A. Ridolfo

Nanopolaritons: vacuum Rabi splitting with a single quantum dot in the center of a dimer nanoantenna.

We demonstrate with accurate scattering calculations that a system constituted by a single quantum emitter (a semiconductor quantum dot) placed in the gap between two metallic nanoparticles can display the vacuum Rabi splitting. The largest dimension of the investigated system is only 36 nm. This nonperturbative regime is highly desirable for many possible applications in quantum information processing or schemes for controlling individual photons. Along this road, it will be possible to implement scalable photonic quantum computation without renouncing to the nanometric size of the classical logic gates of the present most compact electronic technology.

Photon blockade in the ultrastrong coupling regime.

We explore photon coincidence counting statistics in the ultrastrong coupling regime, where the atom-cavity coupling rate becomes comparable to the cavity resonance frequency. In this regime, usual normal order correlation functions fail to describe the output photon statistics. By expressing the electric-field operator in the cavity-emitter dressed basis, we are able to propose correlation functions that are valid for arbitrary degrees of light-matter interaction. Our results show that the standard photon blockade scenario is significantly modified for ultrastrong coupling. We observe parametric processes even for two-level emitters and temporal oscillations of intensity correlation functions at a frequency given by the ultrastrong photon emitter coupling. These effects can be traced back to the presence of two-photon cascade decays induced by counterrotating interaction terms.

Spontaneous conversion from virtual to real photons in the ultrastrong-coupling regime.

We show that a spontaneous release of virtual photon pairs can occur in a quantum optical system in the ultrastrong coupling regime. In this regime, which is attracting interest both in semiconductor and superconducting systems, the light-matter coupling rate Ω(R) becomes comparable to the bare resonance frequency of photons ω(0). In contrast to the dynamical Casimir effect and other pair creation mechanisms, this phenomenon does not require external forces or time dependent parameters in the Hamiltonian.

Nonclassical Radiation from Thermal Cavities in the Ultrastrong Coupling Regime

Thermal or chaotic light sources emit radiation characterized by a slightly enhanced probability of emitting photons in bunches, described by a zero-delay second-order correlation function g((2))(0)=2. Here we explore photon-coincidence counting statistics of thermal cavities in the ultrastrong coupling regime, where the atom-cavity coupling rate becomes comparable to the cavity resonance frequency. We find that, depending on the system temperature and coupling rate, thermal photons escaping the cavity can display very different statistical behaviors, characterized by second-order correlation functions approaching zero or greatly exceeding two.

Fano-Doppler laser cooling of hybrid nanostructures.

Laser cooling the center-of-mass motion of systems that exhibit Fano resonances is discussed. We find that cooling occurs for red or blue detuning of the laser frequency from resonance depending on the Fano factor associated with the resonance. The combination of the Doppler effect with the radiation cross-section quenching typical of quantum interference yields temperatures below the conventional Doppler limit. This scheme opens perspectives for controlling the motion of mesoscopic systems such as hybrid nanostructures at the quantum regime and the exploration of motional nonclassical states at the nanoscale.

Vacuum-induced symmetry breaking in a superconducting quantum circuit

a † acquires a nonzero expectation value in the system ground state. We demonstrate that, in this case, the parity symmetry of an additional artificial atom with an even potential is broken by the interaction with the resonator. Such a mechanism is analogous to the Higgs mechanism where the gauge symmetry of the weak force’s gauge bosons is broken by the nonzero vacuum expectation value of the Higgs field. The results presented here open the way to controllable experiments on symmetry-breaking mechanisms induced by nonzero vacuum expectation values. Moreover, the mechanism proposed here can be used as a probe of the ground-state macroscopic coherence emerging from quantum phase transitions with vacuum degeneracy.

All Optical Switch of Vacuum Rabi Oscillations: The Ultrafast Quantum Eraser

We study the all-optical timexa0control of the strong coupling between a single cascade three-level quantum emitter and a microcavity. We find that only specific arrival times of the control pulses succeed in switching off the Rabi oscillations. Depending on the arrival times of control pulses, a variety of exotic nonadiabatic cavity quantum electrodynamics effects can be observed. We show that control pulses with specific arrival times, performing which-path and quantum-eraser operations, are able to suddenly switch-off and on first-order coherence of cavity photons, without affecting their strongxa0coupling population dynamics.

Parametric Oscillation, Frequency Mixing, and Injection Locking of Strongly Coupled Nanomechanical Resonator Modes

We study locking phenomena of two strongly coupled, high quality factor nanomechanical resonator modes to a common parametric drive at a single drive frequency in different parametric driving regimes. By controlled dielectric gradient forces we tune the resonance frequencies of the flexural in-plane and out-of-plane oscillation of the high stress silicon nitride string through their mutual avoided crossing. For the case of the strong common parametric drive signal-idler generation via nondegenerate parametric two-mode oscillation is observed. Broadband frequency tuning of the very narrow linewidth signal and idler resonances is demonstrated. When the resonance frequencies of the signal and idler get closer to each other, partial injection locking, injection pulling, and complete injection locking to half of the drive frequency occurs depending on the pump strength. Furthermore, satellite resonances, symmetrically offset from the signal and idler by their beat note, are observed, which can be attributed to degenerate four-wave mixing in the highly nonlinear mechanical oscillations.

Delayed-choice quantum control of light-matter interaction

Superposition is the fundamental signature of quantum behavior. A quantum system can exist in a superposition of different eigenstates of an observable. Only after the measurement process, the system will collapse on one of the eigenvalues of the measured observables. We propose the realisation of the counterintuitive superposition of On and Off strong light-matter interaction using an optical resonator and a generic three-level quantum system. Leaked cavity photons can be detected and collected in order to post-select the events and to choose if we want to observe interacting or non-interacting photons. Such proposal can be readily realised within the present technology. Finally we will show that the cavity field can have a morphing behavior between On and Off interaction, by changing the area of the control pulse.

Photoluminescence of single quantum dots in microcavities

We study theoretically the photoluminescence of a single quantum dot in a microcavity under incoherent excitation. Analytical results including pure dephasing show that strong coupling and linewidths are largely independent on the pumping intensity (until saturation effects come into play). We show the reliable predicting character in the analysis of some experiments.