The second quantization of a scalar field in an open cavity is formulated, from first principles, in terms of the quasinormal modes ͑QNMs͒, which are the eigensolutions of the evolution equation that decay exponentially in time as energy leaks to the outside. For any amount of damping, this formulation provides an exact description involving the cavity degrees of freedom only, with the outside acting as a ͑thermal or driven͒ source. Thermal correlation functions and cavity Feynman propagators are thus expressed in terms of the QNMs, labeled by a discrete index rather than a continuous momentum. Single-resonance domination of the density of states and the spontaneous decay rate is then given a proper foundation. This is an essential first step towards the application of QNMs to cavity quantum electrodynamics phenomena, to be reported elsewhere.

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