The quantum world is full of surprises. In a Kerr parametric oscillator, a device where light is trapped in a ring and driven by an external field, researchers recently noticed something strange: pairs of energy levels merge in an embrace. Physicists have dubbed this a "spectral kiss" spectroscopy.

But perfect conditions are rare. There is always friction, leakage, and noise. The authors of the study showed that in the weak dissipation regime, the steady state of the system retains a memory of the kiss. The eigenvalues of the density matrix, which describes the state with friction taken into account, form nearly degenerate pairs, just as in the isolated case. As friction grows, the pairs split apart, and the kiss fades. Analytically, they managed to find the boundaries: where the merging begins and where it disappears at a dissipative phase transition.