arXiv subjects

Kosei Fujiki

Publications and source records attributed to Kosei Fujiki.

7 recordsLinked to original sources

arXiv2026

Detecting one-dimensional bosonic SPT phases via twisted entropic order parameter

Entanglement asymmetry, introduced by F. Ares, S. Murciano and P. Calabrese, provides a density-matrix diagnostic of symmetry breaking and successfully captures the Landau data associated with a broken symmetry pattern. However, it is by now well established that gapped quantum many-body systems can exhibit phases which are not characterized solely by Landau symmetry breaking. A fundamental example is a symmetry-protected topological (SPT) phase, and the ordinary definition of entanglement asymmetry is insensitive to this topological information. In this work we introduce a refined quantity, which we call the twisted entropic order parameter, designed to detect SPT phases from reduced density matrices, particularly focusing on one-dimensional bosonic systems. The key ingredient in our construction is an ancilla degrees of freedom that coherently records the untwisted state and the twisted state associated to a one-ended topological defect of unbroken symmetry, so that the enlarged density matrix retains the charge carried by the defect endpoint. We demonstrate our proposal in concrete lattice models and further generalize it beyond ordinary group symmetries, establishing its ability to diagnose SPT phases. This provides a first step toward a unified entanglement-asymmetry framework for diagnosing quantum phases of matter.

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arXiv2026

Effective Dynamics of Inflationary End-of-the-World Branes in AdS$_3$

We develop an effective description of a two-dimensional cosmological end-of-the-world brane embedded in AdS$_3$, with a scalar field localized on the brane. Integrating out bulk degrees of freedom, especially in the small-gradient regime, the effective action reduces to a Liouville-like theory coupled to the brane scalar. We then construct brane trajectories whose induced geometry realizes slow-roll inflation and reconstruct the associated scalar profile and potential. We also study regular Euclidean brane geometries that admit a smooth continuation to Lorentzian de Sitter and inflationary universes, and evaluate the semiclassical on-shell action for the de Sitter solution. Finally, we analyze linearized inhomogeneous perturbations and find no exponentially growing mode within the regime of validity of the approximation.

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arXiv2026

Saddle Incompatibility Generates Genuine Multientropy in Heavy Local Quenches

We study the time evolution of tripartite genuine multientropy in heavy local quenches via AdS${}_3$/CFT${}_2$. Its linear response around the vacuum cancels pointwise for arbitrary adjacent intervals. At finite backreaction, the vacuum-subtracted genuine multientropy arises only from an incompatibility gap between the triplet of windings selected by the trivalent geodesic network and the windings selected independently by the bipartite geodesics. The heavy-light vacuum-block calculation reproduces the same formula from the boundary perspective. The time dependence is kinematically fixed and is independent of the heavy operator dimension. These results show that the genuine multipartite entanglement is controlled by global saddle transition rather than by local energy response or quasiparticle propagation.

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arXiv2026

de Sitter holography from a Lorentzian torus

We show that the quantum entanglement between the conformal field theories (CFTs) living on the future and past boundaries in the de Sitter/conformal field theory (dS/CFT) correspondence can be described by Wick rotating to a geometry with two timelike directions. We propose a new realization of the dS/CFT correspondence in which quantum gravity on this two-time geometry is holographically dual to a CFT defined on a Lorentzian torus. We show that this duality reproduces key features of dS holography, including the dS entropy, correlation functions, and pseudoentropy. Finally, by extending the framework of path-integral optimization, we explain how dS spacetime emerges from the CFT on the Lorentzian torus.

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arXiv2026

The Entanglement Wedge Polygon

In this work we consider a particular codimension-1 region of a holographic spacetime which we call the entanglement wedge polygon (EWP). For a pure state and a partition of the boundary into a number of regions $A_i$ the EWP is defined as the region external to all the individual homology regions $r_{A_i}$ which consists of the intersection of the entanglement wedge EW($A_i$) with the time slice. In vacuum AdS$_3$ and BTZ spacetime, the quantity is topological as a direct consequence of the Gauss-Bonnet theorem. In higher dimensions we make progress by considering a number of concrete examples including vacuum, black brane, and soliton solutions of AdS$_{d+1}$ as well as spacetime geometries with end of the world branes dual to boundary conformal field theories. We provide a suitable generalization to mixed states and comment on possible connections between the EWP and measures of multi-partite entanglement.

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arXiv2025

Entropic Interpretation of Einstein Equation in dS/CFT

In this paper, we demonstrate that the first law of holographic pseudo-entropy, which is a non-Hermitian generalization of entanglement entropy in a two-dimensional conformal field theory (CFT), is equivalent to the perturbative Einstein equation in three-dimensional de Sitter (dS) space, assuming the dS/CFT correspondence. Our analysis reveals that the geodesic that accurately satisfies the first law of holographic pseudo-entropy consists of a timelike curve and a curve whose coordinates are complex. We also demonstrate that infinitesimal changes to the pseudo entropy satisfy a Klein-Gordon equation in two-dimensional de Sitter space. These imply the emergence of a time coordinate from a Euclidean CFT in dS/CFT.

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arXiv2025

Brane Cosmology from AdS/BCFT

In this paper, we study the time-dependent dynamics of an end-of-the-world (EOW) brane in AdS with a scalar field localized on the brane. We mainly studied several aspects of holography and cosmology. Standard requirements in the AdS$_{d+1}$/CFT$_d$ lead to a constraint on the conformal dimension in the dS$_d$/CFT$_{d-1}$. We also prove a time-like analog of g-theorem using the null energy condition in the context of AdS$_3$/BCFT$_2$. In the cosmological interpretation, we rewrite the equation of motion of the brane as a Friedman-like equation, which enables us to consider its dynamics in analogy with the ordinal cosmology. And then we classify all possible solutions of the brane when the potential takes a constant value. We find that our brane cosmology model can describe a process of creating a universe via a big-bang. Additionally, we show that when the brane is close to a hyperplane, its effective action is given by a Liouville gravity with a scalar field matter. Finally, we also obtain brane solutions with boost symmetry, which are obtained by analytical continuation of Euclidean branes with a torus topology.

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