Graph Picture of Linear Quantum Networks and Entanglement

The indistinguishability of quantum particles is widely used as a resource for the generation of entanglement. Linear quantum networks (LQNs), in which identical particles linearly evolve to arrive at multimode detectors, exploit the indistinguishability to generate various multipartite entangled states by the proper control of transformation operators. However, it is challenging to devise a suitable LQN that carries a specific entangled state or compute the possible entangled state in a given LQN as the particle and mode number increase. This research presents a mapping process of arbitrary LQNs to graphs, which provides a powerful tool for analyzing and designing LQNs to generate multipartite entanglement. We also introduce the perfect matching diagram (PM diagram), which is a refined directed graph that includes all the essential information on the entanglement generation by an LQN. The PM diagram furnishes rigorous criteria for the entanglement of an LQN and solid guidelines for designing suitable LQNs for the genuine entanglement. Based on the structure of PM diagrams, we compose LQNs for fundamental N-partite genuinely entangled states.

Correlations in twisted double-layer graphene with virtual photons in a microcavity

We analyze the entanglement generation of a system composed of two decoupled rotated graphene layers inside a planar microcavity. By considering the electromagnetic field of the cavity in the vacuum state and using time-dependent perturbation theory it is possible to obtain the range of geometric parameters at which the quantum states of electrons in different layers are entangled. By employing the negativity measure, correlations between layers are obtained for time scales smaller than the light-crossing time of the layers. It is shown that the negativity measure is modulated by the rotation angle between layers, allowing manipulation of X states. Finally, an experimental protocol is analyzed in order to detect non-causal effects between layers, by allowing back-voltage switching functions in the two layers with supports that do not overlap in time. By turning off the second-back voltage at a time smaller than the light-crossing time, it is possible to obtain correlations between layers through the independent interaction with virtual photons. The exchange of virtual photons implies that the propagator can be nonzero outside the light cone and this non-causal propagation can create entangled quantum states.

Speeding up the spread of quantum information in chaotic systems

We explore the effect of introducing mild nonlocality into otherwise local, chaotic quantum systems, on the rate of information spreading and associated rates of entanglement generation and operator growth. We consider various forms of nonlocality, both in 1-dimensional spin chain models and in holographic gauge theories, comparing the phenomenology of each. Generically, increasing the level of nonlocality increases the rate of information spreading, but in lattice models we find instances where these rates are slightly suppressed.

Quadrature entanglement in the microwave domain with a phase locking protocol

Entanglement has attracted great attention in the past few decades due to its potential applications in the field of quantum information protocols. From now, achieving excellent phase locking in entanglement generation is significant yet a challenging task in cryogenic quantum technology. In this work, we propose and demonstrate a comprehensive paradigm of phase locking protocol for quadrature entanglement at the microwave wavelengths. We carry out a theoretical derivation of the quadrature entangled microwaves generated based on Josephson Parametric Amplifiers (JPAs), and the phase locking error signal, which is used to lock the relative phase of zero between the two quadrature squeezed microwaves. Simulating data for the phase locking scheme are shown under different parameter settings for comparison. Finally, we use the proposed phase locking scheme to enable a stable output of quadrature entangled microwave.

On safe post-selection for Bell tests with ideal detectors: Causal diagram approach

Reasoning about Bell nonlocality from the correlations observed in post-selected data is always a matter of concern. This is because conditioning on the outcomes is a source of non-causal correlations, known as a selection bias, rising doubts whether the conclusion concerns the actual causal process or maybe it is just an effect of processing the data. Yet, even in the idealised case without detection inefficiencies, post-selection is an integral part of experimental designs, not least because it is a part of the entanglement generation process itself. In this paper we discuss a broad class of scenarios with post-selection on multiple spatially distributed outcomes. A simple criterion is worked out, called the all-but-one principle, showing when the conclusions about nonlocality from breaking Bell inequalities with post-selected data remain in force. Generality of this result, attained by adopting the high-level diagrammatic tools of causal inference, provides safe grounds for systematic reasoning based on the standard form of multipartite Bell inequalities in a wide array of entanglement generation schemes, without worrying about the dangers of selection bias. In particular, it can be applied to post-selection defined by single-particle events in each detection chanel when the number of particles in the system is conserved.