Achieving precise preparation of quantum many-body states is crucial for the practical implementation of quantum computation and quantum simulation. However, the inherent challenges posed by unavoidable excitations at critical points during quench processes necessitate careful design of control fields. In this work, we introduce a promising and versatile dynamic control neural network tailored to optimize control fields. We address the problem of suppressing defect density and enhancing cat-state fidelity during the passage across the critical point in the quantum Ising model. Our method facilitates seamless transitions between different objective functions by adjusting the optimization strategy. In comparison to gradient-based power-law quench methods, our approach demonstrates significant advantages for both small system sizes and long-term evolutions. We provide a detailed analysis of the specific forms of control fields and summarize common features for experimental implementation. Furthermore, numerical simulations demonstrate the robustness of our proposal against random noise and spin number fluctuations. The optimized defect density and cat-state fidelity exhibit a transition at a critical ratio of the quench duration to the system size, coinciding with the quantum speed limit for quantum evolution.

We investigate the radiation damping experienced by a dielectric spherical particle when it is illuminated by an electromagnetic plane wave within the Rayleigh regime. We derive the equivalent electric dipole of the moving particle and subsequently calculate the electromagnetic force acting on it from two different approaches. In the first approach, we calculate the force from the integration of stress tensor and field momentum. In the second one, we calculate the force directly from the integration of the force density. Our derivations reveal that the damping coefficient is equal to $6P_{\mathrm{scat}}/m{c}^2$ along the propagation direction, whereas it is $P_{\mathrm{scat}}/m{c}^2$ along perpendicular directions. Here, $P_{\mathrm{scat}}$ denotes the power scattered by the particle, and $m{c}^2$ represents the particle's mass energy. The radiation damping derived in this paper sets upper limits on the quality factor of optically levitated objects and ensures the existence of a steady-state solution of the particle's dynamics.

An optomechanical microcavity can considerably enhance the interaction between light and mechanical motion by confining light to a subwavelength volume. However, this comes at the cost of an increased optical loss rate. Therefore, microcavity-based optomechanical systems are placed in the unresolved-sideband regime, preventing sideband-based ground-state cooling. A pathway to reduce optical loss in such systems is to engineer the cavity mirrors, i.e., the optical modes that interact with the mechanical resonator. In our work, we analyze such an optomechanical system, whereby one of the mirrors is strongly frequency dependent, i.e., a suspended Fano mirror. This optomechanical system consists of two optical modes that couple to the motion of the suspended Fano mirror. We formulate a quantum-coupled-mode description that includes both the standard dispersive optomechanical coupling as well as dissipative coupling. We solve the Langevin equations of the system dynamics in the linear regime showing that ground-state cooling from room temperature can be achieved even if the cavity is per se not in the resolved-sideband regime, but achieves effective sideband resolution through strong-optical-mode coupling. Importantly, we find that the cavity output spectrum needs to be properly analyzed with respect to the effective laser detuning to infer the phonon occupation of the mechanical resonator. Our work also predicts how to reach the regime of nonlinear quantum optomechanics in a Fano-based microcavity by engineering the properties of the Fano mirror.

The photon blockade based on destructive quantum interference is called the unconventional photon blockade (UPB), which has been intensively studied in Markovian systems but barely explored in the non-Markovian ones. In this paper, we construct a coupled-cavities system to achieve UPB with the non-Markovian effect, where the dissipationless left cavity and Markovian dissipative right cavity are respectively mediated by two-photon pump and single-photon driving field. Through the equivalence between the Markovian master equation and Heisenberg-Langevin equation for the environment initialization in the vacuum state, we can derive the exact non-Markovian Heisenberg-Langevin equation and reduced master equation for the left cavity, which contains the two-photon pump and effective single-photon driving field. In the non-Markovian regime (the dissipation falling below a threshold), the effective single-photon driving field holds nonzero, which can lead to UPB occurring due to a closed quantum interference path forming. When the dissipation exceeds the threshold, the system enters the Markovian regime, where UPB weakens. Especially, if the dissipation approaches infinity, UPB for the left cavity disappears due to the effective single-photon driving field tending to zero. We analytically derive an optimal condition for UPB, which is in good agreement with that obtained by the numerical simulation. We also discuss the situation where both cavities have dissipations. Finally, the above model is extended to a general system involving a dissipationless left cavity (mediated by two-photon pump) coupling with noninteracting dissipative right cavities (driven by single-photon driving fields). Our scheme might pave an avenue towards applications on photon statistics and quantum optics with the non-Markovian effect.

We study multiple scattering of light in a random medium containing both optical turbulence and randomly distributed particles. Based on the form of dielectric correlation function attained, the Dyson equation and Bethe-Salpeter equation for scalar waves are derived. The Bethe-Salpeter equation is then expanded according to the number of scattering times and then simplified to a practical expression for calculations after manipulations. The numerical scheme allows to calculate the field-field correlation function of scalar wave multiple scattering by particles larger than wavelength, and that of propagation in a medium containing both optical turbulence and particles. Theoretical analysis and numerical calculations may have applications in light detection and ranging (LIDAR) and underwater optical systems.

Quantum batteries have significant potential applications for future industry and daily life. The capacity is an important indicator for a battery. Methods to improve the capacity of quantum batteries are important. We consider quantum batteries given by bipartite quantum systems and study the enhancement of the battery capacity under local projective measurements on a subsystem of the quantum state. By using two-qubit Bell-diagonal states and $X$-type states as examples, we show that quantum battery capacity with respect to the whole system or a subsystem can be improved by local projective measurements. Our theoretical analysis will provide ideas for the experimental development of quantum batteries.

Quantum metrology explores quantum effects to improve the measurement accuracy of some physical quantities beyond the classical limit. However, due to the interaction between the system and the environment, the decoherence can significantly reduce the accuracy of the measurement. Many methods have been proposed to restore the accuracy of the measurement in the long-time limit. Recently, it was found that the bound state can help improve measurement accuracy and recover the $t^{-1}$ scaling [Bai et al., Phys. Rev. Lett. 123, 040402 (2019)]. Here, by using $N$ qubits, we propose a method to simulate the open quantum dynamics of a hybrid system including one atom and coupled resonators. We find that the error of the measurement can decrease as the time increases due to the existence of the bound state. With both analytical and numerical simulations, we prove the $t^{-1}$ scaling of the measurement error can be recovered when there is a bound state in the hybrid system. Interestingly, we observe that there are regular oscillations which can be used for the evaluation of the atomic transition frequency. For a finite $N$, the duration of the regular oscillations doubles as one more qubit is involved.

We propose a joint experimental and theoretical approach to measure the self-diffusion in a laser-cooled trapped ion cloud where part of the ions are shelved in a long-lived dark state. The role of the self-diffusion coefficient in the spatial organisation of the ions is deciphered, following from the good agreement between the experimental observations and the theoretical predictions. This comparison furthermore allows us to deduce the temperature of the sample. Protocols to measure the self-diffusion coefficient are discussed, in regard with the control that can be reached on the relevant timescales through the dressing of the atomic levels by laser fields.

A longitudinal radio-frequency (rf) magnetic field resonating at the Larmor frequency along the $\widehat{z}$ direction in the presence of a static transverse magnetic field s investigated. Optical-radio-frequency double resonance in ${}^{85}\mathrm{Rb}$ characterizes the strength of an external magnetic field through the precession frequency of ${}^{85}\mathrm{Rb}$ atoms. The measurements are carried out in a paraffin-coated ${}^{85}\mathrm{Rb}$ cell with a spin alignment configuration. An intriguing discovery of magnetic resonance peaks is observed at subharmonics of the Larmor frequency, accompanied by the emergence of nonlinear effects in the vicinity of the resonance. Notably, when the Larmor frequency reaches zero, the optical rotation signals induced by the rf magnetic field demonstrate a superposition of high-order harmonics of the rf frequency. Based on the simplified density matrix formalism and perturbation treatment, analytical expressions for the optical rotation are consistent with experimental outcomes. These findings hold the potential of extending our understanding on unveiling the dynamics of population and coherence among Zeeman sublevels, thereby advancing our knowledge in the field of alignment magnetometry.

We present a theoretical investigation of the influence of molecular alignment and orientation on elliptically polarized high-order harmonic generation (HHG) from CO molecules exposed to a linearly polarized pulse. The results show that the ellipticity of HHG observed in a partially aligned or oriented CO molecular ensemble significantly deviates from that of the individual response. Additionally, we find that the ellipticity of HHG in the CO molecular ensemble exhibits strong dependence on the degree of molecular alignment/orientation as well as the time delay between the alignment/orientation and probe pulses. More importantly, under the proper alignment/orientation angle, a large HHG ellipticity can be achieved from the partially aligned/oriented molecular ensemble at a moderate degree of alignment/orientation. These findings relax the experimental requirement of a high degree of molecular alignment/orientation for harmonic generation with large ellipticity.

The Efimov phenomenon manifests itself as an emergent discrete scaling symmetry in the quantum three-body problem. In the unitarity limit, it leads to an infinite tower of three-body bound states with energies forming a geometric sequence. In this work, we study the evolution of these so-called Efimov states using relativistic scattering theory. We identify them as poles of the three-particle $S$ matrix in the complex energy plane, and we study how they transform from virtual states through bound states to resonances when we change the interaction strength. We dial the scattering parameters toward the unitarity limit and observe the emergence of the universal scaling of energies and couplings—a behavior known from the nonrelativistic case. We additionally find that Efimov resonances follow unusual, cyclic trajectories accumulating at the three-body threshold and then disappear at some values of the two-body scattering length. We propose a partial resolution to this “missing states” problem.

We employed caustic theory to analyze the propagation dynamics of partially coherent self-accelerating beams such as self-healing of partially coherent Airy beams. Our results indicate that appropriately reducing spatial coherence can effectively enhance the self-healing capability of beams. As the spatial coherence decreases, the self-healing ability of the beams increases. However, below a certain value of coherence, this advantage does not exist, and a longer propagation distance is required for the reconstruction. We meticulously validated these findings through both simulation and experimental data, reinforced by a quantitative similarity analysis between beams with obstacles and their obstacle-free counterparts. The application of caustic theory in the field of partially coherent structured beams not only enhances our understanding of self-healing properties but also carries substantial implications for practical applications in various fields such as optical communication, encryption, and imaging.

We propose a setup to create and detect the atomic counterpart of snake-state trajectories which occur at the interface where the magnetic field reverses direction. Such a synthetic magnetic field is generated by coupling two counterpropagating modes of a ring cavity to a two-level atom. The spatial distribution and the strength of the induced magnetic field are controlled by the transverse- mode profile of the cavity modes and the number of photons in the two modes, respectively. By analyzing the atomic motion in such a magnetic field while including the cavity back-action, we find that the atom follows snake-state trajectories which can be nondestructively detected and reconstructed from the phase and the intensity of the light field leaking from the cavity. We finally show that the system parameters can be tuned to modify the transport properties of the snake states and even amplify the effect of cavity feedback, which can completely alter their topology.

Static disorder in a three-dimensional crystal degrades the ideal ballistic dynamics until it produces a localized regime. This metal-insulator transition is often preceded by coherent diffusion. By studying three paradigmatic one-dimensional models, namely, the Harper-Hofstadter-Aubry-André and Fibonacci tight-binding chains, along with the power-banded random matrix model, we show that whenever coherent diffusion is present, transport is exceptionally stable against decoherent noise. This is completely at odds with what happens for coherently ballistic and localized dynamics, where the diffusion coefficient strongly depends on the environmental decoherence. A universal dependence of the diffusion coefficient on the decoherence strength is analytically derived: The diffusion coefficient remains almost decoherence independent until the coherence time becomes comparable to the mean elastic scattering time. Thus, systems with a quantum diffusive regime could be used to design robust quantum wires. Moreover, our results might shed light on the functionality of many biological systems, which often operate at the border between the ballistic and localized regimes.

Positron binding to neutral atoms, which to this day have not been detected experimentally, is thought to be rather weak. According to a few reliable nonrelativistic calculations reported previously, the positron affinity for small atoms, even if it happens to be positive, is predicted to be very small in magnitude, of the order of just few millihartrees $(<0.1\mathrm{eV})$. In this work, we used a highly accurate variational expansion in terms of explicitly correlated Gaussians to investigate how relativistic effects may affect the stability of bound states of positrons with atoms. We performed calculations of positronic beryllium, $e^{+}\left[\mathrm{Be}\right]$, in its ground singlet $S$ and excited triplet $S$ and $P$ states, all of which are bound. According to our findings, neither the inclusion of scalar relativistic nor spin-dependent corrections alters the predictions regarding the existence of the bound states. When leading-order relativistic effects are taken into account, positron affinities change only by 2.2% or less. Notably, this is so even for the triplet $P$ state of positronic beryllium, where the spin-orbit correction is not at all canceled out when the energy difference with the parent system is computed.

We investigate a noninteracting many-particle bosonic system, placed in an asymmetric double-well potential. We first consider the dynamics of a single particle and determine its time-dependent probabilities to be in the left or the right well of the potential. These probabilities obey the standard Josephson equations, which in their many-particle interpretation also describe a globally coherent system, such as a Bose-Einstein condensate. This system exhibits the widely studied Josephson oscillations of the population imbalance between the wells. In our paper we go beyond the regime of global coherence by developing a formalism based on an effective density matrix. This formalism gives rise to a generalization of Josephson equations, which differ from the standard ones by an additional parameter, that has the meaning of the degree of fragmentation. We first consider the solution of the generalized Josephson equations in the particular case of thermal equilibrium at finite temperatures, and extend our discussion to the nonequilibrium regime afterwards. Our model leads to a constraint on the maximum amplitude of Josephson oscillations for a given temperature and the total number of particles. A detailed analysis of this constraint for typical experimental scenarios is given.

Inspired by the narrow Feshbach resonance in systems with the two-body interaction, we propose a two-channel model of three-component fermions with a three-body interaction that takes into account finite-range effects in low dimensions. Within this model, the $p$-wave Efimov-like effect in the four-body sector is predicted in fractional dimensions between $d=1$ and $d=2$. The impact of the finite-range interaction on the formation of the four-body bound states in $d=1$ is also discussed in detail.

Motivated by recent cold atom experiments exploring the phase structure of Bose-Fermi mixtures, we propose novel emergent bound states. Cold atoms embedded in a degenerate Fermi system interact via a fermionic analog of the Casimir force, which is an attraction of a $-1/r$ form at distances shorter than the Fermi wavelength. Interestingly, the hydrogenic two-body bound states do not form in this regime because the interaction strength is too weak under realistic conditions, and yet the three-body bound states can have a considerably higher degree of stability. As a result, the trimer bound states can form even when the dimer states are unstable. A quasiclassical analysis of quantum states supported by periodic orbits singles out the figure-eight orbits, predicting bound states that are more stable than the ones originating from circular orbits. The discrete energies of these states form families of resonances with a distinct structure, enabling a direct observation of signatures of figure-eight braiding dynamics.