A scheme for the generation of steady-state entanglement between optical and mechanical cavity modes in an optomechanical crystal nanobeam is proposed. Based on finite element simulation and the quantum Langevin equation, we explore the evolution of entanglement with optomechanical properties in various device configurations, and demonstrate the ability of optimal entanglement generation in both blue and red detuned optical pumps. The intrinsic relationship between optomechanical cooperativity and entanglement is further clarified for depicting a clear physical picture of optomechanical interaction. Our work expands the understanding of macroscopic quantum mechanics and is an important step for developing novel quantum optomechanical devices.

Quantum computers require high-fidelity quantum gates. These gates are obtained by routine calibration tasks that eat into the availability of cloud-based devices. Restless circuit execution speeds up characterization and calibration by foregoing qubit reset between circuits. Postprocessing the measured data recovers the desired signal. However, since the qubits are not reset, leakage—typically present at the beginning of the calibration—may cause issues. Here we develop a simulator of restless circuit execution based on a Markov chain to study the effect of leakage. In the context of error-amplifying single-qubit gates sequences, we show that restless calibration tolerates up to 0.5% of leakage, which is large compared to the ${10}^{-4}$ gate fidelity of modern single-qubit gates. Furthermore, we show that restless circuit execution with leaky gates reduces by 33% the sensitivity of the ORBIT cost function developed by J. Kelly et al. which is typically used in closed-loop optimal control [Phys. Rev. Lett. 112, 240504 (2014)]. Our results are obtained with standard qubit state discrimination showing that restless circuit execution is resilient against misclassified noncomputational states. In summary, the restless method is sufficiently robust against leakage in both standard and closed-loop optimal control gate calibration to provided accurate results.

An information-theoretical study of the Dirac oscillator and the usual quantum oscillator is provided, in terms of the respective spatial densities. The comparative analysis is grounded on the values of Shannon entropy, Fisher information, and disequilibrium of both systems in arbitrary states. It is emphasized the dependence of these entropic functionals on the angular frequency and the quantum numbers that characterize the state of the systems, and remarkably also on the different spin-angular momentum coupling schemes. The results are interpreted accordingly with the structural patterns of the corresponding densities, providing information on their spread, uniformity, and disorder.

Electric-field noise stemming from surface adsorption of atoms serves as the main factor of anomalous motional heating and decoherence for trapped ions confined in harmonic potentials of surface-electrode traps. However, this also provides a possible way for ions playing as nanosensors to detect surface defects and explore the mechanism of noise. Taking monatomic vibrations on the surface into account, here we calculate the electric-field noise and discuss its spectral properties by utilizing density-functional theory and the incoherent master equation with fitting parameters. We find that most adatoms lead to universal spectral intensity and frequency scaling. Particularly, we discover a colored feature for the weak-binding Ar adatom with the increase of the temperature, which can be attributed to the anharmonic potential and high-lying vibrational states. Our results could be helpful for understanding the mechanism of the surface noise and developing efficient noise-detection methods.

We study the ground state and low-lying polaritonic excitation states in the Ising-Rabi lattice model (IRLM), where the transverse field in the quantum transverse field Ising model is replaced by a quantized local photon field. The competition among the photon-hopping coupling, the intrasite Rabi interaction, and the Ising interaction drives the system across a quantum phase transition from the insulating phase to the delocalized superradiant phase. Mean-field theory confirms the existence of the quantum phase transition and the analytical expressions for the phase boundary are derived. The spectra of low-lying polaritonic excitations and Bose-Einstein condensation are further explored by a unitary transformation method. The deep strong-coupling analysis reveals a quantum criticality, where the IRLM belongs to the universality class of the $XYZ$ model. When the hopping strength $t=0$, the properties of the ground state in the IRLM are investigated by the symmetry analysis and the density-matrix renormalization-group method. It turns out that the photon field cannot alone serve as a global transverse field in the quantum transverse field Ising model without the presence of hopping coupling since a crossover behavior instead of a quantum phase transition is confirmed.

We theoretically investigated the bistability in a small Fabry-Perot interferometer (FPI) with the optical wavelength size cavity, the nonlinear Kerr medium, and only a few photons, on average, excited by the external quantum field. Analytical expressions for the stationary mean photon number, the bistability domain, the field, and the photon number fluctuation spectra are obtained. Multiple stationary states of the FPI cavity field with different spectra are possible at realistic conditions, for example, in the FPI with the photonic crystal cavity and the semiconductor-doped glass nonlinear medium.

This study investigates the use of orbital angular momentum (OAM) to enhance phase estimation in Mach-Zehnder interferometers by employing non-Gaussian states as input resources in the presence of noise. Our research demonstrates that non-Gaussian states, particularly the photon-subtraction-then-addition state, exhibit the best sensitivity in the presence of symmetric noise. Additionally, a higher order of the Bose operator of non-Gaussian states provides better sensitivity for symmetric noise. OAM can mitigate the deterioration of noise, making it possible to estimate small phase shifts $\theta \to 0$. OAM enhances the resolution and sensitivity of all input states and mitigates the deterioration caused by photon loss. Additionally, OAM enhances the resolution and sensitivity of all input states, enabling the sensitivity to approach the $1/N$ limit even under significant photon loss (e.g., $50\%$ symmetric photon loss). These results hold promise for enhancing the sensitivity and robustness of quantum metrology, particularly in the presence of significant photon loss.

The systematic theoretical study of the cascaded Auger decay following the $3d\to 5p$ excitation in Kr is presented. The origin of the ${\mathrm{Kr}}^{3+}$ ions in the cascade is explained by analyzing the influence of the correlation effects. The study shows that the time dependence of population for the ${\mathrm{Kr}}^{+}$ ion resembles results for the ${\mathrm{Kr}}^{3+}$ ion obtained using ultrafast laser spectroscopy. It is demonstrated that different states of the ${\mathrm{Kr}}^{+}$ and ${\mathrm{Kr}}^{2+}$ ions can contribute to the population of the ${\mathrm{Kr}}^{3+}$ ion observed in the probe-pulse experiments. Furthermore, the deeper states that could be reached by the probe laser pulse produce a peak shift for the population to the higher time delays in accordance with experimental observations for the higher pulse intensities.

The ionization energy of HD has been determined to be $E_{\mathrm{I}}\left(\mathrm{HD}\right)=124568.48466\left(7\right){\text{cm}}^{-1}$ by two-photon Doppler-free vacuum-ultraviolet pulsed laser spectroscopy, near-infrared continuous-wave laser spectroscopy, and Rydberg-series extrapolation by multichannel quantum-defect theory (MQDT). From this value, the dissociation energy of HD is deduced to be $D_0\left(\text{HD}\right)=36405.78253\left(7\right){\text{cm}}^{-1}$, representing a fivefold improvement over previous values and resolving a $3.2\sigma$ disagreement with ab initio calculations of the four-particle nonadiabatic relativistic energy and of quantum-electrodynamic corrections up to order $m{\alpha }^6$. Interactions between $d, f$, and $g$ Rydberg series have been observed and had to be included in the MQDT extrapolation of the Rydberg series.

We investigate the external and internal dynamics of a two-level atom strongly coupled to a weakly pumped nanophotonic cavity. We calculate the dipole force, friction force, and stochastic force due to the cavity pump field and show that a three-dimensional cooling region exists near the surface of a cavity. Using a two-color evanescent field trap as an example, we perform three-dimensional Monte Carlo simulations to demonstrate the efficient loading of single atoms into a trap by momentum diffusion, and the stability of cavity cooling near the trap center. Our analyses show that cavity cooling can be a promising method for directly loading cold atoms from free space into a surface microtrap. We further discuss the impact of pump intensity on atom trapping and loading efficiency.

We present a comprehensive study of stationary states in a coherent medium with a quadratic or Kerr nonlinearity in the presence of localized potentials in one dimension for both positive and negative signs of the nonlinear term as well as for barriers and wells. The description is in terms of the nonlinear Schrödinger equation and hence applicable to a variety of systems, including interacting ultracold atoms in the mean field regime and light propagation in optical fibers. We determine the full landscape of solutions in terms of a potential step and build solutions for rectangular barrier and well potentials. It is shown that all the solutions can be expressed in terms of a Jacobi elliptic function with the inclusion of a complex-valued phase shift. Our solution method relies on the roots of a cubic polynomial associated with a hydrodynamic picture, which provides a simple classification of all the solutions, both bounded and unbounded, while the boundary conditions are intuitively visualized as intersections of phase space curves. We compare solutions for open boundary conditions with those for a barrier potential on a ring, and also show that numerically computed solutions for smooth barriers agree qualitatively with analytical solutions for rectangular barriers. A stability analysis of solutions based on the Bogoliubov equations for fluctuations shows that persistent instabilities are localized at sharp boundaries and are predicated by the relation of the mean density change across the boundary to the value of the derivative of the density at the edge. We examine the scattering of a wave packet by a barrier potential and show that at any instant the scattered states are well described by the stationary solutions we obtain, indicating applications of our results and methods to nonlinear scattering problems.

We study the interplay between non-Hermitian dynamics and classical phase synchronization in a system of $\mathcal{N}$ bosonic modes commonly coupled to an auxiliary, driven mode. For any set of non-Hermitian bipartite interactions between the auxiliary and other modes, the system evolves towards a phase synchronized state. We provide analytical and numerical evidence of such classical phase synchronization for systems ranging from a few modes to the macroscopic limit of large $\mathcal{N}$ and analyze the effects of inhomogeneous frequency broadening and robustness under the action of external thermal noise.

Spin squeezing is vitally important in quantum metrology and quantum information science. The noise reduction resulting from spin squeezing can surpass the standard quantum limit and even reach the Heisenberg limit (HL) in some special circumstances. However, systems that can reach the HL are very limited. Here, we study the spin squeezing in atomic systems with a generic form of the quadratic collective-spin interaction, which can be described by the Lipkin-Meshkov-Glick model. We find that the squeezing properties are determined by the initial states and the anisotropic parameters. Moreover, we propose a pulse rotation scheme to transform the model into a two-axis twisting model with Heisenberg-limited spin squeezing. Our study paves the way for reaching HL in a broad variety of systems.

The nonadiabatic dynamics of the electromagnetic field triggers photon generation from the quantum vacuum. Shortcuts to adiabaticity, instead, are protocols that mimic the field's adiabatic dynamics in a finite time. Here, we show how the counterdiabatic term of the transitionless tracking algorithm cancels out exactly the term responsible for the photon production in the dynamical Casimir effect. This result suggests that the energy of producing photons out of the vacuum is related to the energetic cost of the shortcut. Furthermore, if the system operates under a quantum thermodynamic cycle, we confirm the equivalence between the adiabatic and nonadiabatic work outputs. Finally, our study reveals that identifying these observations can only be possible using the so-called effective Hamiltonian approach.

We simulate the critical behavior of the Ising model utilizing a thermal state prepared using quantum computing techniques. The preparation of the thermal state is based on the variational quantum imaginary time evolution (QITE) algorithm. The initial state of QITE is prepared as a classical product state, and we propose a systematic method to design the variational ansatz for QITE. We calculate the specific heat and susceptibility of the long-range interacting Ising model and observe indications of the Ising criticality on a small lattice size. We find the results derived by the quantum algorithm are well consistent with the ones from exact diagonalization, in both the neighborhood of the critical temperature and the low-temperature region.

We present a theoretical study of the image manipulation induced by an additional oscillating electric field in traditional photodetachment microscopy. We find that the spatial electron interference image on the detecting screen perpendicular to the field is now time modulated and it varies with the oscillating field periodically. With the oscillating-field strength comparable to the static-field strength, we identify an electron bunching effect in the time domain where the electrons detached in a large time window reach the detector in a very narrow time window due to the underlying dynamics, resulting in a high concentration of detached electrons near certain observation time windows. The spatial interference images for any observational time in the narrow window show a high-intensity ring or center depending on the different electron sources. Furthermore, when the period of the oscillating field is much smaller than the flight time of the detached electrons from the negative ions to the detector, the oscillating field has a strong effect on the center of the images. On the other hand, when the oscillating-field period is comparable to or larger than the flight time of the detached electrons, the edge and the size of the interference images on the screen can be greatly modified. The accumulated electron images over time are also studied and explained using Bessel functions and stationary phase approximations.

In this work, we explore the driven dynamics of the one-dimensional localization transitions. By linearly changing the strength of disorder potential, we calculate the evolution of the localization length $\xi$ and the inverse participation ratio $\mathcal{I}$ in a disordered Aubry-André (AA) model, and investigate the dependence of these quantities on the driving rate for a single typical realization of the disorder configuration. At first, we focus on the limit in the absence of the quasiperiodic potential. We find that the driven dynamics from both ground state and excited state can be described by the Kibble-Zurek scaling (KZS). Then, the driven dynamics near the critical point of the AA model is studied. Here, since both the disorder and the quasiperiodic potential are relevant directions, the KZS should include both scaling variables. Our present work not only extends our understanding of the localization transitions but also generalizes the application of the KZS.

We investigate the formation of self-bound quantum droplets in a one-dimensional binary mixture of bosonic atoms, applying the method of numerical diagonalization of the full Hamiltonian. The excitation spectra and ground-state pair correlations signal the formation of a few-boson droplet when crossing the region of critical interspecies interactions. The self-binding affects the rotational excitations, displaying a change in the energy dispersion from negative curvature, associated with superfluidity in the many-body limit, to a nearly parabolic curvature indicative of rigid body rotation. We exploit two global symmetries of the system to further analyze the few-body modes in terms of transition matrix elements and breathing mode dynamics. The ground and excited state energies reported here are exact within the respective Hilbert spaces, which have been adapted according to a given set of single-particle basis states and a specific importance threshold of the employed truncation method. We compare these data to the usual ad hoc inclusion of higher-order contributions in the extended Gross-Pitaevskii equation. For the given parameters we find a remarkable agreement between the few-body regime and the thermodynamic limit in one dimension.

We propose that frequency-tunable terahertz (THz) emissions can be efficiently generated from single-layer graphene at normal incidence by optimizing two-color laser pulses. For three different pulse durations, a genetic algorithm (GA) is applied to search for optimal laser parameters, which show quite different characteristics. Resulted THz spectra have different spectral shapes, and their dependence on the relative phase between two colors is dramatically changed with pulse duration. We develop an interference model to explain the THz emissions for three durations, employ a four-wave mixing model to obtain the optimal wavelength of second color for long duration, and use an asymmetry parameter model to describe the THz yields with the relative phase for short duration. We reveal that the dominant mechanism for the THz generation is shifted from the symmetry of laser pulse to the four-wave mixing when the duration of two-color laser is increased. We identify that the shorter duration is better for generating intense THz emission while the longer duration is preferable for selecting the THz frequency. Furthermore, we optimize the two-color laser parameters for maximizing the THz emissions from gapped graphene with the GA. And we also examine the temporal THz waveforms by varying the parameters in the two-color laser pulses.