We show that the Einstein-Laub formulation of electrodynamics is invalid since it yields a stress-energy-momentum (SEM) tensor that is not frame invariant. Two leading hypotheses for the kinetic formulation of electrodynamics (Chu and Einstein-Laub) are studied by use of the relativistic principle of virtual power, mathematical modeling, Lagrangian methods, and SEM transformations. The relativistic principle of virtual power is used to demonstrate the field dynamics associated with energy relations within a relativistic framework. Lorentz transformations of the respective SEM tensors demonstrate the relativistic frameworks for each studied formulation. Mathematical modeling of stationary and moving media is used to illustrate the differences and discrepancies of specific proposed kinetic formulations, where energy relations and conservation theorems are employed. Lagrangian methods are utilized to derive the field kinetic Maxwell's equations, which are studied with respect to SEM tensor transforms. Within each analysis, the Einstein-Laub formulation violates special relativity, which invalidates the Einstein-Laub SEM tensor.

We study the effect of mode coupling on a single-photon device in which a dipole quantum emitter (DQE) is embedded in a bimodal whispering-gallery-mode cavity. A scatterer is used to induce mode coupling between counterclockwise and clockwise propagating light fields, which interact with the DQE. We find that the antibunching increases when an additional cavity field is coupled to the DQE interacting with a cavity field. In contrast to models for the interaction between a DQE and a (one-mode or two-mode) cavity field, we find that a strong photon antibunching can occur, even for a weaker DQE-field coupling strength and large dephasing of the DQE, when mode coupling is introduced. We also find that mode coupling can make the device robust against either (1) the frequency mismatch between cavity modes and the DQE or (2) the coupling strength mismatch between the DQE and each mode in the two-mode cavity. Moreover, we find that these mismatches can be used to generate better antibunching. Our study shows that mode coupling in a bimodal cavity is very important for the realization of a good single-photon device.

We report the cancellation of the cross-phase-modulation-instability (XPMI) gain over a large spectral window (which we term the XPMI band gap) in a highly birefringent photonic-crystal fiber with zero group birefringence. The XPMI ceases to occur when single-frequency pumping of orthogonally polarized modes takes place in such a spectral band gap whose frequency bandwidth depends on the pump power itself. The suppression of XPMI sidebands is confirmed experimentally when Raman scattering remains negligible. At high powers the Raman Stokes wave, generated by the pump, implies novel dual-frequency pump configurations with large group-velocity mismatch, thus leading to another type of Raman-induced XPMI sidebands. The experimental results are in good agreement with analytical phase-matching calculations and numerical simulations.

The ability to transfer quantum information from one location to another with high probability is of central importance to quantum information science. Unfortunately, for the simplest system of a uniform chain (a spin chain or a particle in a one-dimensional lattice), the state transfer time grows exponentially in the chain length $N$ at fixed transfer probability. In this work we show that the addition of an impurity near each end point, coupled to the uniform chain with strength $w$, is sufficient to ensure efficient and high-probability state transfer. An eigenstate localized in the vicinity of the impurity can be tuned into resonance with chain-extended states by adjusting $w\left(N\right)\propto N^{1/2}$; the resulting avoided crossing yields resonant eigenstates with large amplitudes on the chain end points and approximately equidistant eigenvalues. The state transfer time scales as $t\propto N^{3/2}$, and its transfer probability $P$ approaches unity in the thermodynamic limit $N\to \infty$; the error scales as $1-P\propto N^{-1}$. Thus, with the addition of two impurities, asymptotically perfect efficient state transfer with a uniform chain is possible even in the absence of external control.

We propose a no-go theorem by proving the impossibility of constructing a deterministic quantum circuit that iterates a unitary oracle by calling it only once. Different schemes are provided to bypass this result and to approximately realize the iteration. The optimal scheme is also studied. An interesting observation is that for a large number of iterations, a trivial strategy like using the identity channel has the optimal performance, and preprocessing, postprocessing, or using resources like entanglement does not help at all. Intriguingly, the number of iterations, when being large enough, does not affect the performance of the proposed schemes.

In the quantum adiabatic algorithm, as the adiabatic parameter $s\left(t\right)$ changes slowly from zero to one with finite rate, a transition to excited states inevitably occurs and this induces an intrinsic computational error. We show that this computational error depends not only on the total computation time $T$ but also on the time derivatives of the adiabatic parameter $s\left(t\right)$ at the beginning and the end of evolution. Previous work [A. T. Rezakhani, A. K. Pimachev, and D. A. Lidar, Phys. Rev. A 82, 052305 (2010)] also suggested this result. With six typical paths, we systematically demonstrate how to optimally design an adiabatic path to reduce the computational errors. Our method has a clear physical picture and also explains the pattern of computational error. In this paper we focus on the quantum adiabatic search algorithm although our results are general.

It has been known for more than 15 years that the differential cross section of electrons emitted from diatomic molecules during interaction with energetic charged particles oscillates as a function of electron momentum. The origin of the phenomenon is two-center interference, which naturally relates it back to the Young double-slit experiment. In addition to a characteristic frequency which can be described by lowest-order perturbation theories, the observation and origin of higher-order harmonics of the basic oscillation frequency has been much discussed. Here, we show that high harmonics of the fundamental Young-type oscillation frequency observed in electron spectra in fast ion-molecule collisions can be clearly exposed in numerical solutions of the time-dependent Schrödinger equation within a one-dimensional model. Momentum distribution of the ejected electron is analyzed and shows that the phenomenon emerges when the charged particle beam collides with diatomic molecules with substantial large internuclear distance. Frequency spectra from nonperturbative calculations for electron emission from ${\text{Rb}}_2{}^{+}$ and ${\text{Cs}}_2{}^{+}$ exhibit a pronounced high-order oscillation in contrast to similar close-coupling calculations performed on ${\text{H}}_2$ targets. The electron emission from these heavy molecules contains second- and third-order harmonics which are fully reproduced in an analytic model based on the Born series. Extending to triatomic molecular targets displays an increased range of harmonics. This suggests that electron emission spectra from new experiments on heavy diatomic and linear polyatomic molecular targets may provide a unique insight into competing coherent emission mechanisms and their relative strength.

We derive the large-particle-number limit of the Bethe equations for the ground state of the attractive one-dimensional Bose gas (Lieb-Liniger model) on a ring and solve it for arbitrary coupling. We show that the ground state of this system can be mapped to the large-$N$ saddle point of Euclidean Yang-Mills theory on a two-sphere with a $\text{U}\left(N\right)$ gauge group, and the phase transition that interpolates between the homogeneous and solitonic regime is dual to the Douglas-Kazakov confinement-deconfinement phase transition.

We consider an ideal invisibility cloak which is illuminated by monochromatic light and which moves in vacuum at constant relativistic velocity with respect to the common inertial frame of light source and observer. We show that, in general, the moving cloak becomes detectable by image distortions and by generating a broad frequency spectrum of the scattered light. However, for many special combinations of incident light frequency, wave vector of light, and cloak velocity, ideal cloaking remains possible. It becomes nonreciprocal though. This means that light rays emitted by the light source arrive at the observer as though they have traveled through vacuum, but they take completely different paths after being retroreflected at the observer position.

Entangled atomic states, such as spin-squeezed states, represent a promising resource for a new generation of quantum sensors and atomic clocks. We demonstrate that optimal control techniques can be used to substantially enhance the degree of spin squeezing in strongly interacting many-body systems, even in the presence of noise and imperfections. Specifically, we present a protocol that is robust to noise and outperforms conventional methods. Potential experimental implementations are discussed.

Using the model of parametric interaction based on the spatiospectral Schmidt modes and generalized parametric approximation, we analyze coherence and the mode structure of ultraintense twin beams generated in the regime with pump depletion. We show that the increase of spatial and spectral coherence with the increasing pump power observed for moderate powers is replaced by a decrease for the pump powers at which pump depletion occurs. This behavior of coherence is opposed to that exhibited by a number of spatiospectral modes effectively constituting the twin beam. The conditions for maximal coherence are analyzed by considering pump-beam parameters (spectral width and transverse radius). The existence of additional coherence maxima occurring at even higher pump powers is predicted and explained by the oscillatory evolution of the modes' populations.

Atom chips provide compact and robust platforms towards the implementation of practical quantum technologies. A quick and faithful preparation of arbitrary input states for these devices is crucial but represents a challenging experimental task. This is especially difficult when the dynamical evolution is noisy and unavoidable setup imperfections have to be considered. Here, we experimentally prepare with very high fidelity nontrivial superpositions of internal states of a rubidium Bose-Einstein condensate realized on an atom chip.

Adaptive data collection and analysis, where data are being fed back to update the measurement settings, can greatly increase speed, precision, and reliability of the characterization of quantum systems. However, decoherence tends to make adaptive characterization difficult. As an example, we consider two coupled discrete quantum systems. When one of the systems can be controlled and measured, the standard method to characterize another, with an unknown frequency ${\omega }_{\mathrm{r}}$, is swap spectroscopy. Here, adapting measurements can provide estimates whose error decreases exponentially in the number of measurement shots rather than as a power law in conventional swap spectroscopy. However, when the decoherence time is so short that an excitation oscillating between the two systems can only undergo less than a few tens of vacuum Rabi oscillations, this approach can be marred by a severe limit on accuracy unless carefully designed. We adopt machine learning techniques to search for efficient policies for the characterization of decohering quantum systems. We find, for instance, that when the system undergoes more than 2 Rabi oscillations during its relaxation time $T_1, O\left({10}^3\right)$ measurement shots are sufficient to reduce the squared error of the Bayesian initial prior of the unknown frequency ${\omega }_{\mathrm{r}}$ by a factor $O\left({10}^4\right)$ or larger. We also develop policies optimized for extreme initial parameter uncertainty and for the presence of imperfections in the readout.

We present a theoretical analysis of different methods to synthesize entangled states of two quantum mechanical resonators. These methods are inspired by experimentally demonstrated interactions of superconducting resonators with artificial atoms, and offer efficient routes to generate nonclassical states. Using a two-mode Jaynes-Cummings model, we analyze the theoretical structure of these algorithms and their average performance for arbitrary states and for deterministically preparing NOON and maximally entangled states. Using a new state synthesis algorithm, we show that NOON and maximally entangled states can be prepared in a time linear in the desired photon number and without any state-selective interactions.

We propose a generalization of the superconducting circuit simulation of the dynamical Casimir effect where we consider relativistically moving boundary conditions following different trajectories. We study the feasibility of the setup used in the past to simulate the dynamical Casimir effect to reproduce richer relativistic trajectories differing from purely sinusoidal ones. We show how different relativistic oscillatory trajectories of the boundaries of the same period and similar shape produce a rather different spectrum of particles characteristic of their respective motions.

Fast creation of a single vortex in BEC of alkali-metal atoms at a prescribed position and time is still challenging, even though various methods to create single and multiple vortices have been proposed and demonstrated. Topological vortex formation is advantageous in this respect over other methods in that the position and time of vortex formation is highly controllable. This method requires inversion of the bias magnetic field along the axis of the condensate, which leads to unwanted atom loss due to nonadiabatic transitions when the bias field crosses zero. It is the purpose of this paper to propose a scheme that enables a fast creation of a vortex in much shorter time than needed for adiabatic control time by introducing the counterdiabatic field to avert the atom loss. We further introduce a gauge transformation so that the required magnetic field is generated by manipulating the current of the Ioffe bars, which makes our proposal experimentally feasible.

Potoček and Ferenczi [Phys. Rev. A 92, 023829 (2015)] provided an analysis of the experimental evidence obtained by Danan et al. [Phys. Rev. Lett. 111, 240402 (2013)] for the surprising behavior of photons passing through an interferometer, in particular, motion on disconnected paths. Potoček and Ferenczi reproduced the results of the experiment, but when analyzing its modification, they claimed that the reasoning of Danan et al., which led to disconnected paths, is erroneous. It is argued here that the criticism of Potoček and Ferenczi is unfounded.

We give an analytic construction of a class of two-qubit gate pulse sequences that act on five of the six spin-$\frac{1}{2}$ particles used to encode a pair of exchange-only three-spin qubits. Within this class, the problem of gate construction reduces to that of finding a smaller sequence that acts on four spins and is subject to a simple constraint. The optimal sequence satisfying this constraint yields a two-qubit gate sequence equivalent to that found numerically by Fong and Wandzura. Our construction is sufficiently simple that it can be carried out entirely with pen, paper, and knowledge of a few basic facts about quantum spin.

In this Rapid Communication, we theoretically discuss a large Doppler shift in a signal slow-light pulse in a photonic-crystal waveguide by considering its reflection at a quasilight speed mirror. The mirror is formed by the photonic band-gap shift induced by the high nonlinearity of a control slow-light pulse, which could be possible in a realistic device. In the simulation, the Doppler shift appears at multiple frequencies due to the Bloch nature of the photonic lattice. Larger but inefficient Doppler shifts occur through nonadiabatic processes, whereas the smallest but more efficient shift (i.e., the intraband Doppler shift) occurs through an adiabatic process. The occurrence of the intraband shift depends on whether the adiabatic process produces a complete reflection of the incident pulse, despite the fact that the pulse penetrates the mirror. A large band-gap shift and a moderately slow mirror satisfy this condition; otherwise, the shift ends at the halfway point.