  Losses are ubiquitous and unavoidable in nature inhibiting the performance of
most optical processes. Manipulating losses to adjust the dissipation of
photons is analogous to braking a running car that is as important as
populating photons via a gain medium. Here, we introduce the transient loss
boundary into a photon populated cavity that functions as a photon brake and
probe photon dynamics by engineering the brake timing and brake strength.
Coupled cavity photons can be distinguished by stripping one photonic mode
through controlling the loss boundary, which enables the transition from a
coupled to an uncoupled state. We interpret the transient boundary as a
perturbation by considering both real and imaginary parts of permittivity, and
the dynamic process is modelled with a temporal two-dipole oscillator, one with
the natural resonant polarization and the other with a frequency-shift
polarization. The model unravels the underlying mechanism of concomitant
coherent spectral oscillations and generation of tone-tuning cavity photons in
the braking process. By synthesizing the temporal loss boundary into a photon
populated cavity, a plethora of interesting phenomena and applications are
envisioned such as the observation of quantum squeezed states, low-loss
nonreciprocal waveguides and ultrafast beam scanning devices.
