We investigate spectral singularities in an alkali-metal atomic vapor modeled using four and effectively three hyperfine states. By comparing the eigenvalue spectra of a non-Hermitian Hamiltonian (NHH) and a Liouvillian superoperator, we analyze the emergence and characteristics of both semiclassical and quantum exceptional points. Our results reveal that, for atomic systems, the NHH approach alone may be insufficient to fully capture the system’s spectral properties. While NHHs can yield accurate predictions in certain regimes, a comprehensive description typically requires the Liouvillian formalism, which governs the Lindblad master equation and explicitly incorporates quantum-jump processes responsible for repopulation dynamics. We demonstrate that the inclusion of quantum jumps fundamentally alters the spectral structure of the system. In particular, we present examples in which the existence, location in parameter space, or even the order of spectral degeneracies differs significantly between the two approaches, thereby highlighting the impact of quantum jumps and the limitations of the NHH method. Finally, using the hybrid-Liouvillian formalism, we show how quantum jumps reshape spectral features initially predicted by the NHH, ultimately determining the full Liouvillian spectrum.

For several decades it has been known that divergences arise in the ground-state energy and chemical potential of unitary superfluids, where the scattering length diverges, due to particle-hole scattering. Leading textbooks and research articles recognize that there are serious issues but ignore them due to the lack of an approach that can regularize these divergences. We find a solution to this difficulty by proposing a general method, called the weighted Hartree-Fock-Bogoliubov theory, to handle multiple decomposition channels originating from the same interaction. We distribute the interaction in weighted channels determined by minimization of the action, and we apply this idea to unpolarized Fermi superfluids. Using our method, we solve a long-standing difficulty in the partitioning of the interaction into Hartree, Fock, and Bogoliubov channels for Fermi superfluids, and we obtain a phase diagram at the saddle-point level, which contains multichannel nonperturbative corrections. In particular, we find a previously overlooked superfluid phase for weak interactions, which is dominated by particle-hole processes, in addition to the usual superfluid phase only containing particle-particle physics.

In the pursuit of numerically identifying ground states of quantum many-body systems, approximate quantum wave function ansatzes are commonly employed. This study focuses on the spectral decomposition of these approximate quantum many-body states into exact eigenstates of the target Hamiltonian, which reflects the intricate physics at the interplay between quantum systems and numerical algorithms. Here, we examine various parametrized approximate quantum states constructed from neural networks, tensor networks, and quantum circuits, employing differentiable programming to numerically approximate ground states and imaginary-time evolved states. Our findings reveal a consistent exponential pattern in the energy eigenbasis decomposition contributions of approximate states across different ansatzes, optimization objectives, and quantum systems, characterized by remarkably small decay rates, i.e., high effective temperatures. This finding is counterintuitive for high-fidelity approximate ground states: While the total contribution from excited states can be made sufficiently small, the residual spectral weight does not decay rapidly with energy. This behavior is an intrinsic property of the variational ansatz, independent of the approximation’s overall accuracy. The effective temperature is related to ansatz expressiveness and accuracy and shows phase transition behaviors in learning imaginary-time evolved states. The universal picture and unique features suggest the significance and potential of the effective temperature metric in characterizing approximate quantum states.

We introduce a tensor network method for approximating thermal equilibrium states of quantum many-body systems at low temperatures. Whereas the usual approach starts from infinite temperature and evolves the state in imaginary time (toward lower temperature), our ansatz is constructed from the zero-temperature limit, the ground state, which can be found with a standard tensor network approach. Motivated by properties of the ground state for conformal field theories, our ansatz is especially well suited near criticality. Moreover, it allows an efficient computation of thermodynamic quantities and entanglement properties. We demonstrate the performance of our approach with a tree tensor network ansatz, although it can be extended to other tensor networks, and present results illustrating its effectiveness in capturing the finite-temperature properties in one- and two-dimensional scenarios. In particular, in the critical one-dimensional case we show how the ansatz reproduces the finite temperature scaling of entanglement in a conformal field theory.

The fluxonium qubit is a promising platform for quantum operations due to its large anharmonicity and long coherence time. However, conventional resonant driving methods often require long operation times and complex implementation. As an alternative, nonadiabatic transitions induced by a time-dependent external flux ${\varphi }_{\mathrm{ext}}\left(t\right)$ can enable faster and simpler control by exploiting quantum interference between multiple transition paths. Existing approaches typically impose periodicity or symmetry on ${\varphi }_{\mathrm{ext}}\left(t\right)$, which can limit control efficiency. We propose a design strategy for ${\varphi }_{\mathrm{ext}}\left(t\right)$ that relaxes these constraints. We demonstrate the effectiveness of this approach in two key scenarios: population inversion and adiabatic passage. Numerical simulations show that our method achieves higher control efficiency than existing schemes, emphasizing the advantage of breaking periodicity and symmetry. Furthermore, by incorporating decoherence effects using the Lindblad master equation, we confirm that the proposed scheme remains robust under realistic conditions.

The coupling between magnetism and topological states at a topological insulator (TI)-magnetic insulator interface can induce exchange gap opening and spin texture change, resulting in many emergent transport phenomena, including anisotropic magnetoresistance (AMR) and planar Hall effect (PHE). While AMR/PHE have been reported in several ferromagnetic insulator-TI heterostructures so far, such effects in antiferromagnetic insulator-TI heterostructure remain elusive. Here, we observe unique features of AMR and PHE in an antiferromagnetic insulator-TI heterostructure $\mathrm{MnSe}\text{/}{\left(\mathrm{Bi},\mathrm{Sb}\right)}_2\mathrm{T}{\mathrm{e}}_3$. Specifically, at low magnetic fields ($H$), the in-plane AMR exhibits dips under $H$ applied parallel to the current ($H\text{//}I$) but peaks under the perpendicular geometry ($H\text{⊥}I$), opposite to the zero AMR features in ferromagnetic insulator-TI heterostructures. The low-field PHE is most striking at $H\text{//}I$, but gradually suppressed when $H$ deviates from parallel direction. At high magnetic fields, both AMR and PHE show prominent π-period oscillation. Gate-tuning and temperature-dependence measurements demonstrate that the observed AMR/PHE originate from the interplay between the topological surface state of TI with the spin canting and magnetic phase transition of MnSe. These results not only differentiate the AMR/PHE in TI-based magnetic heterostructure from the pristine TI, but also offer a feasible way to distinguish spin Hall effect and AMR in TI-based heterostructures.

Recovering dynamical equations from observed noisy data is the central challenge of system identification. We develop a statistical mechanics approach to analyze sparse equation discovery algorithms, which typically balance data fit and parsimony via hyperparameter tuning. In this framework, statistical mechanics offers tools to analyze the interplay between complexity and fitness similarly to that of entropy and energy in physical systems. To establish this analogy, we define the hyperparameter optimization procedure as a two-level Bayesian inference problem that separates variable selection from coefficient inference and enables the computation of the posterior parameter distribution in closed form. Our approach provides uncertainty quantification, crucial in the low-data limit that is frequently encountered in real-world applications. A key advantage of employing statistical mechanical concepts, such as free energy and the partition function, is to connect the large-data limit to thermodynamic limit and characterize the sparsity- and noise-induced phase transitions that delineate correct from incorrect identification. We thus provide a method for closed-loop inference, estimating the noise in a given model and checking if the model is tolerant to that noise amount. This perspective of sparse equation discovery is versatile and can be adapted to various other equation discovery algorithms.

Chirality-induced spin selectivity (CISS), a phenomenon wherein chiral structures selectively determine the spin polarization of electron currents flowing through the material, has garnered significant attention due to its potential applications in areas such as spintronics, enantioseparation, and catalysis. The underlying physical effect is the Edelstein effect that converts charge to angular momentum. Besides a spin contribution, there exists a contribution based on the orbital angular momentum but the precise mechanism for its generation remains yet to be understood. Here, we introduce the minimal model for explaining the phenomenon based on the orbital Edelstein effect. We consider nonlocal intersite contributions to the current-induced orbital angular momentum and reveal the underlying mechanism by analytically calculating the Edelstein susceptibilities in a tight-binding and Boltzmann approach. While the orbital angular momentum is directly generated by the chirality of the crystal, the spin contribution of each spin-split band pair relies on spin-orbit coupling. Using tellurium as an example, we show that the orbital contribution surpasses the spin contribution by orders of magnitude.

The quantum approximate optimization algorithm (QAOA) is a quantum heuristic for combinatorial optimization that has been demonstrated to scale better than state-of-the-art classical solvers for some problems. For a given problem instance, QAOA performance depends crucially on the choice of the parameters. While average-case optimal parameters are available in many cases, meaningful performance gains can be obtained by fine-tuning these parameters for a given instance. This task is especially challenging, however, when the number of circuit executions (shots) is limited. In this work, we develop an end-to-end protocol that combines multiple parameter settings and fine-tuning techniques. We use large-scale numerical experiments to optimize the protocol for the shot-limited setting and observe that optimizers with the simplest internal model (linear) perform best. We implement the optimized pipeline on a trapped-ion processor using up to 32 qubits and 5 QAOA layers, and we demonstrate that the pipeline is robust to small amounts of hardware noise. To the best of our knowledge, these are the largest demonstrations of QAOA parameter fine-tuning on a trapped-ion processor in terms of two-qubit gate count.

The generation of energetic neutron beams by $\mathrm{d}{(\mathrm{d},\mathrm{n})}^3\mathrm{He}$ reactions during the interaction of an intense laser with micron-thick deuterated plastic targets is investigated through two-dimensional particle-in-cell simulations equipped with nuclear reaction formalism. The capability of modeling neutron generation along with laser-plasma interaction enables us to identify and quantify multiple distinct and competing regimes of neutron production happening concurrently at the laser irradiated target. The high-energy neutrons are seen to be produced by two different mechanisms of ion acceleration in thick targets, namely, hole boring radiation pressure occurring inside the target bulk and target normal sheath acceleration that occurs on the rear surface of the target. These two regimes contribute two distinctly different components to the final neutron spectrum. Further a narrow-band (2–3 MeV), quasi-isotropic neutron production of significant flux is also seen during the interaction due to shocklike features seeded inside the bulk target plasma by the fast expansion of contaminant species from the target's front and rear surfaces. While the peak neutron energy is found to saturate at high laser intensities, the integrated flux continues to scale favorably.

Architecting mechanisms of damage in metamaterials by leveraging lattice topology and geometry poses a vital yet complex challenge, essential for engineering desirable mechanical responses. Of these metamaterials, Maxwell lattices, which are on the verge of mechanical stability, offer significant potential for advanced functionality. By leveraging their robust topological features, they enable precise control of effective elastic properties, manipulation of stress localization and delocalization across specific domains, and targeted global damage that follows local fracture events. In this work, we identify topology and geometry-dependent parameters that establish a simple, yet precise, framework for designing the behavior of nonidealized Maxwell lattices and their damage processes. We numerically explore the underlying phenomenology to demonstrate how this framework can guide or arrest damage in lattices, both with and without domain walls and additional boundary constraints. Our approach uncovers a robust way to manipulate the mechanisms of damage and the path they follow in metamaterials, with further insight into crack arrest, diversion, and shielding.

Quantum systems can be prepared in an infinite continuum of states, but only some of them can be used as resources for quantum technologies. Discerning whether a specific quantum state falls into this class is often a challenging task. We show that it can be performed by looking at the squeezing of the quantum states—a scenario in which the variance of some observable operator is suppressed below the threshold given by the nonuseful states. We discuss the general concept first; then, we illustrate the approach by evaluating cubic nonlinear squeezing in the system of collective atomic spins.

Optical phase measurements play a key role in the detection of macroscopic parameters such as position, velocity, and displacement. They also permit to qualify the microscopic properties of photonic waveguides such as polarization mode dispersion, refractive index difference, and chromatic dispersion. In the quest for ever-better measurement performance and relevance, we report a quantum nonlinear interferometer based on a Sagnac configuration allowing precise, accurate, self-stabilized, and reproductible optical phase measurement. The potential of this system is demonstrated through the measurement of second-order dispersion, namely, chromatic dispersion of a commercial dispersion-shifted fiber at telecommunication wavelength. We demonstrate a precision of $0.007\%$, surpassing state-of-the-art measurements by more than one order of magnitude. Additionally, the accuracy of the second-order dispersion value is determined through the measurement of the third-order dispersion, showing a quadratic error as low as 5%. Our system promises the development of photonic-based sensors enabling the measurements of optical-material properties in a user-friendly manner.

Shanghai Soft X-ray Free Electron Laser facility has entered user operation, delivering soft x-ray pulses in self-amplified spontaneous emission mode with an intensity of the peak fluence of $\sim 1.0\times {10}^{-2}\mu \mathrm{J}/\mu {\mathrm{m}}^2$ at a focus point. With these conditions, at a photon energy of 260 eV, we expect sequential processes in which ${\mathrm{Ar}}^{2+}$ created by the Auger decay following the $2p$ photoionization may be resonantly excited to the $2p^{-1}3p^{-2}3d$ inner-shell excited state and thus resonant Auger decay produces ${\mathrm{Ar}}^{3+}$, which can also be competitively produced by the direct double Auger decay (DDAD) following the one-photon $2p$ ionization of Ar. ${\mathrm{Ar}}^{4+}$ ions dominantly arise from ${\mathrm{Ar}}^{2+}$ with a resonant excitation and subsequent DDAD, involving two-photon absorption. The measured charge distributions as a function of the free-electron laser (FEL) fluence are well reproduced by ab initio calculations that include DDAD, normally neglected in the previous FEL studies, and resonant Auger decay. The present results demonstrate that the low peak fluence of the FEL pulses can be determined if the competition between the one- and two-photon processes is precisely described along with DDAD and resonant Auger decay.