In a superconducting circuit cooled to below 10 millikelvin, a mechanical oscillator vibrates at 5.33 megahertz. Usually in such systems, the mechanics outlives the light, but here the roles are reversed: the oscillator decays faster than the photons in the resonator. This reversed dissipative regime turns the mechanical mode into a cold conductor, making the vacuum sing nonclassical notes.

When the laser frequency is slowly modulated, quantum vacuum fluctuations are parametrically amplified. Pairs of microwave photons are born: the dynamical Casimir effect. The Kerr nonlinearity, emerging after adiabatic elimination of the mechanics, limits their number and causes oscillations in time. With weak nonlinearity, photons multiply exponentially; with strong nonlinearity, they pulsate.

In a certain regime, photons exhibit sub-Poissonian statistics (Mandel parameter down to -0.5) and simultaneously Wigner function negativity. Quadrature squeezing reaches 4.3 dB in the full model and 5 dB in the weak-coupling approximation. The photon generation rate varies from 10 MHz to 1.23 GHz. Such a combination of nonclassical features in a single scheme is unattainable in standard optomechanical systems.

Until now, the reversed dissipative regime has been used for cooling, amplification, and entanglement, but not for generating nonclassical states of light. The new work shows that slow modulation of the laser frequency turns this regime into a source of photons with two nonclassical signatures at once. The induced Kerr nonlinearity is tunable, set by the mechanical oscillator rather than a fixed material. This paves the way for compact sources of nonclassical microwave radiation for quantum computing, quantum sensing, and quantum communication.