  High-quality cavities are crucial for various fundamental physical studies
and applications. Here we find that by coupling two cavities directly or via a
phase-tunable coupling channel, the photon lifetime of the local field can
exceed that of the bare cavities. The cavity photon lifetime is modified by the
phases of the initial states and the phase accumulation on the coupling channel
which affect the interference between cavities. In experiments, by coupling
superconducting radio-frequency cavities via phase-tunable cables, we realize a
factor of two improvement in the cavity photon lifetime. The results can bring
rich revenue to quantum information science, sensing, and high-energy physics.
