O. Di Stefano

Quantum complementarity of microcavity polaritons

We present an experiment that probes polariton quantum correlations by exploiting quantum complementarity. Specifically, we find that polaritons in two distinct idler modes interfere if and only if they share the same signal mode so that “which-way” information cannot be gathered. The experimental results prove the existence of polariton pair correlations that store the which-way information. This interpretation is confirmed by a theoretical analysis of the measured interference visibility in terms of quantum Langevin equations.

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.

Ultrastrong light-matter coupling in electrically doped microcavity organic light emitting diodes

The coupling of the electromagnetic field with an electronic transition gives rise, for strong enough light-matter interactions, to hybrid states called exciton-polaritons. When the energy exchanged between light and matter becomes a significant fraction of the material transition energy an extreme optical regime called ultrastrong coupling (USC) is achieved. We report a microcavity embedded p-i-n monolithic organic light emitting diode working in USC, employing a thin film of squaraine dye as active layer. A normalized coupling ratio of 30% has been achieved at room temperature. These USC devices exhibit a dispersion-less angle-resolved electroluminescence that can be exploited for the realization of innovative optoelectronic devices. Our results may open the way towards electrically pumped polariton lasers.

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.

Calculation of the local optical density of states in absorbing and gain media

The local optical density of states plays a key role in a wide range of phenomena. Near to structures displaying optical absorption or gain, the definition of the photonic local density of states needs to be revised. In this case two operative different definitions can be adopted to characterize photonic structures. The first (ρ(A)(r, ω)) describes the light intensity at a point r when the material system is illuminated isotropically and corresponds to what can be measured by a near-field microscope. The second (ρ(B)(r, ω)) gives a measure of vacuum fluctuations and coincides with ρ(A)(r, ω) in systems with real susceptibility. Scattering calculations in the presence of dielectric and metallic nanostructures show that these two definitions can give rather different results, the difference being proportional to the thermal emission power of the photonic structure. We present a detailed derivation of this result and numerical calculations for nanostructures displaying optical gain. In the presence of amplifying media, ρ(B)(r, ω) displays regions with negative photon densities, thus failing in describing a power signal. In contrast, ρ(A)(r, ω), positive definite, properly describes the near-field optical properties of these structures.

Dynamics-controlled truncation scheme for quantum optics and nonlinear dynamics in semiconductor microcavities

We present a systematic theory of Coulomb-induced correlation effects in the nonlinear optical processes within the strong-coupling regime. In this paper, we shall set a dynamics controlled truncation scheme microscopic treatment of nonlinear parametric processes in semiconductor microcavities including the electromagnetic field quantization. It represents the starting point for the microscopic approach to quantum optics experiments in the strong-coupling regime without any assumption on the quantum statistics of electronic excitations (excitons) involved. We exploit a previous technique, which was used in the semiclassical context, which, once applied to four-wave mixing in quantum wells, allowed us to understand a wide range of observed phenomena. We end up with dynamical equations for exciton and photon operators, which extend the usual semiclassical description of Coulomb interaction effects in terms of a mean-field term plus a genuine noninstantaneous four-particle correlation to quantum optical effects.

Microscopic quantum theory of spatially resolved photoluminescence in semiconductor quantum structures

We present a microscopic analysis of spatially resolved photoluminescence and photoluminescence excitation spectroscopy in semiconductor quantum structures. Such theoretical and numerical framework provides a general basis for the description of spectroscopic imaging in which the excitation and detection energies and spatial positions can all independently be scanned. The numerical results clarify the impact of the near-field optical setup on the obtained images and resolutions.

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.

Emergent entanglement of microcavity polariton pairs

We show theoretically that polariton pairs with a high degree of polarization entanglement can be produced through parametric scattering. In our proposed scheme, varying the polarizations of the two pumps, we have a complete control over the symmetries of the produced state (i.e. singlet or triplet output). Our microscopic model shows how a tomographic reconstruction, based on two-time correlation functions, can provide a quantitative assessment of the level of entanglement produced under realistic experimental conditions. Our study provides a suggestive perspective towards hybrid all-optical quantum devices where quantum information can be efficiently generated and controlled within the same structure. This result puts forward the robustness of pair correlations in solid-state devices, even when noise dominates one-body correlations.

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.