We investigate the performance and accuracy of digital quantum algorithms for the study of static and dynamic properties of the fermionic Hubbard model at half-filling with next-nearest-neighbor hopping terms. We provide quantum circuits to perform ground and excited state calculations, via the variational quantum eigensolver (VQE) and the quantum equation of motion (qEOM) approach, respectively, as well as product formula decompositions for time evolution. We benchmark our approach on a chain with $L=6$ sites and periodic boundary conditions, computing the charge and spin gaps, the spectral function and spin-spin dynamic correlations. Our results for the ground-state phase diagram are in qualitative agreement with known results in the thermodynamic limit. Finally, we provide concrete scalings for the number of gates needed to implement our protocols on a qubit register with all-to-all connectivities and on a heavy hexagonal coupling map.

We propose a method to realize dynamical phonon blockade in a hybrid spin-mechanical system. Under the weak excitation limit, we demonstrate that by simply controlling the external pump field, antibunching is enhanced in the strong coupling regime and can also be induced in the weak coupling regime. Additionally, we conduct a comprehensive analysis of the proposed scheme and assess the impact of relevant parameters on phonon blockade. Through further optimization of the system using differential evolution algorithms, we achieve a sub-Poissonian phonon source with high purity and strong single-phonon statistics. The results show that the dynamic scheme allows the system to achieve quantum effects at the single-phonon level over a broad parameter range, which improves its robustness. This mechanism also regulates the time interval between consecutive emissions, reducing the requirements for time resolution. Meanwhile, the time window for quantum information is sufficiently broad, providing a practical method for generating single-phonon sources.

Correlated electron dynamics are pivotal in double ionization (DI) induced by ultrashort laser pulses, yet their direct, time-resolved characterization has remained elusive. Here, we apply the reconstruction of attosecond beating by interference of two-photon transitions (RABBITT) technique to directly characterize correlation-driven effects in sequential DI of helium with attosecond resolution. We uncover a previously unobserved scattering phase shift, sensitive to the kinetic energy difference between the two released electrons, corresponding to a time delay of the order of $\sim 10$ as. Moreover, when the DI pathway proceeds via a doubly excited state, Fano resonances introduce nontrivial phase shifts for both the first electron and its correlated partner. By extracting joint angular distributions at each sideband energy, we provide an intuitive picture to understand these correlation-driven effects. Our findings provide direct, time-resolved insights into electron-electron interactions in DI, demonstrating the power of attosecond chronoscopy in unraveling electron-electron interactions on their natural timescale.

Topological photonics provides a powerful and robust way to control the flow of light. Recently, the introduction of ${\chi }^{\left(3\right)}$ nonlinearity made it possible to realize non-Hermitian topological photonics. In this work, we propose a ${\chi }^{\left(2\right)}$-coupled double-chain Su-Schrieffer-Heeger scheme in a periodically poled lithium niobate on insulator (LNOI) waveguide array. By tuning the pump power, one can easily modulate the nonlinear coupling strength, and therefore, manipulate the topological states of signal and idler lights. By designing various configurations based on LNOI waveguide array, we present non-Hermitian phase transition, topological edge state, and photonic zero mode. Our work provides a useful platform to investigate the interaction between ${\chi }^{\left(2\right)}$ nonlinearity, topology, and non-Hermiticity, which have potential applications to manipulate light fields in classical and quantum optics.

The pulsed driving of passive fiber Kerr resonators is an effective approach to generating coherent optical soliton combs with high efficiency. Desynchronization, which is defined as the frequency mismatch between the pumping pulse repetition rate and the cavity round-trip time, is an additional system variable, making the nonlinear system of fiber resonators more complex. Research on the relevant physics of desynchronization is not yet comprehensive. In this article, the complex dynamics of Kerr cavity solitons in a pulsed-pumped normal dispersion fiber Kerr resonator with third-order dispersion (TOD) and desynchronization is studied in simulations. We find that the desynchronization effect plays a critical role in the formation of Kerr cavity solitons. Interestingly, the introduction of desynchronization leads to the formation of breathing Kerr cavity solitons before the onset of stable operation of Kerr cavity solitons. The competition and balance between TOD and desynchronization determine the behavior of Kerr cavity solitons. Without TOD, a strong desynchronization effect results in a breathing state with a sliding shift of pulse peak position. In contrast, a slight desynchronization effect causes the periodic oscillation of pulse peak power without changing the pulse location. The breathing period depends on the absolute value of desynchronization coefficient and the sliding direction is related to the sign of the desynchronization coefficient. Furthermore, a reversible switching between different nonlinear Kerr cavity soliton states is achieved by changing the degree of desynchronization, suggesting that the desynchronization can be used as an additional degree of freedom to control the Kerr cavity soliton dynamics in Kerr resonators. These findings enrich our understanding of Kerr cavity solitons and provide important references for improving the stability of Kerr cavity solitons and the application of efficient resonator frequency combs.

In this Letter, we show how Euclidean three-space uniquely emerges from the structure of quantum temporal correlations associated with sequential measurements of Pauli observables on a single qubit. Quite remarkably, the quantum temporal correlations which give rise to geometry are independent of the initial state of the qubit, which we show enables an observer to extract geometric data from sequential measurements without the observer having any knowledge of initial conditions. Such results suggest the plausibility that space itself may emerge from quantum temporal correlations, and we formulate a toy model of such a hypothetical phenomenon.

Electrons emitted during photoionization provide key insights into light-atom interactions. While conventional laser pulses have been widely studied, the effects of structured light fields, particularly twisted Bessel beams with orbital angular momentum, remain less explored. Previous research has shown that spectral peaks in above-threshold ionization shift with changes in the opening angle and orbital angular momentum of Bessel pulse. However, the precise impact and sensitivity of these parameters on photoelectron momentum distributions (PMD) is still unclear. In this paper, we use the strong-field approximation and saddle-point method to analytically derive the ionization transition amplitude for these pulses. For computational simplicity, we model the atomic target using a hydrogenic $1s$ orbital wave function, with parameters adapted for krypton to capture the essential ionization dynamics. This allows us to compute the PMD while resolving the effects of both the opening angle and orbital angular momentum.

We propose a phase-space Gaussian quantum ensemble to depict the high-order harmonic generation process. Our model takes the wave-particle duality and the uncertainty principle into account. It bridges the ab initio calculation with the semiclassical analysis, enabling us to distinguish different channels and reveal the underlying electron dynamics. We investigate the ionization and recombination time of the short, long, and multi-rescattering trajectory harmonics. Our model also provides access to investigate the position-momentum properties. Results demonstrate that the electron has a nonzero initial velocity at the tunnel exit, revealing the nonadiabatic effect. We prove the validity of our analysis by interpreting the observables in the two-color interference experiments. The nonzero transverse harmonic signal for any two-color delay in the orthogonally polarized fields can be explained by the delocalized distribution of the quantum ensemble. Our model can guide the semiclassical analysis and provide a benchmark for high-order harmonic spectroscopy.

The Jaynes-Cummings and quantum Rabi models are fundamental to cavity and circuit quantum electrodynamics, as they describe the simplest form of light-matter interaction, where a single qubit is coupled to a single bosonic mode. A scenario that is commonly encountered in the experimental practice arises when the bosonic mode interacts with an external dissipative thermal bath, making the qubit-boson system open. In this work, we present a new analytical solution of the Lindblad master equations for the open dispersive Jaynes-Cummings model and a perturbative analytical solution of the open quantum Rabi model in the limit of weak qubit-boson coupling $g$, using the holomorphic formalism in Bargmann space. Specifically, we derive the most general solution of the local Lindblad master equation for the open dispersive Jaynes-Cummings model coupled to a thermal bath, with the only assumptions that the initial state of the qubit-boson system is separable. Additionally, we obtain a perturbative analytical solution for the open quantum Rabi model up to second order in $g$. Notably, our findings include a new formula for the qubit's steady state at zeroth order, showing that the stationary populations depend on both qubit and boson frequencies in the quantum Rabi model, but not in the Jaynes-Cummings model, regardless of the value of $g$. Our results are of general interest to the study of open quantum systems in the context of light-matter interaction.

We demonstrate that the nonequilibrium steady state (NESS) of an open-system Holstein model with linear bias displays extreme sensitivity to the closed-system parameters. This sensitivity is shown to correspond to avoided crossings in the closed-system spectrum, as previously demonstrated in the Rabi model. We then develop a kinetic model to analyze the effects of environmental parameters on NESS hypersensitivity. This reveals that hypersensitivity only exists in intermediate environmental parameter regimes, a prediction that is verified numerically. The inherent spatial character of the Holstein model offers a natural connection to transport, revealing that transport properties in the steady-state regime can be optimized by simultaneously coordinating the closed- and open-system parameters.

Over the past two decades, much progress has been made in the construction of nonlocal sets without entanglement. Recently, Li and Wang [Quantum 7, 1101 (2023)] proposed an open question that an orthogonal product set (OPS) $\mathcal{S}$ is locally stable with minimum cardinality in ${⨂}_{i=1}^n{\mathbb{C}}^{d_i}$, and the numerical lower bound for such a set was given by Wang et al. [Quantum Inf. Process. 22, 15 (2023)], $\left(\right|\mathcal{S}|-1\left)\right|\mathcal{S}|\ge {\sum }_{i=1}^n(d_i^2-1)$, but the existence of the construction remains unresolved. In this work we construct locally stable sets with minimum cardinalities $d^2$ and $d^2+1$ via 1-orthogonal cyclic product sets in ${\left({\mathbb{C}}^d\right)}^{\otimes d^2}$ for odd $d$ and ${\left({\mathbb{C}}^d\right)}^{\otimes d^2+1}$ for even $d$, respectively, thereby partly addressing Li and Wang's open question and confirming the existence of the locally stable OPS with the lower bound. Furthermore, we present a locally stable orthogonal cyclic product set of size $n$ in ${\left({\mathbb{C}}^d\right)}^{\otimes n}$ and a locally stable OPS with smaller size in general multipartite systems. Our results show a perfectly cyclic structure of locally stable OPSs and provide new insight into the role of local stability without entanglement in quantum information theory.

The Leggett-Garg inequalities have been proposed to identify the quantum behavior of a system; specifically, the violation of macrorealism. They are usually implemented by performing two sequential measurements on quantum systems, calculating the correlators of such measurements and then combining them arriving at Leggett-Garg inequalities. However, this approach only provides sufficient conditions for the violation of macrorealism. Recently, an alternative approach was proposed that uses nondemolition measurements and gives both a necessary and sufficient condition for the violation of macrorealism. By storing the information in a quantum detector, it is possible to construct a quasiprobability distribution whose negative regions unequivocally identify the quantum behavior of the system. Here, we perform a detailed comparison between these two approaches. The use of the IBM quantum simulators allows us to evaluate the performance in real-case situations and to include both the statistical and environmental noise. We find that the nondemolition approach is not only able to always identify the quantum features, but it requires fewer resources than the standard Leggett-Garg inequalities. In addition, while the efficiency of the latter is strongly affected by the presence of the noise, the nondemolition approach results incredibly robust and its efficiency remains unchanged by the noise. These results make the nondemolition approach a viable alternative to the Leggett-Garg inequalities to identify the violation of macrorealism.

Here we present the results of benchmarking a quantum processor based on trapped ${}^{171}\mathrm{Yb}^{+}$ ions by performing basic quantum machine learning algorithms. Using a quantum-enhanced support vector machine algorithm with up to five qubits we perform a supervised binary classification on two types of datasets: small binary digit images and weighted graphs with a ring topology. For the first dataset, images are intentionally selected so that they could be classified with 100% accuracy. This allows us to specifically examine different types of quantum encodings of the digit dataset and study the impact of experimental noise. In the second dataset, graphs are divided into two categories based on the spectral structure of their Ising Hamiltonian models, which is related to the NP-hard problem. For this problem we consider an embedding of an exponentially large Hamiltonian spectrum into an entangled state of a linear number of qubits. For both problems, we study various levels of circuit optimization and found that, for all experiments conducted, we achieve classifiers with 100% accuracy on both training and testing datasets. This demonstrates that the quantum processor has the ability to correctly solve the basic classification task under consideration. As we expect, with the increase in the capabilities of quantum processors, they can be utilized for solving machine learning tasks.

This work presents a scheme for engineering quantum entanglement and Einstein-Podolsky-Rosen (EPR) steering with Gaussian measurements based on the quantum Hopfield model that incorporates a common thermal reservoir. We begin by examining quantum correlations, specifically quantum entanglement and EPR steering, in the ground state. These quantum correlations primarily stem from squeezing interactions in weak-coupling and normal strong-coupling regimes. As the coupling strength increases, especially upon entering the ultrastrong-coupling regime, the correlations emerge from the combined effect of squeezing and mix-mode interactions. Importantly, this scenario enables the realization of two-way EPR steering. Moreover, lower optical frequencies enhance both quantum entanglement and EPR steering. Further, when considering thermal effects, the ultrastrong-coupling and deep strong-coupling regimes, paired with lower optical frequencies, lead to improved entanglement. The one-way EPR steering for the resonant case can be effectively controlled in the ultrastrong-coupling and deep strong-coupling regimes which originate from the asymmetry of the subsystem and reservoir coupling induced by the diamagnetic term. Additionally, one-way EPR steering can also be produced for the nonresonant case. In this case, the asymmetry of the subsystem and reservoir originates from the combined effect of nonresonant frequencies and the diamagnetic term. Our findings have the potential to inspire further research into quantum information processing that leverages light-matter entanglement and EPR steering.

Integer factorization remains a significant challenge for classical computers and is fundamental to the security of RSA encryption. Adiabatic quantum algorithms present a promising solution, yet their practical implementation is limited by the short coherence times of current noisy intermediate-scale quantum devices and quantum simulators. In this work, we apply the chopped random-basis (CRAB) optimization technique to enhance adiabatic quantum factorization algorithms. We demonstrate the effectiveness of CRAB by applying it to factor the integers ranging from 21 to 2479, achieving significantly improved fidelity of the target state when the evolution time exceeds the quantum speed limit. Notably, this performance improvement shows resilience in the presence of dephasing noise, highlighting CRAB's practical utility in noisy quantum systems. Our findings suggest that CRAB optimization can serve as a powerful tool for advancing adiabatic quantum algorithms, with broader implications for quantum information processing tasks.

Electron spin polarization in an intense laser field presents a promising avenue for generating highly polarized high-energy electron sources. In this study, we investigate numerically the spin polarization dynamics of an electron in a rotating laser field with a frequency chirp. We consider a standing wave formed by two counterpropagating chirped laser pulses, which have different chirp modes: linear and time-reversed chirp. Our results reveal that the frequency chirp breaks the time symmetry of the field, thereby altering the laser-electron interaction dynamics. Consequently, an electron can gain more net energy from a chirped laser field through optimal phase synchronization between the electron and the laser field. This results in a higher electron quantum parameter and an increased photon emission probability. As a result, the probability of spin flip for an electron during radiation in the magnetic node plane is enhanced, leading to improved electron polarization efficiency. We also find that the polarization degree is highly sensitive to the chirp parameter, and we identify an optimal chirp value that maximizes the polarization degree. By applying this optimal chirp parameter in the time-reversed chirped case, the electron's relative polarization can be enhanced by $20\%$ and reach its maximum value of approximately $85\%$.

Higher-order topological insulators have attracted significant interest in both static single-particle and many-body lattice systems. While periodically driven (Floquet) higher-order topological phases have been explored at the single-particle level, the role of interactions in such systems remains less understood. In this paper, we extend previous studies by investigating interaction-induced higher-order topological phases through Floquet engineering. To achieve this, we construct an extended Bose-Hubbard model on a square lattice subjected to periodic driving. We demonstrate the emergence of interaction-induced normal Floquet second-order topological corner states for doublons (i.e., bound boson pairs) from a trivial phase, which exhibit robustness against disorder. Notably, beyond the normal phase, we reveal an interaction-induced anomalous Floquet second-order topological phase, where in-gap corner states of doublons emerge within the $\pi /T$ gap ($T$ being the driving period). Our model, accessible with state-of-the-art ultracold atom techniques, provides a platform for realizing interaction-driven higher-order topological phases uniquely enabled by periodic driving, with no direct counterparts in static or single-particle systems.

Soliton microcombs (SMCs) spontaneously emerge in coherently pumped high-quality (high-$Q$) microcavities and are widely used as on-chip optical frequency combs for precise measurements and spectroscopic applications. In this work, we present a thermal compensation scheme for generating single SMCs. Compared to other methods, the single SMC generated using our approach exhibits remarkable stability, with a lifetime exceeding 24 hours, and can be deterministically generated over a wide temperature range. Utilizing this feature, we successfully achieve thermal self-starting of single SMCs using a simple temperature control process. Over an experiment lasting more than 3 hours (with a cycle time of approximately 200 seconds between soliton generation, decay by design, and restart), the success rate of single SMC generation was 100%.