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Coupled loss and gain resonators can exhibit parity-time (PT) symmetry and exceptional points, and find rich applications in sensing, lasing and quantum optics. This talk presents an intuitive and powerful quasinormal mode theory for coupled loss and gain resonators, including a quantitatively accurate coupled mode theory using quasinormal modes. As an application of the theory, we demonstrate extremely rich spectral lineshapes and Purcell factors for coupled loss-gain microdisk cavities. We then point out a fundamental flaw in currently adopted theories of spontaneous emission in absorptive and amplifying media, and present the corrected Fermi's golden rule, one that requires a fully quantum mechanical description.
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