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Latest articles for New Journal of Physics
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Superfluid dripping: a new analog for continuous time crystals
Time crystals refer to a state realized in an open system that spontaneously breaks time translation symmetry. There are ongoing discussions, both theoretical and experimental, regarding their realization and potential extensions, which highlights the need to explore new demonstrations in non-equilibrium systems. In this study, we demonstrate that the dripping of a superfluid can exhibit time crystallinity. We visualized pendant droplets of superfluid ⁴He dripping from various shaped surfaces and observed that the dripping period became consistently discretized when the edge of the droplet was free to move along the surface. This edge motion resulted in oscillation periods that were independent of the droplet’s volume, effectively eliminating the influence of variations in volume and flow rates on the timing of the dripping. Consequently, the dripping process became regular. The unimpeded movement of the droplet’s edge is a characteristic of superfluids and is facilitated by a preexisting superfluid thin film on the surface, which minimizes the effects of surface roughness and enhances the mobility of the edge. We examined the stability of the discretized periods as a function of input flow rate and concluded that the superfluid dripping system spontaneously broke continuous time translation symmetry, making it analogous to a continuous time crystal, but with multiple stable phases. In contrast, when the edge of the droplet was pinned, the dripping period displayed a wide distribution, resembling the irregular dripping behavior seen in classical fluids.
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Cavity-Heisenberg spin-j chain quantum battery and reinforcement learning optimization
In the realm of quantum batteries (QBs), model construction and performance optimization are central tasks which can be addressed by exploiting machine learning algorithms. Here, we propose a cavity-Heisenberg spin chain QB model with spin- and investigate the charging performance under both closed and open quantum cases, considering spin–spin interactions, ambient temperature, and cavity dissipation. By employing a reinforcement learning (RL) algorithm to modulate the cavity-battery coupling, we further optimize the QB performance, enhancing the charging capability of the spin chain. It is shown that the charging energy and the power of the QB are significantly improved with the spin size. In particular, the use of a RL algorithm in case of large spin in presence of cavity losses allows for more stability in the optimization of the cavity-spin coupling strength, which in perspective makes an experimental realization more feasible. We analyze the optimization mechanism and find an intrinsic relationship between cavity-spin entanglement and charging performance: while in the closed-system scenario the charging energy increases together with the cavity-spin entanglement, in the open-system scenario the increase of the charging energy can be accompanied by a decrease of entanglement. Our results provide a possible scheme for design and optimization of QBs.
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Dissipative evolution of a two-level system through a geometry-based classical mapping
In this manuscript, we introduce a geometry-based formalism to obtain a Meyer–Miller–Stock–Thoss mapping in order to study the dynamics of both isolated and interacting two-level systems (TLS). After showing the description of the isolated case using classical-like canonically conjugate variables, we implement an interaction model by bilinearly coupling the corresponding population differences à la Caldeira–Leggett, obtaining a Gross–Pitaevskii-like equation. This allows us to analytically find a transition between oscillatory and tunneling block dynamics by varying the coupling constant. Extending our model to the system plus environment case, where the environment is considered as a collection of TLSs, we reveal that the dynamics become chaotic, exhibiting tunneling-suppressed dynamics in the strong coupling limit and the usual damping effect similar to that of a harmonic oscillator bath in the weak coupling one. Finally, we observe that our interacting model turns an isolated symmetric TLS into an environment-assisted asymmetric one.
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Dynamic modulation of toroidal bound states in the continuum with suppressed continuum energy
Bound states in the continuum (BICs) enable ultrahigh-Q photonic resonators critical for advanced optoelectronics, yet their dynamic reconfigurability, particularly modulation depth in single-resonance systems, remains fundamentally limited. We overcome this by synergistically coupling a toroidal dipole quasi-BIC with a Fabry–Pérot resonance in a graphene-integrated metasurface. Through overdamped mode hybridization and graphene’s gate-tunable complex conductivity, the system achieves record modulation depth of 97.7% at 1550 nm while dynamically tuning Q-factors from 240 to 2600. This dual-resonance architecture simultaneously enables spectral agility and critical coupling absorption, resolving the long-standing trade-off between ultrahigh-Q confinement and reconfigurability for intelligent photonic devices.
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High-harmonic spectroscopy of mobility edges in one-dimensional quasicrystals
Quasicrystals occupy a unique position between periodic and disordered systems, where localization phenomena such as Anderson transitions and mobility edges (MEs) can emerge even in the absence of disorder. This distinctive behavior motivates the development of robust, all-optical diagnostic tools capable of probing the structural, topological, and dynamical properties of such systems. In this work, focusing on generalized Aubry–André–Harper models and on an incommensurate potential in the continuum limit, we demonstrate that high-harmonic generation (HHG) phenomenon serves as a powerful probe of localization transitions and MEs in quasicrystals. We introduce a new parameter–dipole mobility–which captures the impact of intraband dipole transitions and enables classification of nonlinear optical regimes, where excitation and HHG yield can differ by orders of magnitude. We show that the cutoff frequency of harmonics is strongly influenced by the position of the ME, providing a robust and experimentally accessible signature of localization transitions in quasicrystals.