We present experimental observations of parity-mixed rotational states in the $a^3{\mathrm{\Pi }}_{1,2}$ states of the CO molecule induced by an electric field of ∼600 V/cm. The $e$- or $f$-symmetry-selected rotational states of $a^3\mathrm{\Pi }$ were prepared using the $R/P$ or $Q$ branches of the $a{}^3\mathrm{\Pi }\leftarrow X{}^1{\mathrm{\Sigma }}^{+}$ transitions, respectively. By employing one-photon resonance-enhanced and one-photon ionization methods, we obtained spectra of the $b{}^3{\mathrm{\Sigma }}^{+}\leftarrow a{}^3\mathrm{\Pi }$ transition, revealing electric-field-induced forbidden rotational transitions. Analysis of the spectra allowed us to determine the mixing ratios between Λ doublet states with opposite total parities, offering a potential method for controlling ratios of parity-mixed states in molecules. Furthermore, we provided line positions for the fundamental vibrational band of the transition $b{}^3{\mathrm{\Sigma }}^{+}\leftarrow a{}^3\mathrm{\Pi }$ and the rotational energy levels of the $b^3{\mathrm{\Sigma }}^{+}$ (υ = 0) state.

Atoms constitute promising quantum sensors for a variety of scenarios including vacuum metrology. Key to this application is knowledge of the collision rate coefficient of the sensor atom with the particles being detected. Prior work demonstrated that, for room-temperature collisions, the total collision rate coefficient and the trap depth dependence of the sensor atom loss rate from shallow traps are both universal, independent of the interaction potential at short range. It was also shown that measurements of the energy transferred to the sensor atom by the collision can be used to estimate the total collision rate coefficient. However, discrepancies found when comparing the results of this and other methods of deducing the rate coefficient call into question its accuracy. Here the universality hypothesis is reexamined and an important correction is presented. We find that measurements of the postcollision recoil energy of sensor atoms held in shallow magnetic traps only provide information about the interaction potential at the very largest interatomic distances (e.g., the value of $C_6$ for a leading order term of $-C_6/r^6$). As other non-negligible terms exist at medium and long ranges, the total collision rate coefficient, even if universal, can differ from that computed solely from the value of $C_6$. By incorporating these other long-range terms into a simple semiclassical (SC) calculation, we find the SC prediction matches that of full, multichannel, quantum mechanical scattering calculations using the complete potential. This work resolves the discrepancies, demonstrates the simplicity of estimating the rate coefficients for universal collision partners, and provides guidance for using atoms as a self-calibrating primary quantum pressure standard.

Dielectronic recombination (DR) of highly charged ions with spin-polarized electrons is studied within the framework of density-matrix theory. Special attention is paid to the question of how spin polarization of the recombined electron affects the polarization behavior of the subsequent photon emission. To address this question, while a general density-matrix formalism applicable to any DR process of arbitrary ions is presented, here, it is applied to a particular DR process (i.e., $1s^{2}2s{J}_i=1/2+\varepsilon e^{-}\to 1s2s^{2}2p_{1/2}{J}_d=1\to 1s^{2}2s^2{J}_f=0+\gamma$) of lithiumlike ${\mathrm{I}}^{50+}$ ions as an example. For this particular process, detailed calculations are carried out for different spin polarization states of the recombined electron by utilizing the multiconfigurational Dirac-Hartree-Fock method. It is found that the spin polarization of the recombined electron hardly affects the linear polarization $P_1$ (i.e., the first Stokes parameter) of the photon emission, whereas the polarization $P_2$ (i.e., the second Stokes parameter) depends strongly on the spin polarization. In addition, owing to big differences in the magnitudes of $P_1$ and $P_2$, a large polarization tilt angle $\chi$ is obtained that is remarkably affected by the spin polarization, especially for the forward and backward photon emission. These polarization behaviors are found to be rather similar to the results obtained in radiative recombination of highly charged ions with polarized electrons [Maiorova et al., Phys. Rev. A 107, 042814 (2023)] and can be readily observed using modern x-ray polarimeters. Based on the strong dependences obtained, we suggest that accurate measurements of the polarization behaviors can be applied to diagnosing the spin polarization state of electron beams, which is more feasible and promising than the case of radiative recombination [Maiorova et al., Phys. Rev. A 107, 042814 (2023)] owing to the resonance nature of DR processes.

We present a theoretical study of the excitations of the two-dimensional supersolid state of a Bose-Einstein condensate with either dipole-dipole interactions or soft-core interactions. This supersolid state has three gapless excitation branches arising from the spontaneously broken continuous symmetries. Two of these branches are related to longitudinal sound waves, similar to those in one-dimensional supersolids. The third branch is a transverse wave arising from the nonzero shear modulus of the two-dimensional crystal. We present the results of numerical calculations for the excitations and dynamic structure factor characterizing the density fluctuations and study their behavior across the discontinuous superfluid to supersolid transition. We show that the speeds of sound are described by a hydrodynamic theory that incorporates generalized elastic parameters, including the shear modulus. Furthermore, we establish that dipolar and soft-core supersolids manifest distinct characteristics, falling into the bulk-incompressible and rigid-lattice limits, respectively.

Homogeneous planar superfluids exhibit a range of low-energy excitations that also appear in highly excited states like superfluid turbulence. In dilute gas Bose-Einstein condensates, the Jones-Roberts soliton family includes vortex dipoles and rarefaction pulses in the low- and high-velocity regimes, respectively. These excitations carry both energy and linear momentum, making their decay characteristics crucial for understanding superfluid dynamics. In this work, we develop the theory of planar soliton decay due to thermal effects, as described by the stochastic projected Gross–Pitaevskii theory of reservoir interactions. We analyze two distinct damping terms involving transfer between the condensate and the noncondensate reservoir: particle transfer that also involves energy and usually drives condensate growth, and number-conserving energy transfer. We provide analytical treatments for both the low- and high-velocity regimes and identify conditions under which either mechanism dominates. Our findings indicate that energy damping prevails at high phase-space density. These theoretical results are supported by numerical studies covering the entire velocity range from vortex dipole to rarefaction pulse. We use interaction energy to characterize rarefaction pulses, analogous to the distance between vortices in vortex dipoles, offering an experimentally accessible test for finite-temperature theory in Bose-Einstein condensates.

We study the generation of three-particle (one photon plus two phonons) states and phonon pairs in a cavity-waveguide system where the optomechanical coupling is quadratic in the mechanical displacement. Both the three-particle state and the phonon pair exhibit quantum entanglement in the continuous frequency domain, and the entanglement can be controlled by adjusting the pump-pulse spectral width. By introducing a phantom waveguide, we can analyze the impact of intrinsic cavity losses on the quantum-state generation, and we find that the cavity losses can actually enhance the entanglement in the successfully generated three-particle state or phonon pair.

Exciton optomechanics, bridging cavity exciton polaritons and optomechanics, opens new opportunities for the study of light-matter strong interactions and nonlinearities, due to the rich nonlinear couplings among excitons, phonons, and photons. Here, we propose to entangle two exciton modes in an exciton-optomechanics system, which consists of a semiconductor microcavity integrated with two quantum wells. The quantum wells support two exciton modes, which simultaneously couple to an optical cavity mode via a linear dipole interaction and to a mechanical vibration mode via a nonlinear deformation potential interaction. We show that by strongly driving the microcavity with a red-detuned laser field and when the two exciton modes are respectively resonant with the Stokes and anti-Stokes sidebands scattered by the mechanical motion, stationary entanglement between the two exciton modes can be established under realistic parameters. The protocol is within reach of current technology and may become a promising approach for preparing excitonic entanglement.

A pure state of fixed Hamming weight is a superposition of computational basis states such that each bitstring in the superposition has the same number of ones. Given a Hilbert space of the form $\mathcal{H}={\left({\mathbb{C}}_2\right)}^{\otimes n}$, or an $n$-qubit system, the identity operator can be decomposed as a sum of projectors onto subspaces of fixed Hamming weight. In this work, we propose several quantum algorithms that realize a coherent Hamming weight projective measurement on an input pure state, meaning that the post-measurement state of the algorithm is the projection of the input state onto the corresponding subspace of fixed Hamming weight. We analyze a depth-width tradeoff for the corresponding quantum circuits, allowing for a depth reduction of the circuits at the cost of more control qubits. For an $n$-qubit input, the depth-optimal algorithm uses $\mathcal{O}\left(n\right)$ control qubits and the corresponding circuit has depth $\mathcal{O}(log\left(n\right))$, assuming that we have the ability to perform qubit resets. Furthermore, the proposed algorithm construction uses only one- and two-qubit gates.

We investigate both classical and quantum chaotic dynamics of a two-mode Bose-Einstein condensate driven by a continuously periodic field. We calculate the quantities, including quantum Fisher information (QFI), fidelity, and linear entropy, to characterize the quantum chaos and compare these quantities with the maximum Lyapunov exponent (MLE) of the corresponding classical system. The behaviors of the quantum quantities and the classical quantity show a strong correspondence regardless of the classical regular and chaotic regions in the parameter space. A quantitative link between the MLE and the short-time dynamics of the QFI is established. Particularly, we find that our continuously driven model is significantly superior to other models in accelerating entanglement generation through chaos, which is evidenced by the shorter time required for the QFI to reach the Heisenberg limit. We briefly discuss the underlying mechanism of this finding and its application in saving time costs and improving measurement accuracy as well.

Illumination is a binary signal-detection protocol to check whether the target exists in a noisy environment; its achievable sensitivity is practically related to the choice of illuminating sources and the methods for measuring echo-wave signals. Here we demonstrate that the achievable sensitivity of the usual coherent-state illumination (CSI) with homodyne detection in phase space can be significantly enhanced by alternatively employing the photon-number-resolvable (PNR) detection in Fock-state space for both single-shot and multiple-shot illumination scenarios. Furthermore, using a typical infrared-light CSI, we show how to optimize the thresholds of PNR detection to improve sensitivity gains for varying illuminating intensities. It is also demonstrated that the present CSI protocol possesses a comparative sensitivity advantage over the CSI using threshold detection without PNR capability. We also discuss feasible strategies to achieve these sensitivity gains in different noise environments and under varying illuminating intensities.

This study examines the performance of finite spin quantum batteries (QBs) using Heisenberg $XX, XY, XXZ$, and $XYZ$ spin models with Dzyaloshinsky-Moriya (DM) and Kaplan–Shekhtman–Entin-Wohlman–Aharony (KSEA) interactions. The QBs are modeled as interacting quantum spins in local inhomogeneous magnetic fields, inducing variable Zeeman splitting. We derive analytical expressions for the maximal extractable work, ergotropy and the capacity of QBs, as recently examined by Yang et al. [Phys. Rev. Lett. 131, 030402 (2023)]. These quantities are analytically linked through certain quantum correlations, as posited in the aforementioned study. Different Heisenberg spin-chain models exhibit distinct behaviors under varying conditions, emphasizing the importance of model selection for optimizing QB performance. In antiferromagnetic (AFM) systems, maximum ergotropy occurs with a Zeeman splitting field applied to either spin, while ferromagnetic (FM) systems benefit from a uniform Zeeman field. Temperature significantly impacts QB performance such that ergotropy in the AFM case is generally more robust against increasing temperature compared to the FM case. Incorporating DM and KSEA couplings can significantly enhance the capacity and ergotropy extraction of QBs. However, there exists a threshold beyond which additional increases in these interactions cause a sharp decline in capacity and ergotropy. This behavior is influenced by temperature and quantum coherence, which signal the occurrence of a sudden phase transition. The resource theory of quantum coherence proposed by Baumgratz et al. [Phys. Rev. Lett. 113, 140401 (2014)] plays a crucial role in enhancing ergotropy and capacity. However, ergotropy is limited by both the system's capacity and the amount of coherence. These findings support the theoretical framework of spin-based QBs and may benefit future research on quantum energy storage devices.

High-order harmonic generation in atomic gases is important for several applications in ultrafast strong-field physics, ranging from attosecond pulse generation to ultrafast spectroscopy and imaging of different forms of matter. In the case of the generation with focused short Gaussian pulses, recent theoretical studies indicate that the conversion efficiency depends on the spatial phase distribution of the driving laser pulse which scales with the Porras factor. Using theoretical analysis and the results of numerical simulations, we find that for positive Porras factors the contribution of the Gouy phase to phase matching can be balanced and the conversion efficiency can be significantly enhanced as compared to a standard laser setup. Specifically, our results indicate that for a Porras factor of $g_0\simeq 1.2$, the conversion efficiency as well as the cutoff of the harmonic spectra can be optimized while the harmonic lines remain narrow, which may be interesting for spectroscopic applications.

We theoretically investigate a nonlinear model with dissipative coupling. In this system, the cavity mode and the subharmonic mode are coupled through a two-photon process. In addition, the subharmonic mode is also dissipatively coupled with the magnons of an yttrium-iron-garnet sphere. By calculating the retarded Green's function, we investigate the response behavior of the system to external probing field. We find the nonlinear processes can be manipulated by the dissipative coupling. In particular, a phenomenon similar to electromagnetically induced transparency has emerged. These results show that the nonlinear system with dissipative coupling is a promising platform for controlling light propagation and light manipulation operations, and has potential applications for the quantum information processes.

The polarization properties and spatial distribution of electromagnetic radiation emitted from a high-energy electron can be calculated using the Lienard-Wiechert potentials. These properties are characterized by changes in electron orbitals. In Thomson or Compton scattering by relativistic electrons, which involves the interaction between electrons and a laser, the transverse motion of the electrons is induced by the laser's electric field. The polarization characteristics and spatial distribution of emitted radiation in linearly and circularly polarized laser fields have been extensively studied. In this paper, using a CdTe image sensor, the spatial distribution of MeV gamma rays generated by a 90-degree collision between a 750 MeV electron beam and an axially symmetric polarized laser was measured. The axially symmetric polarized laser, which includes radially and azimuthally polarized components, was created from a linearly polarized laser using a spatially variant retarder known as an s-waveplate. The spatial distribution of gamma rays exhibited a peak at the center, similar to linearly and circularly polarized gamma rays, but differed in overall contour from both. This result suggested that axially symmetric polarized lasers generated gamma rays with different polarization states. Moreover, the contour was observed to change with the polarization axis of the axially symmetric polarized laser, likely owing to the nonuniform distribution of the laser's polarization intensity. Additionally, the spatial distribution of gamma rays produced by circularly polarized optical vortex lasers was measured, showing that their contours corresponded to those of nonvortex circularly polarized lasers.

We discuss the propagation of electromagnetic waves in a space-time modulated medium, for which neither energy nor momentum remains conserved. We have found a conservation law when modulation moves at a constant speed $v$ in a traveling-wave fashion. We use this law to study how light is scattered inside such a medium. We consider both the subluminal ($v<c$) and superluminal ($v>c$) cases and find that they differ considerably in terms of what remains conserved. We show that the total number of photons is conserved in the subluminal case. In contrast, it is the difference between the forward- and backward-moving photons that remains conserved in the superluminal case. We also study the reciprocity issue for a space-time modulated medium and find that reciprocity holds in all situations. We develop a quantum formulation of our scattering problem and show that the subluminal case is similar to the action of a beam splitter. In the superluminal case, photon pairs can be generated from vacuum in a fashion analogous to the two-mode frequency down-conversion process.

A conventional quantum detection device is unable to distinguish between an input signal and the accompanying noise. This kind of indistinguishability forms a response symmetry to the detection system, and the detection accuracy is limited by the initial signal-to-noise ratio (SNR). We study this limiting problem in an optomechanical angular velocity detection device. By further analyzing the system dynamics in the frequency domain, we find that the indistinguishability between signal and noise can be effectively eliminated by the classical Sagnac effect, which induces the symmetry-breaking response to the detection system. In this case, the SNR can be significantly enhanced, even if the strength of the Sagnac effect is much lower than the input noise. The mechanism of SNR enhancement is illustrated by defining a criterion to quantify the degree of symmetry breaking. In addition, the feasibility of the scheme is also discussed, which offers a path to improve the detection accuracy by breaking the indistinguishability between the signal and noise.

Differential-phase-shift (DPS) quantum key distribution (QKD) is a type of QKD protocol. It is a unique QKD scheme wherein a lasting coherent pulse sequence containing numerous photons is transmitted as an information carrier instead of qubits in traditional QKD protocols. Regarding the security of DPS-QKD, a general individual attack (GIA), which first conducts a beam-splitting attack and then entangles probe states with each photon in a pulse sequence after beam splitting, has been presented and is usually assumed to evaluate the DPS-QKD system performance. Thus, this study reconsidered the GIA scheme, examined its feasibility, and consequently modified it to render it feasible. A formula for the secret key creation rate, assuming a modified GIA, was formulated. Calculations using the presented and conventional formulas indicated that while the key creation rates in systems assuming conventional and modified GIAs were approximately the same, the modified GIA resulted in a higher key creation rate in short- and medium-distance systems to a certain extent.