We investigate single-photon scattering in a system comprising a waveguide coupled to a pair of Rydberg atoms, illuminated by a coherent field. By adjusting the interatomic distance, we can transition between the Rydberg blockade and Rydberg antiblockade regimes, as the van der Waals interaction strength varies with distance. These distinct regimes, manifesting in single-photon scattering, allow flexible reflection control due to their analogy to those of small- and giant-atom interactions with the waveguide. We also derive scattering criteria for Rydberg blockade and Rydberg antiblockade, corresponding to specific features in the single-photon reflection spectrum. Based on these criteria, the blockade and antiblockade distances can be estimated.

This study investigates the entanglement dynamics of a bipartite system comprising two two-level atoms, initially prepared in a pure entangled state and independently coupled to spatially separated cavities under local squeezed thermal environments. We demonstrate that environmental interactions induce entanglement sudden death (ESD), leading to the abrupt loss of quantum correlations. To mitigate this effect, we propose a protocol involving local unitary operations—X gate $({\sigma }_x^{\left(1\right)}\otimes I^{\left(2\right)})$ and XX gate $({\sigma }_x^{\left(1\right)}\otimes {\sigma }_x^{\left(2\right)})$—applied to the time-evolved state. We further analyze the impact of these operations under symmetric and asymmetric probability amplitudes of the atomic states, considering variations in thermal and squeezing parameters across different reservoirs. Our results show that these operations extend entanglement duration in specific reservoir conditions, while in the case of a vacuum reservoir, they transform ESD into asymptotic decay for both symmetric and asymmetric probability amplitudes of the atomic states. Moreover, independent variations of the thermal and squeezing parameters in each reservoir influence the dynamics of local gate operations, thereby affecting the overall entanglement evolution. Additionally, applying these operations at an earlier stage of the system's evolution can further delay ESD. This scheme offers valuable insights into entanglement behavior and its resilience under various noisy environmental conditions.

We study double ionization in a one-dimensional model of a three-electron atom exposed to strong short laser fields. The corresponding ab initio simulations reveal a characteristic “knee” in the ionization yield at lower field intensities than in comparable two-electron systems. To understand the origin of this shift, a quantitative semianalytic model for double ionization of a three-electron atom has been created. In this model, ionization is treated as a combination of different scenarios and channels. Each channel is simulated with either quantitative rescattering theory (QRS) or a two-electron ab initio model. Analysis of the semianalytical results indicates that the shifted part of the “knee” is dominated by a recollision-excitation channel in which the rescattering electron initially escapes the parent atom from an inner orbital. This counterintuitive phenomenon is then discussed and explained.

Singular optics is an important and meaningful subject that has attracted widespread attention. The famous vortex beam and vector beam carrying different singularities have found broad applications, and there are four kinds of optical singularities including one kind of phase singularity and three kinds of polarization singularities. However, the classification of optical singularities remains incomplete, as the polarization singularity with indeterminate ellipticity and determinate orientation has not been defined. Here, we propose a kind of E-point singularity, where the ellipticity of polarization is indeterminate and the orientation is determinate. The amount, positions, and topological charges of the E-point singularities can be theoretically and experimentally manipulated on the wave front of the beam. It is found that the E-point singularity has a function of polarization purification in propagation, and the E-point singularity acts like catalyst when achieving pure transverse spin angular momentum (SAM) in tight focusing process. The proposed E-point singularity not only supplements the theoretical framework of singular optics, but also enables SAM manipulation during propagation and focusing processes, demonstrating broad application potential across various regions.

We generalize a space-time-symmetric (STS) extension of nonrelativistic quantum mechanics (QM) to describe a particle moving in three spatial dimensions. In addition to the conventional time-conditional (Schrödinger) wave function $\psi (x,y,z|t)$, we introduce space-conditional wave functions such as $\varphi (t,y,z|x)$, where $x$ plays the role of the evolution parameter. The function $\varphi (t,y,z|x)$ represents the probability amplitude for the particle to arrive on the plane $x=\text{constant}$ at time $t$ and transverse position $(y,z)$. More generally, within our framework, any coordinate $x^{\mu }\in \{t,x,y,z\}$ can be conveniently chosen as the evolution parameter, depending on the experimental context. This leads to a unified formalism governed by a generalized Schrödinger-type equation ${\widehat{P}}^{\mu }|{\varphi }^{\mu }\left(x^{\mu }\right)\rangle =i\hslash {\eta }^{\mu \nu }\frac{d}{d{x}^{\nu }}|{\varphi }^{\mu }\left(x^{\mu }\right)\rangle$. It reproduces standard QM when $x^{\mu }=t$, with $|{\varphi }^0\left(x^0\right)\rangle =|\psi \left(t\right)\rangle$, and recovers the STS extension when $x^{\mu }=x^i\in \{x,y,z\}$. For a free particle, we show that $\varphi (t,y,z|x)=\langle t,y,z|\varphi \left(x\right)\rangle$ naturally reproduces the same dependence on the momentum wave function as the axiomatic Kijowski distribution. Possible experimental tests of these predictions are discussed. Finally, we demonstrate that the different states $|{\varphi }^{\mu }\left(x^{\mu }\right)\rangle$ can emerge by conditioning (i.e., projecting) a timeless and spaceless physical state onto the eigenstate $|x^{\mu }\rangle$, leading to constraint equations of the form ${\widehat{\mathbb{P}}}^{\mu }|{\mathrm{\Phi }}^{\mu }\rangle =0$. Rather than privileging time as the sole evolution parameter, this formulation treats all coordinates on equal footing.

Quantum machine learning (QML) requires significant quantum resources to address practical real-world problems. When the underlying quantum information exhibits hierarchical structures in the data, limitations persist in training complexity and generalization. Research should prioritize both the efficient design of quantum architectures and the development of learning strategies to optimize resource usage. We propose a framework called quantum curriculum learning (Q-CurL) for quantum data, where the curriculum introduces simpler tasks or data to the learning model before progressing to more challenging ones. Q-CurL exhibits robustness to noise and data limitations, which is particularly relevant for current and near-term noisy intermediate-scale quantum devices. We achieve this through a curriculum design based on quantum data density ratios and a dynamic learning schedule that prioritizes the most informative quantum data. Empirical evidence shows that Q-CurL significantly enhances training convergence and generalization for unitary learning and improves the robustness of quantum phase recognition tasks. Q-CurL is effective with physical learning applications in physics and quantum chemistry.

The polarization mode dispersion (PMD) in optical fibers poses a major challenge for maintaining the fidelity of quantum states for quantum communications. In this work, a comprehensive model linking the probability of quantum measurement errors (infidelity) to PMD is developed and validated by experimental measurements of differential group delay and quantum bit error rate (QBER). Our research proposes effective methods to mitigate PMD effects for broadband entangled photons and evaluates the impact of higher-order PMD effects. The model provides an experimentally verified framework for the optimization of commercial quantum key distribution systems in deployed fiber-optic lines.

Ultracold polar molecules trapped in optical lattices or tweezer arrays are an emerging platform for quantum information processing and quantum simulation, thanks to their rich internal structure and long-range dipolar interactions. Recent experimental breakthroughs have enabled precise control over individual molecules, paving the way for implementing two-qubit quantum gates, based on the iSWAP gate. A key challenge is, however, the sensitivity to variations of the dipole-dipole interaction strength—stemming from motion of the molecules and uncertainty on the precise positioning of external confining potentials—that limits current gate fidelities. To address this, we develop a quantum optimal control framework, based on a perturbative approach, to design gates that are robust with respect to quasistatic deviations of either system Hamiltonian parameters, external control parameters, or both, and provide criteria to evaluate a priori whether a gate can be made robust for a given control Hamiltonian. By applying these criteria to exchange-coupled qubits, as polar molecules, we demonstrate that robustness cannot be achieved with global controls only, but can be attained by breaking the exchange symmetry through local controls, such as a local detuning. We determine the robust time-optimal solution for realizing an iSWAP gate, show that the control pulses can be designed to be smooth functions, and achieve theoretical gate fidelities compatible with error correction using reasonable experimental parameters. Additionally, we show that certain entangled-state preparations, such as Bell states, can be made robust even with global controls only. We demonstrate that, under the adiabatic approximation, i.e., where molecular motion occurs on timescales comparable to, or faster, than that of the exchange interaction, the noise arising from thermal motion can be effectively treated as quasistatic variations of Hamiltonian parameters. This allows us to extend our treatment to the concrete experimental case of polar molecules.

We implement the Fock-space perturbed relativistic coupled-cluster (FSPRCC) theory to compute the electric dipole polarizability of ground and low-lying excited states and nuclear spin-dependent (NSD) parity nonconservation (PNC) transition amplitudes in Cs. Further, by combining our computed NSD-PNC transition amplitudes with a Cs experiment [C. S. Wood et al., Science 275, 1759 (1997)], we extract the value of the nuclear anapole moment and pion-mediated meson-nucleon coupling constant $f_{\pi }$. The deviation in our $f_{\pi }$ from the standard model value could be attributed to the existence of new particles such as $Z^{\prime }$ bosons. By combining our nuclear spin-independent PNC calculation with experiment [Science 275, 1759 (1997)], we extract the mass of the $Z^{\prime }$ boson to be $M_{Z^{\prime }}>1.6$ TeV/${\mathrm{c}}^2$. To check the accuracy of the wave functions used in the calculations, we compute the excitation energies, $E1$ transition amplitudes, and magnetic dipole hyperfine constants for ground and low-lying excited states. To improve the accuracy of the computed properties, we incorporate the corrections from the relativistic and QED effects in our calculations. The contributions from triple excitations are accounted for perturbatively. Our results on excitation energies, $E1$ transition amplitudes, and hyperfine constants are in good agreement with the available experimental results. Our polarizability results using the FSPRCC theory match well with the experimental values. The values of parity-violating transition amplitudes from our calculations are in general on the lower side of the previous values. From a detailed analysis of electron correlations, we find that the corrections from the Breit interaction and QED effects are important to get accurate results of NSD-PNC amplitudes in Cs. The largest cumulative contribution from the Breit and QED corrections is found to be approximately equal to 3.2% of the total value. The upper bound on the theoretical uncertainty in our calculated NSD-PNC amplitudes is estimated to be about 1%.

We perform real-time time-dependent density-functional theory (TDDFT) simulations with Ehrenfest dynamics to investigate how the incident point and initial kinetic energy of hydrogen atom projectiles influence their interaction with a graphenelike structure. To evaluate the role of electronic excitations, we compare these nonadiabatic results against two adiabatic approaches: Born-Oppenheimer molecular dynamics and ground-state energy profiling. We find that the incident point strongly affects the hydrogen atom's postcollision kinetic energy, scattering angle, and the vibrational response of the graphene lattice. Trajectories that avoid direct collisions with carbon atoms lead to longer interaction times and reduced energy transfer, increasing the likelihood of hydrogen adsorption by facilitating penetration of the graphene's potential energy barrier. Comparison with adiabatic methods reveals that electronic excitations and other non-adiabatic effects—captured by TDDFT—play a significant role in modulating energy loss and scattering direction. These results underscore the importance of including electron dynamics to accurately model hydrogen-graphene interactions and highlight how both geometric and energetic factors govern adsorption and scattering outcomes.

We investigate $D$-dimensional atomic Bose-Einstein condensates in a hypercylindrical trap with a vortex core along the $z$ axis and quantized circulation $\hslash m$. We analytically approximate the hypercylindrical Gross-Pitaevskii equation using dimensional perturbation theory with perturbation parameter $\delta =1/(D+2|m|-d)$, where $d$ controls the contribution of kinetic energy at zeroth order. We derive the zeroth-order ($\delta \to 0$) semiclassical approximations for the condensate energy, density, chemical potential, and critical vortex rotation speed in arbitrary dimensions. We investigate the effect of trap anisotropy on lower effective dimensionality and compute properties of vortices in higher dimensions motivated by the study of synthetic dimensions and holographic duality, where a higher-dimensional gravitational model corresponds to a lower-dimensional quantum model. In the zeroth-order approximation, we observe crossings between energy levels for different dimensions as a function of interaction strength and anisotropy parameters.

Nonreciprocal optical devices, allowing transmission of light with different efficiencies in opposite directions, are key elements for modern optical communication and even quantum information technologies, but elusive to be integrated on a chip to date. Such devices exploring nonlinearity can tackle this integration issue without using a magnetic field. However, they are constrained by narrow bandwidth or large signal power because of the weak nonlinear strength of materials. Here, by combining the strong gain-saturation nonlinearity of an Er-doped optical fiber and the asymmetric inputs, we experimentally demonstrate a broadband resonator-free nonreciprocal optical device. This nonmagnetic device operating at telecom wavelength shows nonreciprocal transmission with contrast of $19.97\mathrm{dB}$ over a bandwidth of $33.3\mathrm{nm}$ and reaches a maximum contrast of $23.91\mathrm{dB}$. With the potential of extending to on-chip Er-doped optical waveguides with currently existing experimental techniques, this simple design paves the way for integrated broadband nonreciprocity devices and can boost the functions of photonic chips.

Device-independent quantum key distribution (DI-QKD) enables two remote parties to share an information-theoretically secure key without any assumptions on the inner workings of the devices used. Device-independent conference key agreement (DI-CKA) is multipartite DI-QKD in which more than two parties share a common secure key. The performance of DI-CKA, however, is strictly limited because of its susceptibility to losses due to, e.g., imperfect detection efficiency and channel transmission. Here, we propose a DI-CKA protocol which reduces this limitation by using a heralding scheme to distribute multipartite entanglement. We analyze key rates of our protocol for two different measurement scenarios, and we show that our protocol outperforms a previous DI-CKA protocol even with an experimentally feasible measurement.

Using the bitstring probabilities of ground states of bipartitioned ladders of Rydberg atoms, we calculate the mutual information, which is a lower bound on the corresponding bipartite von Neumann quantum entanglement entropy $S_A^{\text{vN}}$. We show that in many cases these lower bounds can be improved by removing the bitstrings with a probability lower than some value $p_{\text{min}}$ and renormalizing the remaining probabilities (filtering). We propose a heuristic based on the change of the conditional entropy under filtering that very effectively improves the estimate of $S_A^{\text{vN}}$. We consider various sizes, lattice spacings, and bipartitions. Our numerical investigation suggest that the filtered mutual information obtained with samples having just a few thousand bitstrings can provide reasonably close estimates of $S_A^{\text{vN}}$. We briefly discuss practical implementations with QuEra's Aquila device.

In this paper, we present a set of local fermion-to-qudit mappings for simulating fermionic lattice systems. We focus on the use of multilevel qudits, specifically ququarts. Traditional mappings, such as the Jordan-Wigner transformation (JWT), while useful, often result in nonlocal operators that scale unfavorably with system size. To address these challenges, we introduce mappings that efficiently localize fermionic operators on qudits, reducing the nonlocality and operator weights associated with JWT. We propose one mapping for spinless fermions and two mappings for spinful fermions, comparing their performance in terms of qudit weight, circuit depth, and gate complexity. Notably, for superconducting devices, the control complexity of qudits compared to qubits does not increase prohibitively for four-level systems, making them practical for near-term demonstrations, though control challenges grow with increasing dimensionality. Therefore, we provide solutions to the operator decompositions of the Trotterized quantum dynamics into one- and two-qudit gates for all mappings. By leveraging the extended local Hilbert space of qudits, we show that these mappings enable more efficient quantum simulations in terms of two-qudit gates, reducing hardware requirements without increasing computational complexity. We validate our approach by simulating prototypical models such as the spinless $t\text{−}V$ model and the Fermi-Hubbard model in two dimensions, using Trotterized time evolution. Finally, we show the connection between the plaquette constraint of our mapping and the ${\mathbb{Z}}_2$ toric code. This connection can be exploited to prove that the ground state of the localized qudit mapping can be efficiently prepared using quantum circuits or measurement-based feedback control. Our results highlight the potential of qudit-based quantum simulations in achieving scalability and efficiency for fermionic systems on near-term quantum devices.