and the voltage across the junction is monitored. Because this voltage depends on the superconducting phase difference between the two leads of the junction, the SQUID sensitively converts small changes in magnetic flux into measurable voltage signals, enabling spatially resolved imaging of magnetic fields at cryogenic temperatures. Despite its remarkable performance, the DC scanning SQUID faces a key fundamental limitation to being able to sensitively probe systems. Biasing the junction near its critical current introduces dissipation and electromagnetic radiation that can perturb delicate quantum coherent samples. This constraint motivates the development of radio frequency (RF)-based scanning SQUIDs, which offer low-dissipation operation while retaining sensitivity. The RF SQUID represents a promising next generation of scanning SQUID technology. Unlike its DC counterpart, the RF SQUID operates in the zero-voltage branch, making it inherently low-dissipation. Its resonance frequency, set by the SQUID’s inductance and capacitance, can be tuned by magnetic flux, which for our high frequency SQUID is from 3 to 12 GHz. This allows us to target specific probe frequencies to match our system of interest, ranging from probing Andreev spin qubits, collective modes, and superconducting quantum circuits. A key advantage of the RF configuration is the ability to harness its intrinsic nonlinearity of the Josephson Junction for parametric amplification using three-wave mixing processes, enabling gain of 10–20 dB while operating at “Kerr-free” bias points that minimize unwanted frequency shifts. In this regime, our RF scanning SQUID achieves flux noise levels close to the quantum noise limit. Moreover, because the device provides in situ gain, it can directly boost the signal-to-noise ratio before amplification from HEMTs or room-temperature electronics. Together, these features offer a pathway to low-noise, and minimally invasive scanning technology with broad frequency tunability. With its high sensitivity, low noise and minimal invasiveness, potential applications include the readout of delicate quantum states in qubits, BKT transition noise, Andreev Spin qubits and other sensitive condensed matter systems.