We propose a scheme to realize the controlled-phase gates between nitrogen-vacancy (NV) centers in an optically trapped nanodiamond, through a uniform magnetic field-induced coupling between the NV centers and the torsional mode of the levitated nanodiamond. The gates are insensitive to the thermal noise of the torsional mode. By combining the scheme with the dynamical decoupling, it is found that the high-fidelity universal quantum gates are possible with present experimental technology. The proposed scheme is useful for an NV-center-based quantum network and distributed quantum computation.

We investigate triatomic molecules that consist of two ground-state atoms and a highly excited Rydberg atom, bound at large internuclear distances of thousands of angstroms. In the molecular state the Rydberg electron is in a superposition of high angular momentum states whose probability densities resemble the form of trilobite fossils. The associated potential-energy landscape has an oscillatory shape and supports a rich variety of stable geometries with different bond angles and bond lengths. Based on an electronic structure investigation we analyze the molecular geometry systematically and develop a simple building principle that predicts the triatomic equilibrium configurations. As a representative example we focus on ${}^{87}\mathrm{Rb}$ trimers correlated to the $n=30$ Rydberg state. Using an exact diagonalization scheme we determine and characterize localized vibrational states in these potential minima with energy spacings on the order of 100 $\mathrm{MHz}\times h$.

We present a study about vibrational quenching and chemical processes of cold molecular ions immersed in an ultracold atomic gas. In particular, the quasiclassical trajectory method is applied to ${\mathrm{BaRb}}^{+}\left(v\right)+\text{Rb}$ collisions at $T\gtrsim 1\mathrm{mK}$, revealing a large vibrational quenching cross section, which follows the Langevin capture model prediction. These results play a key role in the understanding of relaxation phenomena of cold molecular ions in ultracold gases.

We present a multichannel finite-range three-body scattering theoretical model used for investigating the magnetically tuned atom-dimer collision reaction. We take the ${}^6\mathrm{Li}\text{−}{}^6\mathrm{Li}_2$ system as an example to calculate the atom-dimer scattering length and dimer relaxation rate steered by a magnetic field. When the reactants are prepared in three shallow atom-dimer hyperfine channels, our calculation results agree with two experimental measurements [S. Nakajima et al., Phys. Rev. Lett. 105, 023201 (2010); T. Lompe et al., Phys. Rev. Lett. 105, 103201 (2010)]. We find that the two-body $p$-wave interaction has a significant influence on the atom-dimer collision reaction in the three hyperfine channels. By including the hyperfine and Zeeman interactions, we calculate the hyperfine state-to-state transition probability from the reactant channel to the deep atom-dimer hyperfine channel. We find a universality in the atom-dimer collision reaction, i.e., the universality of deep dimer product. The universality of deep dimer product in the ${}^6\mathrm{Li}\text{−}{}^6\mathrm{Li}_2$ collision reaction is more robust at magnetic field $B>620$ G than that at $B<620$ G.

The laser-induced nonsequential double ionization (NSDI) of rare gas atoms in the near and mid-IR laser fields is studied experimentally and theoretically. We investigate electron-electron correlation at high recollision energies, experimentally achieving ponderomotive energies ($U_p$) above 80 eV. The contribution of the two dominant channels of NSDI in the photoelectron momentum distribution, impact, and excitation ionization, are both shown to scale with ponderomotive energy and are well reproduced by theory. Surprisingly, for a large $U_p$ in mid-IR fields, a noticeable electron-electron anticorrelation signal emerges at low photoelectron momenta, which cannot be explained by these mechanisms within state-of-the-art theoretical approaches.

We propose a method of experimental coherent control that exploits partial and/or prior knowledge of a molecular system to efficiently arrive at a solution by using an artificial neural network (ANN) to generate a control field in consecutive temporal steps based on dynamic experimental feedback. Using a one-dimensional double-well potential model corresponding to the torsional motion of 3,5-difluoro-$3^{\prime },5^{\prime }$-dibromobiphenyl (${\mathrm{F}}_2{\mathrm{H}}_3{\mathrm{C}}_6-{\mathrm{C}}_6{\mathrm{H}}_3{\mathrm{Br}}_2$) to outline and verify our approach, we theoretically demonstrate that an optimized ANN can achieve robust quantum control of nuclear wave-packet transfer between wells despite the addition of random perturbations to the simulated molecular potential energy and polarizability surfaces. We suggest that under certain conditions this will also allow the ANN to achieve the stated control objective in an experimental situation. We show that the number of measurements our method requires to generate an optimized field is equal to the dimensionality of the optimization problem, which is significantly less than a naive closed-loop approach would generally need to achieve the same results.

A coincidence momentum imaging method is used to investigate the dissociative ionization of carbonyl sulfide (OCS) irradiated by linearly polarized 800 nm femtosecond laser pulses. As a typical asymmetric molecule, kinetic-energy release (KER) distributions of two-body Coulomb explosion (CE) along C-S or C-O bond breakage from $\mathrm{OC}{\mathrm{S}}^{q+}$ ($q=2$, 3, 4) exhibit a different behavior when the parent charge state increases. Two processes, i.e., concerted enhanced multiple ionization and ladder-climbing-type sequential ionization, are proposed for the C-S and C-O bond breakage, respectively. With the help of the potential-energy curves (PEC) obtained from the multistate density-functional-theory method, the KER values are calculated for these CE channels of $\mathrm{OC}{\mathrm{S}}^{q+}$. The overall good quantitative agreement between the experiment and calculation confirms the validity of the ionization mechanism assumed. In our approach, the global minimum of cationic PEC for the OCS can be treated as $R_c$ for enhanced ionization. Furthermore, a bimodal KER distribution was observed for the channel of $\mathrm{OC}{\mathrm{S}}^{2+}\to {\mathrm{O}}^{+}+\mathrm{C}{\mathrm{S}}^{+}$, and with the help of PEC, we believe that both of the ionization processes are involved in the dynamics of its formation.

We theoretically demonstrate that a pair of Dirac monopoles with opposite synthetic charges can be created within a single spin-1 Bose-Einstein condensate by steering the spin degrees of freedom by external magnetic fields. Although the net synthetic magnetic charge of this configuration vanishes, both the monopole and the antimonopole are accompanied by vortex filaments carrying opposite angular momenta. Such a Dirac dipole can be realized experimentally by imprinting a spin texture with a nonlinear magnetic field generated by a pair of coils in a modified Helmholtz configuration. We also investigate the case where the initial state for the dipole-creation procedure is pierced by a quantized vortex line with a winding number $\kappa$. It is shown that if $\kappa =-1$, the resulting monopole and antimonopole lie along the core of a singly quantized vortex whose sign is reversed at the locations of the monopoles. For $\kappa =-2$, the monopole and antimonopole are connected by a vortex line segment carrying two quanta of angular momentum, and hence the dipole as a whole is an isolated configuration. In addition, we simulate the long-time evolution of the dipoles in the magnetic field used to create them. For $\kappa =0$, each of the semi-infinite doubly quantized vortices splits into two singly quantized vortices, as in the case of a single Dirac monopole. For $\kappa =-1$ and $\kappa =-2$, the initial vortices deform into a vortex with a kink and a vortex ring, respectively.

We investigate the use of correlated photon pair sources for the improved quantum-level detection of a target in the presence of a noise background. Photon pairs are generated by spontaneous four-wave mixing, one photon from each pair (the herald) is measured locally while the other (the signal) is sent to illuminate the target. Following diffuse reflection from the target, the signal photons are detected by a receiver and nonclassical timing correlations between the signal and herald are measured in the presence of a configurable background noise source. Quantum correlations from the photon pair source can be used to provide an enhanced signal-to-noise ratio when compared to a classical light source of the same intensity.

We present a theoretical scheme for ground-state cooling of a mechanical resonator in a membrane-in-the-middle optomechanical system (OMS) driven by two red-detuned drive fields. The details of dynamical evolution of OMS are provided, and the effect of system conditions on cooling results are systematically studied. Most importantly, the setups with two drives are found to be capable of achieving better cooling results than the theoretical cooling limit with single cavity. Even an improvement by one order of thermal phonon number is possible with proper combination of the cavity damping rate and drive intensity.

Spin-orbit coupling (SOC) light fields with spatially inhomogeneous polarization have attracted increasing research interest within the optical community. In particular, owing to their spin-dependent phase and spatial structures, many nonlinear optical phenomena which we have been familiar with up to now are being reexamined; hence there is a revival of research in nonlinear optics. To fully investigate this topic, knowledge on how the topological structure of the light field evolves is necessary, but, as yet, there are few studies that address the structural evolution of the light field. Here, we present a universal approach for theoretical tomographic treatment of the structural evolution of SOC light in nonlinear optics processes. Based on a Gedanken vector second-harmonic generation, a fine-grained slice of evolving SOC light in a nonlinear interaction and the following diffraction propagation are studied theoretically and verified experimentally, which at the same time reveal several interesting phenomena. The approach provides a useful tool for enhancing our capability to obtain a more nuanced understanding of vector nonlinear optics, and sets a foundation for further broad-based studies in nonlinear systems.

Faraday rotation has become a powerful tool in a large variety of physics applications. Most prominently, Faraday rotation can be used in precision magnetometry. Here we report measurements of gyromagnetic Faraday rotation on a dense, hyperpolarized ${}^3\mathrm{He}$ gas target. Theoretical calculations predict the rotations of linearly polarized light due to the magnetization of spin-1/2 particles on the scale of ${10}^{-7}$ radians. To maximize the signal, a ${}^3\mathrm{He}$ target designed to use with a multipass cavity is combined with a sensitive apparatus for polarimetry that can detect optical rotations on the order of ${10}^{-8}$ radians. Although the expected results are well above the sensitivity for the given experimental conditions, no nuclear-spin-induced rotation was observed.

We demonstrate microwave-to-optical conversion using six-wave mixing in cold ${}^{87}\mathrm{Rb}$ atoms where the microwave field couples to two Rydberg states and propagates collinearly with the converted optical field. Our experiment is performed with a free-space microwave field, and we achieve a conversion efficiency of about $5\%$ for the microwave photons entering the conversion medium. In addition, we theoretically investigate all-resonant six-wave mixing and outline a realistic experimental scheme for reaching an efficiency close to 70%.

We propose a theoretical method to enhance the coherent dipole coupling between two atoms in an optical cavity via parametrically squeezing the cavity mode. In the present scheme, conditions for coherent coupling are derived in detail and diverse dynamics of the system can be obtained by regulating system parameters. In the presence of environmental noise, an auxiliary squeezed field is employed to suppress and even completely eliminate the additional noise induced by squeezing. In addition, we demonstrate that our scheme enables the effective suppression of atomic spontaneous emission. The results in our investigation could be used for diverse applications in quantum technologies.

The optical Yoneda and Brewster scattering phenomena are studied theoretically based on perturbative solutions of the reduced Rayleigh equations. The Yoneda phenomenon is characterized as an enhancement of the intensity of the diffuse light scattered by a randomly rough interface between two dielectric media when the light is observed in the optically denser medium. The intensity enhancement occurs above a critical angle of scattering which is independent of the angle of incidence of the excitation. The Brewster scattering phenomenon is characterized by a zero scattered intensity either in the reflected or transmitted light for an angle of scattering which depends on the angle of incidence. We also describe a generalization of the Brewster scattering phenomenon for outgoing evanescent waves and circularly polarized waves. The physical mechanisms responsible for these phenomena are described in terms of simple notions such as scalar waves, oscillating and rotating dipoles, and geometrical arguments, and are valid in a regime of weakly rough interfaces.

Superresolution is an important feature needed for modern optical microscopy. It can be achieved by using the transmission of evanescent waves in hyperbolic metamaterials. However, such devices suffer from material losses. Here we investigate a recently proposed metamaterial solid immersion lens—an assembly of dielectric nanoparticles. Using the multiple-scattering theory we reveal conversion of evanescent to propagating waves under conditions of coherent scattering. However, efficiency of the conversion is rather low as confirmed by the transmission of the fields of a couple of point sources. Comparing the scattering of light of a spherical cluster of nanospheres with that of a solid sphere of the same radius we find the same far fields in both cases. Generally, our conclusion is that material in the form of the assembly of nanoparticles behaves like an effective medium and does not demonstrate the superresolution in the far field. We believe that the resolution of the metamaterial solid immersion lens has the same origin as that of the conventional sold immersion lens.

Gaussian boson sampling is a promising scheme for demonstrating a quantum computational advantage using photonic states that are accessible in a laboratory and, thus, offer scalable sources of quantum light. In this contribution, we study two-point photon-number correlation functions to gain insight into the interference of Gaussian states in optical networks. We investigate the characteristic features of statistical signatures which enable us to distinguish classical from quantum interference. In contrast to the typical implementation of boson sampling, we find additional contributions to the correlators under study which stem from the phase dependence of Gaussian states and which are not observable when Fock states interfere. Using the first three moments, we formulate the tools required to experimentally observe signatures of quantum interference of Gaussian states using two outputs only. By considering the current architectural limitations in realistic experiments, we further show that a statistically significant discrimination between quantum and classical interference is possible even in the presence of loss, noise, and a finite photon-number resolution. Therefore, we formulate and apply a theoretical framework to benchmark the quantum features of Gaussian boson sampling under realistic conditions.

There is recurrent interest in the orbital angular momentum (OAM) conveyed by optical vortices, which are structured beams with a helically twisted wave front. Particular significance is attached to the issue of how material interactions with light conveying OAM might prove sensitive to the handedness and degree of twist in the optical wave front. As a result of recent experimental and theoretical studies, the supposition that beams with OAM might enable spectroscopic discrimination between oppositely handed forms of matter has become a renewed focus of attention. Some of the tantalizing conclusions that are beginning to emerge from this research have, however, not yet established a definitive basis for a supporting mechanism. To resolve this problem requires the development of theory to support a faithful representation, and a thorough understanding, of the fundamental molecule-photon physics at play in such optical processes—even for processes as basic as absorption. The present analysis establishes mechanisms at play that entail an unconventional manifestation of optical spin-orbit interactions, engaging transition electric-quadrupole moments. Powerful symmetry principles prove to render distinctively different criteria governing the exhibition of two-dimensional (2D) and 3D chirality. These results elucidate the operation of such effects, identifying their responsibility for discriminatory optical interactions of various forms in both chiral and achiral media.

A Bell test can rule out local realistic models and has potential applications in communication and information tasks. For example, a Bell's inequality violation can certify the presence of intrinsic randomness in measurement outcomes, which can be used to generate unpredictable random numbers. Nevertheless, a Bell test requires measurements that are chosen independently of environment in the test, as would be the case if the measurement setting choices were themselves intrinsically random. Such situation seems to create a “bootstrapping problem” recently addressed in the BIG Bell Test, a collection of various Bell tests using human choices. Here, we report in detail our experimental methods and results within the BIG Bell Test, specifically for a special type of Bell inequality, known as the measurement-dependent local inequality. With this inequality, even a small amount of measurement independence makes it possible to disprove local realistic models. The experiment utilizes human-generated random numbers in selecting the measurement settings and is implemented with space-like separation between two distant measurement sites. The experimental result violates a Bell's inequality, which cannot be explained by local hidden variable models with independence parameter (as defined in [G. Pütz et al., Phys. Rev. Lett. 113, 190402 (2014)]) $l>0.10\pm 0.05$. This result further quantifies the degree to which a local hidden variable model would need to constrain human choices, if it is to reproduce the experimental results.

A sequential steering scenario is investigated, where multiple Bobs aim at demonstrating steering using successively the same half of an entangled quantum state. With isotropic entangled states of local dimension $d$, the number of Bobs that can steer Alice is found to be $N_{\mathrm{Bob}}\sim d/logd$, thus leading to an arbitrary large number of successive instances of steering with independently chosen and unbiased inputs. This scaling is achieved when considering a general class of measurements along orthonormal bases, as well as complete sets of mutually unbiased bases. Moreover, we show that similar results can be obtained in an anonymous sequential scenario, where none of the Bobs know their position in the sequence. Finally, we briefly discuss the implication of our results for sequential tests of Bell nonlocality.