Characterizing complex quantum evolutions in the microscopic world is crucial for understanding fundamental physics and advancing quantum information applications. In conventional quantum process tomography, individual process matrix elements are obtained indirectly through reconstruction. Here, we use sequential system-ancilla couplings and quantum erasure to directly access these elements for arbitrary quantum processes. Within the pseudo-density-matrix framework, these operations realize an effective erasure-based closed timelike curve (E-CTC) interaction. During E-CTC-assisted measurements, the protocol combines selective computational-basis addressing with fixed superposition preparation and projections. Although the configuration count is not reduced for general processes, these fixed settings can simplify control when computational-basis states are readily addressable. We experimentally demonstrate our scheme in a photonic platform by characterizing several two- and four-dimensional quantum processes with average fidelities above $99\%$. Our work offers a basis-rotation-free approach to high-dimensional quantum process characterization within the erasure-based CTC framework.

We consider polaritons formed by hybridizing a continuum of interband particle-hole excitations of an insulating phase with a cavity photon at subgap frequencies, where absorption is suppressed. The strength of the hybridization is driven by the Van Hove singularity (VHS) in the joint density of states (JDOS) at the band gap: The stronger the singularity, the more a photon is hybridized with the interband transitions. In order to increase the singularity and thus the polariton hybridization without absorption, we propose to engineer a nonparabolic momentum dispersion of the bands around the gap in order to implement a high-order VHS in the JDOS. Ultracold atoms in tunable optical lattices are an ideal platform to engineer two-dimensional gapped phases with nontrivial band dispersions at the gap. Moreover, the intrinsic noninteracting nature of polarized fermionic atoms prevents the emergence of subgap excitations, which are common in solid-state systems and could otherwise spoil the absence of absorption below the gap. Our findings identify band engineering at the gap edge as a promising route for polariton control with applications in quantum nonlinear optics.

Since the random language model was proposed by De Giuli [Phys. Rev. Lett. 122, 128301 (2019)], language models have been investigated intensively from the viewpoint of statistical mechanics. Recently, the existence of a Berezinskii-Kosterlitz-Thouless transition was numerically reported in models with long-range interactions between symbols. In statistical mechanics, it has long been known that long-range interactions can induce phase transitions. Therefore, it has remained unclear whether phase transitions observed in language models originate from genuinely linguistic properties that are absent in conventional spin models. In this study, we construct a random language model with short-range interactions and numerically investigate its statistical properties. Our model belongs to the class of context-sensitive grammars in the Chomsky hierarchy and allows explicit reference to contexts. We find that a phase transition occurs even when the model refers only to contexts whose length remains constant with respect to the sentence length. This result indicates that finite-temperature phase transitions in language models are genuinely induced by the intrinsic nature of language, rather than by long-range interactions.

We report a comprehensive study of the low-temperature magnetic properties of the insulating rare-earth triangular magnet ${\mathrm{CsErSe}}_2$. We uncover a field-induced level crossing at the crystal electric field ground state, which gives rise to a first-order phase transition as well as to distinctive magnetic properties. This crossing is identified by the accurate determination of the single-ion Hamiltonian, the reliability of which is substantially enhanced by field-dependent optical spectroscopy that directly tracks the Zeeman splittings of Kramers doublets. We also observe spontaneous antiferromagnetic ordering in ${\mathrm{CsErSe}}_2$ below $T_N\approx 110\mathrm{mK}$ and resolve the corresponding magnetic structure using elastic neutron scattering. We discuss how the rich magnetic behavior of ${\mathrm{CsErSe}}_2$ arises from the interplay of nontrivial field-dependent single-ion physics and spontaneous ordering, and highlight the implications of these results for understanding magnetic phenomena across a wide range of insulating rare-earth magnets.

We study the spatiotemporal evolution of small disturbances developing in an unstable dispersive system. At the initial linear stage, the slow modulation of an unstable harmonic is analyzed subject to prescribed initial and boundary conditions. A theoretical model based on a modified linearized Ginzburg-Landau equation predicts the emergence of effectively localized envelope structures that can strongly affect the subsequent nonlinear stage of evolution and provides a simple criterion for the onset of absolute instability. As an illustrative example, we consider wind waves generated by an impulsive forcing in a semi-infinite domain over a viscous shear flow.

We present the first experimental investigation of deuteron fusion at thermal energy performed underground. Using deuterated palladium targets, we measured spontaneous proton emission attributed to nuclear fusion reactions between deuterons. Protons with energies around 3 MeV, consistent with the expected signature of the deutron-deutron fusion reaction, were observed in palladium foils. Despite extensive efforts, we found no evidence for the predicted $0^{+}$ resonance near the reaction threshold.

We introduce graphene nanohelicoids, geometric analogs of graphene nanoribbons, in which the honeycomb lattice is embedded on a helicoidal surface. Starting from the three-dimensional helical structure, we construct effective one-dimensional lattice models with band structures characterized by a momentum-shifted particle-hole relation $E_v\left(k\right)=-E_c(k+\pi )$ that reflects an antichiral symmetry arising from the nonsymmorphic symmetry. A systematic investigation of graphene nanohelicoids using the tight-binding approximation reveals a number of trends upon varying width and edge orientation, for instance, alternating transitions between semiconducting and metallic regimes. As the structure width varies, the band gap periodically closes and reopens, accompanied by an alternating Zak phase that switches between trivial and nontrivial. Using an analytic tight-binding model with a continuous deformation of the graphene nanohelicoids, we explain the origin of width- and termination-dependent band inversion and alternating Zak phase, and derive the relationship between the termination-resolved index and the sublattice symmetry operator in one-dimensional nonsymmorphic systems.

Large optical coherent-state superpositions are essential to advance quantum sensing, quantum repeaters, and error-correction codes. We propose a deterministic feedforward protocol employing qubit-mode dispersive coupling, currently available in cavity quantum electrodynamics (QED). We show this single-mode protocol to outperform the advanced three-mode Gaussian-photon-number-resolving detector scheme in terms of both average fidelity and quantum non-Gaussian phase-space properties, and propose sensitivity to weak displacements of interference fringes as a feasible and conclusive witness of quantum interference. This approach combining QED with electro-optical feedforward is extendable to tailored states for applications and other platforms.

We report a combined microfocused angle-resolved photoemission spectroscopy and first-principles study of the rare-earth tetraborides ${R\mathrm{B}}_4$ $(R=\mathrm{Tb}, \mathrm{Gd})$, which host a Shastry-Sutherland lattice. We identify extended bulk nodal lines at the tetragonal Brillouin-zone boundaries, whose fourfold degeneracies are enforced by the nonsymmorphic glide-mirror symmetries even in the presence of spin-orbit coupling (SOC), unlike conventional nodal lines that SOC typically gaps. Systematically evolving Dirac-like dispersions resolved by momentum-distribution-curve analysis support this symmetry-enforced nodal line. We further identify a related nodal-line feature that disperses toward the Fermi level $\left(E_{\text{F}}\right)$; although the calculated SOC-induced gap for this feature (0.2–4.6 meV) lies below the experimental resolution, its proximity to $E_{\text{F}}$ offers a band-structure basis consistent with the emergence of large magnetoresistance. First-principles Wilson-loop calculations establish a nontrivial $\pi$ Berry phase for the bulk nodal line, and we observe a photon-energy-independent surface-derived feature whose dispersion is consistent with the calculated surface state. Our results identify ${R\mathrm{B}}_4$ as a platform for nonsymmorphic-symmetry-enforced nodal-line physics with potential relevance to large magnetoresistance and topological surface states.

We study the matter and entropy transport between two ultracold neutral Fermi-gas reservoirs linked by a quantum point contact under a chemical-potential gradient. We describe the two leads with a BCS mean-field model and derive the current-bias characteristics for both particle and entropy transport. We compute the out-of-equilibrium steady-state currents by using the Keldysh formalism. In accordance with previous works in the literature, we confirm the well-known behavior for the particle current and extend the computation to the entropy current in the BCS regime. The entropy current shows an oscillatory behavior at low voltage in the ballistic junction limit. We analyze the results for a wide range of values of the junction's transparency. We also compare our findings with experimental results in cold atomic gases in the unitary regime.

Light possesses both spin and orbital angular momentum. In spatially inhomogeneous optical fields, which are characterized by a finite distribution of wave vectors in $k$ space, these properties undergo spontaneous coupling, referred to as optical spin-orbit coupling. The study of the coupling has recently become central in modern optics due to its substantial applications in communications, sensing, and quantum control. A key challenge is to clarify the relationship between the origins of spatially inhomogeneous optical fields and the resulting spin-orbit coupling. Current research focuses on materials and configurations exhibiting spatial inhomogeneity, such as focusing lenses, interfaces, inhomogeneous media, and metasurfaces. However, Maxwell’s equations indicate that matter can introduce both spatial inhomogeneity and temporal asymmetry into optical fields. For instance, magnetic ordering breaks the time-reversal symmetry of interacting optical fields via the magneto-optic effect, introducing nonreciprocity in the resulting optical phenomena. Despite the importance, optical phenomena involving both spatially inhomogeneous and temporally asymmetric optical fields remain unexplored. Here, we demonstrate that combining temporal asymmetry and spatial inhomogeneity through magnons and light focusing, respectively, transforms an input Gaussian beam into a specific optical vortex beam in a nonreciprocal manner. This phenomenon is quantitatively explained by integrating the physics of magnon-induced Brillouin light scattering with optical spin-orbit coupling. The observed conservation of total angular momentum, encompassing both magnons and photons, further indicates that magnons can control both spin and orbital angular momentum of light. Finally, we outline future research directions enabled by optical fields exhibiting both spatial inhomogeneity and temporal asymmetry.

We introduce an experimental technique for achieving an unprecedented energy resolution in attosecond interferometry. Using a second-harmonic bandwidth compressor, we create narrow-band ($<1\mathrm{nm}$), tunable femtosecond pulses that generate high harmonics with a bandwidth of 50–73 meV, achieving the highest energy resolution in any attosecond time-resolved measurement, sufficient to resolve the spin-orbit splitting of the ${}^2\mathrm{P}$ ground state of ${\mathrm{Ar}}^{+}$. The fine tunability of these pulses resolves a 10-as variation of the spin-orbit photoionization delay across the [Ne]$3{\mathit{s}}^{1}3{\mathit{p}}^{6}5\mathit{p}$ resonance of neutral Ar. Calculations based on the relativistic random-phase approximation with exchange, with qualitative inclusion of shakeup configurations, reveal the importance of spin-orbit and electron-correlation effects to accurately reproduce and interpret photoionization delays.

We obtain the exact many-body density operator of a boundary-driven XXZ quantum circuit via a spatially inhomogeneous matrix product ansatz. The ansatz has formally infinite bond dimension and generalizes authors’ previous construction [Popkov and Prosen, Phys. Rev. Lett. 135, 070401 (2025)] for the XXZ interactions. The boundary qubits are coupled to reset quantum channels that project them toward arbitrary pure target states. We find and describe a family of relatively robust separable chiral nonequilibrium steady states, which are elliptic analogs of spin helices for the circuit, and which are particularly attractive from an experimental perspective.

Strong coupling between atomic ensembles and high-quality optical cavities enables collective and nonlinear phenomena that are central to cavity quantum electrodynamics (cQED). Many experiments have been performed on this topic, with most of them focusing on cold atoms. Here, we experimentally demonstrate collective strong coupling between thermal rubidium vapor and high-quality silicon nitride microring resonators (MRRs) on an integrated photonic chip. We observe cavity-mode splitting, with a measured collective coupling strength of $g_N/2\pi \approx 1\mathrm{G}\mathrm{Hz}$ and a collective cooperativity of $C_N\approx 2$ at $110{}^{\circ }\mathrm{C}$, indicating coherent energy exchange between the atomic ensemble and the cavity mode despite rapid decoherence in the thermal vapor system. We infer an average of 20 atoms effectively participating in the collective interaction, yielding a single-atom cooperativity of $C_0=0.1$ and approaching the single-atom strong-coupling regime. Our results establish the integrated thermal vapor MRR platform as a robust, compact, and scalable system for studying collective and nonlinear phenomena in cQED.

Epidemic spreading over population networks has been an important subject of research for several decades, especially during the Covid-19 pandemic. Most epidemic outbreaks are likely to create multiple mutations during their spreading over the population. In this paper, we study the evolution of a pathogen that can mutate continuously during the epidemic spreading. We consider pathogens whose mutating parameter is the mortality mean time and study the evolution of this parameter over the spreading process. We use analytical methods to compute the dynamic equation of the epidemic and the conditions for it to spread. We also use numerical simulations to study the pathogen flow in this case and to understand the mutation phenomena. We show that the natural selection leads to less violent pathogens becoming predominant in the population. We discuss a wide range of network structures and show how different effects are manifested in each case. We also applied our theory in the context of the Covid-19 pandemic, using relevant epidemiological data collected for this outbreak. We provided insights for the variants spreading processes observed throughout this pandemic.

Brownian circuits perform computations using stochastic transitions driven by thermal fluctuations. While the energetic costs of such fluctuation-driven computation have been extensively studied within stochastic thermodynamics, much less is known about its computational complexity, in particular, how computation time scales with circuit size. In this work, the computation time for explicitly designed Brownian circuits is numerically investigated via the first-passage time to a completed state. For arithmetic circuits such as adders, varying the forward transition rate induces a sharp change in the scaling behavior of the mean computation time with circuit size, from linear to exponential. This change can be interpreted as an easy-hard transition in computational time complexity. The transition suggests that, for meaningful computational tasks, achieving efficient polynomial-time computation generally requires a finite forward bias corresponding to a nonzero energy input. As a counterexample, we show that arbitrary logical operations can be reduced to an effective one-dimensional stochastic process in which the zero-bias limit lies within the computationally efficient (easy) regime. However, achieving such a one-dimensional normal form unavoidably leads to an exponential increase in circuit size. These results reveal a fundamental trade-off between computation time, circuit size, and energy input in Brownian circuits and demonstrate that phase transitions in time complexity provide a natural framework for characterizing the cost of fluctuation-driven computation.

Direct experimental access to orbital states in strongly correlated materials remains a major challenge, despite their central role in driving coupled structural and magnetic phase transitions. In systems where electronic correlations, electron-lattice coupling, and relativistic spin-orbit interactions compete on comparable energy scales, first-principles calculations often yield multiple metastable solutions, hindering the unambiguous identification of the ground state. Here, we demonstrate that the orbital states of the spinel oxide ${\mathrm{FeV}}_2{\mathrm{O}}_4$, which possesses active orbital degrees of freedom on both Fe and V ions, are uniquely resolved by combining valence electron density (VED) analysis based on state-of-the-art synchrotron x-ray diffraction with spin-polarized density-functional-theory calculations. Our results reveal that temperature-dependent rearrangements of orbital occupations drive successive structural transitions that accompany collinear and noncoplanar ferrimagnetic ordering, establishing a direct correspondence between orbital anisotropy and spin structure. This work shows that experimentally determined VED provides a decisive real-space constraint on competing theoretical solutions, offering a powerful and broadly applicable framework for elucidating the microscopic mechanisms of complex phase transitions in strongly correlated electron systems.