High-order harmonic generation (HHG) with simultaneously high energy and high photon flux remains a fundamental challenge in strong-field physics. In this study, we introduce a method employing radially chirped annular (RCA) beams to significantly enhance HHG efficiency and coherence. RCA beams, which enable symmetric simultaneous spatio-temporal focusing with precise control, satisfy the essential requirements for efficient HHG: they can maximize laser intensity at the target while spatially separating the driving field from the harmonics in the far field. The 2D $+$ 1 carrier-resolved pulse propagation simulations demonstrate that, in the presence of plasma-induced defocusing, the RCA beam resists lateral spreading and maintains a more consistent phase front across the beam due to its spatio-spectral structure. Interestingly, the beam profile also exhibits dual chirp behavior, with positive chirping observed on axis and negative chirping off axis. RCA-driven harmonics demonstrate position-independent yield stability relative to the focal point, while remaining highly structured, broader in spectrum, more intense, and less divergent than those generated with conventional Gaussian beams. Moreover, harmonics achieve more spectral extension in the cutoff region and can generate attosecond pulses with durations nearly twice as short and intensities 1.5 times higher than conventional methods, along with enhanced phase stability across the generated harmonics. These findings suggest RCA beams hold strong potential for applications requiring high-flux coherent extreme ultraviolet and soft-x-ray radiation, opening transformative possibilities for advancing attosecond science and ultrafast spectroscopy.

Topological insulators are unique physical structures that exhibit insulating properties in their interior while being conductive on their surface. Here, we present an observation of distinct first-order and second-order topological insulators in photonic martini lattices fabricated by femtosecond laser writing technology. By using direct femtosecond laser writing techniques, we experimentally construct two types of photonic nontrivial martini lattices with tailored boundary conditions (i.e., the martini-up lattice and the martini-down lattice), enabling direct experimental observation of topological edge states and corner states with distinct phase structures. Remarkably, these lattices support different second-order topological phases, each giving rise to distinct classes of topological corner states under varying phase transition conditions. Furthermore, we uncover striking differences in their edge state behavior: while the martini-down lattice sustains only in-phase topological edge states, the martini-up lattice hosts both in-phase and out-of-phase configurations. This work not only introduces a novel framework for exploring higher-order topology, but also provides the possibility for innovative approaches to light manipulation and the development of advanced photonic devices.

Arrays of nonlinear waveguides are a powerful integrated photonics platform for studying and manipulating quantum states of light. Also, they are a valuable resource for various quantum technologies. In this work, we employed a supermode approach to obtain an analytic solution to the evolution of degenerate biphoton states in arrays of nonlinear waveguides. The solution accounts for arrays of an arbitrary number of waveguides and coupling profiles. In addition, it provides an explicit analytic expression without the need of computationally expensive steps. Actually, it only relies on the calculation of the eigenvalues and eigenvectors of the coupling matrix. In general, this needs to be performed numerically, but there are relevant instances in which analytic expressions are available. Thus, in certain cases, the procedure proposed here does not require the use of any numerical method. We analyze the general properties of the solution and show some application examples. Particularly, simple solutions that can be obtained by special symmetric injection profiles, the results for small arrays, and the properties of the propagation when only the center waveguide is pumped in a symmetric odd array. In addition, we present a proof-of-principle example on how to use the analytic solution to tackle inversion problems. That is, obtaining the initial conditions required to achieve a desired quantum state—which is a valuable technological application. A relevant aspect of the method presented here is its scalability for large arrays due to the lack of resource-intensive steps—both for the direct and inverse problems.

Classical chaos is usually characterized by the sensitive dependence of trajectories on initial conditions. However, in quantum mechanics, the unitarity of time evolution ensures that the distances between quantum states are preserved in time. It has, therefore, been a challenge in quantum chaos to reconcile the lack of sensitive dependence on initial conditions with classical chaos. Ballentine [Fundamental Problems in Quantum Physics, edited by M. Ferrero and A. van der Merwe (Springer Netherlands, Dordrecht, 1995), pp. 15–28] has suggested a way out by constructing a parallel argument in classical mechanics based on the preservation of the inner product of phase-space densities. He has argued that the exponential separation of nearby states is a good identifier of chaos only at the level of individual trajectories. For statistical states such as quantum states and classical phase-space densities, chaos must instead be identified by some fundamentally statistical signatures. The search for these signatures is the primary goal in quantum chaos research. However, this perspective also naturally motivates the search for classical analogs of these signatures to reveal the inner machinery of chaos in quantum systems. One widely recognized signature of chaos in quantum systems is the dynamical generation of entanglement. Chaos in the classical system is correlated with a greater entanglement production in the corresponding quantum system. One of the most well-studied examples of this is the kicked top model. In this paper, we construct a classical analog of bipartite entanglement in terms of the mutual information between phase-space distributions of subsystems and find completely analogous signatures of chaos as those found in entanglement for the kicked top Hamiltonian.

Quantum counterparts of Schrödinger's classical bridge problem have been around for the better part of half a century. During that time, several quantum approaches to this multifaceted classical problem have been introduced. In the present work, we unify, extend, and interpret several such approaches through a classical large-deviations perspective. To this end, we consider time-symmetric ensembles that are pre- and postselected before and after a Markovian experiment is performed. Then, the Schrödinger bridge problem is that of finding the most likely joint distribution of initial and final outcomes that is consistent with obtained endpoint results. The derived distribution provides quantum Markovian dynamics that bridge the observed endpoint states in the form of density matrices. The solution retains its classical structure in that density matrices can be expressed as the product of forward-evolving and backward-evolving matrices. In addition, the quantum Schrödinger bridge allows inference of the most likely distribution of outcomes of an intervening measurement with unknown results. This distribution may be written as a product of forward- and backward-evolving expressions, in close analogy to the classical setting, and in a time-symmetric way. The derived results are illustrated through a two-level amplitude-damping example.

Gottesman-Kitaev-Preskill (GKP) states have been demonstrated to pose significant advantages when utilized for fault-tolerant all-optical continuous-variable quantum computing as well as for quantum communications links for entanglement distribution. However, interfacing these systems to long-lived solid-state quantum memories has remained an open problem. Here we propose an interface between quantum memories and GKP qubit states based on a cavity-mediated controlled displacement gate. We characterize the quality of memory-GKP entanglement as a function of cavity parameters suggesting optimal regimes of operation for high-quality state transfer between either qubit states. We further extend this protocol to demonstrate the creation of GKP cluster states by avoiding the requirement of ancillary optical quadrature-squeezed light. Utilizing postselected entanglement swapping operations for GKP qubits, we demonstrate the utility of our protocol for high-rate entanglement generation between quantum memories. Extensions and derivatives of our proposal could enable a wide variety of applications by utilizing the operational tradeoffs for qubits encoded in memory and in the GKP basis.

In some quantum computing architectures, Pauli noise is highly biased. Tailoring quantum error-correcting codes to the biased noise may benefit reducing the physical qubit overhead without reducing the logical error rate. In this paper, we propose a family of quantum $XYZ$ cyclic codes, which is the only family of quantum cyclic codes with code distance increasing with code length to the best of our knowledge. We exploit fully decoupled belief propagation combined with ordered statistics decoding (FDBP-OSD) and Tesseract decoder (TD) to explore the error-correcting performance of the codes. Our simulation results show that the quantum $XYZ$ cyclic codes have $50\%$ code-capacity thresholds for all three types of pure Pauli noise. Under depolarizing noise, we observe $13\%$ code-capacity threshold obtained by FDBP-OSD, while the results obtained by TD indicates that the optimal code-capacity threshold may be approximately $19\%$. In the finite-bias regime, when the noise is biased towards Pauli $Z$ errors with noise bias ratios ${\eta }_Z=1000$, the results obtained by FDBP-OSD and TD both indicate the quantum $XYZ$ cyclic codes have around $49\%$ code-capacity threshold. When ${\eta }_Z=10$ and 100, TD indicates that the corresponding code-capacity thresholds may be around $29\%$ and $32\%$, respectively. In addition, we show that to reach the same code distance, the physical qubit overhead of $XYZ$ cyclic code is much less than that of the XZZX surface code.

Quantum operator scrambling describes the spreading of local operators into the whole system in the picture of Heisenberg evolution, which is often quantified by the operator size growth. Here we propose a measure of quantum operator scrambling via Holevo information of operators, by taking its capacity to distinguish operator information locally. We show that the operator size is closely related to a special kind of Holevo information of operators. Moreover, we propose a feasible protocol for measuring Holevo information of operators on digital quantum simulators based on random states. For the mixed-field Ising model, our numerical simulations show that the integrable system can be told apart from the chaotic system by measuring the spatial-temporal patterns of Holevo information. Furthermore, we find that error mitigation is required to restore the time-oscillation behavior of Holevo information for the integrable system, a crucial feature distinct from the chaotic one. Our work provides a new perspective to understand the information scrambling and quantum chaos from aspects of Holevo information of operators.

Superconducting quantum circuits, such as the transmon, have multiple quantum states beyond the computational subspace. These states can be populated during quantum logic operations; residual population in such states is known as leakage. While control methods can eliminate this error in ideal systems, in the presence of dephasing, transient population leads to leakage. This dephasing-induced leakage effect is analyzed, both analytically and numerically, for common single- and two-qubit operations used in transmon-based approaches to quantum information processing.

Simulating the dynamics of complex quantum systems is a central application of quantum devices. Here we propose leveraging the power of measurements to simulate short-time quantum dynamics of physically prepared quantum states in classical postprocessing using a truncated Taylor series approach. While limited to short simulation times, our hybrid quantum-classical method is equipped with rigorous error bounds. It is extendable to estimate low-order Taylor approximations of smooth time-dependent functions of tractable linear combinations of measurable operators. These insights can be made use of in the context of Hamiltonian learning and device verification, short-time imaginary-time evolution, or the application of intractable operations to subuniversal quantum simulators in classical postprocessing.

Creating a soft-Coulomb-type (SC) molecular potential within single-active-electron approximation (SAE) is essential since it allows solving the time-dependent Schrödinger equations with fewer computational resources compared to other multielectron methods. The current available SC potentials can accurately reproduce the energy of the highest occupied molecular orbital (HOMO), which is sufficient for analyzing nonlinear effects in laser-molecule interactions like high-order harmonic generation (HHG). However, recent discoveries of significant effects of deep-lying molecular orbitals call for more precise potentials to analyze them. In this study, we present a fast and accurate method based on machine learning to construct SC potentials that simultaneously reproduce various molecular features, including energies, symmetries, and dipole moments of HOMO, HOMO-1, and HOMO-2. We use this ML model to create SC SAE potentials of the HCN molecule and then comprehensively analyze the fingerprints of lower-lying orbitals in HHG spectra emitted while varying the H-CN distance. Our findings reveal that HOMO-1 plays a role in forming the second HHG plateau. Additionally, as the H-C distance increases, the plateau structure and the smoothness of HHG spectra are altered due to the redistribution of orbital electron density. These results are in line with other theoretical and experimental studies. Lastly, the machine learning approach using deconvolution and convolution neural networks in the present study is so general that it can be applied to construct a molecular potential for other molecules and molecular dynamic processes.

The development of hybrid photonic-atomic chips (PACs) for trapping, guiding, and controlling single atoms on a chip has been limited by the lack of efficient methods for routing atoms between different optical waveguides. Here we propose an approach for extending single-waveguide atom trapping techniques to multiple-waveguide structures, enabling the realization of multiport hybrid PAC devices. We introduce inverted-rib waveguides to mitigate the scattering and interference of dipole trap lasers at waveguide crossings and utilize high-order optical waveguide modes for atom manipulation. Numerical simulations demonstrate stable atom transporting and routing using copropagating dipole trap lasers with tens of milliwatt power, providing a trap depth of 0.51 mK. This work contributes to the development of scalable PACs, opening avenues for advanced matter-wave manipulation and precision measurements.

Simulation of nonperturbative interactions between electric fields and real three-dimensional atoms is demanding. For this reason, many proof-of-principle and method studies replace three-dimensional atoms by suitable one-dimensional soft-core models. Soft-core potentials may be classified by their curvature at the nucleus that can be either infinite, finite, or vanishing. Within each class, models with analytical ground states and correct $1s$ binding energy exist. In this work we examine if the agreement with real atoms extends to perturbations by electric fields for such models. We demonstrate that the static polarizability is generally overestimated. Accordingly, we construct an improved soft-core potential, for which both $1s$ binding energy and polarizability agree with three-dimensional values. Additionally, the hyperpolarizability of this model deviates by as little as 3% from three-dimensional hydrogen, and field-induced ionization is correctly captured. Finally, we show that the new potential leads to significantly improved response to time-dependent fields.

We propose a method to improve the purity of interacting systems through quantum measurement. We find that the previous quantum-energy-teleportation-based cooling scheme is a special case of our method and the local operations are not necessary for purification [N. A. Rodríguez-Briones et al., Phys. Rev. Lett. 119, 050502 (2017)]. We give the analytical results of measurement-based cooling for two-qubit systems and numerical examples in Gibbs states. Our method can exceed the partner pairing algorithm without multiple iterations and additional auxiliary systems. This shows that the measurement-based cooling scheme may be an effective cooling scheme for strongly interacting systems and shows the important role of quantum correlations in algorithmic cooling.

We investigate superfluorescence (SF) in a degenerate system, where two polarization modes of SF were emitted in competition with each other. A circularly polarized femtosecond laser pulse is used to excite Rb atoms in a heated gas cell from the $5S$ ground to $6P$ state, followed by the SF decay process $6P\to 6S\to 5P\to 5S$. Because the $6S$ and $5P$ levels degenerate into two and four states, respectively, four degenerate transitions are optically allowed between the two levels. Therefore, the left- or right-circularly polarized SF is emitted with a relative intensity determined by the Clebsch-Gordan coefficients of all degenerate transitions. We investigate the polarization characteristics of all SF fields emitted during the decay process. Furthermore, we demonstrate the possibility of switching the dominant polarization component of the SF on the $6S\to 5P$ transition by seeding it with a weak resonant circularly polarized continuous laser field.

We have discovered the abnormally strong influence of the magnetic field on the optical properties of an atomic ensemble confined in a waveguide. We demonstrate qualitative changes in the character of spontaneous emission and single-atom susceptibility. Based on the revealed effects, we propose a scheme of a true single-photon source. Furthermore, we propose the highly efficient optical gate using just one atom.