Generation and characterization of more complex non-classical light states

Abstract

In this thesis, two optical systems that generate non-classical states of light were studied: a triply resonant Optical Parametric Oscillator (OPO) operating above the threshold and a cold atomic ensemble of neutral 87Rb atoms obtained from a magneto-optical trap. The main idea in both cases is to prepare entangled states for future use in quantum information protocols. In relation to the OPO, experimental measurements of the complete quantum state for six modes of the electromagnetic field produced by this system are theoretically explained. The investigation involves the sidebands of the intense pump, signal and idler fields generated by stimulated parametric down-conversion inside the resonator. The model takes into account the coupling of the field modes with the phonon bath of the nonlinear crystal, clearly showing the roles of different physical effects in shaping the structure of the quantum correlations between the six optical modes. Moreover, it is theoretically and experimentally studied how these modes are entangled to one another using the positive partial transpose criterion in the continuous variables regime. It was found that the hexapartite entanglement in this system can be thought of as being generated by a combination of two-mode squeezers and beam splitter Hamiltonians acting on six different colors of light. On the other hand, in relation to the cold atomic ensemble, a "write-read" scheme inspired by the Duan-Lukin-Cirac-Zoller (DLCZ) protocol for long-distance quantum communication was implemented in order to experimentally generate an entangled state between individual photons of a mode of the electromagnetic field and atomic excitations in a particular collective mode. By performing photon statistics analysis it was possible to determine that the quantum state of the system corresponds to an entangled two-mode squeezed vacuum state, as expected from the theory. In the experiments, either one or two excitations are initially stored in the atomic medium, which acts as a quantum memory, and are subsequently mapped to one or two photons, respectively. Moreover, measurements and theoretical modeling of the photonic wave packets were realized, observing an acceleration in the photon emission due to the collective nature of the atomic state, a phenomenon known as superradiance. Such progress opens the way to the study and future control of the interaction of non-classical light modes with collective quantum memories at higher photon numbers.