Josephson Inductance Thermometry in Resonantly-Coupled Van-der-Waals Heterostructures

A promising strategy for pushing single-shot energy-resolving detection to the level of individual microwave photons, thermal phonons, and single kBT heat pulses is the development of thermal detectors with miniscule heat capacities. Graphene, with its vanishing heat capacity and diminished electron-phonon coupling at cryogenic temperatures, is an enticing material platform for achieving heat capacities at the level of single-kB in solid-state systems at dilution refrigerator temperatures. Key to the design and operation of a thermal detector is the readout method employed to monitor the temperature of the thermal element. However, to date, existing thermometry methods for Van-der-Waals materials have typically been slow and ill-suited for single-shot calorimetry, limited either by long averaging times, sweep repetition rates, or resetting times of switched Josephson junctions. This dissertation presents Josephson inductance thermometry, a method we have demonstrated for probing the electron temperature of Van-der-Waals materials at milli-Kelvin temperatures. The technique relies upon the inductive loading of a superconducting resonator by a graphene-based Josephson junction, in which increases in electron temperature of the graphene flake are transduced to shifts of the resonant frequency. This technique brings with it many of the benefits of resonant readout, such as fast response times, ease of frequency-division multiplexing, and operation at the lowest temperatures available to a dilution refrigerator where the device parameter regime yields the greatest detector sensitivities. Such a device design is well-suited, for example, to the serving as the fundamental pixel architecture of next-generation dark matter searches. This thesis derives all thermal detector performance metrics and fundamental noise sources from first principles and provides a pedagogical introduction to the superconducting phenomena and low-temperature physics exploited by the detector. Subsequently, a thorough discussion is presented of the device architecture, fabrication procedures, measurement chain, physical characterization via carrier density sweeps, physical characterization via Joule heat sweeps, and noise measurement characterization. It is our hope that researchers interested in pushing the limits of ultrasensitive thermal detectors and calorimetry can use this thesis to delve into details of Josephson inductance thermometry as well as the field of cryogenic thermal detection broadly.