Quantum sensing utilizes the inherent sensitivity of a quantum system to external stimuli. Our goal is to leverage this sensitivity to develop a quantum sensor designed for the detection of ionizing radiation. Here we report on the design, fabrication, and measurement of a new quantum device for hard x-ray and gamma-ray detection. Our quantum device is based on a superconducting quantum bit (qubit) with superconducting tunnel junctions as the core device elements. We describe our experimental investigation directed toward the detection metrics of energy resolution, dynamic range, and active area. In contrast to existing superconducting detectors, the active area per qubit may be much larger than the physical area of the tunnel junctions or the physical area of the qubit device, due to the sensitivity of quantum coherence to ionizing radiation deposition within a radius on the millimeter or centimeter scale. Our experimental design enables an ionizing radiation source at room temperature to be detected by our quantum sensor at low temperature.
We present experimental results from the microfabrication and characterization of nanostructured thin films within prototype devices, which we refer to as Meissner-effect transition-edge-sensors, that may ultimately be utilized for sensing applications including microscale magnetometry and microcalorimetry. For the devices reported here, the change in magnetic flux from a microscale disk of electrodeposited tin may be detected by a planar pickup coil consisting of a niobium nanostructured thin film. The optimization of the nanostructured thin films requires sufficient control of sputtering deposition parameters including chamber pressure, gas flow, and power. Multiple mechanisms provide nanostructured pathways for infiltration of the niobium thin film during deposition. The nanostructured thin films were experimentally characterized in terms of thickness, stress, room-temperature resistivity, and resistance as a function of temperature. We find that the superconducting transition temperature for the niobium nanostructured thin film pickup coils spans a significant range from approximately 4 Kelvin to 11 Kelvin depending on the sputtering deposition parameters. We target operation in the regime where the microscale tin disk is in the transition between the normal and superconducting states but the nanostructured thin film pickup coil is fully superconducting and therefore compatible with Superconducting Quantum Interference Device (SQUID) readout.
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