We observe long-range ${}^{85}\mathrm{Rb}$ and ${}^{87}\mathrm{Rb}$ ($24D_J+5S_{1/2}$) Rydberg molecules for eight different spin couplings, with binding energies up to 440 MHz and subpercent relative uncertainty. Isotopic effects of the molecular binding energies arise from the different masses and nuclear spins. Because the vibrational states involve different spin configurations and cover a wide range of internuclear separations, the states have different dependencies on the $s$-wave and $p$-wave scattering phase shifts for singlet and triplet scattering. Fitting the spectroscopic data, we comprehensively determine all four scattering length functions over the relevant energy range as well as the zero-energy scattering lengths of the two $s$-wave channels. Our unusually high temperature and low density (180 $\mu \mathrm{K}$, $1\times {10}^{11}{\mathrm{cm}}^{-3}$) suggest that the molecule excitation occurs through photoassisted collisions.

We investigate the ground-state properties of a bosonic Harper-Hofstadter model with local interactions on a finite cylindrical lattice with filling fraction $\nu =1/2$. We find that our system supports topologically ordered states by calculating the topological entanglement entropy, and its value is in good agreement with the theoretical value for the $\nu =1/2$ Laughlin state. By exploring the behavior of the density profiles, edge currents, and single-particle correlation functions, we find that the ground state on the cylinder shows all signatures of a fractional quantum Hall state even for large values of the magnetic flux density. Furthermore, we determine the dependence of the correlation functions and edge currents on the interaction strength. We find that depending on the magnetic flux density, the transition toward Laughlin-like behavior can be either smooth or it can happen abruptly for some critical interaction strength.

We consider theoretically ultracold interacting bosonic atoms confined to a wire geometry and coupled to the field of an optical cavity. A spin-orbit coupling is induced via Raman transitions employing a cavity mode and a transverse running-wave pump beam; the transition imprints a spatially dependent phase onto the atomic wave function. Adiabatic elimination of the cavity field leads to an effective Hamiltonian for the atomic degrees of freedom with a self-consistency condition. We map the spin-orbit coupled bosonic wire to a bosonic ladder in a magnetic field, by discretizing the spatial dimension. Using the numerical density-matrix renormalization group method, we show that in the continuum limit the dynamical stabilization of a Meissner superfluid is possible for parameters achievable by current experiments.

By connecting the optical and mechanical degrees of freedom, an optomechanical system can provide schemes for coherent control over the quantum states once entering the strong coupling regime. Well known as a unique substitution of conventional microcavities allowing operating in the deep subwavelength scale, plasmonic nanocavities usually suffer from pronounced dissipative loss that hinders the realization of strong coupling in the weak excitation limit. Here we theoretically predict that weak-excitation plasmomechanical strong coupling can be realized in a system consisting of two cuboid-shaped metal nanoparticles. It takes full advantage of a recorded low-loss nanocavity plasmon which is simultaneously highly sensitive to the interparticle distance and imposes an intense optical binding force between the two cuboids. The theoretical minimal number of plasmon quanta required by strong coupling is 441 for silver at optical frequency and 25 for graphene at near infrared. This plasmomechanical prototype could potentially offer a nanoscale testbed for quantum experiments in strong coupling regimes and further enrich the toolbox for sensing and precise measurements in molecular scale.

The creation of quantum coherences requires a system to be anharmonic. The simplest such continuous one-dimensional quantum system is the Kerr oscillator. It has a number of interesting symmetries we derive. Its quantum dynamics is best studied in phase space, using Wigner's distribution $W$ and the associated Wigner phase space current $\mathbf{J}$. Expressions for the continuity equation governing its time evolution are derived in terms of $\mathbf{J}$ and it is shown that $\mathbf{J}$ for Kerr oscillators follows circles in phase space. Using $\mathbf{J}$ we also show that the evolution's classical shear in phase space is quantum suppressed by an effective “viscosity.” Quantifying this shear suppression provides measures to contrast classical with quantum evolution and allows us to identify special quantum states.

We study the role of the transition rule in high-order harmonic generation from solids driven by linearly polarized laser fields. The orientation-dependent transition dipoles can regulate the emergence of the multiplateau structure. In the multiplateau zone, however, different from the mechanisms of step-by-step excitation reported previously, we find that the emission time and orientation-dependent yields exhibit synchronization and more fine patterns, respectively. In addition, the orientation-dependent yields and their fine patterns can be attributed to the collective contribution of the transition rule and quantum-path interference. Therefore, to better understand the constructive and destructive patterns in the orientation-dependent yields, we propose a scheme of intercycle interference, which can further be utilized as a tool to image the structure of the solids and provide an avenue to optimize the electron dynamics in solids for the production of attosecond pulses in a compact setup.

A flat band is nondispersive and formed under destructive interference. Although flat bands are found in various Hermitian systems, to realize a flat band in non-Hermitian systems is an interesting task. Here, we propose a flat band in a parity-time symmetric non-Hermitian lattice. The proposed flat band has entirely real energy and is formed at an appropriate match between synthetic magnetic flux and non-Hermiticity. The flat band energy is tunable. At a weak intercell coupling, the flat band is isolated, whereas at a strong intercell coupling, it intersects with the dispersive band at the non-Hermitian phase transition point. The eigenstates of the flat band are compact localized states and are confined in one, two, or three unit cells at the edges or inside the non-Hermitian lattice.

Reduced bases are obtained for a single three-level atom interacting dipolarly with a two-mode electromagnetic field in a cavity. A truncation scheme for the infinite dimensional Hilbert space of the system is proposed, which we take it as the exact ground-state solution. This is used to determine the quantum phase diagram of the atomic $\mathrm{\Lambda }$ configuration and to judge the goodness of the reduced bases. This provides us with a mathematical technique that can be used to solve systems where the number of atoms and excitations grow, yielding a Hilbert space with enormous dimensions, more effectively than with the currently available methods. Additionally, the sudden changes suffered by the ground-state solution can be observed through the calculation of the purity and the occupation probabilities.

We consider transfer of optical vortices between laser pulses carrying orbital angular momentum in a cloud of cold atoms characterized by the $\mathrm{\Lambda }$ configuration of the atom-light coupling. The atoms are initially prepared in a coherent superposition of the lower levels, creating a so-called phaseonium medium. If a single vortex beam initially acts on one transition of the scheme, an extra laser beam is subsequently generated with the same vorticity as that of the incident vortex beam. The absorption of the incident probe beam takes place mostly at the beginning of the atomic medium within the absorption length. The losses disappear as the probe beam propagates deeper into the medium where the atoms are transferred to their dark states. The method is extended to a tripod atom-light coupling scheme and a more general ($n+1$)-level scheme containing $n$ ground states and one excited state, allowing for creation of multiple twisted light beams. We also analyze generation of composite optical vortices in the scheme using a superposition of two initial vortex beams and study lossless propagation of such composite vortices.

We study the correlated evolutions of two Rydberg atoms, interacting via a van der Waals (vdW) potential $V_6$ and driven by a laser field of detunings ${\mathrm{\Delta }}_1$ and ${\mathrm{\Delta }}_2$. The two atoms may exhibit the in-phase dynamics with identical Rydberg populations or the antiphase dynamics with complementary Rydberg populations, depending on their initial states. For a moderate vdW potential far from the blockade regime, the in-phase or antiphase dynamics can be attained along two intersecting lines in the parameter space of ${\mathrm{\Delta }}_1$ and ${\mathrm{\Delta }}_2$ with an exact or approximate figure of merit, respectively. Note, in particular, that the exact in-phase dynamics is trivial because it requires identical detunings while the approximate in-phase, exact antiphase, and approximate antiphase dynamics are nontrivial because they require distinct detunings. The specific requirements on ${\mathrm{\Delta }}_1$ and ${\mathrm{\Delta }}_2$ for both in-phase and antiphase dynamics can be understood by considering the balanced transitions from two initially populated states to two initially empty states in the double-atom state basis.

We systematically study mode characteristics in circular-side polygonal microcavities (CSPMs), particularly in these cavities with chaotic ray dynamics, in order to gain insights into the wave chaos in the CSPMs. The circular sides could improve the light confinement of the CSPMs as concave mirrors, in that regular islands are formed around the stable fixed points in the Poincaré surface of sections (SOS). However, the fixed points become unstable under some specific deformations, and global chaos with quasistable “star islands” appears around these fixed points in the Poincaré SOS accordingly. The phenomenon can be well explained by the ray dynamic analysis under the second-order approximation, and the results show that the high-order terms play an important role in the motions of light rays and destroy the regular islands in the phase space leading to chaotic ray dynamics. The destruction of regular islands results in degradation of mode quality factors and dispersed mode field distributions according to the finite-element method simulation of the confined modes. Furthermore, an unusual variation of mode quality factor is observed by varying the refractive index of the outside media for the CSPM with chaotic ray dynamics.

We show how the dispersive regime of the Jaynes-Cummings model may serve as a valuable tool to the study of open quantum systems. We employ it in a bottom-up approach to build an environment that preserves qubit energy and induces varied coherence dynamics. We then present the derivation of a compact expression for the qubit coherence, applied here to the case of a finite number of thermally populated modes in the environment. We also discuss how the model parameters can be adjusted to facilitate the production of short-time monotonic decay (STMD) of the qubit coherence. Our results provide a broadly applicable platform for the investigation of energy-conserving open system dynamics which is fully within the grasp of current quantum technologies.

Local realistic models cannot completely describe all predictions of quantum mechanics. This is known as Bell's theorem that can be revealed either by violations of Bell inequality, or all-versus-nothing proof of nonlocality. Hardy's paradox is an important all-versus-nothing proof and is considered as “the simplest form of Bell's theorem.” In this work, we advance the study of Hardy's paradox based on Bell's inequality. Our formalism is essentially equivalent to the “logical Bell inequality” formalism developed by Abramsky and Hardy, but ours is more easily applied to experimental tests. Remarkably, we find that not only for a two-qubit two-setting case but also for a two-qubit four-setting case, we can construct stronger Hardy-type paradoxes based on Bell's inequality, whose successful probabilities can attain four times and seven times larger than the original one, respectively, thus providing more friendly tests for experiment. Meanwhile, we experimentally test the stronger Hardy-type paradoxes in a two-qubit system. Within the experimental errors, the experimental results coincide with the theoretical predictions.

One of the challenges in practical quantum key distribution is dealing with efficiency mismatch between different threshold single-photon detectors. There are known bounds for the secret key rate for the BB84 protocol with detection-efficiency mismatch provided that the eavesdropper sends no more than one photon to the legitimate receiver. Here we improve these bounds and give tight bounds for the secret key rate with a constant detection-efficiency mismatch under the same single-photon assumption. We propose a method based on the analytical minimization of the relative entropy of coherence, which can be used in other problems in quantum key distribution. Also we propose an adaptation of the decoy state method to prove the security in the case of weak coherent pulses on the source side.

Investigation of the nuclear magnetic quadrupole moment (MQM) is an excellent way to reveal the new physics in the hadron sector of matter. Therefore, we investigate the violation of parity ($\mathcal{P}$) and time-reversal ($\mathcal{T}$) invariance induced by the MQM of the ${}^{201}\mathrm{Hg}$ nucleus in the HgH molecule, which has been proposed as a very promising candidate for the experimental search of the electric dipole moment of electron [M. G. Kozlov and A. Derevianko, Phys. Rev. Lett. 97, 063001 (2006)]. We report the precise value of the molecular parameter, $W_{\text{M}}$, associated with the $\mathcal{P},\mathcal{T}$-odd nuclear MQM-electron interaction in ${}^{201}\mathrm{HgH}$ using the four-component relativistic coupled-cluster method. This parameter is required to interpret the experimental $\mathcal{P},\mathcal{T}$-odd frequency shift in terms of the MQM of nuclei. Furthermore, the magnetic hyperfine structure (HFS) constants of the molecule are computed at the same level of theory. We also study the role of core-correlating functions and the virtual energy functions in the calculations of the HFS constant and $W_{\text{M}}$. The most reliable value of $W_{\text{M}}$ in HgH is obtained as $3.22\times {10}^{33}$ Hz/$e$ ${\mathrm{cm}}^2$ with an uncertainty of around 6%.

We report a measurement of the ratio of electric dipole transition matrix elements of cesium for the $6p{}^2{P}_{1/2}\to 7s{}^2{S}_{1/2}$ and $6p{}^2{P}_{3/2}\to 7s{}^2{S}_{1/2}$ transitions. We determine this ratio of matrix elements through comparisons of two-color, two-photon excitation rates of the $7s{}^2{S}_{1/2}$ state using laser beams with polarizations parallel to one another vs perpendicular to one another. Our result of $R\equiv \langle 7s{}^2{S}_{1/2}\left|\right|r\left|\right|6p{}^2{P}_{3/2}\rangle /\langle 7s{}^2{S}_{1/2}\left|\right|r\left|\right|6p{}^2{P}_{1/2}\rangle =1.5272\left(17\right)$ is in excellent agreement with a theoretical prediction of $R=1.5270\left(27\right)$. Moreover, the accuracy of the experimental ratio is sufficiently high to differentiate between various theoretical approaches. To our knowledge, there are no prior experimental measurements of $R$. Combined with our recent determination of the lifetime of the $7s{}^2{S}_{1/2}$ state, we determine reduced matrix elements for these two transitions, $\langle 7s{}^2{S}_{1/2}\left|\right|r\left|\right|6p{}^2{P}_{3/2}\rangle =-6.489\left(5\right)a_0$ and $\langle 7s{}^2{S}_{1/2}\left|\right|r\left|\right|6p{}^2{P}_{1/2}\rangle =-4.249\left(4\right)a_0$. These matrix elements are also in excellent agreement with theoretical calculations. These measurements improve knowledge of Cs properties needed for parity violation studies and provide benchmarks for tests of high-precision theory.

Resonant Auger spectra (RAS) of core-excited ${\mathrm{N}}_2$ are theoretically investigated by implementing an ultrashort x-ray pump and strong continuous-wave (CW) IR-control scheme. A femtosecond weak x-ray pump resonantly excites the nitrogen core-excited $1s\to {\pi }^{*}$ of the ${\mathrm{N}}_2$ molecule; a strong CW IR control induces dynamic Stark shift of the core-excited level. With the pulse duration of the ultrashort pump comparable to or even shorter than the period of the IR control, the time overlapping between the pump and control pulses reduces into a subcycle of the IR pulse; the efficient pumping process strongly depends on the dynamic Stark shift and the relative phase between the two pulses, resulting in a great manipulation of the core-excited wave-packet dynamics and the subsequent RAS. The results are numerically illustrated for the ${\mathrm{N}}_2$ molecule by a 2-fs x-ray pulse. The RAS also show a strong dependence on the bond-distance-dependent Auger decay probability.

Positronium in the $2{}^3\mathrm{S}$ metastable state exhibits a low electrical polarizability and a long lifetime (1140 ns), making it a promising candidate for interferometry experiments with a neutral matter-antimatter system. In the present work, $2{}^3\mathrm{S}$ positronium is produced, in the absence of an electric field, via spontaneous radiative decay from the $3{}^3\mathrm{P}$ level populated with a 205-nm UV laser pulse. Thanks to the short temporal length of the pulse, 1.5 ns full width at half maximum, different velocity populations of a positronium cloud emitted from a nanochanneled positron-positronium converter were selected by delaying the excitation pulse with respect to the production instant. $2{}^3\mathrm{S}$ positronium atoms with velocity tuned between $7\times {10}^4{\mathrm{ms}}^{-1}$ and $10\times {10}^4{\mathrm{ms}}^{-1}$ were thus produced. Depending on the selected velocity, a $2{}^3\mathrm{S}$ production efficiency ranging from $\sim 0.8\%$ to $\sim 1.7\%$, with respect to the total amount of emitted positronium, was obtained. The observed results give a branching ratio for the $3{}^3\mathrm{P}-2{}^3\mathrm{S}$ spontaneous decay of $(9.7\pm 2.7)\%$. The present velocity selection technique could allow one to produce an almost monochromatic beam of $\sim 1\times {10}^{3}2{}^3\mathrm{S}$ atoms with a velocity spread of $<{10}^4{\mathrm{ms}}^{-1}$ and an angular divergence of $\sim 50$ mrad.

We investigate the possibility of realizing supersolid quantum phases in bosonic Rydberg-excited quantum lattice gases in the presence of nonunitary processes, by simulating the dynamical evolution starting from initial preparation in nondissipative equilibrium states. Within Gutzwiller theory, we first analyze the many-body ground state of a bosonic Rydberg-excited quantum gas in a two-dimensional optical lattice for variable atomic hopping rates and Rabi detunings. Furthermore, we perform time evolution of different supersolid phases using the Lindblad-master equation. With the inclusion of two different nonunitary processes, namely, spontaneous decay from a Rydberg state to the ground state and dephasing of the addressed Rydberg state, we study the effect of nonunitary processes on those quantum phases and observe long-lived states in the presence of decay and dephasing. We find that long-lived supersolid quantum phases are observable within a range of realistic decay and dephasing rates, while high rates cause any initial configuration to homogenize quickly, preventing possible supersolid formation.