The cascade of resonant PT-symmetric topological structures is shown to emit laser light with a frequency comb spectrum. We consider optically active topological lattices supporting edge modes at regularly spaced frequencies. When the amplified resonances in the PT-broken regime match the edge modes of the topological gratings, we predict the emission of discrete laser lines. A proper design enables the engineering of the spectral features for specific applications. Topological protection makes the system very well suited for a novel generation of compact frequency comb emitters for spectroscopy, metrology, and quantum information.

Laura Pilozzi and Claudio Conti, Optics Letters 42, 5174 (2017)

## Quantum Simulation of Rainbow Gravity

Rainbow gravity modifies general relativity by introducing an energy dependent metric, which is expected to have a role in the quantum theory of black holes and in quantum gravity at Planck energy scale. We show that rainbow gravity can be simulated in the laboratory by nonlinear waves in nonlocal media, as those occurring in Bose-condensed gases and nonlinear optics. We reveal that at a classical level, a nonlocal nonlinear Schr\”odinger equation may emulate the curved space time in proximity of a rotating black hole as dictated by the rainbow gravity scenario. We also demonstrate that a fully quantized analysis is possible. By the positive $\mathcal{P}$-representation, we study superradiance and show that the instability of a black-hole and the existence of an event horizon are inhibited by an energy dependent metric. Our results open the way to a number of fascinating experimental tests of quantum gravity theories and quantum field theory in curved manifolds, and also demonstrate that these theories may be novel tools for open problems in nonlinear quantum physics.

The picture below shows spectra and configuration of particles trapped in a quantum simulation of a black-hole.

Braidotti and Conti, in ArXiv:1708.02623

## Squeezing of shock waves

Shock waves emerge in various fields and trigger effects like rogue waves, supercontinuum generation and emission of resonant radiation. Despite shock waves ubiquity, their effect on the coherence properties is barely understood. Among the open problems, the quantum features of shock waves are particularly intriguing for the possibility of generating non-classical states by highly nonlinear regimes. In a manuscript in arXiv, Maria Chiara Braidotti, Antonio Mecozzi, and Claudio Conti theoretically predict classical squeezing of light during shock formation. The picture above shows the Wigner transform of the field in the squeezed regime. By using techniques of rigged Hilbert space quantum mechanics, they show that a coherent state develops squeezing which stops in proximity of the shock point. The analysis holds true for temporal and spatial shock waves in highly nonlocal regime.These results open a new scenario in shock phenomena and link nonlinear propagation in extreme regimes with quantum optics and related applications.