QUANTUM PHENOMENA UNDER EIT CONDITIONS

This thesis is on the study of the mechanisms for the production of nonlinear photon-photon interaction under the condition of Electromagnetically Induced Transparency (EIT), and on the implementation of a Quantum Error Correction (QEC) scheme adopting a Cavity Quantum\ud Electrodynamics (CQED) set-up. The latter part of the thesis will be treated in the last chapter.\ud Large non linear eects are usually accompanied, by a large increase of the absorption. This fact makes classical nonlinear devices not useful for the purposes of the rapid developing fields of Quantum Computation and Information. Strong absorption process results in a complete loss of\ud the information represented by the state of the photons. EIT is a well known process of atomic coherence, largely studied starting from the seventies together with the discovery of Coherent Population Trapping (CPT), Lasing Without Inversion (LWI), slow light propagation.\ud After the recent experimental realization of slowed down light, EIT has experienced a revival in recent years mainly due to the possible application to Quantum Computation and Quantum Information (QC&QI). EIT is a process that makes transparent a previously opaque medium thanks to the interaction with an electromagnetic field. It is a manifestation of the quantum nature of atoms, that exploits a quantum interference process. In this thesis we will show how an EIT medium permits to obtain giant non-linear Kerr interaction (up to eight order of magnitude\ud increase) even at low light level, by the study of several light-atoms scheme. The production and control of giant cross phase modulation (XPM) is analysed and all sources of noise and loss will be included in the treatment.\ud The main goal of this thesis is to study the photon-photon interaction at the level of single photons in several multilevel schemes. In the full quantum limit we shall concentrate to a specific light-atoms interaction scheme, the symmetric M scheme (SMS) under the condition of EIT. We\ud shall study how to implement a Quantum Phase Gate (QPG). Such a gate has been already experimentally realized for a cavity, but here we want to find the optimal condition to implement the QPG between flying qubits. To this end we adopt EIT in order increase the photon-photon\ud interaction: in a cavity such an increase is provided by the small volume of the cavity, while in EIT it is based on the strong dispersion dn/dω at the center of the transparency window. Photons represent the best candidates for the implementation of transmission lines for carrying\ud quantum information, and this is why strong nonlinear hoton-photon interaction is particularly interesting from the point of view of quantum information.\ud In this scenario the principal goal of this thesis is to analyze the potentiality of a EIT based device in full quantum regime, for the implementation of the QPG. The quantities that define the performance of the gate are calculated in detail including decoherence mechanisms and finding a possible set-up for the experimental realization

Implementation of a three-qubit quantum error correction code in a cavity-QED setup

The correction of errors is of fundamental importance for the development of contemporary computing devices and of robust communication protocols. In this paper we propose a scheme for the implementation of the three-qubit quantum repetition code, exploiting the interaction of Rydberg atoms with the quantized mode of a microwave cavity field. Quantum information is encoded within two circular Rydberg states of the atoms and encoding and decoding process are realized within two separate microwave cavities. We show that errors due to phase noise fluctuations could be efficiently corrected using a state-of-the-art apparatus.Comment: 9 pages, 5 figures. This is v2. Some misprints corrected, conclusions section extended, refs added. Accepted for publication on PR

Adiabatic Splitting, Transport, and Self-Trapping of a Bose-Einstein Condensate in a Double-Well Potential

We show that the adiabatic dynamics of a Bose-Einstein condensate (BEC) in a double well potential can be described in terms of a dark variable resulting from the combination of the population imbalance and the spatial atomic coherence between the two wells. By means of this dark variable, we extend, to the non-linear matter wave case, the recent proposal by Vitanov and Shore [Phys. Rev. A 73, 053402 (2006)] on adiabatic passage techniques to coherently control the population of two internal levels of an atom/molecule. We investigate the conditions to adiabatically split or transport a BEC as well as to prepare an adiabatic self trapping state by the optimal delayed temporal variation of the tunneling rate via either the energy bias between the two wells or the BEC non-linearity. The emergence of non-linear eigenstates and unstable stationary solutions of the system as well as their role in the breaking down of the adiabatic dynamics is investigated in detail.Comment: 8 pages, 7 figure

Finite-size analysis of measurement-device-independent quantum cryptography with continuous variables

We study the impact of finite-size effects on the key rate of continuous-variable (CV) measurement-device-independent (MDI) quantum key distribution (QKD). Inspired by the parameter estimation technique developed in [Rupert \textit{et al.} Phys. Rev. A \textbf{90}, 062310 (2014)]~we adapt it to study CV-MDI-QKD and, assuming realistic experimental conditions, we analyze the impact of finite-size effects on the key rate. We find that, increasing the block-size, the performance of the protocol converges towards the ideal one, and that block-sizes between $10^{6}$ and $10^{9}$ data points can already provide a key rate $\sim10^{-2}$ bit/use over metropolitan distances.Comment: 10 pages, 3 figures, typos corrected, abstract and intro modified, references adde

Reducing the meta-emotional problem decreases physiological fear response during exposure in phobics

Anxiety disorders may not only be characterized by specific symptomatology (e.g., tachycardia) in response to the fearful stimulus (primary problem or first-level emotion) but also by the tendency to negatively evaluate oneself for having those symptoms (secondary problem or negative meta-emotion). An exploratory study was conducted driven by the hypothesis that reducing the secondary or meta-emotional problem would also diminish the fear response to the phobic stimulus. Thirty-three phobic participants were exposed to the phobic target before and after undergoing a psychotherapeutic intervention addressed to reduce the meta-emotional problem or a control condition. The electrocardiogram was continuously recorded to derive heart rate (HR) and heart rate variability (HRV) and affect ratings were obtained. Addressing the meta-emotional problem had the effect of reducing the physiological but not the subjective symptoms of anxiety after phobic exposure. Preliminary findings support the role of the metaemotional problem in the maintenance of response to the fearful stimulus (primary problem)