* **Vacuum Fluctuations:** These are the zero-point energy fluctuations of the electromagnetic field, which are always present even in perfect vacuum. They provide a fundamental decay channel, causing spontaneous emission into available modes and affecting T1. The rate is governed by Fermi's Golden Rule and is highly dependent on the local density of states (LDOS) of the electromagnetic vacuum at the qubit's frequency, a phenomenon known as the Purcell effect. Engineering the LDOS via integrated photonic structures (e.g., cavities, photonic crystals) can either enhance (for fast reset) or suppress (for long T1) spontaneous emission into specific modes. The spontaneous emission rate $\Gamma_{sp}$ is proportional to the LDOS at $\omega_q$. Vacuum fluctuations are a form of quantum noise and set a fundamental limit on T1 in the absence of other noise sources.