We prove that given any two general probabilistic theories (GPTs) the following are equivalent: (i) each theory is nonclassical, meaning that neither of their state spaces is a simplex; (ii) each theory satisfies a strong notion of incompatibility equivalent to the existence of “superpositions”; and (iii) the two theories are entangleable, in the sense that their composite exhibits either entangled states or entangled measurements. Intuitively, in the post-quantum GPT setting, a superposition is a set of two binary ensembles of states that are unambiguously distinguishable if the ensemble is revealed before the measurement has occurred, but not if it is revealed after. This notion is important because we show that, just like in quantum theory, superposition in the form of strong incompatibility is sufficient to realize the Bennett-Brassard 1984 protocol for secret key distribution.

A revised experimental arrangement could significantly boost the generation of quantum correlations between particles that interact only gravitationally thus boosting the prospects of testing the quantumness of gravity with tabletop experiments. Consider two particles levitated in vacuum and prepared in a quantum superposition which are let to interact gravitationally for a time long enough to correlate significantly theirmechanical degrees of freedom. A specific type of correlation, called quantum correlation or entanglement, is incompatible with the notion of a classical gravitational field mediating the interaction, that is, with a field that takes a definite value at each point in space. Thus, the observation of such quantum correlations, which can be detected by measuring the position and momentum of the test particles, would rule out classical theories of gravity, and advocate in favor of a quantum description.  The difficulty in realising such a test lies in the necessity of preparing massive particles in a quantum superposition for a time sufficiently long to build detectable correlations. In their recent work, Pedernales et al. prove that such an experiment can be performed in significantly shorter time scales by introducing a third larger mass that interacts with the test particles. This revised arrangement is able to leverage the larger gravitational pull of the mediator mass and considerably enhance the rate at which quantum correlations are built between the test particles. The remarkable aspect of their result is that, under suitable conditions, the large mass does not need to be prepared in a pure quantum state, immediately augmenting the range of masses available for this type of experiments with the same technological capabilities. 

We investigate how we can use numerics to construct quantum encodings that can protect against losses in an optimal way - both probabilistically and deterministically. For this hard problem, which involves a joint optimization both over an optimal encoded state as well as its optimal decoder, we develop various algorithms that provide convincing results. This is a first step in dealing with the problem, as we restrict ourselves to permutationally-invariant subspaces (allowing to scale the problem to a large number of photons); in subsequent work, we will improve on this restriction.

We study the non-equilibrium dynamics of electron transmission from a straight waveguide to a helix with spin-orbit coupling. Transmission is found to be spin-selective and can lead to large spin polarizations of the itinerant electrons. The degree of spin selectivity depends on the width of the interface region, and no polarization is found for single-point couplings. We show that this is due to momentum conservation conditions arising from extended interfaces. We therefore identify interface structure and conservation of momentum as crucial ingredients for chiral-induced spin selectivity, and confirm that this mechanism is robust against static disorder.

Can the interferometric signal of a two-level system that is interacting with an oscillator be used to determine whether the field mediating the interaction requires a quantum description as suggested by a recent proposal? In this work we show that this is not the case and suggest an alternative route highlighting its connection to black hole physics.