The discovery of the non-Hermitian skin effect (NHSE) has revolutionized our understanding of wave propagation in non-Hermitian systems, highlighting unexpected localization effects beyond conventional theories. Here, we discover that NHSE, accompanied by multitype spectral phases, can be induced by manipulating nonconservative couplings. By characterizing the spectra through the windings of the energy bands, we demonstrate that band structures with identical, opposite, and even twisted windings can be achieved. These inequivalent types of spectra originate from the multichannel interference resulting from the interplay between conservative and nonconservative couplings. Associated with the multitype spectra, unipolar and bipolar NHSE with different eigenmode localizations can be observed. Additionally, our findings link the nonreciprocal transmission properties of the system to multiple spectral phases, indicating a connection with the skin modes. This paper paves new pathways for investigating non-Hermitian topological effects and manipulating nonreciprocal energy flow.

We investigate the coherent manipulation of the complex refractive index via the interaction of chiral molecules with Laguerre-Gaussian (LG) vortex beams and the microwave field. The results show that a zero index of refraction associated with zero absorption can be obtained at the vortex core. Moreover, chiral symmetry and symmetry breaking appear in the refractive index distributions, which depend not just on the topological charge $l_j$ ($j=1,2$) of LG beams but also on the phase of the microwave field. We find the following. (i) When the phase value of microwave field is an even multiple of $\pi /2$, if the value of $|l_1-l_2|$ is an odd number, the spatial distributions of the complex refractive index for left-handed and right-handed molecules exhibit chiral symmetry; if $|l_1-l_2|$ is an even number, the refractive index distributions for left-handed and right-handed molecules display chiral symmetry breaking. (ii) When the phase value of the microwave field is an odd multiple of $\pi /2$, the phenomena of chiral symmetry and symmetry breaking of the refractive index for the left-handed and right-handed molecules are just the opposite. These findings originate from the unique optical properties of vortex beams and the quantum interference between different transition pathways. This may provide new opportunities for next-generation chiroptical devices, chiral detection, and chiral molecular switches.

Based on full-wave simulations, we demonstrate that a spectrally isolated toroidal dipole resonance can induce a direction reversal of the lateral optical force (OF) acting on plasmonic nanoparticle clusters illuminated by a linearly polarized plane-wave optical field. The spatial profiles of the electric and magnetic fields, as well as the individual radiation powers from different multipoles on the clusters, are presented to establish the association of the lateral OF reversal with the toroidal dipole resonance. The angular distribution of far-field scattered irradiance is exhibited to elucidate its physical origin. The phenomenon suggests an alternative way of optically manipulating the plasmonic nanoparticle aggregates that support toroidal moments and clusters involving toroidal molecular structures.

The realization of correlated multiphoton processes usually depends on the interaction between nonlinear media and atoms. However, the nonlinearity of optical materials is generally weak, making it still very challenging to achieve correlated multiphoton dynamics at the few-photon level. Meanwhile, giant atoms, with their capability for multipoint coupling, which is a novel paradigm in quantum optics, mostly focus on the single-photon field. In this work, by using the method described in Phys. Rev. Res. 6, 013279 (2024), we reveal that the ladder-type three-level giant atom spontaneously emits strongly correlated photon pairs with high efficiency by designing and optimizing the target function. In addition, by encoding local phases into the optimal coupling sequence, directional two-photon correlated transfer can be achieved. This method does not require a nonlinear waveguide and can be realized in the conventional environment. We show that the photon pairs emitted in both the bidirectional and the chiral case exhibit strong correlation properties in both time and space. Such correlated photon pairs have great potential applications for quantum information processing. For example, numerical results show that our proposal can realize the two-photon mediated cascaded quantum system.

We study theoretically the interplay of spontaneous emission and interactions for the multiple-excited quasistationary eigenstates in a finite periodic array of multilevel atoms coupled to the waveguide. We develop an analytical approach to calculate such eigenstates based on the subradiant dimer basis. Our calculations reveal the peculiar multimerization effect driven by the anharmonicity of the atomic potential: while a general eigenstate is an entangled one, there exist eigenstates that are products of dimers, trimers, or tetramers, depending on the size of the system and the fill factor. At half-filling, these product states acquire a periodic structure with all-to-all connections inside each multimer and become the most subradiant ones.

We study the fractional Langevin equation with fractional $\alpha$-order and linear friction terms of a system coupled to white and colored thermal baths using both analytical and numerical methods. We find analytical expressions for the position and the mean squared displacement (MSD) of the system using the Prabhakar-Mittag-Leffler function. The MSD exhibits long-term subdiffusive regimes $t^{\alpha }$ driven by colored noise and $t^{2\alpha -1}$ driven by white noise. When the linear friction is neglected, periodic ordered phases emerge for small fractional orders $\alpha \lesssim 0.1$. In particular, the zero-linear friction system driven only by colored noise manifests the properties of a transient time-crystal, with a ground state satisfying the fluctuation-dissipation theorem and a periodicity proportional to $2\pi$. On the other hand, the zero-linear friction system driven only by white noise displays an out-of-equilibrium time glass phase with periodicity proportional to $\pi$. A mixed phase with contributions from both phases is encountered when the system couples to both baths. In that case, the periodicity deviates from $2\pi$ due to damping effects. We test all the analytical results numerically by implementing a discrete recursive expression, where the random forces of the system are modeled as the derivative of the fractional Brownian motion. A microscopic description for the system is also provided by an extension of the Caldeira-Leggett model.

The non-Markovian depolarizing channel is explored from the perspective of understanding its non-Markovian behavior as well as the occurrence of singularities. The study brings together the various ways to identify and quantify non-Markovianity. This includes dynamical techniques such as quantum information backflow witness, Breuer-Laine-Piilo, Rivas-Huelga-Plenio, and Hall-Cresser-Li-Andersson measures. In addition, geometrical visualization of non-Markovian effects is presented using the variation in the volume of accessible states during dynamical evolution. Further, a trajectory-based visualization of the dynamical map within the parameter space is presented. The trajectories traced during evolution demonstrate the loss of CP divisibility and the emergence of non-Markovianity under systematic variations of the system parameters. The effects of increasing system dimensions and qubit numbers on singularity and non-Markovianity are presented, with an extension of characterization techniques to higher-dimensional systems.

Dichromatic soliton molecules (DSMs), comprising multiple optical solitons with two carrier frequencies, can exhibit static or breathing states, akin to real chemical molecules. Although these two states have been extensively studied individually, comprehending the intricate transitions between them is crucial for effectively controlling lasing states. Here, we experimentally observe spectral-temporal transition dynamics between these DSM states in a synchronous mode-locked fiber laser, leveraging dispersive Fourier transform and dispersive temporal interferometry techniques. Our findings reveal obvious transient breathing processes during the transition from breathing to static states, while the reverse transformation is not evident. Numerical simulations, utilizing a model integrating equations for population inversion in mode-locked lasers and cross-phase-modulation-coupled complex Ginzburg-Landau equations, reproduce similar dynamics. This investigation elucidates transition dynamics in nonequilibrium systems, providing insights for intelligent manipulation of lasing states.

Investigating the interactions of vortex electrons with electromagnetic fields is crucial for advancing particle acceleration techniques, scattering theory in background fields, and developing novel electron beams for material diagnostics. In this work, we systematically study the dynamics of relativistic vortex electrons during their head-on collisions with linearly polarized (LP) and circularly polarized (CP) laser pulses, as well as their superposition. We develop a theoretical framework using Volkov-Bessel wave functions to describe the spatiotemporal characteristics of vortex electrons in these external fields. We show that the beam center of the vortex electron follows the classical trajectory of a point-charge electron while maintaining the transverse structure of both vortex eigenstates and superposition states. Specifically, CP laser pulses cause the beam center to rotate, while LP laser pulses induce a lateral shift. The combined effect of LP and CP laser pulses in a two-mode field results in a twisted spiral pattern. Our findings demonstrate the potential for versatile control of vortex electron beams using various laser modes, providing a foundation for future experimental and theoretical studies. This work serves as a benchmark reference for investigations into the manipulation of vortex electron beams using more realistic laser or other types of external fields.

The optimal runtime of a quantum computer searching a database is typically cited as the square root of the number of items in the database, which is famously achieved by Grover's algorithm. With parallel oracles, however, it is possible to search faster than this. We consider a many-body quantum system that naturally effects parallel queries, and we show that its parameters can be tuned to search a database in constant time, assuming a sufficient number of interacting particles. In particular, we consider Bose-Einstein condensates with pairwise and three-body interactions in the mean-field limit, which effectively evolve by a nonlinear Schrödinger equation with cubic and quintic nonlinearities. We solve the unstructured search problem formulated as a continuous-time quantum walk searching the complete graph in constant time. Depending on the number of marked vertices, however, the success probability can peak sharply, necessitating high precision time measurement to observe the system at this peak. Overcoming this, we prove that the relative coefficients of the cubic and quintic terms can be tuned to eliminate the need for high time-measurement precision by widening the peak in success probability or having it plateau. Finally, we derive a lower bound on the number of atoms needed for the many-body system to evolve by the effective nonlinearity.

A method is presented for discrete real-time learning (DRTL) of the evolution of a many-body quantum state in the presence of time-dependent control fields. The DRTL method is based on the recent adoption of the artificial neural network (ANN) ansatz within the framework of the time-dependent variational Monte Carlo method in learning large-scale quantum many-body dynamics. The method leverages accurate short-time quantum-state propagation with the Schrödinger equation and the efficient stochastic quantum natural gradient descent algorithm in machine learning. Importantly, the DRTL method is devoid of high numerical costs resulting from the exponentially large many-body Hilbert space by replacing the full state propagation with short-time propagations of a sequence of partial stochastic samples of the ANN state and may be considered as a synergism between the Schrödinger equation and the ANN that streamlines the computation of the many-body controlled quantum dynamics. Specifically, this method minimizes the normed distances between the time-dependent variational ANN ansatz and a set of partial quantum states over a sequence of stochastically sampled subspaces that are connected to each other in small discrete time steps. We demonstrate the high accuracy and stability of the DRTL method for the spin excitation dynamics of $4\times 4,6\times 6, 8\times 8$, and $10\times 10$ two-dimensional (2D) Heisenberg spin-1/2 lattices driven by time-dependent control fields applied to a corner spin along with strong exchange coupling between the nearest-neighbored spins. It is found that the number of ANN parameters for properly following the dynamically occupied subspaces (i.e., starting from the pure tensor-product spin configuration with all spins down) approximately increases quadratically with the number of spins in the system. Importantly, we show that the DRTL method can accurately and efficiently follow the small, yet importantly, occupied parts of the Hilbert space of the 2D spin square lattices during the interaction with the strong time-dependent control fields over a physically significant time period. This finding provides a basis for future work to perform quantum optimal control simulations of 2D spin lattices and other multiparticle systems.

In this work, we present a method for measuring the motional state of a two-level system coupled to a harmonic oscillator. Our technique uses ultranarrowband composite pulses on the blue sideband transition to scan through the populations of the different motional states. Our approach does not assume any previous knowledge of the motional state distribution and is easily implemented. It is applicable both inside and outside of the Lamb-Dicke regime. For higher phonon numbers especially, the composite pulse sequence can be used as a filter for measuring phonon number ranges. We demonstrate this measurement technique using a single trapped ion and show good detection results with the numerically evaluated pulse sequence.

We consider the spin-orbit-coupled Bose gas with repulsive mean-field interparticle interactions. We analyze the phase diagram of the system varying the temperature $T>0$, the chemical potentials, and interparticle and spin-orbit interaction couplings. Our results indicate that, for Rashba- and Weyl-type spin-orbit couplings, condensates featuring an ordering wave vector $\vec{Q}\ne \vec{0}$ are fragile with respect to thermal fluctuations and at $T>0$ the only stable thermodynamic phases involving the Bose-Einstein condensate (BEC) are those of uniform type with $\vec{Q}=\vec{0}$. On the other hand, the presence of the spin-orbit coupling stabilizes the $\vec{Q}=\vec{0}$ BEC state at any dimensionality $d>1$ and modifies either the order or the universality class of the corresponding phase transition. We emphasize the singular nature of the limit of vanishing spin-orbit interaction coupling $v$, sizable shifts of the phase boundaries upon varying $v$, and the role of the relative magnitudes of the interparticle interaction couplings for the character of the condensation transition.

We investigate analytically and numerically the Anderson localization of quasiparticles in binary Bose mixtures in the presence of the Lee-Huang-Yang quantum and thermal corrections subjected to correlated disordered potentials. We calculate the density profiles, the Bogoliubov quasiparticle modes, and the localization length in both the mixture and droplet phases. We show that for one-dimensional speckle potentials, the peculiar interplay of disorder and the Lee-Huang-Yang fluctuations may enhance localization in one component while inducing delocalization in the other. Our results reveal also that thermal fluctuations lead to the emergence of two and multiple localization maxima. In the droplet state, our findings indicate that the Anderson localization of quasiparticles is weak in the flat-top region since its excitation modes are restricted to those below the particle-emission threshold.

We investigate the recoil heating phenomenon experienced by a dielectric spherical particle when it interacts with a propagating or standing plane wave within the Rayleigh regime. We derive the fluctuating force acting on the particle due to the fluctuations of the electromagnetic fields. For the case of the propagating wave, we show that the spectral density of the fluctuating force in the propagation direction is $7\hslash {\omega }_0{P}_{\mathrm{scat}}/5c^2$, while in the direction of the electric and magnetic fields it is $\hslash {\omega }_0{P}_{\mathrm{scat}}/5c^2$ and $2\hslash {\omega }_0{P}_{\mathrm{scat}}/5c^2$, respectively, where $P_{\mathrm{scat}}$ denotes the power scattered by the particle, $\hslash {\omega }_0$ represents the energy of a photon, and $c$ is the speed of light. When the particle interacts with a standing wave, the longitudinal force fluctuation at the trapping position equals $2\hslash {\omega }_0{P}_{\mathrm{scat}}/5c^2$, while the transverse force fluctuations are similar to those in the case of a propagating wave.

We investigate the Aharonov-Anandan phase and dynamical phase of a well-known Akhmediev breather. We find that the Aharonov-Anandan phase has two different evolution forms with an identical modulation period, while the dynamical phase has only one form and its value can be time independent. To understand the two different Aharonov-Anandan phase forms, we uncover that there are three different topological vector potentials defined in various spaces by following the zeros of the density using Dirac's theory. The singularities of vector potentials are always located at the zeros of the density in each parameter space. Our analysis indicates that the root multiplicity of a zero of density is closely related to the topological charge of the vector potential. These results indicate some interesting connections among zeros of the density, singularities, topological properties, and geometric phases.

In a mode-locked fiber laser with a saturable absorber, two types of period doubling of vector solitons are numerically observed: One is the asynchronous period doubling of two orthogonal polarization components, and the other is a synchronous one under different operation conditions. The spectral evolution of the asynchronous period doubling of vector solitons is characterized by the appearance of additional sidebands in the spectrum, with the polarization state locked to multiples of the cavity length. In contrast, the synchronous period doubling of vector solitons evolves more slowly, with their polarization locked at approximately $-\pi /2$, indicating circular polarization. Additionally, using the soliton distillation method removes the continuous-wave background and sidebands, revealing the dynamics of asynchronous and synchronous period doubling of pure vector solitons.

Even though heralded single-photon sources have been generated routinely through spontaneous parametric down conversion, vacuum and multiple photon states are unavoidably involved. With machine learning, we report the experimental implementation of photon number state tomography by directly estimating target parameters. Compared to the Hanbury Brown and Twiss measurements only with clicked events recorded, our neural-network-enhanced quantum-state tomography characterizes the photon number distribution for all possible photon number states from the balanced homodyne detectors. By using the histogram-based architecture, a direct parameter estimation on the negativity in Wigner's quasiprobability phase space is demonstrated. Such a fast, robust, and precise quantum-state tomography provides us a crucial diagnostic toolbox for the applications with incoherent mixture of Fock states.

QBism has long recognized quantum states, positive-operator-valued measure elements, Kraus operators, and even unitary operations to be of the same type: they express aspects of an agent's belief system concerning the consequences (for her) of actions she might take upon her external world. Such action-consequence pairs have conventionally been called “quantum measurements.” The calculus of quantum theory is then viewed as an empirically motivated addition to Bayesian decision theory when brought to this notion of measurement. This radical approach has allowed QBism to eliminate conceptual problems that plague other interpretations of quantum mechanics. However, one issue has remained elusive: If a QBist does not believe in the existence of an ontic (agent-independent) dynamical variable evolving over time, why would there be any constraints on her quantum-state assignment in the absence of performing a measurement? Why would she introduce unitary or open-system quantum dynamics at all? Here, we present a representation theorem based on van Fraassen's reflection principle to answer these questions. Simply put, an agent's assignment of quantum dynamics represents her belief that a measurement action she is contemplating would not change her current odds for future gambles. A corollary to this approach is that one can make sense of “open-system dynamics” without the need to introduce an “environment with a measurement record,” as is common in decoherence accounts of quantum measurement. QBism's understanding instead rests more fundamentally on an agent's beliefs about the system of interest (not system plus environment) and her judgments about measurements she might perform on that system. More broadly, this result establishes QBism's contention that measurement itself is the central concept of quantum theory and thus the framework upon which any future QBist ontology must hang.