Two central concepts of quantum mechanics are Heisenberg’s uncertainty principle and a subtle form of nonlocality that Einstein famously called “spooky action at a distance”. These two fundamental features have thus far been distinct concepts. We show that they are inextricably and quantitatively linked: Quantum mechanics cannot be more nonlocal with measurements that respect the uncertainty principle. In fact, the link between uncertainty and nonlocality holds for all physical theories. More specifically, the degree of nonlocality of any theory is determined by two factors: the strength of the uncertainty principle and the strength of a property called “steering”, which determines which states can be prepared at one location given a measurement at another.
More details see Jonathan Oppenheim and Stephanie Wehner, Science 330, 1072 (2010).
Miles Blencowe’s recent work, PRL 111, 021302 (2013), shows that Gravity induces the rapid decoherence of stationary matter superposition states when the energy differences in the superposition exceed the Planck energy scale. This could kill the possibility of the macroscopic superposition states as the Gravity, unlike environmental thermal noises, can never be eliminated.
More details see Physics 6, 78 (2013), “Gravity Makes the Universe Classical“.
An interesting and fundamental witness based on estimation of the dimension of systems could be applied to the study of the quantum-to-classical transition phenomenon.
“Experimental observations are usually described using theoretical models that make assumptions about the dimensionality of the system under consideration. However, would it be possible to assess the dimension of a completely unknown system only from the results of measurements performed on it, without any extra assumption? The concept of a dimension witness answers this question, as it allows bounding the dimension of an unknown system only from measurement statistics. Here, we report on the experimental demonstration of dimension witnesses in a prepare and measure scenario6. We use photon pairs entangled in polarization and orbital angular momentum to generate ensembles of classical and quantum states of dimensions up to 4. We then use a dimension witness to certify their dimensionality as well as their quantum nature. Our work opens new avenues in quantum information science, where dimension represents a powerful resource, especially for device-independent estimation of quantum systems and quantum communications…”
more details see Nature Physics 8, 588 (2012), Phys. Rev. Lett. 105, 230501 (2010).
A review of recently proposed witnesses of non-Markovianity has been given by Heinz-Peter Breuer in the Special issue of J. Phys. B, “loss of coherence and memory effects in quantum dynamics“:
Foundations and measures of quantum non-Markovianity, J. Phys. B 45 154001 (2012).
Universal nonclassicality witnesses for harmonic oscillators are proposed by T. Kiesel and W. Vogel, Phys. Rev. A 85, 062106 (2012), and a nonclassicality witnessing of a quantum oscillator by coupling it to a qubit is proposed by S. Agarwal and J.H. Eberly, Phys. Rev. A 86, 022341 (2012).