Abstract: Noise sensing in microwave cavities has implications in important tasks such as dark matter search. In this talk, we introduce the fundamental limits and design protocols on achieving the limits. We begin with assuming noiseless ancilla, where ultimate limit is derived and protocols to achieve them are found. When all modes are lossy, it is found that a Rayleigh limit exists on the sensitivity. We discuss transient state sensing to mitigate the Rayleigh limit.