Takahiro Morimoto

Quantum Fisher information (QFI) sets the ultimate precision of optical phase measurements and can reveal multiphoton entanglement. Yet the photon-number fluctuations that encode this sensitivity are difficult to convert into an electrical signal. We theoretically predict that a photodetector utilizing the shot noise of the quantum-geometric shift current of exciton polaritons can measure photon-number statistics and, for pure states, infer the QFI. By solving the Lindblad equation, we obtain the time-dependent nonlinear photocurrent for an arbitrary initial photon state. It turns out that, regardless of the quantum state of the incident light, the integrated current depends only on the mean photon number. In contrast, the shot noise retains information about photon-number fluctuations: Its Fano factor is proportional to the photon-number variance. Numerical calculations confirm these relations for optical Schrödinger cat and squeezed-vacuum states. A photodetector based on shift current noise can therefore provide a solid-state platform for high-precision measurements of quantum-optical statistics without relying on mobile photocarriers.

A gapped Dirac cone in antiferromagnetic topological insulators (AFM-TIs) is essential for realizing the quantized anomalous Hall effect, yet its intrinsic magnitude remains controversial. In the prototypical AFM-TI MnBi ₂ Te ₄ , however, the observed Dirac gap is significantly smaller than theoretical predictions or nearly absent, highlighting a long-standing discrepancy. Here we demonstrate that NdBi hosts a spin-polarized topological surface Dirac cone with a sizable gap of 17 ± 2 meV at the Dirac point, in excellent agreement with density functional theory predictions (12 meV). This direct observation is enabled by high-resolution 7 eV laser-based spin- and angle-resolved photoemission spectroscopy, which unambiguously disentangles the spin-polarized surface Dirac cone from bulk-derived states. Furthermore, through temperature dependence and controlled surface contamination, we establish a direct linkage between surface magnetism and the Dirac gap, demonstrating that the gap opens only in the presence of surface magnetic order. These results provide definitive spectroscopic evidence for an intrinsic Dirac gap in AFM-TIs and establish NdBi as a model platform to resolve the long-standing discrepancy between experiment and theory in AFM-TIs.

We theoretically study the emergence of topological magnons in electron-magnon coupled systems. The magnon dispersion in a ferromagnet usually possesses an antiunitary symmetry consisting of a combination of time-reversal and spin-rotation operations in the absence of a Dzyaloshinskii-Moriya (DM) interaction, preventing the appearance of topological magnons. When a spin system is coupled to itinerant electrons, we find that the magnon band structure of the spin system experiences time-reversal symmetry breaking with the electron-magnon interaction via the exchange coupling, where topological magnons arise without requiring strong DM. Specifically, we consider a heterostructure consisting of a ferromagnetic insulator and a transition metal dichalcogenide monolayer and investigate the topological gap opening in magnon bands. Our findings reveal that even trivial ferromagnets can host topological magnons via coupling to itinerant electronic systems.

We propose a novel method to detect axion dark matter based on a topological phenomenon known as the shift current. We make use of the second-order nonlinearity of the shift current by applying a strong oscillating electric field. This field enhances the axion-induced shift current signal and downconverts its frequency to a more accessible range. The nondissipative nature of the shift current allows us to achieve broadband detection via difference frequency generation. We demonstrate the possibility of probing QCD axions and axion-like particles using the properties of the type-I Weyl semimetal TaAs, targeting an axion mass of O ( 10 ) meV , corresponding to a photon coupling of g a γ γ ≃ O ( 10 − 11 ) GeV − 1 .

Noncoplanar spin structures with scalar spin chirality such as magnetic skyrmions exhibit emergent magnetic field as attested by topological Hall effect (THE). Although the impact of emergent field on electronic system has been extensively explored as transport phenomena of itinerant magnets, it has rarely been addressed in insulating magnets. Here, we show the optical THE in skyrmion-hosting Mott insulator GaV ₄ Se ₈ as detected through the magneto-optical Kerr effect (MOKE). The MOKE is found to be enhanced around optical transition of 0.4 eV near the band edge by the formation of the skyrmion lattice (SkL), manifesting the resonantly enhanced optical THE despite perfect disappearance of THE in dc limit because of insulating nature. The photon energy dependence of THE is captured by the theoretical model incorporating momentum-space Berry curvature induced by SkL formation in real space. Our findings represent the large emergent-field-driven electronic response in the Mott insulating state with strong electron correlation, leading to the development of low energy-consumption skyrmion devices.

Antiferromagnets with parity-time symmetry host intriguing optical and transport phenomena governed by quantum metric, as the counterpart, Berry curvature, vanishes under parity-time symmetry. In antiferromagnets with parity-time symmetry, the intrinsic photovoltaic effect, driven by the interband quantum metric associated with optically allowed transitions, is expected due to the inversion symmetry breaking induced by antiferromagnetic order, but experimental demonstration has remained elusive. Here we report the experimental observation of an intrinsic photovoltaic effect in a two-dimensional antiferromagnet with parity-time symmetry, bilayer CrSBr. Notably, the intrinsic photocurrent reverses sign according to the antiferromagnetic configurations. Moreover, by manipulating the magnetic field and device architecture (the top and bottom contacts), we distinctly identify layer-resolved intrinsic photocurrent responses. A tight-binding model based on the band-resolved quantum-metric-driven magnetic injection current mechanism is proposed to interpret these observations and reveal the layer-localized nature of the quantum metric. Our findings provide a promising strategy for developing switchable photovoltaic devices and engineering the spatial quantum geometry in layered antiferromagnets.