Motivated by the recent observation of real-space edge modes with ultracold atoms [C. Braun et al., Nat. Phys. 20, 1306 (2024)], we investigate the preparation and detection of anomalous counterpropagating edge states, a defining feature of the anomalous Floquet valley-Hall (AFVH) phase, in a two-dimensional periodically driven optical Raman lattice. Modeling the atomic cloud with a Gaussian wave-packet state, we explore, both analytically and numerically, how the population of edge modes depends on the initial-state parameters. In particular, we reveal that, in addition to the internal spin state, the initial momenta parallel and perpendicular to the boundary play essential roles: they independently control the selective population of edge states across distinct momenta and within separate quasienergy gaps. Furthermore, we examine the wave-packet dynamics of counterpropagating edge states and demonstrate that their characteristic motion is robust against long-range disorder. These results establish a theoretical framework for future experimental explorations of the AFVH phase and topological phenomena associated with its unique edge modes.

We theoretically investigate the generation of flat-top frequency combs by the exceptional point (EP) in synthetic frequency dimensions. Two coupled ring resonators under amplitude modulation simulate an anti–parity-time $\left(APT\right)$ symmetric Su-Schrieffer-Heeger frequency lattice. At the phase transition point for the $APT$ symmetry being completely broken, the bands become real and linearly cross at the EP. We find that when the incident light contains a component along the generalized eigenvector at the EP, a flat spectrum with a uniformly increasing width emerges. Further, we prove that the speed of the spectrum broadening and the spectral amplitude can be controlled flexibly via amplitude modulations. The numerical simulations are performed based on temporal coupled-mode theory. The study exemplifies the novel approach that combines non-Hermiticity and synthetic dimensions to generate flat spectra, which may enable new strategies in spectral shaping and flat-top microcomb design.

The development of novel radio-frequency atomic receivers has brought attention to the theoretical description of atom-light interactions in sophisticated multilevel schemes. Of special interest are schemes where several interaction paths interfere with each other, bringing about the phase-sensitive measurement of detected radio fields. In the theoretical modeling of those cases, the common assumptions are often insufficient to determine the boundary detection parameters, such as the receiving bandwidth or saturation point, critical for practical considerations of atomic sensing technology. This evokes the resurfacing of the long-standing problem on how to describe an atom-light interaction in a fractured loop. In such a case, the quantum steady state is not achieved even with constant, continuous interactions. Here we propose a method for modeling such a system, basing our approach on the Fourier expansion of a nonequilibrium steady state. The proposed solution is both numerically effective and able to predict edge cases, such as saturation. Furthermore, as an example, we employ this method to provide a complete description of a Rydberg superheterodyne receiver, obtaining the boundary parameters describing the operation of this atomic detector.

Dynamical decoupling is an effective approach to suppress noise-induced decoherence for the implementation of quantum information processing and quantum technologies. However, current studies of dynamical decoupling mainly focus on Hermitian systems. In this paper, we propose a new approach to implement continuous dynamical decoupling in non-Hermitian two-level systems. For parity-time ($\mathcal{PT}$)-symmetric non-Hermitian Hamiltonians, since the dynamics of the system can be transformed into an effective Hermitian case using the vielbein formalism, we theoretically prove and numerically verify that the constructed driving field succeeds in suppressing the noises. We also demonstrate that this method can be extended to systems without $\mathcal{PT}$ symmetries. Our work is beneficial for further studies and applications of dynamic decoupling and non-Hermitian physics in noisy quantum systems.

Equilibrium probes have been widely used in various noisy quantum metrology schemes. However, such an equilibrium-probe-based metrology scenario severely suffers from the low-temperature-error-divergence problem in the weak-coupling regime. To circumvent this limit, we propose a strategy to eliminate the error-divergence problem by utilizing the strong-coupling effects, which can be captured by the reaction-coordinate mapping. The strong couplings induce a noncanonical equilibrium state which greatly enhances the metrology performance. It is found that our metrology precision behaves as a polynomial-type scaling relation, which suggests the reduction of temperature can be used as a resource to improve the metrology performance. Our result is sharply contrary to that of the weak-coupling case, in which the metrology precision exponentially decays as the temperature decreases. Paving a way to realize a high-precision noisy quantum metrology at low temperatures, our result reveals the importance of the non-Markovianity in quantum technologies.

We develop a vibronic coherence model for multiorbital ionization and excitation of nitrogen molecules, based on an up-to-date concept of coherent ionization into excited states [C. H. Yuen and C. D. Lin, Phys. Rev. A 108, 023123 (2023); Phys. Rev. A 109, 033108 (2024)]. Compared to other models, where the coherence of excitation dynamics is described with fewer details, our model predicts larger populations at excited states, in particular, at levels corresponding to the transition wavelengths 391 or 428 nm relevant to air lasing in the atmospheric environment. The dependence of population inversion on the pump wavelength is investigated, showing that ionic coherence in ionization injection enhances the vibronic three-photon transition in the ${\mathrm{N}}_2$ postionization process.

Quantum vortices play an important role in the physics of two-dimensional quantum many-body systems, though they usually are understood in the single-particle framework as the mean-field approach. Inspired by the study on the relations between solitons and yrast states, we investigate here the emergence of vortices from the eigenstates of an $N$-body Hamiltonian of interacting particles trapped in a disk. We analyze states that appear by consecutively measuring the positions of particles. These states have densities, phases, and energies closely resembling mean-field vortices. We discuss similarities, but also point out purely many-body features, such as vortex smearing due to the quantum fluctuation of their center. The ab initio analysis of the many-body system, and mean-field approaches are supported by the analysis within the Bogoliubov approach.

The transfer-matrix method (TMM) is a widely used technique for modeling linear propagation of electromagnetic waves through stratified layered media. However, since its extension to inhomogeneous and nonlinear systems is not straightforward, much more computationally demanding methods such as finite difference time domain or method of lines are typically used. In this paper, we extend the TMM framework to incorporate the effects of nonlinearity. We consider the case where strong coupling between excitons (electron-hole pairs) and photons leads to the formation of exciton polaritons. This extension is crucial for accurately simulating the behavior of light in polariton microcavities, where nonlinearities arising from exciton-exciton interactions play a key role. We perform efficient simulations of light transmission and reflection in a multidimensional system using the plane-wave basis. Additionally, we compare our extended TMM approach with the state-of-the-art admittance transfer method, and highlight the computational advantage of extended TMM for large-scale systems. The extended TMM not only provides a robust and computationally efficient numerical framework, but also paves the way for the development of future low-power nonlinear optical devices, polariton-based photonic circuits, and quantum photonic technologies.

Temporal quantum correlations provide an intriguing way of testing quantumness at the macroscopic level, with a logical hierarchy present among the quantum correlations associated with nonmacrorealism, temporal steering, and temporal inseparability. By manipulating the dynamics of a superconducting qubit, we observe the full hierarchy of temporal quantum correlations. Moreover, we show that the rich dynamics of the temporal quantum correlations, such as sudden death or revival of temporal steering, provides a useful and unique measure for benchmarking qubits on a realistic circuit. Our work finds applications in identifying the casual structure in a quantum network, the non-Markovianity of open quantum systems, and the security bounds of quantum key distribution. As an example, we demonstrate the non-Markovianity of a single superconducting qubit on the quantum circuit.

Quantum illumination (QI) leverages entangled lights to detect the presence of low-reflectivity objects within a thermal environment. In a related vein, quantum parameter estimation utilizes nonclassical probes to precisely determine unknown system parameters. Although both fields have been studied extensively, their performances have traditionally been assessed using different figures of merit: signal-to-noise ratio for QI and quantum Fisher information for parameter estimation. In this paper, we reveal the intrinsic connection between these two measures in the context of target detection, thereby providing explicit operational criteria for identifying optimal measurements. We further apply this relationship to various target detection protocols that employ exotic non-Gaussian states derived from coherent states and two-mode squeezed vacuum states.

Saddle points constitute a crucial challenge for first-order gradient descent algorithms. In notions of classical machine learning, they are avoided, for example, by means of stochastic gradient descent methods. In this work, we provide evidence that the saddle-points problem can be naturally avoided in variational quantum algorithms by exploiting the presence of stochasticity. We prove convergence guarantees and present practical examples in numerical simulations and on quantum hardware. We argue that the natural stochasticity of variational algorithms can be beneficial for avoiding strict saddle points, i.e., those saddle points with at least one negative Hessian eigenvalue. This insight that some levels of shot noise could help is expected to add a new perspective to notions of near-term variational quantum algorithms.

Critical quantum metrology relies on the extreme sensitivity of a system's eigenstates near the critical point of a quantum phase transition to Hamiltonian perturbations. This means that these eigenstates are extremely sensitive to all the parameters of the Hamiltonian. In realistic settings, there is always some degree of uncertainty in the control parameters used to tune the system to criticality. These uncertainties, while not the target of estimation, can significantly affect the attainable precision, effectively acting as nuisance parameters in the estimation process. Despite being a practically relevant source of noise, their impact on critical metrology has been largely overlooked. In this work we present a general framework that interpolates between single- and multiparameter estimation settings, enabling a systematic analysis of how such uncertainties influence sensitivity. We apply this framework to the paradigmatic transverse field Ising and Lipkin-Meshkov-Glick models, explicitly demonstrating how uncertainty in control parameters affects the metrological performance of critical sensors. For finite-size systems, we identify a fundamental trade-off between robustness to uncertainty and the ability to retain a quantum advantage at the critical point. Our results contribute to a deeper understanding of the practical limitations of critical quantum metrology and provide a route toward its more resilient implementation.

We study polarization effects in photoionization of achiral matter and report on two types of circular dichroism driven by a pair of time-delayed copropagating broadband pulses. Both being of interferometric nature, they manifest in the photoelectron momentum distribution (PMD) and vanish for fully time overlapping pulses having the same carrier-envelope phase. The first type, called atomic interferometric circular dichroism (AICD), is demonstrated in photoionization of the H atom or ${\mathrm{H}}_2$ aligned along the pulse propagation direction $\widehat{\mathbf{k}}\parallel \widehat{\mathbf{R}}$. It is shown to be directly visualized through mirroring effects from two key interferograms created in the PMD within two polarization schemes: (i) spiral patterns produced by a pair of counterrotating circularly polarized pulses and (ii) tuning-fork-like patterns driven by linearly and circularly polarized pulses. The second type is dubbed molecular interferometric circular dichroism (MICD) as it occurs in molecular photoionization within the configuration $\widehat{\mathbf{k}}\perp \widehat{\mathbf{R}}$. However, while the MICD occurs in polarization scheme (i), it is shown to interplay with the molecular-symmetry-mixed dichroism [Phys. Rev. Lett. 123, 143202 (2019)] in scheme (ii). Switching the helicities or ordering of the two pulses is shown to be consistent with time-reversal symmetry, except when $\widehat{\mathbf{k}}\perp \widehat{\mathbf{R}}$ in scheme (ii). Exhibiting an inversion symmetry, characteristic of unpolarized targets, the manifestation of the AICD and MICD in the PMD can serve as target chirality identification in life and chemical science. Our predictions based on nonstationary quantum-mechanical calculations are within the reach of current attosecond or femtosecond laser technology, making this study timely for experimental verification.

We study the zero-energy collision of three fermions, two of which are in the spin-down ($\downarrow$) state and one of which is in the spin-up ($\uparrow$) state. Assuming that the two-body and three-body interactions have a finite range, we find a parameter $D$ called the three-body scattering hypervolume. We study the three-body wave function asymptotically when three fermions are far apart or one spin-$\uparrow$ (spin-$\downarrow$) fermion and one pair, formed by the other two fermions, are far apart and derive three asymptotic expansions of the wave function. The three-body scattering hypervolume $D$ appears in the coefficients of such expansions at the order of $B^{-5}$, where $B={\left[(s_1^2+s_2^2+s_3^2)/2\right]}^{1/2}$ is the hyperradius of the triangle formed by the three fermions (we assume that the three fermions have the same mass), and $s_1, s_2, s_3$ are the sides of the triangle. We compute the $T$-matrix element for three such fermions colliding at low energy in terms of $D$ in the absence of two-body interactions. When the interactions are weak, we calculate $D$ approximately using the Born expansion. We also analyze the energy shift of three two-component fermions in a large periodic cube due to $D$ and generalize this result to the many-fermion system. $D$ also determines the three-body recombination rates in two-component Fermi gases, and we calculate the three-body recombination rates in terms of $D$ and the density and temperature of the gas.

We investigate the specific changes of each soliton during the period-doubling of bound solitons in Er-doped mode-locked fiber lasers. Numerical simulations reveal that each soliton in the bound-state soliton can independently undergo period doubling. The different initial states of lasers may lead to in-phase and out-of-phase period doubling of bound-state solitons. These findings are experimentally verified by constructing an Er-doped fiber laser. The real-time spectrum evolution of bound-state solitons was measured using the time-stretched dispersion Fourier transform technique, confirming the existence of in-phase and out-of-phase period-doubling mechanisms in bound-state solitons. This study provides deeper insight into the complex dynamics of solitons in fiber lasers and contributes to the stability analysis of nonlinear systems and the detection of small signals.

An optimal local quantum thermometer is a quantum many-body system that saturates the fundamental lower bound for the thermal state temperature estimation accuracy [L. Correa et al., Phys. Rev. Lett. 114, 220405 (2015)]. Such a thermometer has a particular energy-level structure with a single ground state and highly degenerated excited-state manifold, with an energy gap proportional to the estimated temperature. In this paper, we show that the optimal local quantum thermometer can be realized in an experimentally feasible system of spinless fermions confined in a one-dimensional optical lattice described by the Rice-Mele model. We characterize the system's sensitivity to temperature changes in terms of quantum Fisher information and the classical Fisher information obtained from experimentally available site occupation measurements.

Larger multiqubit quantum gates allow shallower, more efficient quantum circuits, which could decrease the prohibitive effect of noise on algorithms for noisy intermediate-scale quantum (NISQ) devices and fault-tolerant error correction schemes. Such multiqubit gates can potentially be generated by time-independent Hamiltonians comprising only physical (one- and two-local) interaction terms. Here, we present an algorithm that finds the time-independent Hamiltonian for a target quantum gate on $n$ qubits by using the geodesic on the Riemannian manifold of $\mathrm{SU}\left(2^n\right)$. Differential programming is used to determine how the Hamiltonian should be updated to follow the geodesic to the target unitary as closely as possible. We show that our geodesic algorithm outperforms gradient descent methods for standard multiqubit gates such as Toffoli and Fredkin. The geodesic algorithm is then used to find previously unavailable multiqubit gates implementing high-fidelity parity checks, which could be used in a wide array of quantum codes and increase the clock speed of fault-tolerant quantum computers. The geodesic algorithm is demonstrated on an example relevant to current experimental hardware, illustrating a circuit speed up.