We report on the experimental and theoretical investigation of alignment echoes of ${\mathrm{N}}_2\mathrm{O}$ molecules induced by a pair of time-delayed femtosecond laser pulses with different polarizations. When circularly polarized kick pulses are applied, the alignment echoes in the laser propagation direction are activated, giving rise to $k$-alignment echoes. In these cases, the conventional 2π phase-space analysis, where molecules are restricted to rotate in the plane perpendicular to the laser propagation direction, cannot capture the complete echo information. Thus, we consider the rotational dynamics of molecules in the three-dimensional space and adopt a full phase-space analysis involving evolution in both azimuthal and polar directions, termed 4π phase-space analysis, to reveal the underlying physical mechanism of $k$-alignment echoes. Its mechanism results from a joint contribution of the filamentation in both azimuthal and polar phase spaces. Our classical and quantum simulations are consistent with the experimental observations.

We report experimental measurements of the dynamical polarizability of dysprosium, at a wavelength of 532 nm. We measure all three components (scalar, vector, and tensor) of the anisotropic polarizability for the ground and the excited manifolds of the intercombination transition of Dy at 626 nm. The apparatus on which the measurements are performed is presented. We obtain with this setup imaging of single Dy atoms with fidelity above 99% and losses below 2.5% induced by imaging. We then describe the methods used to extract the polarizability. In particular, we combine a measurement of trap frequency and trap depth on single atoms in optical tweezers, allowing us to obtain a measurement of the ground-state polarizability free of errors in trap waist calibration. The obtained values give a magic condition between two Zeeman states in the ground and excited manifolds, which is used to image single atoms in optical tweezer arrays. The scalar polarizability of the ground state is in disagreement with theoretical expectations, calling for future investigations to resolve the discrepancy.

Temporal-coupled-mode theory (TCMT) is an important tool that is widely used for analyzing and designing integrated photonic systems that consist of multiple connected resonators. In microresonator systems with well-separated modes (relatively large free spectral range compared to the modal width), standard TCMT accounts only for the resonant excitation and is known to give excellent results in the weak-coupling limit. In this paper, we reveal a peculiar situation where TCMT breaks down under multiple excitation conditions. As we demonstrate, this can be explained by the destructive and the constructive interference of the excitations associated to the resonant and off-resonant modes respectively. In this case, we show that a multimode TCMT should be employed to account for these effects. Our results thus suggest that, while it is safe to use the standard TCMT under a single excitation, it must be applied with care when multiple excitations are involved.

The antiresonant effects, which are opposite to the resonant effects, describe the destructive interferences that occur in optical resonators. There is a growing trend in the recent development of photonic devices that explore antiresonant effects such as topology structures and photonic molecules with asymmetrical geometries. For the design of coupled optical resonators, temporal-coupled-mode theory (TCMT) has been widely applied, which provides a simple yet effective physical picture of such systems. However, TCMT is valid for optical frequencies that resonate well with the cavities. It becomes powerless to handle mode coupling at optical frequencies that resonate in part of the resonators but antiresonate in the rest. In this work, we extend TCMT to antiresonant regimes for investigating mode coupling between antiresonances and resonances in coupled-optical-cavity systems via taking the analogy of the mode splitting. Our theory agrees well with the transfer matrix method as well as the experimental results. A concrete example is given to show that our work offers an opportunity to extend the application of TCMT in the design of advanced coupled-cavity structures.

Levitated nanoparticles provide a controllable and isolated platform for probing fundamental quantum phenomena at the macroscopic scale. In this work we introduce an optimization method to determine optimal static potentials for the generation of largely delocalized and non-Gaussian quantum states of levitated nanoparticles. Our optimization strategy accounts for position-dependent noise sources originating from the fluctuations of the potential. We provide key figures of merit that allow for fast computation and capture relevant features of the dynamics, mitigating the computational demands associated with the multiscale simulation of this system. Specifically, we introduce coherence length and coherent cubicity as signatures of large delocalization and quantum non-Gaussian states, respectively. As a proof of principle, we apply the optimization approach to a family of quartic potentials and show that the optimal configuration depends on the strength and nature of the noise in the system. Additionally, we benchmark our results with a full quantum dynamics simulations of the system for the optimal potentials.

A $J=0$ to $J=1$ to $J=0$ ladder scheme can be used as a prototype for probing optical coherence transfer in an atom array. Two incident ${\sigma }_{-}$ polarized fields drive transitions from the ground state to the intermediate state and the intermediate state to the uppermost state in the atoms. The uppermost state is taken to be a long-lived Rydberg level. In the absence of any atom-atom interactions, excitation to the Rydberg level is not allowed, owing to the selection rules. However, dipole-dipole interactions can provide a transfer of the state amplitude associated with the $m=-1$ sublevel in one atom to the $m=+1$ sublevel in another atom. The overall process of field excitation and optical coherence transfer can then lead to an excitation of the Rydberg level. One can then apply a readout pulse to obtain phase-matched emission from the array. For a linear array of atoms, the phase-matched signal can increase more rapidly than $N^2$, where $N$ is the number of atoms in the array. For arrays that are invariant under a rotation $\delta \varphi <\pi$ about the $z$ axis, however, it turns out that the total field intensity vanishes identically in the phase-matched direction. For such arrays, the Rydberg state population can serve as a measure of the optical coherence transfer.

It is well established that the optimal spin squeezing under a one-axis-twisting Hamiltonian follows a scaling law of $J^{-2/3}$ for $J$ interacting atoms after a quench dynamics. Here, we prove analytically and numerically that the spin squeezing of the ground state of the one-axis-twisting Hamiltonian actually reaches the Heisenberg limit $J^{-1}$. By constructing a bilinear Bogoliubov Hamiltonian with the raising and lowering spin operators, we exactly diagonalize the spin Bogoliubov Hamiltonian, which includes the one-axis twisting Hamiltonian as a limiting case. The ground state of the spin Bogoliubov Hamiltonian exhibits excellent spin squeezing, which approaches the Heisenberg limit in the case of the one-axis-twisting Hamiltonian. It is possible to realize experimentally the spin-squeezed ground state of the one-axis-twisting Hamiltonian in dipolar spinor condensates, ultracold atoms in optical lattices, spins in a cavity, or alkali atoms in a vapor cell.

We present a model describing the transmission of light through atomic media with a vanishing index of refraction. Zero-index materials are of particular interest as the infinite phase velocity of light within the material offers the potential to manipulate electromagnetic waves to mediate dipole-dipole interactions over extended distances. We focus on the preparation of near-zero-index (NZI) conditions based on atomic coherence using two distinct atomic media as exemplary of generic NZI materials. We establish a model based on the Maxwell-Bloch equations to describe the propagation of a light pulse through these media. To investigate the sustainability of NZI under minimal light conditions, we assume single-photon intensity of the propagating pulse. Specifically, we examine whether the spatial phase change of the photon remains near zero as it traverses the medium. We employ a finite-element numerical approach to solve the coupled Maxwell-Bloch equations describing the photon propagation. Our results indicate that the presence of a photon within the medium will disrupt the NZI state, thus disallowing the establishment of enhanced dipole-dipole interactions over large distances.

We propose a scheme to manipulate entanglement and the direction of asymmetric Einstein-Podolsky-Rosen (EPR) steering between two interacting bosonic modes in a nonlinear optical apparatus, in which a pumped Fabry-Pérot cavity system containing two nonlinear waveguides is coupled to two feedback loops. It is found that Markovian feedback plays a key role in controlling both entanglement and EPR steering, the maximal entanglement can only be achieved through two symmetric feedback loops, while one-way EPR steering can only occur when both asymmetric feedback loops and an unbalanced beam-splitter are satisfied simultaneously. Furthermore, in terms of an unequal beam-splitter and two different feedback loops, it is demonstrated that the directionality and extent of the asymmetric EPR steering can be readily controlled under certain conditions. A simple and sufficient condition is also revealed for generating two-way EPR steering. Moreover, the entanglement and EPR steering can be effectively enhanced by performing Markovian feedback. In addition, By choosing appropriate feedback strengths, two cavity modes can be driven into a pure two-mode squeezed vacuum state rather than a mixed two-mode Gaussian entangled state or a pure entangled dark state. We hope the proposed scheme will enhance our understanding of the fundamental asymmetry of EPR steering, and it may have potential applications in quantum computation and quantum communication.

We study the separability in multiqubit pure and mixed Greenberger-Horne-Zeilinger (GHZ) and $W$ states with an accelerating qubit using the Abe-Rajagopal (AR) $q$-conditional entropy. We observe that the pure multiqubit GHZ and $W$ states in the inertial-noninertial bipartition with one of their qubits accelerated will remain nonseparable irrespective of the qubit's acceleration. In these systems, we capture the variation of their nonseparability with respect to the acceleration of the qubit and the AR $q$-conditional entropy parameter $q$. However, in the corresponding multiqubit mixed states obtained by introducing a global noise to the above pure states, we could get stronger conditions on their separability in the inertial-noninertial bipartition, in terms of the acceleration of the qubit, the noise parameter, and the number of qubits in the system, in the asymptotic limit of the parameter $q$. These conditions obtained from the AR $q$-conditional entropy serve as necessary conditions for separability in such multiqubit states with a relativistic qubit.

The measurement and characterization of noise is a flourishing area of research in mesoscopic physics. In this work, we propose interaction-free measurements as a noise-detection technique, exploring two conceptually different schemes: the coherent and the projective realizations. These detectors consist of a qutrit whose second transition is resonantly coupled to an oscillatory field that may have noise in amplitude or phase. For comparison, we consider a more standard detector previously discussed in this context: a qubit coupled in a similar way to the noise source. We find that the qutrit scheme offers clear advantages, allowing precise detection and characterization of the noise, while the qubit does not. Finally, we study the signature of noise correlations in the detector's signal.

In recent years, efficient quantum circuit simulations incorporating ideal noise assumptions have relied on tensor network simulators, particularly leveraging the matrix product density operator (MPDO) framework. However, experiments on real noisy intermediate-scale quantum (NISQ) devices often involve complex noise profiles, encompassing uncontrollable elements and instrument-specific effects such as crosstalk. To address these challenges, we employ quantum process tomography (QPT) techniques to directly capture the operational characteristics of the experimental setup and integrate them into numerical simulations using MPDOs. Our QPT-assisted MPDO simulator is then applied to explore a variational approach for generating noisy entangled states, comparing the results with standard noise numerical simulations and demonstrations conducted on the Quafu cloud quantum computation platform. Additionally, we investigate noisy MaxCut problems, as well as the effects of crosstalk and noise truncation. Our results provide valuable insights into the impact of noise on NISQ devices and lay the foundation for enhanced design and assessment of quantum algorithms in complex noise environments.

We leverage random phase approximation and unbiased auxiliary-field quantum Monte Carlo methods to compute dynamical correlations for a dilute homogeneous two-dimensional attractive Fermi gas. Our main purpose is to quantitatively study the collective excitations of the system to generate robust benchmark results and to shed light into fermionic superfluidity in the strongly correlated regime. In particular we are motivated by a recent paper suggesting that the Higgs mode can be detected in the spectrum of spin fluctuations. Despite the fact that we are somewhat limited by finite-size effects, our study indeed pinpoints a clear peak in the spin channel at low momentum, but a detailed analysis suggests that such a peak, though certainly interesting, does not correspond to the Higgs mode. We propose a different explanation for the shape of the spin structure factor. However, our results clearly show that the Higgs mode can be detected in the density channel at very small wave vectors, although very good resolution is necessary.

In atomic Bose-Einstein condensates in optical lattices, mean-field energy can support the existence of period-doubled density waves, which are similar to Bloch waves but have the double periodicity of the underlying lattice potentials. However, they are dynamically unstable. Here, we propose to use the spin-orbit coupling to stabilize the period-doubled density waves. The stabilization mechanism is revealed to relate to interaction-induced spontaneous symmetry breaking of the spin-flip parity symmetry.

The aim of this paper is to derive the hydrodynamics for a cold Bose gas from the microscopic platform based on the many-body Schrödinger equation and general assumptions of the hydrodynamic approach (HA) applicable to any dimension. We develop a general HA for a cold spatially inhomogeneous Bose gas assuming two different temporal and spatial scales and obtain the energy as a functional of both fast inner quantum mode and slow macroscopic mode. The equations governing the fast and slow modes are obtained from this functional by their independent variations. The fast mode is the wave function in the stationary state at local density which can be ground, excited with a nonzero atom momenta, or a superposition of more than one state. The energy eigenvalue (or expectation value) of this local wave function universally enters the hydrodynamic equation for the slow mode in the form of the local chemical potential which incorporates the inner local momentum. For zero inner momenta and particular choices of this eigenvalue as a function of gas density, this equation reduces to the known equations based on the local density approximation. If, however, the inner momenta are nonzero, the equation includes the interaction between these momenta and the slow mode velocity. Relation between this general HA and the standard local density approximation is elaborated. Two effects of the local momenta and their density dependence on the soliton solutions are demonstrated.

We present a quantum-mechanical model that describes fiber-based frequency conversion by four-wave-mixing Bragg scattering in the presence of Raman interactions. In the case of continuous-wave pumps, we find closed-form expressions for the conversion efficiency and photon statistics characterized by the second-order correlation function. For pulsed pumps, we derive a highly general model based on Green functions and provide a numerical solution method using a split-step scheme. In both cases, we find that noise from spontaneous Raman scattering can pose a serious challenge to this type of frequency conversion if the pumps are less than 30 THz from the quantum fields. However, this impact can be mitigated with cross-polarized pumps and, on the anti-Stokes side, through fiber cooling.

We propose a scheme to achieve the magnon blockade (MB) effect in a system consisting of a giant spin ensemble and a giant atom coupled to a waveguide. We show a physical mechanism for generating the MB through non-Hermitian anharmonicity. The non-Hermitian magnon blockade (NHMB) is realized in braided, nested, and separate coupling configurations. The NHMB is shown to be useful for the preparation of a high-efficiency and high-purity single magnon. Remarkably, the coexistence phenomenon of the conventional magnon blockade and the unconventional magnon blockade occurs and their conversion can be completed in the braided and nested coupling configurations. This result realizes the modulation between efficiency and purity of single-magnon generation. For the separate coupling configuration, the frequency-tunable NHMB is obtained due to the variation of Lamb shifts. The phase caused by the photon propagating between neighboring coupling points offers a simple and flexible method to modulate the conversion of MB effects induced by multiple physical mechanisms. Our results open a pathway to manipulate the generation of the MB on demand, which has potential applications in producing a single-magnon source and performing magnon-based quantum information processing.