Akira Matsumura

We investigate the phenomenon of gravity-induced entanglement in optomechanical systems. Assuming photon number conservation and the Newtonian potential expanded up to the quadratic order of the oscillator positions, we exactly solve the dynamics of the optomehcanical systems. Then, we find that the phase difference due to the Newtonian gravity leads to the large entanglement of photons in separated cavities. We clarify the generating mechanism of large gravity-induced entanglements in optomecha

In this study, we determine a violation of the Leggett-Garg inequalities due to gravitational interaction in a hybrid system consisting of a harmonic oscillator and a spatially localized superposed particle. The violation of the Leggett-Garg inequalities is discussed using the two-time quasiprobability in connection with the entanglement negativity generated by gravitational interaction. It is demonstrated that the entanglement suppresses the violation of the Leggett-Garg inequalities when one o

We investigate quantum entanglement between two symmetric spatial regions in de Sitter space with the Bunch-Davies vacuum. As a discretized model of the scalar field for numerical simulation, we consider a harmonic chain model. Using the coarse-grained variables for the scalar field, it is shown that the multipartite entanglement on the superhorizon scale exists by checking the monogamy relation for the negativity which quantifies the entanglement between the two regions. Further, we consider th

Thermal effects in the characteristics of photovoltaic currents of Pb(Zr, Ti)O 3 (PZT) thin films were studied from room temperature up to 360° C. The photovoltaic currents at a steady state increased with increasing temperature and then decreased above 250° C. By conducting annealing for positively or negatively poled samples, the traces of photovoltaic currents gradually became symmetric, suggesting the improvement of crystallinity of PZT thin films and/or the formation of a symmetric pair of

We discuss the generation of field-induced entanglement between two objects, each in a superposition of two trajectories. The objects have currents coupled to local quantum fields, and the currents are evaluated around each trajectory of the objects. The fields have only dynamical degrees of freedom and satisfy the microcausality condition. We find that the superposed state of trajectories cannot be entangled when the objects are spacelike separated. This means that the quantum fields do not gen

We consider how the reduced dynamics of an open quantum system coupled to an environment is realized in the Poincar\'e symmetry. The reduced dynamics is described by a dynamical map, which is a quantum channel (a completely positive and trace-preserving linear map) given by tracing out the environment from the total unitary evolution without initial correlations. We investigate the dynamical map invariant under the Poincar\'e transformations and discuss how the invariance constrains the form of

We consider a quantum circuit model describing the evaporation process of black holes. We specifically examine the behavior of the multipartite entanglement represented by this model, and find that the entanglement structure depends on the black hole mass $M$ and the frequency of the Hawking radiation $\omega$. For sufficiently small values of $M\omega$, the black hole and the radiation system becomes a separable state after the Page time and a firewall-like structure appears. On the contrary, f

We explore a general feature of the interaction mediated by the gravitational fields of spatially superposed masses. For this purpose, based on quantum information theory, we characterize the evolution of two particles each in a superposition state of paths. The evolution is assumed to be given by a completely positive trace-preserving (CPTP) map. We further assume that the probability of a particle being on each path is unchanged during the evolution. This property is called population preservi

We investigate quantum correlations appearing for two-qubit detectors which are initially uncorrelated and locally coupled to a massless scalar field in a vacuum state. Under the perturbation up to the second order in the coupling, the state of the detectors can be entangled through the interaction with the scalar field but satisfies the Bell-CHSH inequality. The violation of the Bell-CHSH inequality for such an entangled state is revealed by local filtering operations. In this paper, we constru

We investigate the quantum nature of gravity in terms of the coherence of quantum objects. As a basic setting, we consider two gravitating objects each in a superposition state of two paths. The evolution of objects is described by the completely positive and trace-preserving (CPTP) map with a population-preserving property. This property reflects that the probability of objects being on each path is preserved. We use the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi

Quantum coherence is one of the most striking features of quantum mechanics rooted in the superposition principle. Recently, it has been demonstrated that it is possible to harvest the quantum coherence from a coherent scalar field. In order to explore a new method of detecting axion dark matter, we consider a point-like Unruh–DeWitt detector coupled to the axion field and quantify a coherent measure of the detector. We show that the detector can harvest the quantum coherence from the axion dark