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A theory governing the metric and matter fields in spacetime is {\it locally causal} if the probability distribution for the fields in any region is determined solely by physical data in its past, i.e. it is independent of events at space-like separated points. This is the case according to general relativity, and it is natural to hypothesise that it should also hold true in any theory in which the fundamental description of space-time is classical and geometric --- for instance, some hypothetical theory which stochastically couples a classical spacetime geometry to a quantum field theory of matter. On the other hand, a quantum theory of gravity should allow the creation of spacetimes which macroscopically violate local causality. I describe a feasible experiment to test the local causality of spacetime, and hence to test whether gravity is better described, in this respect, by general relativity or by quantum theory. The experiment will either identify a definite limit to the domain of validity of quantum theory or else produce significant evidence for the hypothesis that gravity is described by a quantum theory.

Abner Shimony is well-known for, among other contributions, his seminal work on Bell inequalities, turning a philosophical question into an experimental one. In my presentation I like to remind us how this experimental field is nowadays feeding into applied science. This is happening both in terms of the involved technologies and in the conceptual tools.

Entanglement swapping is such a powerful technique for dealing with EPR problems, that it can handle inefficient counters and Bell Theorems without inequalities, even for two particles. We will examine some of the results and pitfalls.

An experimental realization of our spin-1/2 particle version of the Einstein-Podolsky-Rosen (EPR) experiment will be briefly reviewed. In the proposed experiment, two 199Hg atoms in the ground 1S0 electronic state, each with nuclear spin I=1/2, are generated in an entangled state with total nuclear spin zero. Such a state can be obtained by dissociation of a 199Hg2 molecule (dimer) using a spectroscopically selective stimulated Raman process. From symmetry considerations, the nuclear spin singlet state is guaranteed if the initial 199Hg2 molecule is in a rotational state with an even quantum number. Consequently, a thorough investigation and analysis of the rotational structure of the 199Hg2 molecule is required; results of this analysis will be presented.

Feynman was probably correct to say that the only mystery of quantum mechanics is the principle of superposition. Although we may never know which slit a photon has been passing in a Young’s double-slit experiment, we do have a corresponding classical concept in classical electromagnetic theory: the superposition of electromagnetic fields at a local space-time point is a solution of the Maxwell equations. In the case of joint photo-detection measurement of two photons, however, the superposition involves the addition of two-photon amplitudes, different yet indistinguishable alternatives resulting in a click-click joint photo-detection event. There is no counterpart of such concept in classical electromagnetic theory and the superposition may happen at distance. It is the two-photon superposition responsible for the mysteries of EPR by means of reality and causality. This talk will analyze the physics of based on several recent experiments.

According to a widely accepted view, the emergence of macroscopic behavior is related to the loss of quantum mechanical coherence. Opinions on the possible cause of this loss diverge. In the present talk it will be shown how a small, assessable amount of indeterminacy in the structure of space-time may lead to the emergence of macroscopic behavior, in agreement with empirical evidence.

After giving an introduction to the Continuous Spontaneous Localization (CSL) theory of dynamical wave function collapse, I shall discuss 10 problems of dynamical collapse models, 5 of which were resolved by CSL's advent, and 5 of which have been subsequently attacked with varying success.

If one is worried by the quantum measurement problem,a natural question to ask is: Does the quantum-mechanical description of the world retain its validity when its application leads to superpositions of states which by some reasonable criterion are _macroscopically distinct_? Or rather, does any such superposition automatically get "collapsed", even in the absence of "measurement" by a human observer, into one or other of its branches? Scenarios which predict the latter (for example the GRWP theory) may be denoted generically by the term "macrorealistic". Even if one believes that QM remains the whole truth at the macrolevel, it is clear that to the extent that environmental decoherence destroys the delicate phase relations characterizing the superposition, the predictions of QM will be indistinguishable experimentally from those of the class of macrorealistic theories (a remark which is often taken, in my opinion quite erroneously, as "solving" the measurement problem). Thus, to distinguish experimentally between QM and macrorealism one needs a system in which decoherence is low enough that (given that QM is correct) one has a realistic chance of observing _quantum interference of macroscopically distinct states_ ("QIMDS"). Over the last few years, a surprising variety of candidate systems has emerged; however, while all experiments to date have been consistent with the continued validity of QM, none has so far refuted macrorealism outright. In this talk I review the systems in question and discuss the prospects for a truly definitive experiment.