We demonstrate that the spin state of solid-state emitters inside micropillar cavities can serve as measure qubits in syndrome measurements. The photons, acting as data qubits, interact with the spin state in the microcavity and the total state of the system evolves conditionally due to the resulting circular birefringence. By performing a quantum nondemolition measurement on the spin state the syndrome of the optical state can be obtained. Furthermore, due to the symmetry of the interaction, we can alternatively choose to employ the optical states as measure qubits. This protocol can be adapted to various resource requirements, including spectral discrepancies between the data qubits, by considering entangled measure qubits. Finally, we discuss how spin-systems with dissimilar characteristic energies can still be entangled with high levels of fidelity and tolerance to cavity losses in the strong coupling regime.

We theoretically propose and investigate a quantum Otto engine that works with a single-mode radiation field inside an optical cavity and is driven alternately by a hot reservoir and a cold reservoir. The hot reservoir is realized using a beam composed of thermally entangled pairs of two-level atoms that interacts resonantly with the cavity, and the cold reservoir is composed of a collection of noninteracting boson modes. In terms of the quantum discord of the pair of atoms, we derive analytical expressions for the performance parameters (i.e., power and efficiency) and the stability measure (the coefficient of variation of power). We show that nonclassical correlations boost the quantum engine's performance and efficiency, and they may even change the operation mode at specified values of the two bath temperatures. We also demonstrate that the nonclassical correlations improve the stability of the machine by reducing the coefficient of variation of power that satisfies the generalized thermodynamic uncertainty relation. Finally, we find that these results can be transferred to another quantum Otto engine model, in which the optical cavity is coupled alternately to a hot thermal bosonic bath and to a beam composed of pairs of the two correlated atoms that plays the role of a cold reservoir.

We study the electron-impact dissociative ionization of ${\mathrm{SF}}_6$ using a scattered electron-ion coincidence technique. The ion-yield spectra are obtained at an incident electron energy of 1.4 keV for scattering angles ranging from $2.2^{\circ }$ to $8.2^{\circ }$ to investigate the momentum transfer dependences of the fragment-ion yields and shape resonance features. It is found that the $4t_{1u}\to \varepsilon t_{2g}$ resonance is evident over a wide momentum transfer range, indicating its strong influence on the ${{\mathrm{SF}}_3}^{+}$ production by electron impact, while resonance bands in the ${{\mathrm{SF}}_5}^{+}$ and ${{\mathrm{SF}}_4}^{+}$ yield spectra rapidly diminish with increasing momentum transfer. In addition, the angular distribution of ${{\mathrm{SF}}_5}^{+}$ reveals the significant difference in stereodynamics between electron-impact-induced and photon-induced ionization of ${\mathrm{SF}}_6$. We also discuss the dissociation mechanisms of ${{\mathrm{SF}}_6}^{+}$ using the kinetic energy distributions of the fragment ions. Analysis of the data strongly suggests that many of the ${{\mathrm{SF}}_6}^{+}$ ions in the $D{}^{2}T_{2\mathrm{g}}$ state decay by internal conversion to a lower electronic state and dissociate to ${{\mathrm{SF}}_5}^{+}+\mathrm{F}$, followed by statistical emission of F or ${\mathrm{F}}_2$.

The far-field backscattering amplitude of an electric field from a relativistically moving sphere is analyzed. Contrary to prior research, we do so by expressing the fields in the helicity basis and we highlight here its advantages when compared to the commonly considered parity basis. With the purpose of exploring specific scattering phenomena considering relativistic effects, we identify conditions that minimize the backscattered field, leading to a relativistic formulation of the first Kerker condition. The requirements to be satisfied by the sphere are expressed in terms of Mie angles, which constitute an effective parametrization of any possible optical response a sphere might have. By considering different speeds of the sphere and angles of incidence, we are able to identify multiple combinations of Mie angles up to octupolar order via gradient-based optimization that satisfy our relativistic Kerker condition, that is, where the backscattered energy is at most 0.1% of the average scattered energy. Our results can be extended to involve multiple particles forming a metasurface, potentially having direct implications on the design of light sails as considered by the Breakthrough Starshot Initiative.

Network nonlocality allows one to demonstrate nonclassicality in networks with fixed joint measurements, that is, without random measurement settings. The simplest network in a loop, the triangle, with four outputs per party is especially intriguing. The “elegant distribution” [Gisin, Entropy 21, 325 (2019)] still resists analytic proofs, despite its many symmetries. In particular, this distribution is invariant under any output permutation. The Finner inequality, which holds for all local and quantum distributions, has been conjectured to be also valid for all no-signaling distributions with independent sources (NSI distributions). Here we provide evidence that this conjecture is false by constructing a four-output network box that violates the Finner inequality and prove that it satisfies all NSI inflations up to the enneagon. As a first step toward the proof of the nonlocality of the elegant distribution, we prove the nonlocality of the distributions that saturates the Finner inequality by using geometrical arguments.

Recently, it was shown that the qudits used in circuits with a convolutional form (e.g., matrix product states and the multiscale entanglement renormalization Ansatz) can be reset unitarily [Anikeeva et al., Phys. Rev. A 103, 042613 (2021)], even without measurement. We analyze the fidelity of this protocol for a family of quantum circuits that interpolates between such circuits and local quantum circuits, averaged over Haar-random gates. We establish a connection between this problem and counting directed paths on a graph, which is determined by the shape of the quantum circuit. This connection leads to an exact expression for the fidelity of the protocol for the entire family that interpolates between a convolutional circuit and a random quantum circuit. For convolutional circuits with a constant window size, the rate of convergence to unit fidelity is shown to be $\frac{q^2}{q^2+1}$, independent of the window size, where $q$ is the local qudit dimension. Since most applications of convolutional circuits use constant-size windows, our result suggests that the unitary reset protocol will likely work well in such a regime. We also derive two extra results in the convolutional limit, which may be of independent interest. First, we derive exact expressions for the correlations between reset qudits and show that they decays exponentially in the distance. Second, we derive an expression for the fidelity in the presence of noise, expressed in terms of the quantities that define the property of the channel, such as the entanglement fidelity.

Here, we show that partial transposition, which is initially introduced to study entanglement, can also inspire many results on quantum discord, including (I) a discord criterion of a spectrum invariant under partial transposition, stating that one state must contain discord if its spectrum is changed by the action of partial transposition, and (II) an approach to estimate the geometric quantum discord and the one-way deficit based on the change of the spectrum. To compare with entanglement theory, we also lower bound the geometric quantum entanglement and the entanglement of relative entropy. Thus, on one hand, we illustrate an approach to specify and estimate discord based on partial transposition. On the other hand, we show that, entanglement and discord, two basic notions of nonclassical correlations, can be placed on the same ground such that their interplay and distinction can be illustrated within a universal framework.

The relativistic energies and widths of hydrogenlike ions exposed to the uniform electric field are calculated. The calculations are performed for the ground and lowest excited states using the complex-scaling technique in combination with a finite-basis method. The obtained results are compared with the nonrelativistic values. The role of relativistic effects is investigated.

Two species of mutually interacting ultracold bosonic atoms are studied in a ring-shaped trap with a species-selective azimuthal lattice which may rotate. We examine the spectrum and the states in a collective spin formalism. The system can be modeled as a pair of coupled Lipkin-Meshkov-Glick Hamiltonians, and can be used to generate a high degree of entanglement. The Hamiltonian has two components: a linear part that can be controlled by manipulating the azimuthal lattice, and an interaction-dependent quadratic part. Exact solutions are found for the quadratic part for equal strengths of intraspecies and interspecies interactions. In different regimes the Hamiltonian can emulate a beam splitter or a two-mode squeezer of quantum optical systems. We study entanglement properties of the ground state of the Hamiltonian in dependence on various parameters with the prospect of possible quantum information and metrology applications.

We investigate the effect of the presence of a dielectric medium on the stimulated transition processes of a uniformly moving atom in a thermal radiation field. The cases of atomic subluminal and superluminal motions are considered. The calculated results indicate that the atomic stimulated transition rates depend crucially on the atomic velocity, the temperature of the thermal radiation, the refractive index of medium, and the atomic polarizability. The transition rates are expanded by series in some limiting cases, such as the extreme temperature of thermal radiation, the extreme motion of the atom, and the extremely large refractive index. Our analytical and numerical analyses show that the presence of the dielectric medium has profoundly changed the behaviors and characteristics of the atomic stimulated transition rates, especially when the atom moves faster than the speed of light in the medium. Moreover, the behavior of the transition rates for an atom polarized parallel to the direction of atomic motion is quite distinct from that of an atom polarized perpendicularly to the direction of atomic motion.

We consider the problem of quantum temporal imaging in the case where the time lens is implemented by a sum frequency generation nonlinear process, in particular when the device is operated in high conversion efficiency regimes. In the general case where the time lens also presents a finite aperture and a nonperfect phase matching, the relevant figures of merit, as for example the temporal resolution, do not have an explicit expression. As a consequence, the performances of the imaging scheme are difficult to assess. We show that this problem can be solved for conversion efficiencies up to 80% in terms of the eigenmodes of the imaging scheme, approximated by chirped Gauss-Hermite functions, and we show how its relevant figures of merit can be extracted from the modal description of the imaging scheme. As a consequence, we obtain criteria allowing one to design imaging schemes with high efficiencies.

Superlocality and superunsteerability provide operational characterization of quantum correlations in certain local and unsteerable states, respectively. Such quantum correlated states have a nonzero quantum discord. A two-way nonzero quantum discord is necessary for quantum correlations pointed out by superlocality. On the other hand, in this work, we demonstrate that a two-way nonzero quantum discord is not necessary to demonstrate superunsteerability. To this end, we demonstrate superunsteerability for one-way quantum discordant states. This in turn implies the existence of one-way superunsteerability and also the presence of superunsteerability without superlocality. Superunsteerability for nonzero quantum discord states implies the occurrence of steerability in a one-sided semi-device-independent way. Just like one-way steerability occurs for certain Bell-local states in a one-sided device-independent way, our result shows that one-way steerability can also occur for certain nonsuperlocal states but in a one-sided semi-device-independent way.

In various fields from quantum physics to biology, the open quantum dynamics of a system consisting of interacting subsystems emphasizes its fundamental functionality. The local approach, deriving a dissipator in a master equation by ignoring the intersubsystem interaction, has been widely used to describe the reduced dynamics due to its robustness to keep the positivity of a density operator. However, one critique is that a stationary state obtained by the approach in the limit of weak system-environment coupling is written in the form of the Gibbs state for the partial Hamiltonian by excluding the intersubsystem interaction from the total one of the relevant system. As an alternative, the global approach, deriving a dissipator with including the intersubsystem interaction, under the Born-Markov and secular approximations has attracted much attention, and there is debate concerning its violation of positivity in the short-time region and/or limited parameter region for the Bohr frequencies of the subsystems. In this paper, we present a formalism that leads to the time-convolutionless (time-local) master equation obtained by extending the global approach beyond the Born-Markov and secular approximations. We apply it to the excitation energy transfer between interacting sites in which only the terminal site weakly interacts with a bosonic environment of finite temperature in a manner beyond the rotating-wave approximation. We find that the formulation (1) gives the short-time behavior while preserving positivity, (2) shows the oscillatory features that the secular approximation would obscure, and (3) leads to a stationary state very near to the Gibbs state for the total Hamiltonian of the relevant system.

We report theoretical calculations of the diamagnetic susceptibility ${\chi }_0$ of helium atom. We determined the complete relativistic correction to ${\chi }_0$ of the order of ${\alpha }^4$, where $\alpha$ is the fine-structure constant, by including all ${\alpha }^4$ terms originating from the Dirac and Breit equations for a helium atom in a static magnetic field. Finite-nuclear-mass corrections to ${\chi }_0$ were also evaluated. To obtain very accurate results and reliable uncertainty estimates we used a sequence of explicitly correlated basis sets of fully optimized Slater geminals. We found that ${\chi }_0=-2.119070\left(34\right)\times {10}^{-5}{a}_0^3$ and ${\chi }_0=-2.119365\left(34\right)\times {10}^{-5}{a}_0^3$ for ${}^4\mathrm{He}$ and ${}^3\mathrm{He}$ isotopes, respectively, where $a_0$ is the Bohr radius and the uncertainties shown in the parentheses are due entirely to the very conservative estimate of the neglected QED corrections of the order of ${\alpha }^5$. Our results are compared with the available experimental data and with previous, incomplete theoretical determinations of the ${\alpha }^4$ contributions to the diamagnetic susceptibility of helium.

In this paper we study the emergence of two-electron wave packets following the two-photon ionization of helium with intense near-femtosecond pulses. Monitoring the dynamics of the doubly ionized portion of the joint radial and energy distributions during the pulse's lifetime, we observe and examine the effects of strong Coulombic interactions which take place between electrons with partial waves other than the dominant $pp$ channels. The generated doubly ionized wave packets correspond to configurations where the two electrons remain close to the core and roughly equidistant until the pulse's peak; it is at later times that the sequential or nonsequential nature of the double-ionization process leads to distinctly different patterns. Moreover, we also show the difference in the evolution into the corresponding final-state distributions as the pulse's central photon energy varies between 42 and 70 eV and, thus, as the ionization mechanism transforms from the so-called nonsequential to the sequential ionization regime, respectively.

We investigate the optical propagation of reorientational solitary waves in nonuniformly oriented planar samples of thermotropic liquid crystals in the nematic phase. A nonsymmetric distribution of the optic axis, across either the longitudinal or transverse coordinates, entails nematicon paths affected by the side of the cell where the excitation is launched, i.e., direction-sensitive trajectories. We analyze the effect with reference to realistic samples encompassing a linearly modulated orientation vs either length or width, presenting nonlinear models and the outcome of numerical experiments. We briefly discuss the resulting nonspecular transmission in terms of optical nonreciprocity and isolation.

We investigate models of nonlinear qubit evolution based on mappings to an $n$-qubit central-spin model (CSM) in the large-$n$ limit, where mean-field theory is exact. Extending a theorem of Erdös and Schlein [J. Stat. Phys. 134, 859 (2009)], we establish that the CSM is rigorously dual to a nonlinear qubit when $n\to \infty$. The duality supports a type of nonlinear quantum computation in systems, such as a condensate, where a large number of ancillas couple symmetrically to a central qubit. It also enables a gate-model implementation of nonlinear quantum simulation with a rigorous error bound. Two variants of the model, with and without coupling between ancillas, map to effective models with different nonlinearity and symmetry. Without coupling the CSM simulates initial-condition nonlinearity, where the Hamiltonian is a linear combination of $\mathrm{tr}\left({\rho }_0{\sigma }^x\right){\sigma }^x, \mathrm{tr}\left({\rho }_0{\sigma }^y\right){\sigma }^y$, and $\mathrm{tr}\left({\rho }_0{\sigma }^z\right){\sigma }^z$, where ${\sigma }^x, {\sigma }^y$, and ${\sigma }^z$ are Pauli matrices and ${\rho }_0$ is the initial density matrix. With symmetric ancillas coupling it simulates linear combinations of $\mathrm{tr}\left(\rho {\sigma }^x\right){\sigma }^x, \mathrm{tr}\left(\rho {\sigma }^y\right){\sigma }^y$, and $\mathrm{tr}\left(\rho {\sigma }^z\right){\sigma }^z$, where $\rho$ is the current state. This case can simulate qubit torsion, which has been shown by Abrams and Lloyd [Phys. Rev. Lett. 81, 3992 (1998)] to enable an exponential speedup for state discrimination in an idealized setting. The duality discussed here might also be interesting from a quantum foundation perspective. There has long been interest in whether quantum mechanics might possess some type of small unobserved nonlinearity. If not, what is the principle prohibiting it? The duality implies that there is not a sharp distinction between universes evolving according to linear and nonlinear quantum mechanics: A one-qubit universe prepared in a pure state $|\phi \rangle$ at the time of the big bang and symmetrically coupled to ancillas prepared in the same state would appear to evolve nonlinearly for any finite time $t>0$ as long as there are exponentially many ancillas $n\gg \exp \left[O\right(t\left)\right]$.

We introduce a general framework, based on collision models and discrete completely positive maps, to describe on an equal footing coherent and measurement-based feedback control of quantum mechanical systems. We apply our framework to prominent tasks in quantum control, ranging from cooling to Hamiltonian control. Unlike other proposed comparisons, where coherent feedback always proves superior, we find that either measurements or coherent manipulations of the controller can be advantageous depending on the task at hand. Measurement-based feedback is typically superior in cooling, while coherent feedback is better at assisting quantum operations. Furthermore, we show that both coherent and measurement-based feedback loops allow one to simulate arbitrary Hamiltonian evolutions, and discuss their respective effectiveness in this regard.