Dr. Claudiu Genes

We discuss Floquet engineering of dissipative molecular systems through periodic driving of an infrared-active vibrational transition, either directly or via a cavity mode. Following a polaron quantum Langevin equations approach, we derive correlation functions and stationary quantities showing strongly modified optical response<br>of the infrared-dressed molecule. The coherent excitation of molecular vibrational modes, in combination with the modulation of electronic degrees of freedom due to vibronic coupling can lead to both enhanced<br>vibronic coherence as well as control over vibrational sideband amplitudes. The additional coupling to an infrared cavity allows for the controlled suppression of undesired sidebands, an effect stemming from the Purcell enhancement of vibrational relaxation rates.

We investigate the role of time delay in cold-damping optomechanics with multiple mechanical resonances.<br>For instantaneous electronic response, it was recently shown by C. Sommer and C. Genes [Phys. Rev. Lett. 123,<br>203605 (2019)] that a single feedback loop is sufficient to simultaneously remove thermal noise from many<br>mechanical modes. While the intrinsic delayed response of the electronics can induce single-mode and mutual<br>heating between adjacent modes, we propose to counteract such detrimental effects by introducing an additional<br>time delay to the feedback loop. For lossy cavities and broadband feedback, we derive analytical results for the<br>final occupancies of the mechanical modes within the formalism of quantum Langevin equations. For modes<br>that are frequency degenerate collective effects dominate, mimicking behavior similar to Dicke super- and<br>subradiance. These analytical results, corroborated with numerical simulations of both transient and steady state<br>dynamics, allow us to find suitable conditions and strategies for efficient single-mode or multimode feedback<br>optomechanics.

Molecules constitute compact hybrid quantum optical systems that can interface photons, electronic degrees of freedom, localized mechanical vibrations, and phonons. In particular, the strong vibronic interaction between electrons and nuclear motion in a molecule resembles the optomechanical radiation pressure Hamiltonian. While molecular vibrations are often in the ground state even at elevated temperatures, one still needs to get a handle on decoherence channels associated with phonons before an efficient quantum optical network based on optovibrational interactions in solid-state molecular systems could be realized. As a step towards a better understanding of decoherence in phononic environments, we take here an open quantum system approach to the nonequilibrium dynamics of guest molecules embedded in a crystal, identifying regimes of Markovian versus non-Markovian vibrational relaxation. A stochastic treatment, based on quantum Langevin equations, predicts collective vibron-vibron dynamics that resembles processes of sub- and super-radiance for radiative transitions. This in turn leads to the possibility of decoupling intramolecular vibrations from the phononic bath, allowing for enhanced coherence times of collective vibrations. For molecular polaritonics in strongly confined geometries, we also show that the imprint of optovibrational couplings onto the emerging output field results in effective polariton cross-talk rates for finite bath occupancies.

We investigate the influence of light-matter interactions on correlated quantum matter by studying the<br>paradigmatic Dicke-Ising model. This type of coupling to a confined, spatially delocalized bosonic light mode,<br>such as provided by an optical resonator, resembles a quantized transverse magnetic field of tunable strength. As<br>a consequence, the symmetry-broken magnetic state breaks down for strong enough light-matter interactions to<br>a paramagnetic state. The nonlocal character of the bosonic mode can change the quantum phase transition in<br>a drastic manner, which we analyze quantitatively for the simplest case of the Dicke-Ising chain geometry.<br>The results show a direct transition between a magnetically ordered phase with zero photon density and a<br>magnetically polarized phase with superradiant behavior of the light. Our predictions are equally valid for the<br>dual quantized Ising chain in a conventional transverse magnetic field.

We discuss a technique to strongly couple a single target quantum emitter to a cavity mode, which is enabled by virtual excitations of a nearby mesoscopic ensemble of emitters. A collective coupling of the latter to both the cavity and the target emitter induces strong photon nonlinearities in addition to polariton formation, in contrast to common schemes for ensemble strong coupling. We demonstrate that strong coupling at the level of a single emitter can be engineered via coherent and dissipative dipolar interactions with the ensemble, and provide realistic parameters for a possible implementation with <br>SiV− defects in diamond. Our scheme can find applications, amongst others, in quantum information processing or in the field of cavity-assisted quantum chemistry.

We apply adaptive feedback for the partial refrigeration of a mechanical resonator, i.e. with the aim to<br>simultaneously cool the classical thermal motion of more than one vibrational degree of freedom. The<br>feedback is obtained from a neural network parametrized policy trained via a reinforcement learning<br>strategy to choose the correct sequence of actions from a fnite set in order to simultaneously reduce<br>the energy of many modes of vibration. The actions are realized either as optical modulations of the<br>spring constants in the so-called quadratic optomechanical coupling regime or as radiation pressure<br>induced momentum kicks in the linear coupling regime. As a proof of principle we numerically illustrate<br>efcient simultaneous cooling of four independent modes with an overall strong reduction of the total<br>system temperature.