The efficient transfer of quantum states into a long-lived storage unit such as solid-state spin ensembles is widely recognized as a critical challenge with significant implications for quantum communication, sensing, and computing applications. Here, we experimentally investigate the interaction of propagating squeezed microwaves with an electron-spin-resonance transition in order to evaluate the use of spin ensembles as quantum memories for gigahertz signals. We generate continuous-variable microwave states with squeezing levels up to $5\mathrm{dB}$ below the vacuum level and let this signal interrogate a spin ensemble, which is inductively coupled to a lumped-element superconducting microwave resonator with a cooperativity of $C=0.3$. By analyzing this signal using Wigner tomography and modeling our experimental results with a dedicated steady-state model based on the quantum input-output formalism, we extract a transfer efficiency of around $61\%$ between the squeezed microwaves and the spin excitations, resulting in squeezing levels of the spin mode up to 0.5 dB below the vacuum level.

We present a theoretical study of Compton scattering of x-rays and $\gamma$ rays by a $K$-shell electron. Special attention is paid to the double-differential cross section and polarization of the scattered photons for linearly polarized incident photons. To investigate these observables, we employ the scattering matrix $\left(S\text{−}\text{matrix}\right)$ approach based on relativistic Green's functions. The $S\text{−}\text{matrix}$ results are moreover compared with predictions of the free-electron and impulse approximations, allowing us to assess the role of electron binding effects. Detailed calculations are carried out for hydrogenlike ${\mathrm{Ne}}^{9+}$ and ${\mathrm{Pb}}^{81+}$ targets over a wide range of incident photon energies and scattering angles. The calculations reveal kinematic regimes in which the impulse approximation agrees reasonably well with the $S\text{−}\text{matrix}$ results. We also explore the polarization of scattered photons for slightly depolarized incident radiation, including the highly sensitive case of scattering at ${90}^{\circ }$.

Cold-atom experiments that measure Fermi-gas properties near unitarity confine fermionic atoms to a region of space using trapping potentials of various shapes. The presence of a trapping potential introduces a new characteristic physical scale in the superfluid effective-field theory (EFT) description of the unitary Fermi gas which, among other things, describes the acoustic branch of excitations in the far infrared well below the scale of the superfluid gap. In this EFT there is a clear hierarchy of scales, and corrections to the homogeneous system due to the trapping potential may be organized into three regions with distinct power counting that relies on both the EFT derivative expansion and the Wentzel–Kramers–Brillouin approximation, which is an expansion in gradients of the trapping potential. The energy spectrum of the superfluid system is obtained in each of the regions by explicit computation of the phonon-field fluctuations and by the modifications to the dynamic structure factor due to the corresponding density fluctuations. This work presents a systematic and quantitative method for treating the presence of a trapping potential, which is essential for interpreting experimental realizations of the unitary Fermi gas. It provides clear predictions for how the trapping potential modifies the dispersion relation’s curvature, a key factor in characterizing the relaxation mechanisms of the superfluid. The most significant deviations from linear dispersion due to the trapping potential are found in the far-infrared region of the superfluid EFT.

Open-shell lanthanide atoms, and dysprosium in particular, combine a large ground-state angular momentum with dense electronic spectra, making their dynamical polarizability strongly dependent on wavelength and internal state, and therefore challenging to calculate and characterize experimentally. This issue has become especially relevant with the recent development of single-atom trapping of dysprosium in optical-tweezer arrays, where precise knowledge of the polarizability is needed to design optimized trapping architectures. Here, we exploit the strong spin-dependent light shift near the $J^{\prime }=J-1$ intercombination line at 530.306 nm to determine the background scalar and vector polarizabilities of ${}^{162}\mathrm{Dy}$ in its ground state near this wavelength. Our measurements quantitatively agree with atomic-structure calculations and provide new insight into the contributions of nearby transitions in a spectral region relevant to emerging dysprosium tweezer platforms.

We investigate both theoretically and numerically the Riemann pulse (simple Riemann wave) in optical media featuring strong nonlinearity and weak normal higher-order dispersion (HOD). We find that the Riemann pulse under dispersion with arbitrary order $k$ undergoes a progressive steepening process until a shock forms. Employing the semiclassical approximation approach, we derive generalized expressions for the HOD Riemann pulse, whose envelope amplitude is linear to the $k\text{/}2$ power of the pulse chirp. The shock distance and shock time decrease with the dispersion order $k$, while the shock amplitude increases with $k$. Besides, we reveal that loss can delay the shock formation of the HOD Riemann pulse, compared to the lossless case. In the HOD regime, the evolution of the Riemann pulse is significantly influenced by the initial pulse profile and the chirp parameter $C$. Particularly, contrary to the scenario of second-order dispersion, the shock distance, shock time, and shock amplitude of the HOD Riemann pulse exhibit distinct behaviors in the ranges of $C\le 0$ and $C>1$. Theoretical predictions show an excellent agreement with numerical results. Our findings will enrich the fundamental comprehension of shock-wave phenomena in nonlinear optics with complicated dispersion and may inspire potential applications on other physical systems.

Periodic driving greatly expands the physical scope and tunability of non-Hermitian systems, giving rise to rich dynamical phenomena that have no static counterpart. Here we construct a periodically quenched non-Hermitian Su-Schrieffer-Heeger frequency lattice by two coupled ring resonators with periodically dynamic complex modulations. We prove that the quenched lattice is parity-time symmetric as the two durations are equal, and Floquet exceptional points (EPs) will arise with suitable driving period. Meanwhile, the quasienergy bands are approximately linear around the Floquet EPs. With theoretical analysis and numerical simulations, we demonstrate that a flat spectrum with uniformly increasing width can emerge in the quenched frequency lattice, which stems from the linear band structures near the Floquet EPs. The spectrum evolution governed by Floquet EPs shows periodicity and possesses intricate dynamic. Moreover, the broadening speed and power of the flat-top frequency comb can be adjusted by the amplitudes of modulation conveniently, rendering the frequency spectrum more flexible and tunable. This study reveals the intriguing physical phenomena arising from Floquet EPs and demonstrates that the periodically quenched frequency lattice can have applications on spectral manipulation and flat-top microcombs generation.

We present a strategy for obtaining optimal probe states and measurement schemes in a class of noiseless multiparameter estimation problems with symmetry among the generators. The key to the framework is the introduction of a set of quantum metrology conditions, analogous to the quantum error correction conditions of Knill and Laflamme, which are utilized to identify probe states that saturate the multiparameter quantum Cramér-Rao bound. Similar to finding two-dimensional irreps for encoding a logical qubit in error correction, we identify trivial irreps of finite groups that guarantee the satisfaction of the quantum metrology conditions. To demonstrate our framework, we analyze the $\mathrm{SU}\left(2\right)$ estimation with symmetric states in which three parameters define a global rotation of an ensemble of $N$ qubits. For even $N$, we find that tetrahedral symmetry and, with fine-tuning, $S_3$ symmetry, are minimal symmetry groups providing optimal probe states for $\mathrm{SU}\left(2\right)$ estimation, but that the quantum metrology conditions can also be satisfied in an entanglement-assisted setting by using a maximally entangled state of two spin-$N/2$ representations for any $N$. By extending the multiparameter method of moments to noncommuting observables, we use the quantum metrology conditions to construct a measurement scheme that saturates the multiparameter quantum Cramér-Rao bound for small rotation angles.

Spectral density functions quantify how environmental modes couple to quantum systems and govern their open dynamics. Inferring such frequency-dependent functions from time-domain measurements is an ill-conditioned inverse problem. Here, we use exactly solvable spin-boson models with pure-dephasing and amplitude-damping channels to reconstruct spectral density functions from noisy simulated data. First, we introduce a parameter estimation approach based on machine learning regressors to infer Lorentzian and Ohmic-like spectral density parameters, quantifying robustness to noise. Second, we show that a cosine transform inversion yields a physics-consistent spectral prior estimation, which is refined by a constrained neural network enforcing positivity and correct asymptotic behavior. Our neural-network framework robustly reconstructs structured spectral densities by filtering simulated noisy signals and learning general functional dependencies.

Quantum Energy Teleportation (QET) is a protocol in which a remote measurement activates locally inaccessible ergotropy stored in the ground state of a many-body system by consuming preexisting entanglement and a classical communication channel. Nevertheless, the implementation of QET is severely impeded by the fundamental locality of gapped many-body systems, where the exponential clustering of ground-state correlations restricts energy extraction to microscopic scales. In this work we address this scalability challenge within the framework of the one-dimensional anisotropic XY model. We provide a rigorous characterization of a monolithic measurement-induced strategy, demonstrating that while bulk projective measurements can theoretically induce long-range couplings, the approach is rendered physically untenable by exponentially diverging thermodynamic costs and vanishing success probabilities. To circumvent this impasse, we propose and analyze a hierarchical quantum repeater architecture adapted for energy teleportation. By orchestrating heralded entanglement generation, iterative entanglement purification, and nested entanglement swapping, our protocol effectively counteracts the fidelity degradation inherent in noisy quantum channels. We establish that this architecture fundamentally alters the operational resource scaling from exponential to polynomial, providing a physically permissible pathway for ground-state ergotropy activation at arbitrary distances. The significance lies not in net energy gain but in establishing long-range QET as a viable protocol for remote quantum control and resource distribution.

This paper presents a comprehensive analysis of the link budget for free-space optical systems involving low-earth-orbit (LEO) and medium-earth-orbit (MEO) satellites. We develop a detailed model of the satellite-to-ground channel that accounts for the primary physical processes affecting transmittance: atmospheric absorption and scattering, free-space diffraction, and turbulence-induced fluctuations. The study introduces a general method for computing transmittance along a slant path between a satellite and an optical ground station, incorporating zenith angle, slant range, and altitude-dependent attenuation. The proposed framework is intended to support the design and evaluation of space-based optical links and serves as a critical tool for defining technical specifications in satellite communication demonstrators and simulations. Numerical estimates are provided to illustrate the magnitude of losses under typical operational conditions, including the role of aperture averaging. In addition to the link budget analysis, we introduce a satellite-based quantum use case. We propose a scheme for quantum state tomography performed on states generated by an onboard photon source on an LEO or MEO satellite and transmitted to the optical ground station. This approach enables continuous verification of the quality of quantum resources that can be used to perform quantum protocols within quantum information networks.

It is well-known that the inclusion of the Breit interaction, alongside correlations, is important for an accurate theoretical understanding of the properties of heavy ions. In this work we include the Breit interaction in all-order calculations of energy levels and fine-structure intervals of ions in the Cs and Fr isoelectronic sequences using a Feynman-diagram technique. This requires modifying the electron Green's function to account for the Breit interaction within the all-order correlation potential method, which sums a dominating series of perturbative effects exactly. We find that Breit corrections to the energies of moderately charged ions along these sequences are very large, particularly for the $f$ states. Incorporating the Breit interaction into the all-order correlation potential provides a significant additional contribution beyond including it at the second-order level alone, substantially improving the fine-structure intervals. Furthermore, since the static Breit effect is so large for these systems, we also account for the frequency-dependent Breit interaction at the Dirac-Fock level and find that it does not significantly modify calculated atomic properties.

We develop a multireference perturbation theory for electronic structure calculations based on symmetries of the Hamiltonian. The reference Hamiltonian in the symmetry-based perturbation theory (SBPT) is chosen such that it possesses more symmetries than the original Hamiltonian, leading to a larger reduction of computational resources in terms of both the number of configurations in the configuration-interaction expansion and the number of required qubits in quantum computing applications. We provide approximate, scalable solutions for the second-order correction, as well as an application to selected configuration interaction. We show that SBPT is an extension of other existing multireference perturbation theories and that it can give better results for some molecular systems in a robust way.

The angular distribution of Auger electrons following dielectronic capture and the resonance strength of dielectronic recombination for hydrogenlike $\mathrm{S}{\mathrm{n}}^{49+}, \mathrm{X}{\mathrm{e}}^{53+}, \mathrm{N}{\mathrm{d}}^{59+}, \mathrm{H}{\mathrm{o}}^{66+}$, and ${\mathrm{W}}^{73+}$ ions are calculated using the Flexible Atomic Code under the conditions with and without the inclusion of the generalized Breit interaction (GBI), respectively. The doubly excited states are generated by all KLL resonances, and the calculated population of magnetic sublevels with and without the GBI is used to analyze the effect of the GBI on the angular distribution of Auger electrons and the resonance strength. This study has shown that the effect of the GBI on the resonance strength of the single ${\left[2s_{1/2}2p_{1/2}\right]}_1$ resonance is negligible but induces a remarkable modification exceeding $50\%$ on the angular distribution of Auger electrons emitted from ${\left[2s_{1/2}2p_{1/2}\right]}_1$ at 0° and 180°, significantly reducing the anisotropy of Auger electrons. This effect on the resonance strength and the angular distribution stems from the GBI exerting a pronounced opposite effect on the population of magnetic sublevels $M_J=\pm 1$ and $M_J=0$. Additionally, the GBI is particularly strong for the $K{L}_{1/2}{L}_{1/2}$ resonance group and the effect of the GBI on the resonance strength of KLL increases with the increase of the nuclear charge $Z$. These findings reveal the critical role of the GBI in the excitation and decay dynamics of highly charged ions, which is particularly non-negligible in the study of angular distribution.

Structured x-rays carrying an orbital angular momentum break spatial inversion symmetry and have been proposed as probes of chirality. We theoretically investigate twisted nonresonant x-ray diffraction from chiral molecules and demonstrate that no dichroic signal can arise from randomly oriented molecules, irrespective of the beam spatial profile. However, a dichroic response emerges for oriented molecules. Our results establish the beam and sample conditions for which a measurable dichroic scattering signal survives axial and focal averaging.

We calculate the phase diagram of the Bose-Hubbard model on a half-filled ladder lattice including the effect of finite on-site interactions. This shows that the rung-Mott insulator (RMI) phase persists for finite interaction strength, and we calculate the RMI-superfluid phase boundary in the thermodynamic limit. We show that the phases can still be distinguished using the number and parity variances, which are observables accessible in a quantum-gas microscope. Phases analogous to the RMI were found to exist in other quasi-one-dimensional lattice structures, with the lattice connectivity modifying the phase boundaries. This shows that the presence of these phases is the result of states with one-dimensional structures being mapped onto higher-dimensional systems, driven by the reduction of hopping rates along different directions.

Continuously operating atom-light interfaces represent a key prerequisite for steady-state quantum sensors and efficient quantum processors. Here, we demonstrate continuous accumulation of sub-Doppler-cooled atoms in a shallow intracavity dipole trap, realizing this regime. The key ingredient is a light-shift manipulation that creates spatially varying cooling parameters, enabling efficient capture and accumulation of atoms within a cavity mode. Demonstrated with rubidium atoms, a continuous flux from a source cell is funneled through the magneto-optical trap into the cavity mode, where the atoms are cooled and maintained below $10\mu \text{K}$ in steady state without time-sequenced operation. We characterize the resulting continuously maintained ensemble of millions of atoms and its collective coupling to the cavity field, establishing a route toward continuously operated cavity-QED systems and long-duration atomic and hybrid quantum sensors.

Quantum gases confined in reduced dimensions have enabled the observation of many exotic quantum phenomena. While existing experiments primarily focus on homonuclear systems, we report here on the efficient preparation of a quasi-two-dimensional heteronuclear quantum degenerate mixture of ultracold ${}^{23}\mathrm{Na}$ and ${}^{87}\mathrm{Rb}$. We describe the apparatus design and detail the procedures for preparing the two-dimensional (2D) quantum mixture. The apparatus has several unique features, including compact and modular 2D magneto-optical trap sources, a science chamber that accommodates various lattice geometries, a precision in-vacuum electrode assembly, and high-resolution imaging for both atomic species. After loading the dual-species condensate into a single layer of a vertical optical lattice, we prepare a 2D gas mixture and observe quantum immiscibility in the in situ equilibrium density profiles. The observed density profiles reasonably agree with mean-field theories. The apparatus provides a versatile platform for investigating several interesting problems, including quantum impurities, quantum droplets, and polar molecules in low dimensions.

Symmetry is fundamental to photonic systems. External (spatial) symmetries and internal symmetries—Lorentz reciprocity, energy conservation, and time-reversal symmetry—constrain the electromagnetic response. Photonic systems can also possess compound symmetries that combine external and internal transformations, exemplified by parity-time $\left(\mathcal{PT}\right)$ symmetry. However, a unified framework for general compound symmetries involving reciprocity, energy conservation, and time reversal remains lacking, leaving their classification and physical implications unexplored. In this paper, we present such a framework for linear photonic systems. We define compound transformations and symmetries, and derive their constraints on electromagnetic fields and scattering matrices. We show that internal, external, and compound symmetries are naturally described by the theory of double antisymmetry groups. This theory classifies linear time-invariant photonic systems into twelve symmetry categories, each imposing characteristic constraints on the electromagnetic response. We illustrate two representative categories with numerical examples of photonic crystal slabs and apply the theory to examine Kirchhoff's law of thermal radiation for a gyrotropic sphere. Our work provides a systematic foundation for analyzing and engineering symmetry in photonic systems.