We demonstrate the feasibility of implementing a photon source with sub-Poissonian emission statistics through temporal multiplexing of a continuous wave heralded photon source in the optical communications wavelength range. We use the time arrival information of a heralding photon to actively modify the delay of the heralded photon in an all-fiber assembly, in order to synchronize the output with respect to an external clock. Within this synchronized operating regime we show that the addition of a single temporal correcting stage can improve the figure of merit for single-photon emission of a heralded photon source. We obtain a brightness improvement factor of approximately 1.8 and an enhancement of the signal-to-noise ratio, quantified by the coincidence-to-accidental counts ratio. These results clear the way for integrated optics nonclassical photon sources in the optical communication band.

The geometric quantum discord of a two-qudit state has been studied in many papers; however, its exact analytical value in the explicit form is known only for a general two-qubit state, a general qubit-qudit state, and some special families of two-qudit states. Based on the general Bloch vectors formalism [E. R. Loubenets et al., J. Phys. A: Math. Theor. 54, 195301 (2021)], we find the explicit exact analytical value of the geometric quantum discord for an arbitrary two-qudit state of any dimension via the parameters of its correlation matrix and the Bloch vectors of its reduced states. This general analytical result includes all the known exact results on the geometric quantum discord only as particular cases and proves rigorously that the lower bound on the geometric discord presented in [S. Rana et al., Phys. Rev. A 85, 024102 (2012)] constitutes its exact value for each two-qudit state. Moreover, our general result allows us to find for an arbitrary two-qudit state, pure or mixed, the upper and lower bounds on its geometric quantum discord, expressed via the Hilbert space characteristics of this state.

State preparation is a fundamental routine in quantum computation, for which many algorithms have been proposed. Among them, perhaps the simplest one is the Grover-Rudolph algorithm. In this paper we analyze the performance of this algorithm when the state to prepare is sparse. We show that the gate complexity is linear in the number of nonzero amplitudes in the state and quadratic in the number of qubits. We then introduce a simple modification of the algorithm, which makes the dependence on the number of qubits also linear. This is competitive with the best known algorithms for sparse state preparation.

As one kind of ubiquitous collision system, ${\mathrm{O}}^{6+}+\mathrm{He}$ deserves attentive study due to its theoretical significance in fundamental physics and applied value in fields like astrophysics or plasma physics. Specifically, in the case of the electron capture process, while a considerable number of measurements and calculations have focused on single electron capture, research is still inadequate on the double electron capture which contributes nearly 10% to the overall electron capture. In this work, a two-active-electron semiclassical asymptotic-state close-coupling method is used to calculate the total and $l-\mathrm{resolved}$ state-selective double electron capture cross sections of ${\mathrm{O}}^{6+}+\mathrm{He}$ collisions in the energy range 0.5–100 keV/u, accompanied by experimental measurements in the energy range 2.63–37.5 keV/u with an uncertainty of 16%, in good consistency. These theoretical and experimental data can fill gaps in the database of double electron capture in ${\mathrm{O}}^{6+}+\mathrm{He}$ collisions and provide insights for improving theoretical models in further research.

We have measured transition frequencies, isotope shifts, and hyperfine-structure splittings in the $3d^2{(}^{3}F)4s{}^4{F}_J\to 3d^2{(}^{3}F)4p{}^4{G}_{J+1}^o$ transitions in stable ${\mathrm{Ti}}^{+}$ ions for $J=\frac{3}{2},\frac{5}{2},\frac{7}{2}$ using collinear laser spectroscopy. Ions were generated by laser ablation in a buffer-gas atmosphere and extracted into vacuum through a nozzle and a pair of radio-frequency funnels. The results obtained are of interest as reference values for on-line measurements of short-lived titanium isotopes and for astrophysical searches for temporal or spatial variations of the fine-structure constant $\alpha$ using quasar absorption spectra.

The motion of atoms through a magnetic field (due to a change in reference frame) will result in an additional electric field felt by the electrons in that atom. The motional Stark effect is a well-established effect that has been approximately included in calculations of spectra of magnetized plasmas, usually through diagonalizing the Hamiltonian. The motional Stark effect for continuum states is poorly defined due to an overlap integral that results in a Dirac delta function. This paper presents a workaround by evaluating the motional Stark effect in the Green's function within the scattering formalism. We report on some results pertaining to bound-free spectra for white dwarf and neutron star magnetic field strengths. In most cases, resonances in the continuum are shifted and broadened. This behavior has the effect of raising the Rosseland mean opacity in white dwarfs and neutron star atmospheres.

Noise-induced synchronizations have led to diverse phenomena and applications. Here, we propose and demonstrate an approach towards inducing synchronizations between non-overlapping laser beams by intrinsic spin noise. Two linearly polarized and non-overlapping laser beams propagate through a vapor of flying unpolarized alkali atoms and capture their correlated noise properties by the atom-light interaction, resulting in synchronization between the two laser beams. Using correlation spectra, we demonstrate the nearly in phase synchronization at the atomic Larmor frequency between the two laser beams, which can easily be varied across a wide frequency range by the magnetic-field strength. Moreover, we show that the degree of synchronization increases with the atomic density, and the synchronization remains effective when the Larmor frequencies in the regions of the two laser beams are different. Our method may be useful for applications like secure key distribution and secure communication.

Wave-function effects in uncorrelated systems are characterized by the Berry curvature and quantum metric. Beyond those, we propose gauge-independent tensors describing Bloch wave-function effects on local interaction between correlated particles. We derive an effective hydrodynamic theory for ultracold bosons in optical lattices. Ground states and collective modes of superfluids in isotropic harmonic traps are solved for highly symmetric lattices. In a dynamic process, the wave-function effects are featured by the eigenfrequency, amplitude, and phase shift of an excited breathing mode and can be observed in experiments. We also give a tight-binding model of a bipartite square lattice with nontrivial wave-function effects, where results are estimated with typical experimental parameters. Our discovery advances the connections between the modern band theory and quantum many-body physics.

We study the decoherence of a system of $N$ noninteracting heavy particles (atoms) due to coherent scattering with a background gas. We introduce a framework for computing the induced phase shift and loss of contrast for arbitrary preparations of $N$-particle quantum states. We find phase shifts that are inherently $(N\ge 2)$-body effects and may be searched for in future experiments. We analyze simple setups, including a two-mode approximation of an interferometer. We study fully entangled $N00N$ states, which resemble the correlated positions in a matter interferometer, as well as totally uncorrelated product states that are representative of a typical state in an atom interferometer. We find that the extent to which coherent enhancements increase the rate of decoherence depends on the observable of interest, state preparation, and details of the experimental design. In the context of future ultralow-recoil (e.g., light dark matter) searches with atom interferometers we conclude that (i) there exists a coherently enhanced scattering phase which can be searched for using standard (i.e., contrast/visibility and phase) interferometer observables; (ii) although decoherence rates of one-body observables are not coherently enhanced, a coherently enhanced loss of contrast can still arise from dephasing; and (iii) higher statistical moments (which are immediately accessible in a counting experiment) are coherently enhanced and may offer a new tool with which to probe the soft scattering of otherwise undetectable particles in the laboratory.

In systems with periodic potential fields, building relatively local Wannier functions can significantly simplify the Hamiltonian and enhance our understanding of the system's ground state and dynamic properties. In this work, we improve the current method of building the Wannier functions of ultracold atomic systems, including the case in the presence or absence of interactions. In noninteracting systems, we propose a method to directly obtain the real-valued maximally localized Wannier functions (MLWFs) by using real-valued eigenstates, and verify the effectiveness of this method in a two-dimensional (2D) degenerate system. In interacting systems, we obtain the effect of high-energy bands on the lowest-energy band by using the accurate calculation results of the two-particle system. In the two-particle system, we consider the effect of the entanglement between the particles and obtain the optimal two-particle Wannier functions. These Wannier functions are then further utilized to obtain the parameters of the extended Bose-Hubbard model. The effectiveness of the method is verified by taking a one-dimensional (1D) system with contact interaction as an example. In the three-particle and four-particle systems, compared calculation results with the original system and the unmodified two-band Bose-Hubbard model, we find that the effective Hamiltonian is more accurate than the unmodified two-band model. This verifies the effectiveness of our method, and the parameters obtained can reflect the original system well, which provides an effective method for accurate modeling of interacting systems.

We examine the features of electromagnetic bulk and surface modes in a one-dimensional photonic crystal made up of lossless gyroelectric and uniaxial layers. We find a configuration that supports the propagation of a type of surface mode that can have either positive or negative group velocity, depending on the signs of the uniaxial permittivities. We also show how the introduction of gyrotropy qualitatively alters the photonic band structure, making it possible to modify the allowed frequency ranges within the structure just by varying the value of an external magnetic field. Exploiting an Otto configuration, we provide the analysis of a finite system where near-zero reflectivity values correspond to large Goos-Hänchen shifts. We also explore low-symmetry configurations where waves exhibit nonreciprocal behaviors.

We study the geometric phase (GP) of a two-level atom coupled to an environment composed of free space and a dielectric nanosphere in thermal and out of thermal equilibrium. We analytically and numerically analyze the optical properties and loss of the dielectric medium, along with the nonequilibrium effects of the environment on the GP. In the weak coupling limit, we find that the correction to the GP depends on the partial local density of photonic states at the atom position, and an effective parameter that emerges out of the nonequilibrium configuration of the system. The GP exhibits a significant enhancement due to the excitation of evanescent surface waves at its resonance frequency. It is shown that the GP acquired by the atomic system out of thermal equilibrium is always bounded between the thermal-equilibrium counterparts. Furthermore, the temperature difference between the nanosphere and free space can play an important role in the GP only at moderate atomic distances from the nanosphere. Our results elegantly demonstrate properties of the GP near material media that can support phononic modes and pave the way for further research of GP as a resource for quantum computation.

We provide a framework for understanding dynamical metastability in open many-body systems of free bosons, whereby the dynamical stability properties of the system in the infinite-size (thermodynamic) limit may sharply differ from those of any finite-size truncation, and anomalous transient dynamics may arise. By leveraging pseudospectral techniques, we trace the discrepancy between asymptotic and transient dynamics to the non-normality of the underlying quadratic bosonic Lindbladian (QBL) generator and show that two distinct flavors of dynamical metastability can arise. QBLs exhibiting type I dynamical metastability, previously discussed in the context of anomalous transient amplification [Phys. Rev. Lett. 127, 245701 (2021)], are dynamically unstable in the infinite-size limit yet stable once open boundaries are imposed. In contrast, type II dynamically metastable QBLs, which we uncover in this work, are dynamically stable for infinite size but become unstable under open boundary conditions for arbitrary finite system size. We exhibit representative models for both types of metastability in the dissipative, as well as the closed-system (Hamiltonian) settings, and analyze distinctive behavior they can engender. We show that dynamical metastability manifests itself in the generation of entanglement entropy by way of a transient which reflects the stability phase of the infinite (rather than the actual finite) system and, as a result, is directly tied to the emergence of supervolume entanglement scaling in type I systems. Finally, we demonstrate how, even in Hermitian, and especially in highly non-normal regimes, the spectral properties of an infinite-size QBL are reflected in the linear response functions of the corresponding finite QBLs by way of resonant pseudospectral modes.

The quantum rotor represents, after the harmonic oscillator, the next obvious quantum system to study the complementary pair of variables: the angular momentum and the unitary shift operator in angular momentum. Proper quantification of uncertainties and the incompatibility of these two operators are thus essential for applications of rotorlike quantum systems. While angular momentum uncertainty is characterized by variance, several uncertainty measures have been proposed for the shift operator, with dispersion the simplest example. We establish a hierarchy of those measures and corresponding uncertainty relations which are all perfectly or almost perfectly saturated by a tomographically complete set of von Mises states. Building on the interpretation of dispersion as the moment of inertia of the unit ring we then show that the other measures also possess the same mechanical interpretation. This unifying perspective allows us to express all measures as a particular instance of a single generic angular uncertainty measure. The importance of these measures is then highlighted by applying the simplest two of them to derive optimal simultaneous measurements of the angular momentum and the shift operator. Finally, we argue that the model of quantum rotor extends beyond its mechanical meaning with promising applications in the fields of singular optics, superconductive circuits with a Josephson junction, or optimal pulse shaping in the time-frequency domain. Our findings lay the groundwork for quantum-information and metrological applications of the quantum rotor and point to its interdisciplinary nature.

Diagrammatic techniques simplify a weakly interacting many-body problem into an effective few-quasiparticle problem within a system of interest (SOI). If scattering events, mediated by a bath, between those quasiparticles can be approximated as density-density interactions, the bath behaves like an effective external potential. On the other hand, exchange interactions could entangle those quasiparticles and the bath, leading to an open quantum system that induces quantum decoherence and spectral broadening. We investigate the renormalized interaction between the SOI and the bath, employing a projection operator technique similar to the one used in the Nakajima-Zwanzig method. We find that the frequency variation of this renormalized interaction is analogous to the quasiparticle residue and provides a measure of the SOI-bath separability that serves as the lower bound of the SOI-bath entanglement entropy. In the weak-coupling regime and continuum limit, we demonstrate that the degree of SOI-bath separability corresponds to the quasiparticle spectral weight in the single-impurity Anderson model and find that the loss of quantum information to the continuum of the bath can be understood as a decay process where an initial single-impurity state escapes to a thermal bath. This work provides a direction for connecting energy dissipation in quasiparticles propagation to the loss of quantum information in open quantum systems.

We consider the isotropic Heisenberg model in a magnetic field in the strong-rung-coupling limit on a two-dimensional (2D) rectangular zig-zag lattice of arbitrary size, and determine the one-dimensional (1D) effective model representing the low-energy manifold of the 2D model up to second order in perturbation theory. We consider a number of Hermitian operators defined on the Hilbert space of the 2D model, and systematically work out their action on the low-energy manifold, which are operators on the Hilbert space of the 1D effective model. For a class of operators among them, we demonstrate that the expectation values computed in the low-energy manifold of the 2D model can be mimicked by the expectation values of the corresponding operators in the 1D effective model even beyond the perturbation regime of the system parameters. We further argue that quantitatively estimating partial trace-based measures of entanglement in the 2D model may be done in the same fashion only in the perturbation regime. Our results and approach are expected to be useful in investigating observables and entanglement in the 2D models with large system sizes due to the advantage of using the effective 1D model with a smaller Hilbert space as a proxy.

Graph states play an important role in quantum information theory through their connection to measurement-based computing and error correction. Prior work revealed elegant connections between the graph structure of these states and their multipartite entanglement. We continue this line of investigation by identifying additional entanglement properties for certain types of graph states. From the perspective of tensor theory, we tighten both upper and lower bounds on the tensor rank of odd ring states ($|R_{2n+1}\rangle$) to read $2^n+1\le \mathrm{rank}\left(|R_{2n+1}\rangle \right)\le 3\times 2^{n-1}$. Next we show that several multipartite extensions of bipartite entanglement measures are dichotomous for graph states based on the connectivity of the corresponding graph. Finally, we give a simple graph rule for computing the $n$-tangle ${\tau }_n$.

In this work, an improved classical trajectory Monte Carlo method is introduced to describe collisions of multiply charged ions with ${\mathrm{H}}_2$ molecules by merging two hydrogenic three-body models, conceived to improve the H($1s$) electronic radial distribution, into the five-body treatment of Wood and Olson. Present results are contrasted against recently reported laboratory data and the multichannel Landau-Zener method for ${\mathrm{Ne}}^{9+}$ and ${\mathrm{O}}^{6+}$ projectiles at intermediate to low impact energies. A reasonable agreement with the data is obtained at the $n$-state selective level when considering a hydrogenic model that relies on an expansion over different nuclear charges. Complementary results for ${\mathrm{He}}^{2+}$ and ${\mathrm{Fe}}^{26+}$ projectiles suggest that discrepancies among these models accentuate as the projectile charge is increased and the collision energy is lowered, and highlight the need for further joint experimental and theoretical studies.

In the photonic spin Hall effect (SHE), also known as the transverse shift, incident light photons with opposite spins are spatially separated in the transverse direction due to the spin-orbit interaction of light. Here, we propose a gain-assisted model to control the SHE in the reflected probe light. In this model, a probe light is incident on a cavity containing a three-level dilute gaseous atomic medium, where the interaction between the atom and the control field follows two-photon Raman transitions. We show that the direction of photonic spin accumulations can be switched between positive and negative values across the Brewster angle in both the anomalous and normal dispersion regimes. For the same magnitude of control fields, the peak value of the photonic SHE is higher in the anomalous dispersion region compared to the normal dispersion regime. Additionally, the angular range around the Brewster angle is wider in the normal dispersion regime than in the anomalous dispersion region. Furthermore, the peak value of the photonic SHE and the angular range can be controlled by changing the Rabi frequencies of the control fields and the probe field detuning. The measurement of photonic SHE based on gain assistance may enable spin-related applications such as optical sensing.