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We report a proposal which exploits an optical parametric amplification to boost interferometry sensitivity in the presence of losses in a minimally invasive scenario. By performing the amplification process on the microscopic probe after the interaction with the sample, we can beat the losses detrimental effect on the phase measurement which affects the single photon state after its interaction with the sample, and thus improve the achievable sensitivity. [ABSTRACT FROM AUTHOR]

We investigate multiphoton states generated by high-gain optical parametric amplification of a single injected photon, polarization encoded as a “qubit”. Two different experimental configurations were adopted in order to investigate two different quantum processes: the optimal universal quantum cloning and the optimal phase-covariant cloning. The output states of the two apparatuses were found to show the quantum superposition property of mesoscopic multi-photon assemblies and the entanglement characteristics of the “Schroedinger Cat State”. This work represents an experimental attempt to test several fundamental quantum processes in a mesoscopic, or macroscopic frameworks. As it is well known, this is indeed one of the most challenging endavours of modern science. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

We report on the first experimental realization of the single qubits purification protocol for two copies of a qubit sent through a depolarizing channel. The purification of the polarization-encoded photonic qubits is achieved with passive linear optical elements. Our experimental results suggest that the present approach may represent a viable alternative to quantum error correction or entanglement distillation. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

In the last years a thorough research effort has been devoted to the experimental realization of the optimal approximations of the quantum cloning and the quantum flipping of an unknown qubit. Even if these two processes are unrealizable in their exact forms, they can be optimally approximated by the universal quantum machines: the universal optimal quantum cloning machine (UOQCM) and the universal-NOT (U-NOT) gate. The 1->1 U-NOT gate and the 1->2 UOQCM have been experimentally demonstrated by adopting the process of stimulated emission into an optical parametric amplifier and by slightly modifying the quantum state teleportation protocol. In this perspective, a thorough analysis of the spin-flipping action induced by the Faraday mirror on any qubit realized by a polarization-encoded single photon has recently been carried out. The results of the theory show that the device acts as a non-universal NOT gate, in full agreement with quantum mechanical expectations. All theoretical results have been substantiated experimentally by the modern methods of Quantum State Tomography and of the Entanglement assisted Quantum Process Tomography. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

We report on the experimental realization of hyper-entangled two photon states, entangled both in polarization and momentum. These states are produced by a high brilliance parametric source of entangled photon pairs with peculiar characteristics of flexibilty of state generation. The entanglement in the two degrees of freedom has been tested by Hong-Ou-Mandel interferometry. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Two quantum interferometry processes with possible significant applications in quantum information technology are presented. A two frequency Mach-Zehnder interferometer realizes the nonlinear frequency conversion of a single photon qubit. The information-preserving character of the nonlinear process allows to transfer the coherence of the input to the output state. Furthermore, we describe a novel high brilliance universal source of entangled photon pairs with peculiar spatial characteristics, which allows to generate two particle entangled states, either pure or mixed of any structure. By this source we have synthesized and fully characterized by quantum tomography Werner states and maximally entangled mixed states (MEMS). The non local properties of these states have been tested by Bell measurements. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]