Two photons provide us with a number of phenomena where we can expose classically non-reproducible new effects that quantum entanglement brings, all the way to the quantum gates for the optical quantum computers.
The scope of this thesis is to explore different ways of bringing the phenomenon of quantum entanglement to our classical world. A first manner is to make entanglement an element of everyone's cultural background. I show that nontrivial quantum nonlocality tests can be performed with do-it-yourself optical sources. Furthermore, entanglement can be brought to the real world as a technological resource for improving everyday activities, like communication. Sources of entangled photons, suited to long distance quantum communication, need to be developed. I present a source of entangled photons realized via Spontaneous Parametric Down Conversion in an integrated cavity-waveguide. I show how this kind of sources can be engineered. I also report the realization of low-loss sources for the implementation of an experiment of faithful entanglement swapping adopting a nonlinear interaction between two single photons. Last, one can also attempt to physically generate and observe entanglement at macroscopic scales. I analyze the possibility of cloning a photon of an entangled pair and detecting the amplified state with human eyes. Bringing entanglement to scales one can see allows one to investigate ways of measuring light in the quantum-to-classical regime. I report the experimental demonstration of a method, based on optimal quantum cloning, for measuring low optical radiances in an absolute way. Moreover, I present the realization of photon number resolving detectors at infrared wavelengths and room temperature operation.
There are a number of different strategies to measure the phase shift between two pathways of light more efficiently than suggested by the standard quantum limit. One way is to use highly entangled photons. Another way is to expose photons to a non-linear or interacting Hamiltonian. This paper emphasises that the conditional dynamics of open quantum systems provides an interesting additional tool for quantum-enhanced metrology. As a concrete example, we review a recent scheme which exploits the conditional dynamics of a laser-driven optical cavity with spontaneous photon emission inside a quantum feedback loop. Deducing information from second-order photon correlation measurements requires neither optical non-linearities nor entangled photons and should therefore be of immediate practical interest.
We analyze numerically the dynamical generation of quantum entanglement in a system of two interacting particles, started in a coherent separable state, for decreasing values of ℏ. As ℏ→0 the entanglement entropy, computed at any finite time, converges to a finite nonzero value. The limit law that rules the time dependence of entropy is well reproduced by purely classical computations. Its general features can be explained by simple classical arguments, which expose the different ways entanglement is generated in systems that are classically chaotic or regular.