Light matter interaction in mesoscopic systems

The study of light-matter interaction has led to many fundamental discoveries as well as to the development of new technology. In this thesis, we investigate the interaction between light and matter in different mesoscopic systems such us Fabry-Perot cavities with fixed and/or moving mirrors (optomechanical cavities) and superconducting circuits. In the context of optomechanical cavities, we isolate genuine quantum contributions of the interaction between an optical field and a mechanical mirror and study how to probe nonlinearities of the mechanical motion. We also investigate dynamical corrections, arising from an initial non-equilibrium configuration of the system, to the Casimir energy induced by the interaction between a quantum multimode field and the quantum fluctuations of the movable mirror. In a cavity scenario, we further consider such kind of dynamical corrections for the Casimir-Polder force between an excited atom and a perfectly conducting mirror, finding new features that can allow for an easier way to single-out the dynamical Casimir-Polder effect. In the context of superconducting circuits, we explore the light-matter interaction between microwave fields and artificial atoms in the ultrastrong coupling regime, where the system displays a high degree of entanglement. We show how to extract these (otherwise inaccessible) quantum correlations, and how such correlations can potentially be exploited as a resource for entanglement-based applications. In all these investigations we provide feasible experimental scenarios where such new effects can be probed.Open Acces

Nonequilibrium dressing in a cavity with a movable reflecting mirror

We consider a movable mirror coupled to a one-dimensional massless scalar field in a cavity. Both the field and the mirror's mechanical degrees of freedom are described quantum-mechanically, and they can interact each other via the radiation pressure operator. We investigate the dynamical evolution of mirror and field starting from a nonequilibrium initial state, and their local interaction which brings the system to a stationary configuration for long times. This allows us to study the time-dependent dressing process of the movable mirror interacting with the field, and its dynamics leading to a local equilibrium dressed configuration. Also, in order to explore the effect of the radiation pressure on both sides of the movable mirror, we generalize the effective field-mirror Hamiltonian and previous results to the case of two cavities sharing the same mobile boundary. This leads us to address, in the appropriate limit, the dynamical dressing problem of a single mobile wall, bounded by a harmonic potential, in the vacuum space.Comment: 10 pages, 4 figure

Probing anharmonicity of a quantum oscillator in an optomechanical cavity

We present a way of measuring with high precision the anharmonicity of a quantum oscillator coupled to an optical field via radiation pressure. Our protocol uses a sequence of pulsed interactions to perform a loop in the phase space of the mechanical oscillator, which is prepared in a thermal state. We show how the optical field acquires a phase depending on the anharmonicity. Remarkably, one only needs small initial cooling of the mechanical motion to probe even small anharmonicities. Finally, by applying tools from quantum estimation theory, we calculate the ultimate bound on the estimation precision posed by quantum mechanics and compare it with the precision obtainable with feasible measurements such as homodyne and heterodyne detection on the cavity field. In particular we demonstrate that homodyne detection is nearly optimal in the limit of a large number of photons of the field and we discuss the estimation precision of small anharmonicities in terms of its signal-to-noise ratio.Comment: 8 pages, 2 figures, RevTeX

Field observables near a fluctuating boundary

We review several aspects related to the confinement of a massless scalar field in a cavity with a movable conducting wall of finite mass, free to move around its equilibrium position to which it is bound by a harmonic potential, and whose mechanical degrees of freedom are described quantum mechanically. This system, for small displacements of the movable wall from its equilibrium position, can be described by an effective interaction Hamiltonian between the field and the mirror, quadratic in the field operators and linear in the mirror operators. In the interacting, i.e. dressed, ground state, we first consider local field observables such as the field energy density: we evaluate changes of the field energy density in the cavity with the movable wall with respect to the case of a fixed wall, and corrections to the usual Casimir forces between the two walls. We then investigate the case of two one-dimensional cavities separated by a movable wall of finite mass, with two massless scalar fields defined in the two cavities. We show that in this case correlations between the squared fields in the two cavities exist, mediated by the movable wall, at variance with the fixed-wall case.Comment: 7 pages, Proceedings of the DICE 2022 Conference, Castiglioncello, Ital

Quantum and Classical Phases in Optomechanics

The control of quantum systems requires the ability to change and read-out the phase of a system. The non-commutativity of canonical conjugate operators can induce phases on quantum systems, which can be employed for implementing phase gates and for precision measurements. Here we study the phase acquired by a radiation field after its radiation pressure interaction with a mechanical oscillator, and compare the classical and quantum contributions. The classical description can reproduce the nonlinearity induced by the mechanical oscillator and the loss of correlations between mechanics and optical field at certain interaction times. Such features alone are therefore insufficient for probing the quantum nature of the interaction. Our results thus isolate genuine quantum contributions of the optomechanical interaction that could be probed in current experiments.Comment: 10 pages, 3 figure

Spatial correlations of field observables in two half-spaces separated by a movable perfect mirror

We consider a system of two cavities separated by a reflecting boundary of finite mass that is free to move, and bounded to its equilibrium position by a harmonic potential. This yields an effective mirror-field interaction, as well as an effective interaction between the field modes mediated by the movable boundary. Two massless scalar fields are defined in each cavity. We consider the second-order interacting ground state of the system, that contains virtual excitations of both mirror's degrees of freedom and of the scalar fields. We investigate the correlation functions between field observables in the two cavities, and find that the squared scalar fields in the two cavities, in the interacting ground state, are anti-correlated. We discuss the dependence of the correlation on the distance of the two points considered from the mirror's average position, and on its mass and oscillation angular frequency. These results show a sort of communication between the two half-spaces separated by the movable mirror, mediated by its position fluctuations. Observability of this new phenomenon is also discussed.Comment: 6 pages, 1 figur