Hyper-hybrid entanglement for two indistinguishable bosons has been recently proposed by Li et al. [Y. Li, M. Gessner, W. Li, and A. Smerzi, Phys. Rev. Lett. 120, 050404 (2018)]. In the current paper, we show that this entanglement exists for two indistinguishable fermions also. Next, we establish two no-go results: no hyper-hybrid entanglement for two distinguishable particles, and no unit fidelity quantum teleportation using indistinguishable particles. If either of these is possible, then the no-signaling principle would be violated. While several earlier works have attempted extending many results on distinguishable particles to indistinguishable ones, and vice versa, the above two no-go results establish a nontrivial separation between the two domains. Finally, we propose an efficient entanglement swapping using only two indistinguishable particles, whereas a minimum number of either three distinguishable or four indistinguishable particles is necessary for existing protocols.

We achieve fast, nondestructive quantum-state readout via fluorescence detection of a single ${}^{87}\mathrm{Rb}$ atom in the $5S_{1/2}$ $(F=2)$ ground state held in an optical dipole trap. The atom is driven by linearly polarized readout laser beams, making the scheme insensitive to the distribution of atomic population in magnetic sublevels. We demonstrate a readout fidelity of $97.6\pm 0.2\%$ in a readout time of $160\pm 20\mu \mathrm{s}$ with the atom retained in $>97\%$ of the trials, representing an advancement over other magnetic-state-insensitive techniques. We demonstrate that the $F=2$ state is partially protected from optical pumping by the distribution of the dipole matrix elements for the various transitions and the ac-Stark shifts from the optical trap. Our results are likely to find application in neutral-atom quantum computing and simulation.

We investigated experimentally and theoretically angular momentum alignment-to-orientation conversion created by the joint interaction of laser radiation and an external magnetic field with atomic rubidium at room temperature. In particular, we were interested in alignment-to-orientation conversion in the atomic ground state. In the experiment the laser frequency was fixed to the hyperfine transitions of the $D_1$ line of rubidium. To simulate the measured signals we used a theoretical model that takes into account all neighboring hyperfine levels, the mixing of magnetic sublevels in an external magnetic field, the coherence properties of the exciting laser radiation, and the Doppler effect. The experiments were carried out by exciting the atoms with linearly polarized laser radiation. Two orthogonal circularly polarized laser induced fluorescence (LIF) components were detected and afterward their difference was taken. The combined LIF signals originating from the hyperfine magnetic sublevel transitions of ${}^{85}\mathrm{Rb}$ and ${}^{87}\mathrm{Rb}$ rubidium isotopes were included. The alignment-to-orientation conversion can be incontrovertibly identified in the difference signals for various laser frequencies, and a change in signal shapes can be observed when the laser power density is increased. We studied the formation of and the underlying physical processes behind the observed signals of the LIF components and their difference by performing an analysis of the influence of incoherent and coherent effects. We performed simulations of theoretical signals that showed the influence of ground-state coherent effects on the LIF difference signal.

We study the ionization dynamics of oriented ${\mathrm{HeH}}^{+}$ in strong linearly polarized laser fields by numerically solving the time-dependent Schrödinger equation. The calculated photoelectron momentum distributions for parallel orientation show a striking asymmetric structure. With a developed model pertinent to polar molecules, we trace the electron motion in real time. We show that this asymmetric structure arises from the interplay of the Coulomb effect and the permanent dipole in strong laser fields. This structure can be used to probe the degree of orientation which is important in ultrafast experiments for polar molecules. We also check our results for other polar molecules such as CO and BF.

We discuss the decay of a two-level system into an engineered reservoir of coupled harmonic oscillators in the single-excitation manifold and propose its optical simulation with a homogeneous chain of coupled waveguides where individual elements couple to an external waveguide. We use two approaches to study the decay of the optical analog for the probability amplitude of the two-level system being in the excited state. A Born approximation allows us to provide analytic closed-form amplitudes valid for small propagation distances. A Fourier-Laplace approach allows us to estimate an effective decay rate valid for long propagation distances. In general, our two analytic approximations match our numerical simulations using coupled mode theory and show non-Markovian decay into the engineered reservoir. In particular, we focus on two examples that provide enhancement or suppression of the decay rate using flat-top or Gaussian coupling distributions.

Exploring enhanced nonlinear optics based on electromagnetically induced transparency (EIT) has always been one of the striking issues in cavity quantum electrodynamics. However, it is not clear whether the validity of EIT in enhancing nonlinear processes is universal. In this paper we demonstrate an efficient frequency conversion scheme via Autler-Townes splitting, instead of the so-called EIT mechanism, in a three-level quantum system. Because the selective system can be used to implement the conventional three-level EIT phenomenon, we present a direct evidence for the breakdown of nonlinear processes assisted by EIT. This strongly suggests that previously physical insight into the application of the standard EIT technique in nonlinear optics is not sufficient enough.

Radial-basis-function (RBF) networks are third-layered neural networks that are widely used in function approximation and data classification. Here we propose a quantum model for RBF networks. As in the classical case, we use radial basis functions as activation functions; quantum linear algebraic techniques and coherent states can be applied to implement these functions. We define the state of the weight as a tensor product of single-qubit states. This gives a simple approach to implement the quantum RBF network in quantum circuits. In this model we prove that training is almost quadratically faster than in the classical case. We demonstrate numerically that the quantum RBF network can solve binary classification problems as effectively as the classical RBF network, while the time and number of iterations used for training are much smaller.

We report a direct measurement of the $7s{}^2{S}_{1/2}\to 7p{}^2{P}_{3/2}$ electric dipole transition frequency in ${}^{226}\mathrm{Ra}^{+}$. With a single laser-cooled radium ion we determine the transition frequency to be $785722.11\left(3\right)$ GHz by directly driving the transition with frequency-doubled light and measuring the frequency of the undoubled light with an iodine reference. This measurement addresses a discrepancy of five combined standard deviations between previously reported values.

The static and dynamic electric-dipole polarizabilities and tune-out wavelengths for the ground state of Cs atoms are calculated by using a semiempirical relativistic configuration interaction plus core polarization approach. By considering the hyperfine splittings, the static and dynamic polarizabilities, hyperfine Stark shifts, and tune-out wavelengths of the hyperfine components of the ground level of ${}^{133}\mathrm{Cs}$ atoms are determined. It is found that the hyperfine shifts of the first primary tune-out wavelengths are about $-0.0135$ and 0.0106 nm, which are very close to the hyperfine interaction energies. Additionally, the contribution of the tensor polarizability to the tune-out wavelengths is determined to be about ${10}^{-5}$–${10}^{-6}$ nm.

Based on the multiconfiguration Dirac-Hartree-Fock method, the accurate line strengths, transition rates, branching ratio, branching fraction, and lifetime of the $2p^{3}3s{}^{5}S_2^{\circ }$ metastable state of neutral oxygen atom (O i), together with the uncertainties evaluated from the residual electron correlation effects, are determined by taking into account the effects of core-core, core-valence, and valence correlations, the Breit interaction, and QED effect. It is shown that the core correlation effect has a certain influence if the separation between ${}^{3}S^{\circ }$ and ${}^{5}S^{\circ }$ in the $p^{3}s$ configuration is accurately calculated. It is found that the calculations of branching ratio of ${}^{5}S_2^{\circ }$ remain stable in different electron correlation models. We infer that it is a peculiar intrinsic property leading to results useful for plasma diagnostics and other applications. The transition rates from ${}^{5}S_2^{\circ }$ change considerably as a result of the Breit interaction, because the line strengths of ${}^{3}P_2$–${}^{5}S_2^{\circ }$ ($\lambda 135.560$ nm) and ${}^{3}P_1$–${}^{5}S_2^{\circ }$ ($\lambda 135.851$ nm) are different in sensitivity to this effect. This affects the calculations of the branching ratio and lifetime. There exist large discrepancies among several calculations of lifetimes, such as the MCHF calculation by [C. Froese Fischer and G. Tachiev, At. Data Nucl. Data Tables 87, 1 (2004)]. The main reasons are ascribed to the neglected electron correlation and relativistic effects. In addition, by checking different calculations with various sets of multireference configurations, we find that the calculations of the electric dipole matrix elements for ${}^{5}S_2^{\circ }$ in the length gauge are much more sensitive to electron correlation effects than in the velocity gauge. This may also lead to some errors in the calculations of the lifetime of ${}^{5}S_2^{\circ }$ in the length gauge.

The electron-capture process was studied for ${\mathrm{Xe}}^{54+}$ colliding with ${\mathrm{H}}_2$ molecules at the internal gas target of the Experimental Storage Ring (ESR) at GSI, Darmstadt. Cross-section values for electron capture into excited projectile states were deduced from the observed emission cross section of Lyman radiation, being emitted by the hydrogenlike ions subsequent to the capture of a target electron. The ion beam energy range was varied between 5.5 and 30.9 MeV/u by applying the deceleration mode of the ESR. Thus, electron-capture data were recorded at the intermediate and, in particular, the low-collision-energy regime, well below the beam energy necessary to produce bare xenon ions. The obtained data are found to be in reasonable qualitative agreement with theoretical approaches, while a commonly applied empirical formula significantly overestimates the experimental findings.

We theoretically analyze the behavior of single optical bistability (OB), double OB, and tristability (OT) in a three-level V-type atomic system confined in a unidirectional optical ring cavity. The physics behind the emergence of various bistable and tristable states, and switching from the absorptive to the dispersive characteristics, are explored. The role of probe- and control-field detunings on the input-output characteristics is probed. Further, it is found that the switching between various bistable phenomena is defined by the ratio between the absolute peak values of the parameters describing dispersion and absorption, respectively. This ratio sets a criterion for obtaining absorptive OB, OT, double OB, and dispersive OB. The role of control-field intensity and detuning as well as the cooperation parameter on the input-output characteristics are explored. Finally, a scheme is discussed to control the width of the middle branch of double OB and OT as well as single OB.

The transfer of entangled, quantum correlated, and flying photons from a squeezed field to single-mode cavities is investigated. It is shown that while the transfer of photons begins immediately after the input squeezed field is turned on, the time at which quantum correlations start to be transferred to the cavities is strongly dependent on the initial population of the cavities. For the initially empty cavities, the transfer of quantum correlations begins immediately after the squeezed field is turned on but is delayed by a certain time interval when the cavities are initially populated. We find that the transfer of the quantum correlations is postponed until the one-photon states of the system are almost completely depopulated. In other words, the system “waits” for the population of the single-photon states to decay out before starting to build up the quantum correlation between the cavities. The delay time interval is independent of the number of photons initially present in the system but is dependent on the decay rates of the cavities and can be varied (controlled) when the cavities decay with different rates. It is shown that the delayed transfer of the quantum correlation is directly related to the presence of quantum jumps which transfer the population from the entangled to incoherent mixture states.

In this paper, we generalize the concept of strong quantum nonlocality from two aspects. First, in a tripartite quantum system, we present a construction of strongly nonlocal quantum states containing $6{(d-1)}^2$ orthogonal product states in ${\mathbb{C}}^d\otimes {\mathbb{C}}^d\otimes {\mathbb{C}}^d$ and build $6d^2-8d+4$ product states in ${\mathbb{C}}^d\otimes {\mathbb{C}}^d\otimes {\mathbb{C}}^{d+1}$, which have been proved to be strongly nonlocal. Obviously, both results turn out to be one order of magnitude smaller than the number of basis states $d^3$ for $d\ge 3$. Second, we give explicit forms of strongly nonlocal orthogonal product basis in ${\mathbb{C}}^3\otimes {\mathbb{C}}^3\otimes {\mathbb{C}}^3\otimes {\mathbb{C}}^3$ and ${\mathbb{C}}^4\otimes {\mathbb{C}}^4\otimes {\mathbb{C}}^4\otimes {\mathbb{C}}^4$ quantum systems, where four is the largest known number of subsystems in which there exists strong quantum nonlocality without entanglement up to now. All the results positively answer the open problems raised by Halder et al. [Phys. Rev. Lett. 122, 040403 (2019)]; that is, there do exist a small number of quantum states that can demonstrate strong quantum nonlocality without entanglement.

In this paper, we study quantum computing with twists. Twists are yet another form of defects in a lattice. They arise from dislocations in the lattice and can be used to encode and process quantum information. Surface codes with twists are also called dislocation codes. Hastings and Geller showed that dislocation codes could provide gains in space-time complexity of quantum computation. In this paper, we undertake a detailed study of generalized dislocation codes. We develop the theory of qubit dislocation codes over arbitrary four-valent and bicolorable lattices. We give a construction to introduce twists in such lattices and also study the structure of logical operators. We then study dislocation codes over odd prime dimensions in square lattices. Using the theory developed, we present protocols for implementing a universal gate set in qubit dislocation codes. We also show how to implement the generalized Clifford group in qudit dislocation codes in odd prime dimensions.

Waveguides allow for direct coupling of emitters separated by large distances, offering a path to connect remote quantum systems. However, when facing the distances needed for practical applications, retardation effects due to the finite speed of light are often overlooked. Previous works studied the non-Markovian dynamics of emitters with retardation, but the properties of the radiated field remain mostly unexplored. By considering a toy model of two distant two-level atoms coupled through a waveguide, we study the spectrum of the radiated field that exhibits non-Markovian features such as linewidth broadening beyond standard superradiance, or narrow Fano-resonance-like peaks. We also observe modifications to the energy-exchange dynamics in the presence of retardation. We discuss a proof-of-concept implementation of our results in a superconducting circuit platform.

Within the framework of statistical learning theory it is possible to bound the minimum number of samples required by a learner to reach a target accuracy. We show that if the bound on the accuracy is taken into account, quantum machine learning algorithms for supervised learning—for which statistical guarantees are available—cannot achieve polylogarithmic runtimes in the input dimension. We conclude that, when no further assumptions on the problem are made, quantum machine learning algorithms for supervised learning can have at most polynomial speedups over efficient classical algorithms, even in cases where quantum access to the data is naturally available.

Manipulating dynamical evolution is an important task in quantum information. The sudden death phenomenon (SDP), which is predicted to be unattainable beyond entanglement [T. Yu and J. H. Eberly, Sudden death of entanglement, Science 323, 598 (2009)], would be an especially surprising feature in quantum coherence. In this paper, we modulate the spatial modes of photons and build a spatially inclined channel using a series of specially made quartz plate pairs. Within this channel, sudden changes in coherence occur in open-quantum systems. We, then, describe the SDP within a general theoretical framework of sudden quantum-coherence changes and by defining the $n\mathrm{th}$-order sudden coherence change. Moreover, by adding non-Markovian noise, we find that coherence can rebirth after a sudden death. We further compare the sudden change features in coherence and quantum entanglement. We find that both coherence and entanglement perform identical dynamical processes upon Bell diagonal states, although their dynamical characteristics change when the initial state is partially entangled. Our results provide insights into the sudden change phenomena of quantum correlations beyond entanglement along with intriguing perspectives on quantum coherence dynamics.

We present a thorough investigation of problems that can be solved exactly with the level-1 quantum approximate optimization algorithm (QAOA). To this end, we implicitly define a class of problem Hamiltonians that are employed as phase separators in a level-1 QAOA circuit and provide unit overlap with a target subspace spanned by a set of computational basis states. For one-dimensional target subspaces, we identify instances within the implicitly defined class of Hamiltonians for which quantum annealing (QA) and simulated annealing (SA) have an exponentially small probability of finding the solution. Consequently, our results define a demarcation line between QAOA on one hand and QA and SA on the other, and highlight the fundamental differences between an interference-based search heuristic such as QAOA and heuristics that are based on thermal and quantum fluctuations like SA and QA respectively. Moreover, for two-dimensional solution subspaces, we are able to show that the depth of the QAOA circuit grows linearly with the Hamming distance between the two target states. We further show that there are no genuine solutions for target subspaces of dimension higher than 2 and smaller than $2^n$. We also transfer these results to instantaneous quantum polynomial (IQP) circuits.