Measuring spatial correlations of photon pairs by automated raster scanning with spatial light modulators.

We demonstrate the use of a phase-only spatial light modulator for the measurement of transverse spatial distributions of coincidence counts between twin photon beams, in a fully automated fashion. This is accomplished by means of the polarization dependence of the modulator, which allows the conversion of a phase pattern into an amplitude pattern. We also present a correction procedure, that accounts for unwanted coincidence counts due to polarization decoherence effects.

Ancilla-assisted measurement of photonic spatial correlations and entanglement.

We report an experiment in which the moments of spatial coordinates are measured in down-converted photons directly, without having to reconstruct any marginal probability distributions. We use a spatial light modulator to couple the spatial degrees of freedom and the polarization of the fields, which acts as an ancilla system. Information about the spatial correlations is obtained via measurements on the ancilla qubit.

Emergence of the pointer basis through the dynamics of correlations.

We use the classical correlation between a quantum system being measured and its measurement apparatus to analyze the amount of information being retrieved in a quantum measurement process. Accounting for decoherence of the apparatus, we show that these correlations may have a sudden transition from a decay regime to a constant level. This transition characterizes a nonasymptotic emergence of the pointer basis, while the system apparatus can still be quantum correlated.

Environment-induced sudden death of entanglement.

We demonstrate the difference between local, single-particle dynamics and global dynamics of entangled quantum systems coupled to independent environments. Using an all-optical experimental setup, we showed that, even when the environment-induced decay of each system is asymptotic, quantum entanglement may suddenly disappear. This "sudden death" constitutes yet another distinct and counterintuitive trait of entanglement.