Nonreciprocal Phonon Laser in an Asymmetric Cavity with an Atomic Ensemble

Phonon lasers,as a novel kind of lasers for generating coherent sound oscillation,has attracted extensive attention.Here,we theoretically propose a nonreciprocal phonon laser in a hybrid optomechanical system,which is composed of an asymmetric Fabry-Pérot cavity,an ensemble of N identical two-level atoms,and a mechanical oscillator.The effective driving amplitude related to driving direction leads to an obvious difference in mechanical gain and threshold power,bringing about a nonreciprocal phonon laser.In addition,the dependence of the phonon laser on the atomic parameters is also discussed,including the decay rate of the atoms and the coupling strength between the atoms and the cavity field,which provides an additional degree of freedom to control the phonon laser action.Our work provides a path to realizing a phonon laser in an atoms-cavity optomechanical system and may aid the manufacture of directional coherent phonon sources.

Collective radiance of giant atoms in non-Markovian regime

We investigate the non-Markovian dynamics of two giant artificial atoms interacting with a continuum of bosonic modes in a onedimensional(1D) waveguide.Based on the diagrammatic method,we present the exact analytical solutions,which predict the rich phenomena of collective radiance.For the certain collective states,the decay rates are found to be far beyond that predicted in the the Dicke model and standard Markovian framework,which indicates the occurrence of super-superradiance.The superadiance-to-subradiance transition could be realized by adjusting the exchange symmetry of giant atoms.Moreover,there exist multiple bound states in continuum(BICs),with photons/phonons bouncing back and forth in the cavity-like geometries formed by the coupling points.The trapped photons/phonons in the BICs can also be re-released conveniently by changing the energy level splitting of giant atoms.The mechanism relies on the joint effects of the coherent time-delayed feedback and the interference between the coupling points of giant atoms.This work fundamentally broadens the fields of giant atom collective radiance by introducing non-Markovianity.It also paves the way for a clean analytical description of the nonlinear open quantum system with more complex retardation.