A double-heralding technique is presented for producing heralded entangled photon pairs from spontaneous parametric down-conversion (SPDC). Compared to the swap-heralded schemes studied in previous cascaded SPDC and zero-added-loss multiplexing (ZALM) proposals, this double-heralding technique is found to yield the most resource-efficient implementation in terms of minimizing the total number of sources and detectors required to achieve a specified rate and fidelity. This is achieved by reducing the number of modes and mode-sorting optics needed on the heralding path. Specifically, by immediately detecting any two signal photons from an array of down-converters, the corresponding idler photons can be projected onto an anticorrelated pair state which is shown to be unitarily equivalent to the state produced by swap-heralded sources, and hence can be used directly for long-range entanglement distribution in a ZALM architecture. Quasideterministic operation through two distinct multiplexing techniques is analyzed. The analysis derives expressions for the heralded pair probability and fidelity assuming realistic detectors with losses, dark counts, and partial photon number resolution, providing a framework for implementation of the source on a photonic integrated circuit.

Many-body quantum systems can exhibit collective effects that enhance the sensitivity of parameter estimation protocols. An example is provided by resonantly driven two-level atoms subject to collective dissipation, which can display a transition between a stationary phase and a time-crystal one. Previous work has shown that the light emitted in the time-crystal phase can be harnessed for parameter estimation using continuous monitoring protocols, such as photon counting or homodyne detection, which under ideal conditions yield a quadratic enhancement of sensitivity with the number of particles. In this work, we explore what is the minimal information about the emission field that needs to be accessed in order to resolve collective effects and exploit them for parameter estimation. We show that, for short probing times, a single-time measurement of the emission field already captures the collective behavior emerging at the nonequilibrium transition. In contrast, within the time-crystal phase, exploiting collective effects requires at least two-time measurements. To this end, we introduce a family of correlated intensity measurements that extract the relevant information and can be implemented using an interferometric setup. While the ultimate sensitivity bound remains size independent, as recently established within the framework of noisy quantum metrology, our analysis shows that these protocols utilize collective effects to yield a transient increase in sensitivity with particle number.

The contribution of weak interaction to the hyperfine splitting of the ground state of muonium is investigated. The amplitudes of the one- and two-quantum exchanges determined by the Z and W bosons are calculated. One-loop corrections in the photon and Z-boson propagators and their contribution to the hyperfine structure of the spectrum are obtained on the basis of the dispersion method in quantum field theory.

Radiative transition data for open-shell lanthanide ions are essential for large-scale opacity calculations in astrophysical plasmas, yet fully converged configuration-interaction (CI) treatments remain computationally impractical for such systems because of the rapid growth of configuration spaces. Consequently, practical calculations rely on truncated CI models, making it important to identify which correlation contributions dominate transition amplitudes. In the present work, systematically controlled CI calculations for Gd v are performed within the multiconfiguration Dirac-Hartree-Fock framework to investigate the correlation dependence of radiative transition properties. Significant and highly selective variations in transition strengths are observed between single- and double-excitation models, particularly for transitions associated with the $5p\text{−}5d$ and $4f\text{−}5d$ pathways. Through selective inclusion and removal of specific configurations, the $5p^2\to 5d^2$ pair-excitation correlation is identified as the dominant higher-order correlation contribution affecting the redistribution of transition strengths between different CI models. The analysis further shows that higher-order correlation effects primarily modify strongly allowed transition pathways, while the resulting changes propagate to weaker transitions through configuration mixing between different pathways. These results demonstrate that CI-dependent transition behavior in open-shell ions is governed not only by the overall size of the CI expansion, but also by the selective contributions of specific correlation effects, providing guidance for constructing efficient CI expansions in large-scale atomic structure calculations.

In the presence of a magnetic field, hyperfine clock qubits can acquire a vector differential light shift that can be tuned via polarization to suppress the total differential light shift of high-power, off-resonant laser light. We experimentally measure this “magic” polarization condition, suppressing differential light shifts in both the ${}^{2}S_{1/2}$ ground and ${}^{2}F_{7/2}^o$ metastable clock qubits of ${}^{171}\mathrm{Yb}^{+}$. We present calculations of the minimum bias magnetic fields required to suppress differential light shifts in the ground-state clock qubits of commonly trapped-ion species, finding that they are below the strengths of fields already typically present in experiments. We further present methods for metastable clock-qubit control in ${}^{171}\mathrm{Yb}^{+}$, demonstrating a state preparation and measurement infidelity of $2.9_{-1.5}^{+3.0}\times {10}^{-4}$ ($-35\pm 4\mathrm{dB}$).

Resonance laser ionization spectroscopy was conducted on stable chromium isotopes ${}^{50,52,53,54}\mathrm{Cr}$ using the RILIS laser ion source and the CRIS experimental apparatus at the ISOLDE/CERN facility. A unique titanium:sapphire laser-based three-step laser ionization scheme was developed to selectively and efficiently ionize and extract the Cr isotopes. Over 350 electronic transitions were observed, including several efficient transitions to autoionizing states. High-resolution spectroscopic measurements were performed with the $3d^5{(}^{6}S)4s a^7{S}_3\to 3d^5{(}^{6}S)4p z^7{P}_3$ atomic ground-state transition to characterize the scheme and benchmark the performance of the CRIS setup. The isotope shifts $\delta {\nu }_{50,52}=-121\left(7\right)\mathrm{MHz}, \delta {\nu }_{50,53}=-137\left(6\right)\mathrm{MHz}$, and $\delta {\nu }_{54,52}=-14\left(6\right)\mathrm{MHz}$ were measured, and the hyperfine structure parameter $A_{S_3}=-82.64\left(16\right)\mathrm{MHz}$ of ${}^{53}\mathrm{Cr}$ was extracted. A King plot analysis allowed reevaluation of the field shift $F=-271\left(11\right)\mathrm{MHz}{\text{fm}}^{-2}$ and mass shift $M=105.3\left(30\right)\mathrm{GHz}\text{u}$ factors of the transition with improved precision and accuracy. High-resolution measurements agree with the existing literature, supporting the accuracy and the reliability of the method for future studies involving radioactive isotopes.

Within the framework of quantum electrodynamics, the interaction between two atoms at large distances is analyzed. Using the S-matrix formalism, an expression for the magnetic interaction potential is derived, which agrees with the well-known result of classical electrodynamics. However, quantum electrodynamics goes beyond this conventional result and allows one to treat a wide range of problems related to the structure of atomic energy levels. In particular, it is shown that the asymptotic behavior of the interaction potential can deviate from the classical prediction, depending on the atomic states involved. As an example, dispersion coefficients are calculated for the $s$-states of hydrogen atoms, where the long-range potential reduces to a spin-spin interaction. The results obtained open up the possibility of a straightforward comparative analysis of long-range interaction potentials between atoms of matter and antimatter. The applicability of this approach is demonstrated for the hydrogen-antihydrogen system.

The detection dead zone is an important systematic limitation in scalar atomic magnetometers, constraining their practical utility. In this work, we demonstrate a sensitive dead-zone-free scalar magnetometer by integrating previously established techniques into a FID magnetometer based on Bell-Bloom optical pumping. The dead zone is eliminated by inserting a reflecting mirror within a multipass-cavity-assisted atomic cell, which folds the optical beam into orthogonal paths. Our analysis reveals that interregion cross-talk is essential for interpreting the resulted experimental signals. The sensor exhibits oscillation signal amplitude variations within a factor of three across all orientations in three-dimensional space, and a field sensitivity better than 80 ${\mathrm{fT}/\mathrm{Hz}}^{1/2}$ over the full space. We further characterize the heading error in the geomagnetic field range, where experimental results agree with theoretical predictions within 0.7 nT. Additionally, the sensor can operate in a closed-loop mode by feeding back the real-time frequency extracted from the FID signal to modulate the pump beam, we characterize the magnetic field slew rate of the sensor. This work pave the way towards a sensitive vector FID magnetometer.

We theoretically investigate the influence of molecular alignment on elliptically polarized high-order harmonic generation (HHG) from ${\mathrm{H}}_2^{+}$ driven by an orthogonally polarized two-color laser field by numerically solving the three-dimensional time-dependent Schrödinger equation. Our findings indicate that the alignment of ${\mathrm{H}}_2^{+}$ along the laser propagation direction $(R\parallel z)$ leads to an enhancement in both the harmonic intensity and ellipticity compared with alignment along the laser polarization direction $(R\parallel x)$. Increasing the frequency ratio enhances both the ionization and recollision probabilities, thereby increasing the harmonic intensity. Furthermore, the relative phase enables control over the harmonic intensity, ellipticity, and quantum-path selection. We further demonstrate that the relative phase induces asymmetry in the electric field waveform, leading to a redshift of the harmonic spectrum that is closely associated with the enhancement of harmonic ellipticity. Finally, we synthesize a near-circularly polarized attosecond pulse train with an ellipticity of 0.99. Our study provides an effective route for experimentally controlling the polarization and intensity of molecular HHG.

We derive the canonical-ensemble scaling of Tan's contact for $N$ harmonically trapped Tonks-Girardeau bosons at finite temperature in the large-$N$ limit. The leading scaling coefficient reproduces the local-density-approximation result and is obtained from a contour-integral representation of the canonical partition function followed by a saddle-point reduction to a phase-space integral with a self-consistent scaled chemical potential. The subleading coefficient is the central object of this work: it admits an explicit representation in terms of universal phase-space integrals of the Fermi factor, having closed-form Sommerfeld and virial limits, and is identified with the canonical-versus-grand-canonical ensemble difference at fixed mean particle number. In the high-temperature Boltzmann regime the ratio of subleading to leading coefficients collapses to a universal value, traceable to the Poissonian particle-number statistics of the dilute grand-canonical gas. We construct Padé approximants for both scaling functions that interpolate uniformly between the low-temperature Sommerfeld and high-temperature virial regimes; for the subleading coefficient we report a form that is uniformly accurate on our working range of temperatures and asymptotically correct beyond. The scaling law is verified against canonical contour-integration data across the full temperature range, and the experimental visibility of the ensemble correction is quantified: for the mesoscopic particle numbers and temperatures of current one-dimensional experiments it reaches the several-percent level.

Upstream motion, a typical form of nonequilibrium dynamics, is generally exhibited by active matter that is closely correlated with nonreciprocal interactions. This classic picture is likely to be extended into the quantum regime, benefiting from recent advances that endowed the interactions of quantum entities with a nonreciprocal feature. In this work, we demonstrate the upstream motion of a swimmer—a narrow optical beam—in a quantum fluid of light. The swimmer undergoes nonreciprocal mutual interactions with the optical fluid, acquiring a counterintuitive recoil inverted relative to that in equilibrium systems. Experimental validation via an optical platform enabling nonreciprocal light-light interactions confirms the upstream motion. Our findings further reveal that the maximum upstream capability is achieved at moderate fluid-flow velocity and density. Our results unveil another quantum form of active behavior induced by nonreciprocal interactions, providing key inspiration for achieving the goal of designing novel quantum devices with active functionalities.

Quantum-optical simulations of inhomogeneous media typically rely on a global eigenmode basis that must be recomputed for each geometry. We show that, for lossless, nondispersive media, the dynamics can instead be formulated without such a basis. From the source-free Maxwell equations we identify a self-adjoint real-space generator of single-photon dynamics and directly construct bosonic field operators, a single-particle Hamiltonian, and a Liouville–von Neumann equation for the single-photon density matrix. A finite integration technique discretization transfers this operator structure to the numerical grid while preserving self-adjointness. This ensures unitary evolution and exact photon-number conservation up to projection accuracy and machine precision. Numerical examples demonstrate single-photon propagation in a dielectric waveguide and reproduce the two-photon Hong-Ou-Mandel dip in a 50-50 coupler. The few-photon dynamics in this formulation is fully determined by the bosonic symmetry of the state space and the linear single-particle propagation, without explicit photon-photon interaction terms. The approach therefore enables direct quantum-optical simulation of inhomogeneous and geometrically complex photonic structures.

High-fidelity spin readout is a crucial component for quantum information processing with optically interfaced solid-state spins. Here, we propose and investigate two theoretical protocols for fast single-shot readout of cavity-coupled single T center electronic spins. For fluorescence-based readout, we selectively couple one of the T center spin-conserving transitions to a single-mode photonic cavity, exploiting the enhancement of the fluorescence emission and cyclicity. For reflection-based readout, we leverage the spin-dependent cavity reflection contrast to generate the qubit readout signal. We show that the cavity reflection approach enables high-fidelity spin readout even when the T center only has a modest cyclicity. With realistic system parameters, such as cavity quality factor $Q=2\times {10}^5$ and T-center optical linewidth $\mathrm{\Gamma }/2\pi =100\mathrm{MHz}$, we calculate a single-shot readout fidelity exceeding 99% within 10 $\mu \mathrm{s}$ for both spin readout protocols.

Multiphoton resonances provide an important manifestation of counterrotating interactions in light-matter systems. These resonances, however, are sensitive to detuning errors, making the phenomena challenging to experimentally observe. In this manuscript, we introduce an optimization strategy to address this problem. By using an optimized parameter segmented sequence, the sensitivity to quasistatic detuning errors of the high-order quantum state transfers can be substantially relaxed. We prove the versatility of our strategy against frequency detunings by demonstrating the evolution of two specific models. In both cases, the parameter window for maintaining a high state-transfer fidelity is substantially expanded. We further analyze the output photon flux of the optimized system and, taking the three-photon resonance as an example, demonstrate that the system remains capable of generating a stable output photon flux even in the presence of detuning errors.

The open anisotropic quantum Rabi model (QRM) is studied in this work, with an explicit $A^2$ term incorporated. It is shown that, in the presence of cavity loss, anisotropy allows the steady-state superradiant transition to occur even when the $A^2$ term prohibits the transition in the isotropic case. The $A^2$ term reshapes and enriches the known steady-state phase diagram of the open anisotropic QRM, leading to an asymmetric structure composed of normal, superradiant, and bistable phases. In particular, the nonequilibrium tricritical points are generally separated from the merger point of the two critical branches that bound the superradiant region. Isolated bistable regions also emerge, and the number and positions of tricritical points are reduced. Notably, the anomalous photon-number fluctuation scaling is associated with the merger of the two critical branches, rather than with the tricritical points. These results clarify the role of the $A^2$ term contribution in the steady-state structure of the open anisotropic QRM and provide a refined extension of the previously known dissipative phase diagram, offering a useful theoretical reference for future studies of related open light-matter systems.

Encoding continuous-variable quantum information in the optical domain has recently enabled the generation of large entangled states, yet robust implementation remains a challenge. Here we present a straightforward protocol for generating multipartite entanglement based on spontaneous parametric down-conversion in a circular array of quadratic nonlinear waveguides. We provide a rigorous theoretical framework, including comprehensive derivations of the propagation equations and the identification of regimes where analytical solutions are possible. Crucially, our approach identifies the pump and detection configurations required to sustain and measure multipartite full inseparability across arbitrary propagation distances and for any number of waveguides $N=4n$. This regime, elusive to standard numerical methods, represents a key requirement for scalable quantum protocols. Our scheme is inherently robust as it relies on phase-matched propagation eigenmodes, making it resilient against variations in sample length, coupling, and nonlinearity.

Optical asynchronous light-intensity transmission (OALT) is a counterintuitive nonlinear phenomenon where output field intensity declines with increasing input intensity. In this work we theoretically establish a controllable, all-optical, and passive OALT mechanism in an atom-cavity coupling system via disruptive inverted-S-shaped bistability (ISB). Solving the stationary input-output relation for a driven-dissipative Tavis-Cummings model predicts an OALT phase in the reflection field that is distinct from traditional monostable and bistable regimes. The demonstration of controllable optical negative differential transmission, which is governed by disruptive ISB, confirms an efficient OALT response. Our findings suggest a promising paradigm for self-adaptive optical protection systems and self-stabilized generation of quantum light sources.

Optical systems are a promising platform for realizing universal quantum computation. However, photons do not naturally interact, presenting a major challenge for implementing efficient and deterministic multiqubit gates. Here, we propose a coupling scheme between photons and Rydberg atoms that induces strong effective photon-photon interactions with a negligibly small imaginary component. Leveraging this coherent photonic nonlinearity and combining it with the stationary light approach, we design a deterministic two-photon controlled-not gate with significantly improved fidelity compared to previous schemes. This approach could enable additional avenues for universal quantum computation and quantum information processing based on optical systems.