Stimulated Raman scattering on $\mathrm{\Lambda }$ atoms is a promising tool for generating optical photons from a microwave excitation in the ground-state hyperfine manifold. We consider an atomic Bose-Einstein condensate coupled to a microwave field that scatters photons into the guided modes of a nearby optical fiber. Due to momentum transfer to the condensate, stimulated photon scattering can occur outside of the phase-matched direction, which can be used to separate the converted photons from the strong Raman readout pulse. Conversely, in the phase-matched direction, superradiant scattering due to bosonic enhancement leads to an increased collection efficiency in the guided modes of the fiber, for which we determine optimal conditions.

The reduced density matrix ${\rho }_r(x,x^{\prime })$ of the transverse biphoton state beyond the near zone is found to be real on and only on diagonals in the plane of the photon's transverse coordinates $(x,x^{\prime })$. This remarkable feature is found to occur at any distances of photon propagation with diffraction spreading completely taken into account. The functions of the reduced density matrix at two orthogonal diagonals, ${\rho }_r(x,x)$ and ${\rho }_r(x,-x)$, are considered as the only two complementary single-particle distributions generated by the reduced density matrix. The ratio of widths of these two distributions is interpreted as the new entanglement parameter $R_{\mathrm{diag}}$ which is found to be very close to and completely compatible with the Schmidt entanglement parameter $K$.

Digital quantum simulation is a promising application of quantum computers where quantum dynamics is simulated by using quantum gate operations. Many techniques for decomposing a time-evolution operator of quantum dynamics into simulatable quantum gate operations have been proposed, whereas these methods cause some errors. To evaluate these errors, we derive a lower bound for overlap between true dynamics and digital simulated dynamics at the final time. Our result enables us to guarantee how obtained digital simulated dynamics is close to unknown true dynamics. We also extend our formalism to error evaluation of digital quantum simulation on noisy quantum computers.

We quantitatively analyze and evaluate the control power in a high-dimensional noisy quantum channel for transmitting classical information. We calculate the control power of the controlled dense coding scheme in a four-dimensional extended Greenberger-Horne-Zeilinger (GHZ) class state channel. Different from controlled teleportation, there is no tradeoff between the control power and the classical capacity of the optimal four-dimensional GHZ state quantum channel. Only when the channel is noisy and the sender is allowed to gain some control authority can the tradeoff between the control power and the classical capacity be activated, and the overall control power for transmitting classical information will be enhanced by reducing the classical capacity of the high-dimensional quantum channel. This noise induced characteristic is very different from that of transmitting quantum information, and it may provide new ideas and methods to develop the resource theory of quantum channels.

Topological quantum interference emerges from the interplay between quantum mechanics and topology. We present evidence for two types of such interference phenomenon that can result from the quantum dynamics of initial topological states. We realize both types of topological quantum interference in a pumped non-Hermitian Su-Schrieffer-Heeger lattice that can be implemented by creation and coherent control of excitonic states of trapped neutral atoms. On quenching the system from the topological to the gapless phases and then back again, we find that interference patterns develop in the gapless phase and also after switching back to the topological phase. These patterns occur both as many-excitation interferences generated in the presence of pumping the atoms at the end sites, and as one- and two-excitation interferences seen in the absence of pumping when starting with edge excitations. Investigation of the excitation dynamics shows that these interference patterns originate from the topological nature of the initial states and are very different from quantum interferences originating from nontopological states of the lattice. Our results also reveal that unlike well-known situations where topological states are protected against local perturbations, in the non-Hermitian SSH systems resulting from driving the excited-state populations, a local dissipation at each lattice site can suppress both the topological interference and the total population of the lattice.

We establish a relationship between the notion of universal quantum gates and the notion of unitary $t$-designs. We show that a set of qudit gates $\mathcal{S}\subset U\left(d\right)$ is universal if and only if $\mathcal{S}$ forms a $\delta$-approximate $t\left(d\right)$-design, where $\delta <1, t\left(2\right)=6$, and $t\left(d\right)=4$ for $d\ge 3$. Moreover, we argue that from the application point of view sets $\mathcal{S}$ with the $\delta$ close to 1 should be regarded as nonuniversal. We also provide a second, more algebraic, criterion for the universality verification. It says that $\mathcal{S}\subset U\left(d\right)$ is universal if and only if the matrices that commute with $\{U^{\otimes t\left(d\right)}\otimes {\overline{U}}^{\otimes t\left(d\right)}|U\in \mathcal{S}\}$ commute also with $\{U^{\otimes t\left(d\right)}\otimes {\overline{U}}^{\otimes t\left(d\right)}|U\in U\left(d\right)\}$, where $t\left(2\right)=3$, and $t\left(d\right)=2$ for $d\ge 3$. Finally, we show that the complexity of checking this algebraic criterion scales polynomially with the dimension $d$.

The strong-field many-electron dynamics is studied via the resonance-enhanced high-order harmonic generation (HHG) of niobium. In intense IR lights excited states of Nb with the configuration $4d^{3}5snp$ play a vital role in the enhanced HHG below the ionization potential, even though the electron initially occupying the $5s$ orbital is the most active. The energies of these states increase linearly with the laser intensity, which causes the effective ionization potential to increase as well. Above the ionization potential, many-electron effects may cause an electronic state of Nb to be in resonance with that of ${\mathrm{Nb}}^{+}$, which leads to enhanced HHG and which influences the tunneling ionization as well.

In our recent letter [D. B. Milošević, Opt. Lett. 47, 1669 (2022)] we proposed to apply a terahertz pulse to modify the ionization process induced by a strong midinfrared laser pulse. If the vector potentials of the laser and THz fields are comparable, the yield and the maximum energy of the photoelectrons emitted in the strong-field ionization process can be significantly increased. The time delay $\mathrm{\Delta }t$ between the THz pulse and the laser pulse is used as a control parameter. In the present work we further analyze this process, with particular emphasis on high-order above-threshold ionization (HATI). Using our improved strong-field approximation we calculate the ionization amplitude by numerical integration over the electron's ionization and travel times. We apply the quantum-orbit theory and the modified saddle-point method to provide a physical explanation. In addition to the previously considered ionization of Cs atoms by a 3100-nm midinfrared laser, we now show that qualitatively similar results can be obtained for HATI of Ar atoms by a much stronger 800-nm laser pulse. Furthermore, for HATI of Ar by a 3100-nm laser, assisted by a THz pulse of a few-MV/cm strength, we show that the photoelectron energy can be in the keV region.

We studied the fragmentation of planar formic acid (HCOOH) molecules following their double ionization by intense ultrashort laser pulses. Deuterium tagging (i.e., HCOOD) combined with coincidence momentum imaging measurements of all fragment ions enabled determination of the role of the hydroxyl and carboxyl hydrogen atoms in the breakup. Specifically, we observe a strong preference for the hydroxyl (OD) group to remain intact in a ${\mathrm{HCOOD}}^{2+}\to {\mathrm{OD}}^{+}+{\mathrm{HCO}}^{+}$ fragmentation, which is an order of magnitude more likely than ${\mathrm{OH}}^{+}+{\mathrm{DCO}}^{+}$. An even larger preference for breaking the H-C bond over the O-H bond is observed in the ${\mathrm{H}}^{+}+{\mathrm{DCO}}_2^{+}$ and ${\mathrm{D}}^{+}+{\mathrm{HCO}}_2^{+}$ deprotonation channels. Bond rearrangement, leading to ${\mathrm{H}}_2^{+}$ or ${\mathrm{H}}_2{\mathrm{O}}^{+}$ formation, exhibits no isotopic preference. The kinetic-energy-release distributions of the ${\mathrm{OH}}^{+}+{\mathrm{DCO}}^{+}$ and ${\mathrm{O}}^{+}+{\mathrm{H}}_2{\mathrm{CO}}^{+}$ breakup channels suggest that more than one process contributes to these final products, although further theoretical work is needed to identify the specific paths.

In the vicinity of a narrow Feshbach resonance Efimov features are expected to be characterized by the resonance's properties rather than the van der Waals length of the interatomic potential. Although this theoretical prediction is well established by now, it still lacks experimental confirmation. Here, we apply our recently developed three-channel model [Y. Yudkin and L. Khaykovich, Efimov scenario for overlapping narrow Feshbach resonances, Phys. Rev. A 103, 063303 (2021)] to the experimental result obtained in a mass-imbalanced ${}^6\mathrm{Li}\text{−}{}^{133}\mathrm{Cs}$ mixture in the vicinity of the narrowest resonance explored to date [J. Johansen, B. J. DeSalvo, K. Patel, and C. Chin, Testing universality of Efimov physics across broad and narrow Feshbach resonances, Nat. Phys. 13, 731 (2017)]. Our analysis suggests that the observed position of the Efimov resonance is dictated mainly by the resonance physics while the influence of the van der Waals tail of the interatomic potential is minor. We show that the resonance position is strongly influenced by the presence of another Feshbach resonance which significantly alters the effective background scattering length at the narrow resonance position.

Polarization singularities are ubiquitous in most of the vector beams and can be divided into different morphologies as characterized by the polarization singularity index. A precise identification of different polarization topologies may provide abundant polarization morphology for singular optics and increase the information-carrying capacity with a vector beam. However, discrimination of the fractional polarization singularity indices is still an untapped study area. Here, based on deep learning of the Stokes polarimetry, an efficient method is proposed to accurately identify the fractional $C$-point indices in polarization singularity for the vector light fields. Various vector light fields carrying different fractional $C$-point indices are prepared by a polarization-controlled Sagnac interferometer with an embedded spatial light modulator. Then, by analyzing the measured Stokes parameters with a training set of the residual convolutional neural network, the identification of a fractional polarization singularity index with an accuracy up to 0.01 is achieved. The method applies to all $C$-point degenerate beams and may hold promise for increasing the information channel capacity with vector light.

We present a design of a non-Hermitian photonic Chern insulator whose band topology can be tuned via gain and loss. It is found that, by continuously varying the gain and loss parameter, a topological phase transition can take place in the photonic crystal, accompanied by the closing and reopening of a Dirac cone. We demonstrate this phenomenon through numerical simulations of the bulk and the edge dispersions, and steady field distributions excited by a point source. Our results show the possibility of non-Hermitian control over band topology in a Chern insulator setting and may find applications in active topological photonic devices.

Quantum transport in a class of nonlinear extensions of the Rudner-Levitov model is numerically studied in this paper. We show that the quantization of the mean displacement, which embodies the quantum coherence and the topological characteristics of the model, is markedly modified by nonlinearities. Peculiar effects such as a “trivial-nontrivial” transition and unidirectional long-range quantum transport are observed. These phenomena can be understood on the basis of the dynamic behavior of the effective hopping terms, which are time and position dependent, containing contributions of both the linear and nonlinear couplings.

Graph states are a central resource in measurement-based quantum information processing. In the photonic qubit architecture based on Gottesman-Kitaev-Preskill (GKP) encoding, the generation of high-fidelity graph states composed of realistic, finite-energy approximate GKP-encoded qubits thus constitutes a key task. We consider the finite-energy approximation of GKP-qubit states given by a coherent superposition of shifted finite-squeezed vacuum states, where the displacements are Gaussian distributed. We present an exact description of graph states composed of such approximate GKP qubits as a coherent superposition of a Gaussian ensemble of randomly displaced ideal GKP-qubit graph states. Using standard Gaussian dynamics, we track the transformation of the covariance matrix and the mean-displacement vector elements of the Gaussian distribution of the ensemble under tools such as GKP-Steane error-correction and fusion operations that can be used to grow large high-fidelity GKP-qubit graph states. The covariance matrix elements capture the noise in the graph state due to the finite-energy approximation of GKP qubits, while the mean displacements relate to the possible absolute shift errors on the individual qubits arising conditionally from the homodyne measurements that are a part of these tools. Our work thus pins down an exact coherent error model for graph states generated from truly finite-energy GKP qubits, which can shed light on their error-correction properties.

The critical nuclear charge $Z_c$ of the two-electron atoms below which the ground state transforms into a shape resonance and the critical stability of the system around $Z_c$ have been well established. However, the behavior of the shape resonance below $Z_c$ is still a mystery. By employing the complex-scaling method using Hylleraas configuration-interaction basis functions, we trace the trajectory of the shape resonance from $Z_c$ down to a very small nuclear charge. It is shown that at specific values of $Z$ far below $Z_c$ the resonance crosses over higher-lying one-electron thresholds, and when $Z$ is decreased below 0.316, the shape resonance lies above the three-body breakup threshold. We finally show that the imaginary part of the resonance energy at small nuclear charges can be modeled by the dispersion relation with a high-order Padé approximant correction.

We probe the time-dependent ionization dynamics of impulsively excited rotational wave packets of ${\mathrm{N}}_2, {\mathrm{CO}}_2$, and ${\mathrm{C}}_2{\mathrm{H}}_4$ using broadband ultraviolet pulses centered at 262 nm. Photoelectron momentum distributions recorded by velocity-map imaging show a strong dependence on alignment, on multiphoton order, and on the electronic and vibrational states of the cation. We show that substantial information about the molecular-frame photoelectron angular distribution can be obtained from the high-order laboratory-frame asymmetry parameters without any prior knowledge of the photoionization process. We also compare few-photon ionization with one-photon ionization and strong-field ionization.

In this paper. we study the existence, stability, and dynamics of select topological points and line defects in antiferromagnetic, polar phase, $F=1$ ${}^{23}\mathrm{Na}$ spinor condensates. Specifically, we leverage fixed-point and numerical continuation techniques in three spatial dimensions to identify solution families of monopole and Alice rings as the chemical potential (number of atoms) and trapping strengths are varied within intervals of realizable experimental parameters. We are able to follow the monopole from the linear limit of small atom number all the way to the Thomas-Fermi regime of large atom number. Additionally, and importantly, our studies reveal the existence of two Alice ring solution branches, corresponding to, relatively, smaller and larger ring radii, that bifurcate from each other in a saddle-center bifurcation as the chemical potential is varied. We find that the monopole solution is always dynamically unstable in the regimes considered. In contrast, we find that the larger Alice ring is indeed stable close to the bifurcation point until it destabilizes from an oscillatory instability bubble for a larger value of the chemical potential. We also report on the possibility of dramatically reducing, yet not completely eliminating, the instability rates for the smaller Alice ring by varying the trapping strengths. The dynamical evolution of the different unstable waveforms is also probed via direct numerical simulations.

We explore the connections between dissipative quantum phase transitions and non-Hermitian random matrix theory. For this, we work in the framework of the dissipative Dicke model which is archetypal of symmetry-breaking phase transitions in open quantum systems. We establish that the Liouvillian describing the quantum dynamics exhibits distinct spectral features of integrable and chaotic character on the two sides of the critical point. We follow the distribution of the spacings of the complex Liouvillian eigenvalues across the critical point. In the normal and superradiant phases, the distributions are two-dimensional Poisson and that of the Ginibre unitary random matrix ensemble, respectively. Our results are corroborated by computing a recently introduced complex-plane generalization of the consecutive level-spacing ratio distribution. Our approach can be readily adapted for classifying the nature of quantum dynamics across dissipative critical points in other open quantum systems.

Variational quantum algorithms (VQAs) are promising methods to demonstrate quantum advantage on near-term devices as the required resources are divided between a quantum simulator and a classical optimizer. As such, designing a VQA which is resource-efficient and robust against noise is a key factor to achieve a potential advantage with the existing noisy quantum simulators. It turns out that a fixed VQA circuit design, such as the widely used hardware-efficient ansatz, is not necessarily robust against imperfections. In this work, we propose a genome-length-adjustable evolutionary algorithm to design a robust VQA circuit that is optimized over variations of both circuit ansatz and gate parameters, without any prior assumptions on circuit structure or depth. Remarkably, our method not only generates a noise-effect-minimized circuit with shallow depth, but also accelerates the classical optimization by substantially reducing the number of parameters. In this regard, the optimized circuit is far more resource-efficient with respect to both quantum and classical resources. As applications, based on two typical error models in VQA, we apply our method to calculate the ground energy of the hydrogen and the water molecules as well as the Heisenberg model. Simulations suggest that, compared with conventional hardware-efficient ansatz, our circuit-structure-tunable method can generate circuits apparently more robust against both coherent and incoherent noise and hence is more likely to be implemented on near-term devices.

The variational quantum eigensolver (VQE), which has attracted attention as a promising application of noisy intermediate-scale quantum devices, finds a ground state of a given Hamiltonian by variationally optimizing the parameters of quantum circuits called Ansätze. Since the difficulty of the optimization depends on the complexity of the problem Hamiltonian and the structure of the Ansätze, it has been difficult to systematically analyze the performance of optimizers for the VQE. To resolve this problem, we propose a technique to construct a benchmark problem whose ground state is guaranteed to be achievable with a given Ansatz by using the idea of a parent Hamiltonian of a low-depth parametrized quantum circuit. We compare the convergence of several optimizers by varying the distance of the initial parameters from the solution and find that the converged energies showed a thresholdlike behavior depending on the distance. This work provides a systematic way to analyze optimizers for the VQE and contribute to the design of the Ansatz and its initial parameters.