Effects of Shielding and Filtering on Qubit Coherence and Effective Temperatures at QUIET

The Quantum Underground Instrumentation Experimental Testbed (QUIET) is a laboratory 100 meters underground at Fermilab designed to support advanced research in quantum computing and cryogenic detectors. Equipped with a 10 mK dilution refrigerator, QUIET enables the low-background characterization of superconducting qubits and detectors in a controlled environment. Over a series of cooldowns, we systematically adjusted fridge attenuation, shielding, and filtering to significantly reduce qubit effective temperatures, increase single-shot readout fidelities, and increase coherence times in a six-transmon chip. By monitoring the populations of higher excited states and calculating qubit effective temperatures, we can evaluate the efficacy of our noise-mitigation measures in comparative tests from run to run. In this talk, I will present the hardware modifications that led to improved coherence and reduced thermal populations, describe the methods used to extract effective qubit temperatures, and discuss our strategies moving forward.