We investigate the quantum dynamics of two bosons, trapped in a two-dimensional harmonic trap, upon quenching arbitrarily their interaction strength and thereby covering the entire energy spectrum. Utilizing the exact analytical solution of the stationary system, we derive a closed analytical form of the expansion coefficients of the time-evolved two-body wave function, whose dynamics is determined by an expansion over the postquench eigenstates. The emergent dynamical response of the system is analyzed in detail by inspecting several observables such as the fidelity, the reduced one-body densities, the radial probability density of the relative wave function in both real and momentum space, and the Tan contact, which unveils the existence of short range two-body correlations. When the system is initialized in its bound state, it is perturbed in the most efficient manner as compared to any other initial configuration. Moreover, starting from an interacting ground state, the two-boson response is enhanced for quenches toward the noninteracting limit.

Antibunching and entanglement play important roles in quantum information processing as antibunching is an essential ingredient for the production of single photons (phonons) and entanglement is a crucial resource for quantum communication and metrology. In this paper, we propose a atom-photon-phonon (tripartite) interaction in a hybrid cavity-atom-mechanics system, and show that both photon and phonon antibunching can be observed simultaneously under the resonant atomic driving. More importantly, the generated single photons and phonons are strongly correlated and entangled with each other, i.e., single entangled photon-phonon pairs are generated via the atom-photon-phonon interaction. The generation of single entangled photon-phonon pairs is the first step to implement entanglement-based quantum state transfer, which is essential for connecting mechanical and optical systems to build hybrid quantum networks.

The energy-time entangled biphoton source plays a great role in quantum communication, quantum metrology, and quantum cryptography due to its strong temporal correlation and capability of nonlocal dispersion cancellation. As a quantum effect, nonlocal dispersion cancellation is further proposed as an alternative way for nonlocality testing of continuous variable entanglement via the violation of the Bell-like inequality proposed by Wasak et al. [Phys. Rev. A 82, 052120 (2010)]. However, to date there is no experimental report either on the inequality violation or on a nonlocal detection with single-photon detectors at a long-distance transmission channel, which is key for a true nonlocality test. In this paper, we report an experimental realization of a violation of the inequality after 62-km optical fiber transmission at telecom wavelengths with a nonlocal detection based on event timers and a cross-correlation algorithm, which indicates a successful nonlocal test of energy-time entanglement. This work provides feasibility for the strict test of the nonlocality for continuous variables in both long-distance communication fiber channel and free space.

We theoretically and experimentally studied the coupling of the magnetic Mie resonance of a dielectric cuboid and the guide mode resonance of an alumina slab in an all-dielectric metamaterial. It is found that the excitation of the discrete guide mode resonance or substrate resonance of the alumina slab can be properly controlled by designing the geometry of the dielectric cuboid. Moreover, it is found that the coupling of substrate resonance and the Mie resonance of the cuboid can be tailored to achieve electromagnetically induced transparency (EIT)–like spectrum. The tunability of the EIT-like spectrum based on the geometric parameters was also studied. Our results utilizing the crucial contribution of substrate derive a realization of EIT in all-dielectric metamaterial and thus pave a way for optical spectral control with all-dielectric coupled system.

Unidirectional reflectionless propagation (or transmission) is an interesting wave phenomenon observed in many $\mathcal{PT}$-symmetric optical structures. Theoretical studies on unidirectional reflectionless transmission often use simple coupled-mode models. The coupled-mode theory can reveal the most important physical mechanism for this wave phenomenon, but it is only an approximate theory and it does not provide accurate quantitative predictions with respect to geometric and material parameters of the structure. In this paper we rigorously study unidirectional reflectionless transmission for two-dimensional $\mathcal{PT}$-symmetric structures (with one-dimensional periodicity) sandwiched between two homogeneous media. Using a scattering matrix formalism and a perturbation method for the case where only one diffraction channel is open, we show that real zero-reflection frequencies are robust under $\mathcal{PT}$-symmetric perturbations and unidirectional reflectionless transmission is guaranteed to occur if the perturbation (of the dielectric function) satisfies a simple condition. Numerical examples are presented to validate the analytical results and to demonstrate unidirectional invisibility by tuning the amplitude of balanced gain and loss.

Generation of high-fidelity photonic non-Gaussian states is a crucial ingredient for universal quantum computation using continuous-variable platforms, yet it remains a challenge to do this efficiently. We present a general framework for a probabilistic production of multimode non-Gaussian states by measuring a few modes of multimode Gaussian states via photon-number-resolving detectors. We use Gaussian elements consisting of squeezed displaced vacuum states and interferometers, the only non-Gaussian elements consisting of photon-number-resolving detectors. We derive analytic expressions for the output Wigner function, and the probability of generating the states in terms of the mean and the covariance matrix of the Gaussian state and the photon detection pattern. We find that the output states can be written as a Fock-basis superposition state followed by a Gaussian gate, and we derive explicit expressions for these parameters. These analytic expressions show exactly what non-Gaussian states can be generated by this probabilistic scheme. Further, it provides a method to search for the Gaussian circuit and measurement pattern that produce a target non-Gaussian state with optimal fidelity and success probability. We present specific examples such as the generation of cat states, ON states, Gottesman-Kitaev-Preskill states, NOON states, and bosonic-code states. The proposed framework has potentially far-reaching implications for the generation of bosonic error-correction codes that require non-Gaussian states and resource states for the implementation of non-Gaussian gates needed for universal quantum computation, among other applications requiring non-Gaussianity. The tools developed here could also prove useful for the quantum resource theory of non-Gaussianity.

We propose a resource-efficient error-rejecting entangled-state analyzer for polarization-encoded multiphoton systems. Our analyzer is based on two single-photon quantum-nondemolition detectors, where each of them is implemented with a four-level emitter (e.g., a quantum dot) coupled to a one-dimensional system (such as a micropillar cavity or a photonic nanocrystal waveguide). The analyzer works in a passive way and can completely distinguish $2^n$ Greenberger-Horne-Zeilinger (GHZ) states of $n$ photons without using any active operation or fast switching. The efficiency and fidelity of the GHZ-state analysis can, in principle, be close to unity, when an ideal single-photon scattering condition is fulfilled. For a nonideal scattering, which typically reduces the fidelity of a GHZ-state analysis, we introduce a passively error-rejecting circuit to enable a near-perfect fidelity at the expense of a slight decrease of its efficiency. Furthermore, the protocol can be directly used to perform a two-photon Bell-state analysis. This passive, resource-efficient, and error-rejecting protocol can, therefore, be useful for practical quantum networks.

Many-electron relativistic quantum theories of stationary molecular electronic states have been developed in so-called quantum chemistry, in which nuclear configuration is frozen in space-time under the Born-Oppenheimer approximation. These time-independent methods are concerned with energetics, which are supposed to determine molecular structures and dominate low-energy chemical reactions. Yet, rapid progress in laser technology demands that theoretical chemistry should get prepared for relativistic electron-nucleus coupled dynamics driven by unconventional ultrastrong laser pulses. We therefore generalize our previously developed path-integral formalism of nonadiabatic electron dynamics [Hanasaki and Takatsuka, Phys. Rev. A 81, 052514 (2010)] to cover the relativistic regime in radiation fields. Starting from a formal relativistic path-integral formulation of electron-nucleus coupled systems interacting with quantum radiation fields, we reduce it to a tractable level of approximations to set a theoretical foundation for future applications.

We report an observation of phononic Schwinger angular momenta, which fully represent two-mode states in a micromechanical resonator. This observation is based on simultaneous optical detection of the mechanical response at the sum and difference frequency of the two mechanical modes. A postselection process for the measured signals allows us to extract a component of phononic Schwinger angular momenta. This postselection scheme with the nonlinear measurement enables us to conditionally prepare two-mode states, which shows a noise reduction along a cross-phase quadrature and a finite correlation, from a randomly excited (uncorrelated) state. The phononic Schwinger angular momenta could be extended to high-dimensional symmetry [e.g., SU($N$) group] for studying multipartite correlations in nonequilibrium dynamics with macroscopic objects.

We investigated the transverse properties of 420-nm yoked superfluorescence (YSF) emitted from the atomic vapor of rubidium by driving the $5S\text{−}5D$ two-photon transition with an ultrashort laser pulse. From the observed interferograms of the superfluorescent pulses, we derived both the amplitude and phase properties of their electric fields. By increasing the pump-pulse intensity, the spatial profile of the 420-nm YSF changed from a central bright spot into a ring-shaped radial profile, accompanied by a simultaneous distortion of the wave front. This behavior can be explained by considering the intensity-dependent phase shifts of the $5S\text{−}5D$ wave packets during the excitation process, which was further imprinted to the 420-nm YSF wave front. The single-shot experiments enabled us to clarify the locally coherent regions of the 420-nm YSF as well as their stochastic feature.

Quantum entanglement is an important resource for next-generation technologies. We show that diffracting systems can supplant beam splitters and, more generally, interferometric networks for entanglement generation—systems as simple as screens with pinholes can create entanglement. We then discuss the necessary and sufficient conditions for entanglement to be generated by states input to any passive linear interferometric network. Entanglement generated in free space can now be harnessed in quantum-optical applications ranging from quantum computation and communication to quantum metrology.

We investigate the transient dynamics of quantum coherence for a system of two central spins in a spin-star environment by employing a numerical procedure based on a Laguerre polynomial expansion scheme. The dynamics of the total, local, and global coherence are calculated for different values of the anisotropy parameter, the system-bath interaction strengths, and temperature for different initial bipartite states. Significant dynamical features of quantum coherence are found as follows: (i) an $X$ state can only have global coherence; (ii) a state with only initial local coherence gains global coherence during the course of evolution by the induced correlations between the two-qubit system and the common bath; (iii) an incoherent state gains coherence by interacting with an external bath. We find there are two primary ways to gain coherence for an incoherent state: one is by interacting with the external quantum bath and the other is through interconversion of other quantum properties such as purity into coherence. Finally, we demonstrate that our results for the system in an infinite bath also hold qualitatively when the system is in contact with a finite bath.

We show how to achieve full spectral characterization of general multiaxis additive noise on a single qubit, including arbitrary cross-axis noise correlations. Our pulsed spectral estimation technique is based on sequence repetition and frequency-comb sampling and is applicable in principle even to models where a large qubit energy splitting is present, as long as the noise is stationary and a second-order (Gaussian) approximation to the controlled reduced dynamics is viable. A key innovation in our approach is a spherical representation of the noise in terms of operators that couple directly to raising and lowering qubit operators, which is instrumental to show that only three suitably defined spectra effectively contribute in the large-splitting regime. Our result is crucial to extend the applicability of comb-based spectral estimation, which has been so far employed under the assumption of dephasing-dominated dynamics, to experimental platforms where both $T_1$ and $T_2$ processes may occur on comparable timescales or be otherwise significant, such as superconducting qubits.

We present calculations for attosecond atomic delays in the photoionization of noble-gas atoms based on the full two-color two-photon random-phase approximation with exchange in both the length and velocity gauge. Gauge-invariant atomic delays are demonstrated for the complete set of diagrams. The results are used to investigate the validity of the common assumption that the measured atomic delays can be interpreted as a one-photon Wigner delay and a universal continuum-continuum contribution that depends only on the kinetic energy of the photoelectron, the laser frequency, and the charge of the remaining ion, but not on the specific atom or the orbital from which the electron is ionized. Here, we find that although effects beyond the universal IR-photoelectron continuum-continuum transitions are rare, they do occur in special cases such as around the $3s$ Cooper minimum in argon. We conclude also that in general the convergence in terms of many-body diagrams is considerably faster in the length gauge than in the velocity gauge.

Experimental and theoretical investigations of the field-free alignment of the nonrigid methanol molecule are reported. The molecule is subject to a $140-\mathrm{TW}/{\mathrm{cm}}^2$-intensity laser pulse with a 100-fs duration. The experimental signal displays a constant permanent alignment and a fast decaying transient alignment consistent with a prolatelike molecule with $(B+C)/2$ on the order of $0.808{\mathrm{cm}}^{-1}$. The theoretical model takes into account the large-amplitude internal rotation of the methyl group with respect to the hydroxyl group. In the case of a continuous-wave laser field, a rotational alignment close to that of a rigid molecule is predicted. Torsional alignment also occurs even though there is no explicit dependence of the polarizability tensor on the angle of internal rotation. In the case of a strong short laser pulse, the theoretical approach shows that permanent and transient rotational alignment take place. The latter displays an exponential-like decay due to the high density of rotation-torsion levels. Torsional alignment also occurs and depends on the temperature. The theoretical model allows us to reproduce the experimental signal provided one component of the polarizability tensor is adjusted and dissipation effects due to molecular collisions are taken into account.

We study suppression of nonadiabatic transitions during adiabatic generation of a catlike state (a superposition of different-sized cat states) and a spin squeezed state in a bosonic Josephson junction. In order to minimize the adiabatic error, we use the quantum adiabatic brachistochrone, which enables us to track a geometrically efficient path in parameter space under given conditions without requiring additional terms. For the creation of a catlike state, divergence of the quantum geometric tensor associated with gap closing at the critical point is avoided because of the parity conservation. The resulting schedules of parameters are smooth and monotonically decreasing curves. Use of these schedules offers a reduction in the time to generate both a catlike state and a spin squeezed state.

A scheme for heralded generation of frequency-bin photonic qubits via spontaneous four-wave mixing in a system of coupled microring resonators (a photonic molecule) is developed so that the qubit state is fully controlled by the frequency amplitude of the pump field. It is shown that coupling parameters of the photonic molecule can be optimized to provide generation of nearly pure heralded photons. The heralded qubit state does not depend on the microring resonator mode frequency, thereby allowing one to take advantage of frequency multiplexing for improving efficiency of quantum information processing.

We study cross-relaxation-based magnetometry in ensembles of nitrogen-vacancy (NV) centers using the protocol suggested by Akhmedzhanov et al. [R. Akhmedzhanov, L. Gushchin, N. Nizov, V. Nizov, D. Sobgayda, I. Zelensky, and P. Hemmer, Microwave-free magnetometry based on cross-relaxation resonances in diamond nitrogen-vacancy centers, Phys. Rev. A 96, 013806 (2017)]. We suggest a processing scheme that allows us to perform measurements without precise sample orientation. It is shown theoretically and experimentally that using an additional magnetic field can significantly simplify vector magnetic-field measurements.

When time-reversal symmetry is broken on its surface, topological insulators exhibit a magnetoelectric response which is described by axion electrodynamics. A direct consequence of this theory is the appearance of a magnetic field that resembles the one produced by a magnetic image monopole when a pointlike electric charge is located near the surface of the material. In this paper, we investigate the more realistic problem when the pointlike charge is replaced by a finite-size sphere at constant potential. We calculate the electromagnetic fields using the potential formulation in a particular bispherical coordinate system. We find that the electromagnetic fields can be interpreted in terms of point electric and image magnetic charges as if the medium were the vacuum. As a manifestation of the magnetoelectric effect, we highlight the resulting magnetic field, which we analyze in detail along the symmetry axis, since such estimates could be useful in evaluating the experimental possibility of its measurement via sensible magnetometers. Our numerical estimates show that the proposed setup provides a magnetic field strength in the range of 10–100 mG, which is attainable with present day sensitivities in NV center diamond magnetometers, for example.