Whispering gallery modes (WGMs), circulating modes near the surface of a spheroidal material, have been known to exhibit high-quality factors for both acoustic and electromagnetic waves. Here, we report an electro-optomechanical system, where the overlapping WGMs of acoustic and optical waves along the equator of a dielectric sphere strongly couple to each other. The triple-resonance phase-matching condition provides a large enhancement of the Brillouin scattering only in a single sideband, and conversion from the input radio-frequency signal exciting the acoustic mode to the output optical signal is observed.

One of the most direct preparations of a Gottesman-Kitaev-Preskill (GKP) qubit in an oscillator uses a tunable photon-pressure (also called optomechanical) coupling of the form $\widehat{q}{\widehat{b}}^{†}\widehat{b}$, enabling us to imprint the modular value of the position $\widehat{q}$ of one oscillator onto the state of an ancilla oscillator. We analyze the practical feasibility of executing such modular quadrature measurements in a parametric circuit-QED realization of this coupling. We provide estimates for the expected GKP squeezing induced by the protocol and discuss the effect of photon loss and other errors on the resulting squeezing.

We study the quantum state of twin photons generated by parametric down-conversion in a two-dimensional nonlinear photonic crystal, driven by a dual pump beam. The interplay between the grating of the ${\chi }^{\left(2\right)}$ nonlinear response and the transverse spatial modulation of the pump opens a rich scenario of possibilities for controlling the quantum state. We focus on a spatial resonance condition between the pump and the nonlinear grating, where a quadripartite entangled state is generated in proper light modes. The device is shown to implement the mixing of a pair of independent two-mode squeezed states onto four separate spatial modes, without the need of any external beam-splitter. Remarkably, both the squeezing parameters and the mixing coefficients can be reconfigured by modulating the relative phase and intensity of the two pumps, enabling thus the generation of a variety of quantum states in a single device.

Exceptional points (EPs) in the optical whispering-gallery-mode (WGM) microcavity perturbed by Rayleigh scatterers have shown great potential in the areas of on-chip chiral photonics and enhanced sensors, creating a huge demand for deep theoretical insight into the unique physical properties of EPs. Here we propose an intuitive and first-principles-based model to clarify the formation of the EPs in the WGM microcavity perturbed by two nanoparticles. The model is constructed by considering a dynamical multiple-scattering process involving the clockwise and counterclockwise azimuthally propagating modes (APMs) at the nanoparticles. The complex resonance frequencies and resonant modes of the system can be comprehensively reproduced with the model. The conditions for the determination of EPs are derived, and the system parameters at the EP can be quantitatively predicted. The model unveils the relationship between the spatial chirality of the resonant modes near the EP and the scattering of APMs at the nanoparticles. The model quantitatively predicts the difference between the APM coefficients within different azimuthal regions divided by the two nanoparticles, which were considered to be identical in previous theoretical formalisms, and it exhibits the contributions of the APMs, other than those considered in the model, to forming the resonant modes. The proposition of the theoretical model will benefit the investigations of EPs in WGM microcavities and their applications.

The possibility to manage and control spectral and spatial properties of optical-terahertz nonclassical fields generated in strongly nondegenerate parametric down-conversion is demonstrated experimentally using Mach-Zehnder nonlinear interferometer and analyzed theoretically in terms of Schmidt modes. The interferometer can be used for measuring the dispersion properties of different materials in the terahertz frequency range. The key point is that the information about these properties is extracted from the measurements in the optical channel due to strong frequency-angular correlations between optical and terahertz photons. The developed theoretical approach is used to predict properties of the optical-terahertz fields and photon correlations in different regimes of nondegenerate parametric down-conversion. The considered interferometer is shown to be a promising source of terahertz fields with new features and frequency-filtered terahertz Schmidt modes.

Ghost imaging is demonstrated using a polyenergetic reactor source of thermal neutrons. This enables position resolution to be incorporated into a variety of neutron instruments that are not position resolving. Such a proof of concept enables several further applications. For example, in an imaging context, neutron ghost imaging can be beneficial for dose reduction and resolution enhancement. We explore the principle of resolution enhancement by employing a variant of the method in which each pixel of a position-sensitive detector is regarded as an independent bucket detector; a neutron ghost image is then computed for each pixel. We demonstrate the principle that this parallel form of neutron ghost imaging can significantly increase the spatial resolution of a pixelated detector such as a CCD or CMOS camera. Further applications and extensions of our neutron ghost-imaging protocol are discussed. These include neutron ghost tomography, neutron ghost microscopy, dark-field neutron ghost imaging, and isotope-resolved color neutron ghost imaging via prompt-gamma-ray bucket detection.

We propose a machine-learning-based approach to develop the exchange-correlation potential of time-dependent density-functional theory (TDDFT). The neural network projection from the time-varying electron densities to the corresponding correlation potentials in the time-dependent Kohn-Sham equation is trained using a few exact datasets for a model system of electron-hydrogen scattering. We demonstrate that this neural network potential can capture the complex structures in the time-dependent correlation potential during the scattering process and provide correct scattering dynamics, which are not obtained by the standard adiabatic functionals. We also show that it is possible to incorporate the nonadiabatic (or memory) effect in the potential with this machine learning technique, which significantly improves the accuracy of the dynamics. The method developed here offers a way to improve the exchange-correlation potential of TDDFT, which makes the theory a more powerful tool to study various excited state phenomena.

We show how to implement quantum transport, generate entangled state, and achieve spatial entanglement distribution via topological Thouless pumping in one-dimensional disordered lattices. We introduce the on-site disorders to suppress the high-order resonant tunneling (which cause wave-packet dispersion) and realize dispersionless Thouless pumping. The interplay between the on-site disorders and the band topology enables robust unidirectional transport. We also demonstrate how to prepare a spatially entangled two-particle NOON state via Hong-Ou-Mandel interference assisted by the Thouless pumping. The quantum entanglement can be well preserved during the distribution between spatially separated sites. Our system paves a way to wide applications of topology in quantum information process.

Photon pairs entangled in high-dimensional orbital angular momentum (OAM) degrees of freedom (DOF) have been widely regarded as a possible source for improving the capacity of quantum information processing. The generation of a high-dimensional maximally entangled state in the OAM DOF is therefore much desired. In this work, we demonstrate a simple method to generate a broader and flatter OAM spectrum, i.e., a larger spiral bandwidth (SB), of entangled photon pairs generated through spontaneous parametric down-conversion by modifying the pump beam profile. By investigating both experimentally and theoretically, we find that an exponential pump profile that is roughly the inverse of the mode profiles of the single-mode fibers used for OAM detection will provide a much larger SB compared to a Gaussian-shaped pump.

We propose two kinds of distinguishing parameter regimes to induce a topological Su-Schrieffer-Heeger (SSH) phase in a one-dimensional (1D) multiresonator cavity optomechanical system via modulation of the frequencies of both cavity fields and resonators. The introduction of frequency modulations allows us to eliminate the Stokes heating process for mapping of the tight-binding Hamiltonian without the usual rotating-wave approximation, which is totally different from the traditional mapping of the topological tight-binding model. We find that the tight-binding Hamiltonian can be mapped into a topological SSH phase via modification of the Bessel function originating from the frequency modulations of cavity fields and resonators, and the induced SSH phase is independent of the effective optomechanical coupling strength. On the other hand, the insensitivity of the system to the effective optomechanical coupling provides us another new path to induce the topological SSH phase based on the present 1D cavity optomechanical system. And we show that the system can exhibit a topological SSH phase via variation of the effective optomechanical coupling strength in an alternative way, which is much easier to achieve in experiments. Furthermore, we also construct an analogous bosonic Kitaev model with trivial topology by keeping the Stokes heating processes. Our scheme provides a steerable platform for investigation of the effects of next-nearest-neighbor interactions on the topology of the system.

The storage of broadband single photons from a parametric-down-conversion source is a capability with the potential to foster significant development in the field of quantum information. A particular challenge to this problem, however, is the mismatch between the short-lived photon and the long-lived memories, which translates into quite different frequency bands for the two systems. Ultimately, this difficulty can be mapped into the problem of how a narrowband medium can efficiently absorb a broadband pulse of light. Here we present a detailed approach to this problem focusing on the absorption of photons at 800 nm, a common choice for parametric-down-conversion sources, by hot vapors of rubidium atoms. For this, we employ a stronger control field to drive a sequential two-photon transition on the atoms, which is intrinsically broadband, together with a weak signal field consisting of a femtosecond pulse of light. We describe then how to measure small absorptions of the signal pulse and how to improve this absorption through the various parameters of the problem. Our results are modeled by a perturbative theory suitable to our present weak-absorption regime. Finally, we provide a road map with different strategies to achieve larger absorptions.

$X$ waves are a special type of wave packet that can maintain their transverse profile of $X$ shape during propagation, and they are of much interest for the study of fundamental physics and practical applications. Here we present a scheme to generate nonlinear $X$ waves and realize their active control by using a cold gas of Rydberg atoms via electromagnetically induced transparency (EIT). We show that, due to the EIT effect contributed by a control laser field and the strong, nonlocal Kerr nonlinearity contributed by Rydberg-Rydberg interaction between atoms, the system supports high-dimensional, nonlocal, and nonlinear optical $X$ waves, which have low propagation loss, ultraslow propagation velocity, and ultralow generation power. We also show that the stability domain of such $X$ waves can be greatly enlarged by increasing the nonlocality degree of the Kerr nonlinearity, and their motion trajectory can be manipulated by using an external magnetic field. Our study opens a route for generating and controlling nonlocal ultraslow nonlinear optical $X$ waves, which may have promising applications in optical information processing and transmission.

A pertinent question in cavity optomechanics is whether reaching the regime of large single-photon cooperativity, where the single-photon coupling rate exceeds the geometric mean of the cavity and mechanical decay rates, can enable any new phenomena. We show that in some multimode optomechanical systems, the single-photon cooperativity can indeed be a figure of merit. We first study a system with one cavity mode and two mechanical oscillators, which combines the concepts of levitated optomechanics and coherent scattering with standard dispersive optomechanics. Later, we study a more complicated setup involving three cavity modes which does not rely on levitated optomechanics and only features dispersive optomechanical interactions with direct cavity driving. These systems can effectively realize the degenerate or the nondegenerate parametric oscillator models known from quantum optics, but in the unusual finite-size regime for the fundamental mode(s) when the single-photon cooperativity is large. We show that the response of these systems to a coherent optical probe can be highly nonlinear in probe power even for average photon occupation numbers below unity. The nonlinear optomechanical interaction has the peculiar consequence that the probe drive will effectively amplitude-squeeze itself. For large single-photon cooperativity, this occurs for small occupation numbers, which enables observation of nonclassical antibunching of the transmitted probe photons due to a destructive interference effect. Finally, we show that as the probe power is increased even further, the system enters a critical regime characterized by intrinsically nonlinear dynamics and non-Gaussian states.

Multipartite Einstein-Podolsky-Rosen (EPR) steering has been recognized as an essential resource for secure quantum communication tasks composed of several spatially separated parties who cannot be fully trusted. Nevertheless, this resource cannot be distributed arbitrarily over many parties; for instance, two independent players cannot simultaneously steer the third party by two-setting measurements. This feature is referred to as monogamy of steering, which ensures the security of quantum cryptographic protocols and is thus a very desired property to investigate for multipartite steering. Here, we propose symmetric and asymmetric structures of cascaded four-wave mixing of rubidium atoms to generate versatile quadripartite EPR steering and investigate four distinct types of monogamy relations of Gaussian steering. We find that the distribution constraint described by one of the monogamy relations can be lifted only for the quadripartite steering created by the symmetric setup. This result paves the way for a better understanding of the distribution rules of multipartite EPR steering and its potential applications for secure quantum communication.

We propose a simple yet surprisingly effective mechanical squeezing scheme in a standard optomechanical system that is beneficial for the amplitude modulation of the pump laser. By merely introducing a specific kind of periodic modulation into the single-tone driving field to cool down the mechanical Bogoliubov mode, strong mechanical squeezing, far beyond 3 dB, can be engineered without the requirement for any additional techniques. Specifically, we find that the amount of squeezing is not simply dependent on the order of magnitude of the effective optomechanical coupling but strongly on the ratio of sideband strengths for it. To maximize the mechanical squeezing, we numerically and analytically optimize this ratio in the steady-state regime, respectively. The mechanical squeezing engineered in our scheme also is extremely robust and can survive at a high bath temperature. Compared with previous schemes based on the two-tone pump technique, our scheme involves fewer external control laser sources and can be extended to other quantum systems to achieve a strong squeezing effect.

A method for the systematic construction of few-body damped harmonic oscillator networks accurately reproducing the effect of general bosonic environments in open quantum systems is presented. Under the sole assumptions of a Gaussian environment and regardless of the system coupled to it, an algorithm to determine the parameters of an equivalent set of interacting damped oscillators obeying a Markovian quantum master equation is introduced. By choosing a suitable coupling to the system and minimizing an appropriate distance between the two-time correlation function of this effective bath and that of the target environment, the error induced in the reduced dynamics of the system is brought under rigorous control. The interactions among the effective modes provide remarkable flexibility in replicating non-Markovian effects on the system even with a small number of oscillators, and the resulting Lindblad equation for the system and the modes may therefore be integrated at a very reasonable computational cost using standard methods for Markovian problems, even in strongly nonperturbative coupling regimes and at arbitrary temperatures including zero. We apply the method to an exactly solvable problem in order to demonstrate its accuracy, and present two studies based on current research in the context of coherent transport in biological aggregates and organic photovoltaics as more realistic examples of its use and potential; performance and versatility are highlighted, and theoretical and numerical advantages over existing methods, as well as possible future improvements, are discussed.

We present a comprehensive numerical analysis of violations of local realism by tripartite generalized Greenberger-Horne-Zeilinger states. As an indicator of nonlocality we use the nonlocal fraction which describes the probability of violation of local realism under randomly sampled observables. We compare two kinds of local realism based on standard and hybrid local–nonlocal models. As a result, we show a great disproportion of the results determined for both models. Although the nonlocal fraction for standard local realism and tripartite generalized Greenberger-Horne-Zeilinger states significantly increases compared to the bipartite Clauser-Horne-Shimony-Holt scenario, the genuine tripartite nonlocality (the strongest form of nonlocal correlations) is observed with probability much smaller than its bipartite counterpart. Furthermore, when the effects of decoherence on these states are introduced, such disproportion becomes significantly greater. Finally, we present the statistical relevance of various classes of tight Bell inequalities as they are of paramount importance for practical experimental investigation of all problems discussed in this paper. We also propose the nonlocal fraction of hybrid local–nonlocal realism as a measure of genuine tripartite entanglement.