February 2022

Observation of gravitationally induced entanglement between two massive particles can be viewed as implying the existence of the nonclassical nature of gravity. However, weak interaction in the gravitational field is extremely small so that gravitationally induced entanglement is exceptionally challenging to test in practice. For addressing this key challenge, here we propose a criterion based on the logical contradictions of weak entanglement, which may boost the sensitivity of the signal due to the gravitationally induced entanglement. Specifically, we make use of the weak-value scenario and Einstein-Podolsky-Rosen steering. We prove that it is impossible for a classical mediator to act on two local quantum objects to simulate amplified-weak-value phenomenon in two-setting Einstein-Podolsky-Rosen steering. Our approach can amplify the signal of gravitationally induced entanglement that were previously impossible to observe by any desired factor that depends on the magnitude of the weak value. Our results not only open up the possibility of exploring nonclassical nature of gravity in the near future, but they also pave the way for weak entanglement criterion of a more general nature.

Deep neural networks are a powerful tool for the characterization of quantum states. Existing networks are typically trained with experimental data gathered from the specific quantum state that needs to be characterized. But is it possible to train a neural network offline and to make predictions about quantum states other than the ones used for the training? Here we introduce a model of network that can be trained with classically simulated data from a fiducial set of states and measurements, and can later be used to characterize quantum states that share structural similarities with the states in the fiducial set. With little guidance of quantum physics, the network builds its own data-driven representation of quantum states, and then uses it to predict the outcome statistics of quantum measurements that have not been performed yet. The state representation produced by the network can also be used for tasks beyond the prediction of outcome statistics, including clustering of quantum states and identification of different phases of matter. Our network model provides a flexible approach that can be applied to online learning scenarios, where predictions must be generated as soon as experimental data become available, and to blind learning scenarios where the learner has only access to an encrypted description of the quantum hardware.

Spin foam theory is a concrete framework for quantum gravity where numerical calculations of transition amplitudes are possible. Recently, the field became very active, but the entry barrier is steep, mainly because of its unusual language and notions scattered around the literature. This paper is a pedagogical guide to spin foam transition amplitude calculations. We show how to write an EPRL-FK transition amplitude, from the definition of the 2-completo its numerical implementation using texttt{sl2cfoam-next}. We guide the reader using an explicit example balancing mathematical rigor with a practical approach. We discuss the advantages and disadvantages of this approach and provide a novel look at a recently proposed approximation scheme.

Observing entanglement generation mediated by a local field certifies that the field cannot be classical. This information-theoretic argument is at the heart of the race to observe gravity-mediated entanglement in a `table-top’ experiment. Previous derivations of the effect assume the locality of interactions, while using an instantaneous interaction to derive the effect. We correct this by giving a first principles derivation of mediated entanglement using linearised gravity. The framework is Lorentz covariant — thus local — and yields Lorentz and gauge invariant expressions for the relevant quantum observables. For completeness we also cover the electromagnetic case. An experimental consequence of our analysis is the possibility to observe *retarded* mediated entanglement, which avoids the need of taking relativistic locality as an assumption. This is a difficult experiment for gravity, but could be feasible for electromagnetism. Our results confirm that the entanglement is dynamically mediated by the gravitational field.