We address the propagation of light beams in longitudinally modulated $\mathcal{P}\mathcal{T}$-symmetric lattices, built as arrays of couplers with periodically varying separation between their channels, and show a number of possibilities for efficient diffraction control available in such nonconservative structures. The dynamics of light in such lattices crucially depends on the ratio of the switching length for the straight segments of each coupler and the longitudinal lattice period. Depending on the longitudinal period, one can achieve either beam rectification when the input light propagates at a fixed angle across the structure without diffractive broadening or dynamic localization when the initial intensity distribution is periodically restored after each longitudinal period. Importantly, the transition between these two different propagation regimes can be achieved by tuning only gain and losses acting in the system, provided that the $\mathcal{P}\mathcal{T}$ symmetry remains unbroken. The impact of Kerr nonlinearity is also discussed.

An orthonormal basis consisting of unentangled (pure tensor) elements in a tensor product of Hilbert spaces is an unentangled orthonormal basis (UOB). In general, for $n$ qubits, we prove that in its natural structure as a real variety, the space of UOB is a bouquet of products of Riemann spheres parametrized by a class of edge colorings of hypercubes. Its irreducible components of maximum dimension are products of $2^n-1$ two spheres. Using a theorem of Walgate and Hardy, we observe that the UOB whose elements are distinguishable by local operations and classical communication (called locally distinguishable or LOCC distinguishable UOB) are exactly those in the maximum dimensional components. Bennett et al. [Phys. Rev. A 59, 1070 (1999).], in their in-depth study of quantum nonlocality without entanglement, include a specific three-qubit example UOB which is not LOCC distinguishable; we construct certain generalized counterparts of this UOB in $n$ qubits.

Single-photon sources (SPSs) are a fundamental building block for optical implementations of quantum information protocols. Among SPSs, multiple crystal heralded single-photon sources seem to give the best compromise between high pair production rate and low multiple photon events. In this work, we study their performance in a practical quantum-key-distribution experiment, by evaluating the achievable key rates. The analysis focuses on the two different schemes, symmetric and asymmetric, proposed for the practical implementation of heralded single-photon sources, with attention on the performance of their composing elements. The analysis is based on the protocol proposed by Bennett and Brassard in 1984 and on its improvement exploiting decoy state technique. Finally, a simple way of exploiting the postselection mechanism for a passive, one decoy state scheme is evaluated.

A single photon encoded in both the spin and the orbital angular momentum has recently been experimentally demonstrated [X.-L. Wang et al., Nature 518, 516 (2015)] with linear optics using the hyperentangled state, which can be viewed as a bipartite four-dimensional (ququart) entanglement. Here, we investigate this process from a general point of view. By exploring a controlled phase flip induced by atomic ensembles in one-side optical microcavities, we propose teleportations of general ququart systems including a two-atomic-ensemble system, a two-polarized-photon system, one photon with the polarization and spatial degrees of freedom (DOFs), and a hybrid photon-ensemble system using two hyperentangled photons. The output information may also be encoded by different physical systems up to the special requirements of a receiver. These schemes are also adapted to teleportation of a ququart system with only phases or real probability amplitudes, which is beyond previous superdense teleportation [Nature Commun. 6, 7185 (2015)]. With these restrictions, half of the classical communication cost may be saved and experimental complexities are also reduced. Our theoretical schemes are feasible in modern physics and show the possibilities of transferring complex quantum systems for scalable quantum applications.

Quantum cellular automata (QCA) constitute space and time homogeneous discrete models for quantum field theories (QFTs). Although QFTs are defined without reference to particles, computations are done in terms of Feynman diagrams, which are explicitly interpreted in terms of interacting particles. Similarly, the easiest QCA to construct are quantum lattice gas automata (QLGA). A natural question then is, which QCA are not QLGA? Here we construct a nontrivial example of such a QCA; it provides a simple model in $1+1$ dimensions with no particle interpretation at the scale where the QCA dynamics are homogeneous.

We investigate the quantification of coherence in the infinite-dimensional systems, and especially, we focus on the infinite-dimensional bosonic systems in the Fock space. We find that given the average energy constraints, the relative entropy of coherence serves as a well-defined quantification of coherence in the infinite-dimensional systems, however, the $l_1$ norm of coherence fails. Via using the relative entropy of coherence as the quantification of coherence, we generalize the case to multimode Fock spaces, and some special examples are considered. It is shown that with a finite average particle number, increasing the number of modes of light can enhance the relative entropy of coherence. With the mean energy constraint, our results can also be extended to other infinite-dimensional systems.

We present systematic and large-scale calculations for the fine-structure energy splitting and transition rate between the $3d^9{}^{2}D_{3/2,5/2}$ levels of Co-like ions with $28\le Z\le 100$. Two different fully relativistic approaches are used, based on the multiconfiguration Dirac-Hartree-Fock (MCDHF) theory and the relativistic many-body-perturbation theory (RMBPT). Especially the former gives results of similar accuracy as experiments for a large range of ions. Our calculations are therefore accurate enough to probe Breit and quantum-electro-dynamic effects. To obtain spectroscopic accuracy, we show that it is important to include deep core-valence correlation, down to and including the $n=2$ shell. We estimate that the uncertainties of our wavelengths are within the uncertainty of experiments, i.e., 0.02%. We also show that the frequently used flexible atomic code has an inaccurate treatment of the self-energy (SE) contribution and of the $M1$-transition properties for lower-$Z$ ions. After correcting for the SE calculation, the resulting RMBPT transition energies are in good agreement with the MCDHF ones, especially for the high-$Z$ end of the Co-like sequence.

Normalized experimental differential and integral cross sections for elastic and vibrationally inelastic scattering of low-energy electrons from ethylene $\left({\mathrm{C}}_2{\mathrm{H}}_4\right)$ have been measured over a large number of incident electron energies and angles. The differential cross sections are measured at incident energies from 0.5 to 100 eV and scattering angles from $5^{\circ }$ to ${130}^{\circ }$. These measurements are made to monitor the role of the ${}^{2}B_{2\mathrm{g}}\left(\approx 1.8\mathrm{eV}\right)$ and the higher ${}^{2}B_{2\mathrm{u}}+{}^{2}B_{1\mathrm{u}}+{}^{2}A_{\mathrm{g}}\left(\approx 7.5\mathrm{eV}\right)$ resonances in the scattering dynamics. Our differential cross section measurements are in very good to excellent agreement with past measurements, and in reasonable agreement with theory as regards forward scattering. A feature in the elastic cross section at ${90}^{\circ }$ scattering angle at $\approx 3.5\mathrm{eV}$ is tentatively associated with the onset of excitation of the $\sim a{}^{3}B_{1\mathrm{u}}$ triplet electronic state. Differential cross sections for vibrational excitation of four composite energy features in ethylene are also presented from incident energies of 1.25–15 eV. These results are compared to previous measurements with satisfactory results regarding resonant behavior of these features also concerning the role of the ${}^{2}B_{2\mathrm{g}}\left(\approx 1.8\mathrm{eV}\right)$ and the higher ${}^{2}B_{2\mathrm{u}}+{}^{2}B_{1\mathrm{u}}+{}^{2}A_{\mathrm{g}}\left(\approx 7.5\mathrm{eV}\right)$ resonances in the scattering dynamics.

Coherent optical bichromatic forces have been shown to be effective tools for rapidly slowing and cooling simple atomic systems. While previous estimates suggest that these forces may also be effective for rapidly decelerating molecules or complex atoms, a quantitative treatment for multilevel systems has been lacking. We describe detailed numerical modeling of bichromatic forces by direct numerical solution for the time-dependent density matrix in the rotating-wave approximation. We describe both the general phenomenology of an arbitrary few-level system and the specific requirements for slowing and cooling on a many-level transition in calcium monofluoride (CaF), one of the molecules of greatest current experimental interest. We show that it should be possible to decelerate a cryogenic buffer-gas-cooled beam of CaF nearly to rest without a repumping laser and within a longitudinal distance of about 1 cm. We also compare a full 16-level simulation for the CaF $B\leftrightarrow X$ system with a simplified numerical model and with a semiquantitative estimate based on two-level systems. The simplified model performs nearly as well as the complete version, whereas the two-level model is useful for making order-of-magnitude estimates, but nothing more.

We investigate the prospects of creating broad rotational wave packets by means of molecular interaction with long sequences of intense femtosecond pulses. Using state-resolved rotational Raman spectroscopy of oxygen, subject to a sequence of more than 20 laser pulses with peak intensities exceeding ${10}^{13}\mathrm{W}/{\mathrm{cm}}^2$ per pulse, we show that the centrifugal distortion is the main obstacle on the way to reaching high rotational states. We demonstrate that the timing of the pulses can be optimized to partially mitigate the centrifugal limit. The cumulative effect of a long pulse sequence results in a high degree of rotational coherence, which is shown to cause an efficient spectral broadening of probe light via cascaded Raman transitions.

We present fully nonlinear dissipative fluid dynamics simulations of a strongly interacting trapped two-dimensional Fermi gas using a lattice Boltzmann algorithm. We are able to simulate nonharmonic trapping potentials, temperature-dependent viscosities, as well as a discretized version of the ballistic (noninteracting) behavior. Our approach lends itself to direct comparison with experimental data, opening up the possibility of a precision determination of transport coefficients in the strongly interacting Fermi gas. Furthermore, we predict the presence of a strongly damped (“nonhydrodynamic”) component in the quadrupole mode, which should be observable experimentally.

We present numerical analysis of steady states in a two-component (spinor) driven-dissipative quantum fluid formed by condensed exciton polaritons in an annular optically induced trap. We demonstrate that an incoherent ring-shaped optical pump creating the exciton-polariton confinement supports the existence of stationary and rotating azimuthon steady states with azimuthally modulated density associated with Josephson vortices. Such states can be imprinted by coherent light pulses with a defined orbital angular momentum, as well as generated spontaneously in the presence of thermal noise.

Conditional preparation is a well-established technique for quantum state engineering of light. A general trend is to increase the number of heralding detection events in such a realization to reach larger photon-number states or their arbitrary superpositions. In contrast to pulsed implementations, where detections only occur within the pulse window, for continuous-wave light the temporal separation of the conditioning detections is an additional degree of freedom and a critical parameter. Based on a theoretical study by Nielsen and Mølmer [A. E. B. Nielsen and K. Mølmer, Phys. Rev. A 75, 043801 (2007)] and on a continuous-wave two-mode squeezed vacuum from a nondegenerate optical parametric oscillator, we experimentally investigate the generation of two-photon state with tunable delay between the heralding events. The present work illustrates the temporal multimode features in play for conditional state generation based on continuous-wave light sources and quantifies the compromise between preparation rate and fidelity in this scenario.

Entanglement drives nearly all proposed quantum information technologies. By up-converting part of a 1550 nm two-mode squeezed vacuum state to 532 nm, we demonstrate the generation of strong continuous-variable entanglement between widely separated frequencies. Nonclassical correlations were observed in joint quadrature measurements of the 1550 and 532 nm fields, showing a maximum noise suppression 5.5 dB below vacuum. Our versatile technique combines strong nonclassical correlations, large bandwidth, and in principle, the ability to entangle the telecommunication wavelength of 1550 nm with any optical wavelength.

We develop a method of modified hyper-Ramsey spectroscopy in optical clocks, achieving complete immunity to the frequency shifts induced by the probing fields themselves. Using particular pulse sequences with tailored phases, frequencies, and durations, we can derive an error signal centered exactly at the unperturbed atomic resonance with a steep discriminant which is robust against variations in the probe shift. We experimentally investigate the scheme using the magnetically induced ${}^1{S}_0\text{−}{}^3{P}_0$ transition in ${}^{88}\mathrm{Sr}$, demonstrating automatic suppression of a sizable $2\times {10}^{-13}$ probe Stark shift to below $1\times {10}^{-16}$ even with very large errors in shift compensation.

We calculate the mean-field phase diagram of a zero-temperature, binary Bose mixture on a square optical lattice, where one species possesses a non-negligible dipole moment. Remarkably, this system exhibits supersolidity for anomalously weak dipolar interaction strengths, which are readily accessible with current experimental capabilities. The supersolid phases are robust, in that they occupy large regions in the parameter space. Further, we identify a first-order quantum phase transition between supersolid and superfluid phases. Our results demonstrate the rich features of the dipolar Bose mixture, and suggest that this system is well suited for exploring supersolidity in the experimental setting.

The Kondo effect in condensed-matter systems manifests itself most sharply in their transport measurements. Here we propose an analogous transport signature of the orbital Kondo effect realized with ultracold atoms. Our system consists of imbalanced Fermi seas of two components of fermions and an impurity atom of different species which is confined by an isotropic potential. We first apply a $\pi /2$ pulse to transform two components of fermions into two superposition states. Their interactions with the impurity atom then cause a “transport” of fermions from majority to minority superposition states, whose numbers can be measured after applying another $3\pi /2$ pulse. In particular, when the interaction of one component of fermions with the impurity atom is tuned close to a confinement-induced $p$-wave or higher partial-wave resonance, the resulting conductance is shown to exhibit the Kondo signature, i.e., universal logarithmic growth by lowering the temperature. The proposed transport measurement will thus provide a clear evidence of the orbital Kondo effect accessible in ultracold atom experiments and pave the way for developing new insights into Kondo physics.

Mixed states of a quantum system, represented by density operators, can be decomposed as a statistical mixture of pure states in a number of ways where each decomposition can be viewed as a different preparation recipe. However the fact that the density matrix contains full information about the ensemble makes it impossible to estimate the preparation basis for the quantum system. Here we present a measurement scheme to (seemingly) improve the performance of unsharp measurements. We argue that in some situations this scheme is capable of providing statistics from a single copy of the quantum system, thus making it possible to perform state tomography from a single copy. One of the by-products of the scheme is a way to distinguish between different preparation methods used to prepare the state of the quantum system. However, our numerical simulations disagree with our intuitive predictions. We show that a counterintuitive property of a biased classical random walk is responsible for the proposed mechanism not working.