Motivated by recent experimental advances, including the ongoing development of an optical atomic clock in singly ionized radium, we perform a detailed theoretical characterization of ${\mathrm{Ra}}^{+}$ and its lighter analog ${\mathrm{Ba}}^{+}$. Both ions are of interest for precision studies, including for atomic parity violation and searches for new physics beyond the standard model. Using the all-order correlation potential method, including Breit and radiative quantum electrodynamics corrections, we perform high-accuracy calculations of electric dipole ($E1$), electric quadrupole ($E2$), and magnetic dipole ($M1$) transition matrix elements between the low-lying $s, p$, and $d$ states of these ions, as well as the excited-state lifetimes, polarizabilities, magic wavelengths, and magnetic dipole hyperfine-structure constants $A$. By combining lifetime measurements with accurate theoretical ratios, we extract high-precision determinations of the $s\text{−}{d}_{1/2,3/2} E2$ matrix elements. By combining hyperfine measurements with atomic theory, we extract parameters of the nuclear magnetization distribution (the Bohr-Weisskopf effect) for ${}^{135,137}\mathrm{Ba}$ and ${}^{223,225}\mathrm{Ra}$. These results provide theoretical input for ongoing and future experimental programs in fundamental physics and precision metrology.

The low-charged ${\mathrm{Sn}}^{2+}$ ion has emerged as an ideal candidate for developing Coulomb-crystal optical clocks capable of achieving precision below ${10}^{-19}$ [Leibrandt et al., Nat. Commun. 15, 5663 (2024)]. To systematically investigate the structure properties of the ${\mathrm{Sn}}^{2+}$ ion, we employ the $B$-spline-based many-body perturbation theory combined with the configuration-interaction method. We conducted comprehensive calculations of energies, matrix elements, and electric and magnetic dipole polarizabilities. Furthermore, we determined the magic wavelengths and the sensitivity factor $K$ for variations in the fine-structure constant $\alpha$. Detailed comparisons with existing results demonstrate good agreement, validating the reliability of our method. Additionally, we derive a general formula to incorporate the contribution of negative-energy states to the magnetic dipole tensor polarizability, thereby elucidating their non-negligible impact on the calculation of high-accuracy magnetic polarizability.

To characterize the aging process of quantum devices, the concept of quantum reliability has recently been proposed as a process-based metric that quantifies the duration during which operations remain unaffected by noise. This duration is referred to as the lifetime of a quantum system. In this paper, we investigate the lifetime of a two-level atomic system undergoing spontaneous emission. We construct an exact statistical ensemble of the lifetimes for two-level systems using quantum reliability theory, incorporating comprehensive information such as mean values, fluctuations, and non-classical properties. Notably, we demonstrate that the ensemble exhibits can be represented by a density matrix, which is recorded in the monitoring apparatus by an observation process. Under the Born-Markov approximation, the diagonal elements decay exponentially at a state-dependent rate related to quantum fidelity. Beyond this approximation, the off-diagonal elements arise from interference between radiation pathways and are determined by the first-order coherence of the field. This feature indicates that the off-diagonal elements encode spectral information beyond the Markovian decay rate, suggesting a potential protocol for probing the environmental spectral density using two-level atoms.

The fact that, in the course of open evolution, the creation of system-environment entanglement out of the initial coherence occurs in the vast majority of cases has been known for many years. However, how much entanglement can be created under general assumptions has not been analyzed yet. In this work, we investigate system-environment entanglement generation under a broad and important class of interactions known as pure-dephasing interactions for arbitrary initial conditions. Our main results are the following bounds on the relative entropy of entanglement $E_r\left({\sigma }_{SE}\right)$ after evolution: $C_r\left({\rho }_S\right)-H\left(\mathcal{I}\right|\mathcal{M})\le E_r\left({\sigma }_{SE}\right)\le C_r\left({\rho }_S\right)-C_r\left({\sigma }_S\right)+H\left(\mathcal{I}\right)$, connecting it on one side to the initial relative entropy of coherence $C_r\left({\rho }_S\right)$ and information extractable from the environment $H\left(\mathcal{I}\right|\mathcal{M})$ and on the other side to the decoherence strength, $C_r\left({\rho }_S\right)-C_r\left({\sigma }_S\right)$, making the statement that larger entanglement leads to more decoherence quantitatively precise. As an illustration, we apply our results to the spin-boson model, where a qubit interacts with a bosonic bath, showing the time evolution of the entanglement and the crossover between Markovian and non-Markovian dynamics. Our results help us better understand and quantify the complicated correlations produced during open evolution, which will hopefully lead to better and new decoherence-mitigation techniques.

We find that the $m$-separability and $k$-partite entanglement of a multipartite quantum system is correlated with quantum coherence of the same with respect to complete orthonormal bases, distinguishable under local operations and classical communication in certain partitions. In particular, we show that the geometric measure of $m$-inseparable entanglement of a multipartite quantum state is equal to the square of minimum fidelity-based quantum coherence of the state with respect to complete orthonormal bases, which are locally distinguishable in a partition into $m$ parties.

In the paper, we develop a bosonic quantum error correction code based on squeezed Fock states. We compare our proposed code with one based on squeezed Schrödinger's cat states using the Knill-Laflamme cost function and the Petz map fidelity. We demonstrate that squeezed Fock states are competitive in protecting information in a channel with particle loss and dephasing.

Qudits can be described by a state vector in a $q$-dimensional Hilbert space, enabling a more extensive encoding and manipulation of information compared to qubits. This implies that conducting fault-tolerant quantum computations using qudits rather than qubits might entail less overhead. In this work, we investigate the viability of qudits in error-correction codes by creating and simulating the quantum circuitry for the smallest qudit error-correction code with a multidimensional circuit-level noise model and specifically adapted decoders. After introducing a flag qudit to protect the code from hook errors, comparable error thresholds of the order of ${10}^{-4}$ are obtained for qudits of dimensions 2, 3, and 5.

Determining low-energy eigenstates in electronic many-body quantum systems is a key challenge in computational chemistry and condensed-matter physics. Hybrid quantum-classical approaches, such as the Variational Quantum Eigensolver and Quantum Subspace Methods, offer practical solutions but face limitations in circuit depth and measurement overhead. In this article, we propose a variational strategy based on symmetry-preserving cost functions to iteratively construct a reduced subspace for the extraction of low-lying energy states. We show that, under certain conditions, our approach leads to a tridiagonal representation similar to that obtained with the Lanczos algorithm. The iterative process allows control over the trade-off between circuit depth, the number of variational parameters, and the number of measurements required to achieve the desired accuracy, making it suitable for current quantum hardware. As a proof of concept, we test the proposed algorithms on a ${\mathrm{H}}_4$ chain and ring, targeting both the ground-state energy and the charge gap.

Classical communication over lossy quantum channels is an essential topic in quantum information theory, with practical implications for optical-fiber and free-space communications. Multiphase Hadamard codes, based on coherent-state binary phase-shift keying (BPSK) modulation and decoded using vacuum-or-pulse (VP) detectors, offer a promising approach for achieving high communication rates while relying only on linear optics and single-photon detectors (SPDs). However, their performance does not reach the ultimate Holevo limit. In this work, we propose a generalization of Hadamard codes that distributes the signal across multiple modes with optimized probabilities, rather than concentrating it in a single mode, dubbed probabilistic pulse-position modulation (PPPM). We derive an achievable communication rate, demonstrating that our PPPM can outperform traditional Hadamard codes in certain intermediate energy regimes.

Remote quantum state preparation allows one party to prepare a known quantum state at a distant node using local operations and classical communication. Conventional approaches for remotely preparing entangled states typically require joint operations on multiple particles at the local site, which poses practical challenges in scalable quantum networks. Here, we propose a general remote quantum state preparation scheme based on an asymmetric maximally entangled state, where only single-particle operations are needed at the local node to prepare arbitrary multiqubit states at the remote site. We demonstrate this scheme experimentally on a linear optical platform, using a three-photon asymmetric entangled state as a resource. By applying single-particle operations on a four-dimensional photonic ququart, we remotely prepare arbitrary two-qubit states with fidelities ranging from 0.8534 to 0.9444, all surpassing the classical threshold. This approach simplifies local control requirements and offers a scalable route for remote entanglement generation in quantum networks and distributed quantum computing.

Vortex states of photons or electrons are a novel and promising experimental tool across atomic, nuclear, and particle physics. Various experimental schemes to generate high-energy vortex particles have been proposed. However, diagnosing the characteristics of vortex states at high energies remains a significant challenge, as traditional low-energy detection schemes become impractical for high-energy vortex particles due to their extremely short de Broglie wavelength. We recently proposed a novel experimental detection scheme based on a mechanism called superkick that is free from many drawbacks of the traditional methods and can reveal the vortex phase characteristics. In this paper, we present a complete theoretical framework for calculating the superkick effect in elastic Møller scattering and systematically investigate the impact of various factors on its visibility. In particular, we argue that the vortex phase can be identified either by detecting the two scattered electrons in coincidence or by analyzing the characteristic azimuthal asymmetry in individual final particles.

We report the generation of self-frequency-shifting Raman quasisolitons in a pulse-pumped high-$Q$ fiber Fabry-Perot resonator in the weak normal dispersion regime. They are induced by modulation instability mediated by fourth-order dispersion in a regime where the Raman gain overwhelms the cavity losses. The resulting spectrum, spanning over 50 THz, is reminiscent of the supercontinuum generated in single-pass waveguides. We clearly identify this process using a dispersive Fourier transform experiment. Additionally, we demonstrate the suppression of modulation instability by tuning the synchronization mismatch between the pump repetition rate and the cavity round-trip time, enabling the generation of a standard dissipative Kerr soliton in this system. These observations align remarkably well with numerical simulations based on a generalized Lugiato-Lefever equation, incorporating the Raman response of the optical fiber.

The non-Hermitian skin effect (NHSE) is an intriguing phenomenon in which an extensive number of bulk eigenstates localize at the boundaries of a non-Hermitian system with nonreciprocal hoppings. Here, we study the interplay of this effect and a defect in nonreciprocal one-dimensional lattices. We show that the interplay of the NHSE and defects is size-dependent. We demonstrate a novel class of hybrid skin-defect states in finite-size systems resulting from the coupling between the skin and defect states. Next, we consider a single defect in a topologically nontrivial lattice with time-reversal symmetry based on the nonreciprocal Su-Schrieffer-Heeger configuration. We unveil how topologically nontrivial defect states and non-Hermiticity interplay by competing with each other, exhibiting a transition from topologically nontrivial defect states to skin states. In addition, we show that decreasing the defect strength can result in a transition from trivial defect states to skin states. Our work promotes the understanding of the interplay between defects and the NHSE and especially the importance of the energy spectra of the system with the defect under periodic boundary conditions.

Errors in superconducting quantum chips are inevitable for developing robust logical qubits and implementing complex quantum algorithms. These errors arise from various physical mechanisms, each with unique dependence on qubit frequencies. Here, we propose an algorithm for optimizing qubit frequencies to effectively mitigate multiple types of frequency-dependent errors. We employ multichannel message-passing neural networks, with each channel tailored to address a specific error type. Our neural-network model is trained to minimize the weighted sum of different errors, based on frequency-dependent error models, e.g., two-level-system defects that affect individual qubits, quantum crosstalk that depends on the graph structure formed by qubit couplings, and microwave crosstalk that relies on the spatial arrangement of qubits. The trained neural-network model is adaptable to chips with varying qubit arrangements and scales. Recognizing the diverse experimental conditions of different chips, we compare our algorithm with the snake algorithm across various scales and error weights, providing valuable guidance for experimenters in algorithm selection. Our results show that, in most scenarios, our algorithm achieves lower overall errors than the snake algorithm.

We compute the isotope shifts of the total electron binding energy of neutral atoms and singly charged ions up to element $Z=120$, using the relativistic Hartree-Fock method including the Breit interaction. Field shift coefficients are extracted by varying the nuclear charge radius; a small quadratic term is retained to cover large radius changes relevant to superheavy nuclei. We tabulate isotope shift coefficients for closed-shell systems from Ne to Og and benchmark selected open-shell cases, used to test the interpolation formula. A simple power-law interpolation $b{Z}^k$ reproduces calculated field shifts to within about 1% across the table, with the effective exponent $k$ growing from roughly 5 near $Z\sim 50$ to about 12 at $Z\sim 118$. Due to the domination of inner shells, differences between neutrals and singly charged ions does not exceed a few percent, becoming noticeable mainly when an outer $s$ electron is removed. Therefore, these results may also be used for higher charge ions.

We propose a novel nondestructive method for the detection of single Rydberg excitations in a mesoscopic ensemble. The protocol achieves high fidelities on a microsecond timescale and is robust against changes in the probe laser frequency. The technique relies on optical pumping in Autler Townes configuration, whose efficiency is controlled by the presence or absence of a Rydberg excitation. Taking rubidium atoms as an example, we give realistic estimates for the achievable fidelities and parameters. However, our protocol can be transferred to any other atomic species that features multiple stable states. Our protocol is applicable in quantum simulation and quantum information processing with mesoscopic ensembles requiring fast and high-fidelity Rydberg state detection.

This study investigates spin-squeezed states in nuclear magnetic resonance quadrupolar systems with spins $I=3/2$ and 7/2 at room temperature, taking into account the effects of relaxation on the dynamics. The origin of spin squeezing is attributed to the interaction between the nuclear quadrupole moment and the electric-field gradients in the molecular environment. The formal description of the nonlinear operators responsible for spin squeezing is achieved using the spin angular momentum representation via the one-axis twisting mechanism. This approach provides a framework for quantum control and metrology over the spin squeezing process while accounting for the influence of relaxation phenomena. Non-Cartesian angular momentum operators are proposed, with their variance products catching the quantum effects during the dynamics. An upper bound for the squeezing parameter and the Heisenberg uncertainty at steady state are also predicted for any spin quantum number.

We consider a quantum Otto-type heat engine constructed in an optomechanical system with which the cavity is chosen as the working substance. The cavity can effectively be coupled with hot thermal baths in nonequilibrium steady states via optomechanical interaction. The mechanical mode with parametric drive fuels the cavity, and the utilization efficiency of energy is discussed. To obtain higher efficiency in finite time evolution, we use the shortcuts-to-adiabaticity method in work generation processes. The modified thermal efficiencies are obtained by numerical simulation. Such a system provides potential applications in quantum heat transfer and energy utilization in quantum devices.

At present, the ellipticity and orientation dependence of solid harmonic yield have been widely studied, but the influence of excitation mechanism on them remains relatively unexplored. Here, taking graphene as an example, we investigate the ellipticity and orientation dependence of harmonic yield at different laser frequencies by solving the semiconductor Bloch equations. Our results reveal a transition from anomalous to normal in the ellipticity dependence as the laser frequency increases. Simultaneously, the structure of the orientation dependence undergoes a transformation. By analyzing the conduction-band electron populations, we attribute these changes to the transformation of the dominant excitation mechanism. At low frequencies, tunneling ionization dominates, whereas at high frequencies, multiphoton ionization prevails. Furthermore, the limitation of the solid Keldysh parameter is discussed. These findings underscore the crucial role of the dominant excitation mechanism in solid high-order harmonic generation.