We report the computational discovery of complex, topologically charged, and spectrally stable states in three-dimensional multicomponent nonlinear wave systems of nonlinear Schrödinger type. While our computations relate to two-component atomic Bose-Einstein condensates in parabolic traps, our methods can be broadly applied to high-dimensional nonlinear systems of partial differential equations. The combination of the so-called deflation technique with a careful selection of initial guesses enables the computation of an breadth of patterns, including ones combining vortex lines, rings, stars, and vortex labyrinths. Despite their complexity, they may be dynamically robust and amenable to experimental observation, as confirmed by Bogoliubov–de Gennes spectral analysis and numerical evolution simulations.

Achieving spin squeezing within solid-state devices is a long-standing research goal due to the promise of their particularities, for instance, their long coherence times, the possibility of low-temperature experiments, or the integration of entanglement-assisted sensors on-chip. In this work, we investigate an interferometer-free four-color scheme to achieve spin squeezing of rare-earth-ion-doped crystals. The proposal relies on an analytic derivation that starts from a Tavis-Cummings model for light-matter interaction, providing microscopic insights onto spin-squeezing generation. We evince the spin-squeezing signature in the light intensity variance. We consider the two particular cases of europium- and praseodymium-doped yttrium orthosilicates, workhorses of quantum technology developments. We show that up to 8 dB of spin squeezing can be obtained with readily accessible experimental resources, including noise due to photon scattering. Our results for rare-earth-ion-doped crystals add to the promising properties of these platforms for manipulating many-body entangled states and for high-precision measurements.

We study the dispersion relation of Kelvin waves propagating along single- and half-quantum vortices in miscible two-component Bose-Einstein condensates based on the analysis of the Bogoliubov–de Gennes equation. With the help of the interpolating formula connecting the dispersion relations in low- and high-wave-number regimes, we reveal the nontrivial dependence of the dispersion relation on the intercomponent interaction through the change in the vortex-core size of the vortical component. We also find the splitting of the Kelvin wave dispersion into gapless and gapped branches when both components have overlapping single-quantized vortices.

Phase plays an essential role in both classical and quantum wave dynamics, which has motivated many scientists to study the phases of many different nonlinear waves. Among those nonlinear waves, the ones described by rational solutions have been given much attention, partly because some of them can describe rogue wave dynamics. We revisit the phase of several well-known one-dimensional rational solutions and clarify that sudden phase inversion processes indeed exist for rogue waves, in contrast to the ones reported before. Moreover, we investigate the phase properties of rogue waves and rational W-shaped solitons in the Hirota and Sasa-Satsuma model, for which some high-order physical effects are considered, i.e., the third-order dispersion delayed nonlinear response and self-steeping effects. The quite distinctive phases are uncovered for the rational W-shaped solitons with similar density profiles in the two models, by limitation analysis on the underlying topological vector potentials of other related nonlinear waves. These results indicate that the underlying topological vector potentials can be used to identify the phases of nonlinear waves, especially for localized waves with abrupt phase jumps.

We predict a wide class of two-dimensional vector dissipative solitons in wide-aperture semiconductor vertical-cavity surface-emitting lasers with a saturable absorber. For them, polarization structure varies over the laser cross section and includes singularities: $\mathsf{V}$ points with zero total intensity, $\mathsf{C}$ points with purely circular polarization, and $L$ lines with purely linear polarization. We show that the transverse motion of these solitons obeys the Eulerian mechanics dictated by soliton symmetry, taking into account the vector nature of laser solitons; presented are scenarios of soliton transverse rotation and “lunar motion.”

“It is well-known to those who know it” that single-photon interference experiments can be modeled classically [S. Barnett, Phys. Scr. 97, 114004 (2022)]. When a single-photon light pulse was split by a biprism, good agreement with a classical fit was obtained and the photon was counted only once, consistent with a probabilistic interpretation [V. Jacques et al., Eur. Phys. J. D 35, 561 (2002)]. A justification for this “well know result of quantum optics” is implicit in the work by Hawton [Phys. Rev A 104, 052211 (2021)] where a real covariant field describing a single photon is first quantized. Here the theoretical basis of this result is reviewed and the theory is extended to multiphoton states. The crucial role of the charge-parity-time theorem in coupling to charged matter and resolution of the photon localization problem is discussed.

In this work, we deal with the relaxation of two central assumptions in standard locally realistic hidden variable (LRHV) inequalities: free will in choosing measurement settings, and the presence of perfect detectors at the measurement devices. Quantum correlations violating LRHV inequalities are called quantum nonlocal correlations. In principle, in an adversarial situation, there could be a hidden variable introducing bias in the selection of measurement settings, but observers with no access to that hidden variable could be unaware of the bias. In practice, however, detectors do not have perfect efficiency. A main focus of this paper is the introduction of the framework in which given a quantum state with nonlocal behavior under constrained free will, we can determine the threshold values of detector parameters (detector inefficiency and dark counts) such that the detectors are robust enough to certify nonlocality. We also introduce a class of LRHV inequalities with constrained free will, and we discuss their implications in the testing of quantum nonlocal correlations.

We explore the origin of the arrow of time in an isolated quantum system described by the Schrödinger equation. We provide an explanation from weak values in the configuration space, which are understood as operational properties obtained in the laboratory following a well-defined protocol. We show that quantum systems satisfying the eigenstate thermalization hypothesis can simultaneously provide thermalized ensemble expectation values and nonthermalized weak values of the momentum, both from the same operational probability distribution. The reason why weak values of the momentum may escape from the eigenstate thermalization hypothesis is because they are linked only to off-diagonal elements of the density matrix in the energy representation. Therefore, we state that the origin of the arrow of time in isolated quantum systems described by the Schrödinger equation comes from dealing with properties obtained by averaging (tracing out) some degrees of freedom of the configuration space. We then argue that thermalization does not occur in the properties defined in the configuration space, and our argument is compatible with defending that thermalization is a real phenomenon in the properties defined in the physical space. All of these conclusions are testable in the laboratory through many-body weak values.

Non-Gaussian operations have been shown to enhance the fidelity of continuous-variable quantum teleportation. However, a disadvantage of these non-Gaussian operations is that they are probabilistic in nature. In this article we study the trade-off between teleportation fidelity and success probability for optimal performance of the teleportation protocol. To this end, we first derive a unified expression for the Wigner characteristic function describing three non-Gaussian states: photon-subtracted, photon-added, and photon-catalyzed two-mode squeezed vacuum states. We then utilize it to obtain the fidelity of teleportation for input coherent and squeezed vacuum states using the aforementioned non-Gaussian resource states. We optimize the product of the relative enhancement in fidelity and the probability of state preparation by tuning the transmissivity of the beam splitters involved in implementing non-Gaussian operations. This leads to a scenario that can be effectively implemented in a laboratory to enhance fidelity. It turns out that among all the non-Gaussian resource states considered, the symmetric one-photon-subtracted two-mode squeezed vacuum state is the most advantageous. We provide the associated optimal squeezing and beam splitter transmissivity values for the non-Gaussian resource states considered, which will be of significant interest to the experimental community. We also consider the effect of imperfect photon detectors on teleportation fidelity. Further, we expect the derived Wigner characteristic function to be useful in state characterization and other quantum information processing protocols.

Quantum particle statistics fundamentally controls the way particles interact and plays an essential role in determining the properties of the system at low temperature. Here we study how the quantum statistics affects the computational power of quantum annealing. We propose an annealing Hamiltonian describing quantum particles moving on a square lattice and compare the computational performances of the atomic quantum annealers between two statistically different components: spinless fermions and hard-core bosons. In addition, we take an Ising quantum annealer driven by traditional transverse-field quantum fluctuations as a baseline. The potential of our quantum annealers to solve combinatorial optimization problems is demonstrated on random 3-regular graph partitioning. We find that the bosonic quantum annealer outperforms the fermionic case. The superior performance of the bosonic quantum annealer is attributed to larger excitation gaps and the consequent smoother adiabatic transformation of its instantaneous quantum ground states. Along our annealing schedule, the bosonic quantum annealer is less affected by the glass order and explores the Hilbert space more efficiently. Our theoretical finding could shed light on constructing atomic quantum annealers using Rydberg atoms in optical lattices.

We study the performance (rate and fidelity) of distributing multipartite entangled states in a quantum network through the use of a central node. Specifically, we consider the scenario where the multipartite entangled state is first prepared locally at a central node and then transmitted to the end nodes of the network through quantum teleportation. As our first result, we present leading-order analytical expressions and lower bounds for both the rate and fidelity at which a specific class of multipartite entangled states, namely, Greenberger-Horne-Zeilinger (GHZ) states, are distributed. Our analytical expressions for the fidelity accurately account for time-dependent depolarizing noise encountered by individual quantum bits while stored in quantum memory, as verified using Monte Carlo simulations. As our second result, we compare the performance to the case where the central node is an entanglement switch and the GHZ state is created by the end nodes in a distributed fashion. Apart from these two results, we outline how the teleportation-based scheme could be physically implemented using trapped ions or nitrogen-vacancy centers in diamond.

We perform ab initio calculations of the QED effects of order $m{\alpha }^7$ for the $2{}^{3}S$ and $2{}^{3}P$ states of He-like ions. The computed effects are combined with previously calculated energies [V. A. Yerokhin and K. Pachucki, Phys. Rev. A 81, 022507 (2010)], thus improving the theoretical accuracy by an order of magnitude. The obtained theoretical values for the $2{}^{3}S-2{}^{3}P_{0,2}$ transition energies are in good agreement with available experimental results and with previous calculations performed to all orders in the nuclear binding strength parameter $Z\alpha$. For the ionization energies, however, we find some inconsistency between the $Z\alpha$-expansion and all-order calculations, which might be related to a similar discrepancy between the theoretical and experimental results for the ionization energies of helium [V. Patkóš et al., Phys. Rev. A 103, 042809 (2021)].

Photoelectron circular dichroism (PECD) of fenchone induced by short coherent broadband laser pulses is studied theoretically using the time-dependent single-center method. In particular, we investigate the chiral response to circularly polarized pulses with a sine-squared energy spectrum between 1 and 3 eV. We consider nonchirped and differently chirped pulses of a moderate peak intensity of $3\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2$ and a nonchirped pulse of a higher peak intensity of $1\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ leading to a strong-field ionization. Thereby, effects of the frequency ordering and pulse intensity are investigated in some detail. The present theoretical study predicts a substantial multiphoton PECD of fenchone, which can be manipulated by tailoring a spectral phase and intensity profile of a single coherent broadband laser pulse.

We show that a dissipative two-dimensional Raman lattice can be engineered in a two-component ultracold atomic gas, where the interplay of the two-dimensional spin-orbit coupling and light-induced atom loss gives rise to a density flow diagonal to the underlying square lattice. The flow is driven by the non-Hermitian corner skin effect, under which eigenstates localize toward one corner of the system. We illustrate that the topological edge states of the system can only be accounted for by the non-Bloch band theory where the deformation of the bulk eigenstates are explicitly considered. The directional flow can be detected through the dynamic evolution of an initially localized condensate in the lattice, or by introducing an immobile impurity species that interact spin selectively with a condensate in the ground state of the Raman lattice.

We report on the preparation of a quantum degenerate mixture of ${}^{23}\mathrm{Na}{}^{40}\mathrm{K}$ molecules and ${}^{40}\mathrm{K}$ atoms. A deeply degenerate atomic mixture of ${}^{23}\mathrm{Na}$ and ${}^{40}\mathrm{K}$ atoms with a large number ratio ($N_F/N_B\approx 6$) is prepared in a large-volume optical dipole trap. About $3.0\times {10}^4{}^{23}\mathrm{Na}{}^{40}\mathrm{K}$ ground-state molecules are created through magnetoassociation followed by stimulated Raman adiabatic passage. The pure molecular gas is in the moderately degenerate regime. In the atom-molecule mixture, the elastic collisions provide a thermalization mechanism for the molecules. The calculated thermalization time constant is smaller than the lifetime of the molecules in the atom-molecule mixture. This indicates that the molecules may reach thermal equilibrium due to elastic atom-molecule collisions before notable losses take place. In a few tens of milliseconds, the degeneracy of the molecules is maintained. This time interval is long enough for the study of strongly interacting atom-molecule mixtures and the production of ultracold triatomic molecular gases.

We show that a variety of nonequilibrium dynamics of interacting many-body systems are universally characterized by an elegant relation, which we call the dynamic virial theorem. The out-of-equilibrium dynamics of quantum correlations is entirely governed by Tan' s contact. It gives rise to a series of observable consequences and is closely related to experiments with ultracold atoms. In particular, we show that the dynamic virial theorem provides an experimentally accessible verification of the maximum energy growth theorem [R. Qi et al., Phys. Rev. Lett. 126, 240401 (2021)], which is encoded in the evolution of the atomic cloud size during expansion. In addition, the dynamic virial theorem leads to a simple thermodynamic relation of strongly interacting quantum gases in the framework of two-fluid hydrodynamic theory, which holds in a wide range of temperature. This thermodynamic relation is a type of out-of-equilibrium analog of Tan's pressure relation at equilibrium. Our results provide a fundamental understanding of the generic behaviors of interacting many-body systems at nonequilibrium and are readily examined in experiments with ultracold atoms.

Coherent mode-locking (CML) represents a method for ultrashort pulse generation in lasers based on the self-induced transparency (SIT) phenomena, which allows us to overcome the principal limitations on the pulse duration imposed by the active medium's gain bandwidth in standard passively mode-locked lasers. So far, this regime was only studied in lasers with transition dipole moments in the absorber medium being twice larger than in the gain medium. This fact ensures that the $2\pi$-SIT-soliton in the absorber also forms the stable $\pi$-soliton in the gain but can be barely realized in experiments. In this paper we demonstrate theoretically that the self-starting stable coherent mode-locking regime can also arise if the same medium is used both in the gain and absorber laser sections, i.e., the transition dipole moments are equal. These results are both obtained analytically using the area theorem and confirmed by the numerical solution of the Maxwell-Bloch equations. Our findings can enable new opportunities in the area of ultrafast lasers.

In this paper, we propose a sensing mechanism to enhance the sensitivity of a quantum system to nonlinearities by homodyning the amplitude quadrature of the cavity field. The system consists of two dissipatively coupled cavity modes, one of which is subject to single- and two-photon drives. In the regime of low two-photon driving strength, the spectrum of the system acquires a real spectral singularity. We find that this singularity is very sensitive to the two-photon drive and nonlinearity of the system, and compared to the previous nonlinearity sensor, the proposed sensor achieves an unprecedented sensitivity around the singularity point. Moreover, the scheme is robust against fabrication imperfections. This work would open a different avenue for quantum sensors, which could find applications in many fields, such as precise measurement and quantum metrology.

Giant atoms, quantum emitters that couple to light at multiple discrete points, are emerging as a new paradigm in quantum optics due to their many promising properties, such as decoherence-free interaction. While most previous work has considered giant atoms coupled to open continuous waveguides or a single giant atom coupled to a structured bath, here we study the interaction between two giant atoms mediated by a structured waveguide, e.g., a photonic crystal waveguide. This environment is characterized by a finite energy band and a band gap, which affect atomic dynamics beyond the Markovian regime. Here we show that, inside the band, decoherence-free interaction is possible for different atom-cavity detunings, but is degraded from the continuous-waveguide case by time delay and other non-Markovian effects. Outside the band, where atoms interact through the overlap of bound states, we find that giant atoms can interact more strongly and over longer distances than small atoms for some parameters, for instance, when restricting the maximum coupling strength achievable per coupling point. The results presented here may find applications in quantum simulation and quantum gate implementation.