Omar Di Stefano

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.

Ultrastrong Coupling of Plasmons and Excitons in a Nanoshell

The strong coupling regime of hybrid plasmonic-molecular systems is a subject of great interest for its potential to control and engineer light-matter interactions at the nanoscale. Recently, the so-called ultrastrong coupling regime, which is achieved when the light-matter coupling rate reaches a considerable fraction of the emitter transition frequency, has been realized in semiconductor and superconducting systems and in organic molecules embedded in planar microcavities or coupled to surface plasmons. Here we explore the possibility to achieve this regime of light-matter interaction at nanoscale dimensions. We demonstrate by accurate scattering calculations that this regime can be reached in nanoshells constituted by a core of organic molecules surrounded by a silver or gold shell. These hybrid nanoparticles can be exploited for the design of all-optical ultrafast plasmonic nanocircuits and -devices.

One Photon Can Simultaneously Excite Two or More Atoms.

We consider two separate atoms interacting with a single-mode optical or microwave resonator. When the frequency of the resonator field is twice the atomic transition frequency, we show that there exists a resonant coupling between one photon and two atoms, via intermediate virtual states connected by counterrotating processes. If the resonator is prepared in its one-photon state, the photon can be jointly absorbed by the two atoms in their ground state which will both reach their excited state with a probability close to one. Like ordinary quantum Rabi oscillations, this process is coherent and reversible, so that two atoms in their excited state will undergo a downward transition jointly emitting a single cavity photon. This joint absorption and emission process can also occur with three atoms. The parameters used to investigate this process correspond to experimentally demonstrated values in circuit quantum electrodynamics systems.

Quantum nonlinear optics without photons

Spontaneous parametric down-conversion is a well-known process in quantum nonlinear optics in which a photon incident on a nonlinear crystal spontaneously splits into two photons. Here we propose an analogous physical process where one excited atom directly transfers its excitation to a pair of spatially-separated atoms with probability approaching one. The interaction is mediated by the exchange of virtual rather than real photons. This nonlinear atomic process is coherent and reversible, so the pair of excited atoms can transfer the excitation back to the first one: the atomic analogue of sum-frequency generation of light. The parameters used to investigate this process correspond to experimentally-demonstrated values in ultrastrong circuit quantum electrodynamics. This approach can be extended to realize other nonlinear inter-atomic processes, such as four-atom mixing, and is an attractive architecture for the realization of quantum devices on a chip. We show that four-qubit mixing can efficiently implement quantum repetition codes and, thus, can be used for error-correction codes.

Imaging spectroscopy of quantum wells with interfacial fluctuations: A theoretical description

We present a theoretical approach for the simulation of scanning local optical spectroscopy in disordered quantum wells (QWs). After a single realization of the disorder potential, we calculate spectra on a mesh of points on the QW plane, thus obtaining a three-dimensional matrix of data from which we construct two-dimensional spectroscopic images of excitons laterally localized at interface fluctuations. Our simulations are in close agreement with the experimental findings, and contribute to the interpretation of spatially resolved spectra in QWs.

Feynman-diagrams approach to the quantum Rabi model for ultrastrong cavity QED: stimulated emission and reabsorption of virtual particles dressing a physical excitation

In quantum field theory, bare particles are dressed by a cloud of virtual particles to form physical particles. The virtual particles affect properties such as the mass and charge of the physical particles, and it is only these modified properties that can be measured in experiments, not the properties of the bare particles. The influence of virtual particles is prominent in the ultrastrong-coupling regime of cavity quantum electrodynamics (QED), which has recently been realised in several condensed-matter systems. In some of these systems, the effective interaction between atom-like transitions and the cavity photons can be switched on or off by external control pulses. This offers unprecedented possibilities for exploring quantum vacuum fluctuations and the relation between physical and bare particles. We consider a single three-level quantum system coupled to an optical resonator. Here we show that, by applying external electromagnetic pulses of suitable amplitude and frequency, each virtual photon dressing a physical excitation in cavity-QED systems can be converted into a physical observable photon, and back again. In this way, the hidden relationship between the bare and the physical excitations can be unravelled and becomes experimentally testable. The conversion between virtual and physical photons can be clearly pictured using Feynman diagrams with cut loops.

Theory of local optical spectroscopy of quantum wires with interface fluctuations

We present a theory of local optical spectroscopy in quantum wires taking into account structural disorder. The calculated spatially resolved spectra show the individual spectral lines due to the exciton states localized by the disordered potential in agreement with experimental findings. We investigate systematically the influence of the potential profile and of the spatial resolution on the local optical spectra. Several line scans along the wire axis are obtained for different spatial correlations and strength of the disorder potential and for different spatial resolutions ranging from the subwavelength to the diffraction limit. Lowering the spatial resolution causes the disappearance of many spectral lines due to destructive spatial interference. However, our results show that information on the individual eigenstates of this quasi one-dimensional quantum system can be obtained at also resolutions significantly lower than the correlation length of interface fluctuations.

Mode expansion and photon operators in dispersive and absorbing dielectrics

Abstract We present a one-dimensional quantization scheme for the electromagnetic field in arbitrary planar absorbing and dispersive dielectrics, taking into account explicitly the finite extent of media. The complete form of the electric field operator includes a part that corresponds to the free fields incident from the vacuum towards the medium and a particular solution which can be expressed by using the classical Green-function integral representation of the electromagnetic field. By expressing the classical Green function in terms of the light modes of the dielectric structure, we obtain a natural generalization to absorbing media of the familiar method of mode expansion which strictly applies to non-absorbing media. As an application of our mode expansion, we obtain general quantum-optical input-output relations relating the output photon operators in vacuum to the input photon operators and to the reservoir noise operators. The input-output relations obtained are uniquely determined by the knowledge of the classical light modes of the planar system. We finally show that our quantization scheme for arbitrary planar dielectrics satisfies the prescribed equal-time commutation relations of QED.

Many-body and correlation effects in semiconductor microcavities

We discuss the influence of many-body and correlation effects on the transient optical response of semiconductor microcavities. The complexity induced by the Coulomb interaction between electrons determines the non-instantaneous character of exciton–exciton collisions. We show that the exciton–photon coupling in semiconductor microcavities is able to alter the exciton dynamics during collisions strongly affecting the effective scattering rates. This influence determines the very different nonlinear absorption rates of the two polariton branches observed in many experiments. This analysis also clarifies the origin of the great enhancement of polariton parametric amplification observed when increasing the polariton splitting. It demonstrates that exciton–exciton collisions in semiconductor microcavities can be controlled and engineered to produce almost decoherence-free collisions for the realization of all-optical microscopic devices.

Microscopic calculation of noise current operators for electromagnetic field quantization in absorbing material systems

We present a microscopic quantization scheme for the electromagnetic field in dispersive and lossy dielectrics of arbitrary geometry. This method also describes anisotropic media and media driven by light field via a spatially nonlocal permittivity. The method removes the need for complicated diagonalization of material, reservoir and field variables and allows us to include the effect of all the material excitations. Dissipation inside the medium is described by considering the coupling of the polarization quanta of the system with the reservoir oscillators in the usual Langevin approach.