Robust quantum optical states for quantum sensing and entanglement tests

Abstract

In the last few decades the emergence of a new field of research dealing with information at the quantum level has led to a “second quantum revolution”, that promises new technologies whose design is based on the principles of quantum mechanics. A relevant example is provided by the fascinating project of the quantum computer, originally proposed by R. Feynman and subsequently formulated by Deutsch. This proposal is based on developing a new theory of computation based on the rules of quantum mechanics, including peculiar features such as entanglement and superposition, which can lead to improved performances with respect to a classical approach. In parallel, the tools developed within quantum information have found application in the investigation of the foundations of quantum mechanical theory. In this context, an open question is related with the fundamental mechanisms leading to the transition from the quantum dynamics of the microscopic world to the classical dynamics of the macroscopic world. The present thesis is aimed at investigating the possibility of performing both quantum mechanical tests and quantum information protocols with multiphoton states. We adopt as a platform the quantum states generated by an optical parametric amplification process. The main idea beyond this approach is given by the capability of the amplification process to broadcast the features of the input state into a system with a larger number of photons. This property will be applied to analyze the possibility of observing quantum effects when the number of particles in the system progressively increases. We begin by considering a single-photon input into the amplifier, and we show that the states generated in this configuration present a significant resilience with respect to the action of detection losses. From a fundamental point of view, the resilience to losses of such states represents a tool for the investigation of quantum phenomena in a system of increasing size, thus allowing to explore the transfer of quantum properties from a single particle state to a collective multi-particle one. As a second system, we analyze the bipartite system which is obtained by amplification of a single-photon belonging to an entangled pair, thus generating an hybrid microscopic-mesoscopic system. We consider the possibility of detecting the entanglement in this configuration when the number of photons in the mesoscopic part progressively increases. This investigation requires a detailed analysis on the various classes of entanglement and nonlocality tests that can be performed in a joint microscopic-macroscopic bipartite system. After the development of a first insight on this problem with a discrete variable approach, continuous-variables collective measurements will be investigated. Specific attention will be devoted to the entanglement criteria based on the quadrature phase-space operators. The tools developed with this continuous-variables approach will be applied in a different configuration, where both the two subsystems are composed by a multiphoton field. This system can be generated by adopting a parametric amplification process in a noncollinear configuration in the spontaneous emission regime. We investigate the possibility of observing nonlocal features in this class of states when both coarse-grained and high efficiency continuousvariables measurements are adopted. The possible applications in quantum information tasks of the quantum states generated through the optical parametric amplifier will be then 4 investigated. Among the various fields, attention will be devoted to quantum metrology in presence of a lossy apparatus. We show that by performing an amplification process we can preserve the information on the optical phase to be measured from the action of losses, unavoidable in any experimental implementation. More specifically, this approach relies on amplifying the probe state after the phase information has been acquired, increasing its robustness with respect to losses.