Plasmonic structures such as metal nanoparticles (MNPs) provide a promising way to explore and control light in a solid-state environment. Here, we study a hybrid system composed of a localized surface plasmon mode of a MNP interacting with a semiconductor quantum dot (QD) subject to a Faraday magnetic field. The MNP-QD system is driven by an externally applied probe laser field and then its scattering photons are detected in the far field. To this end, employing a full quantum optical model, we theoretically calculate and demonstrate the scattering spectrum of the system via a combination of analytical and numerical techniques with realistic parameters. It is found that the applied magnetic field can be utilized to efficiently regulate and control the scattering line shapes. In particular, a class of optical transparency is induced in the scattering spectrum of the plasmon-excition (plexcitonic) system that we refer to as magnetically induced plexcitonic transparency. At the same time, a sharp double-Fano resonance profile is generated in the presence of the magnetic field. The predicted features of the spectra have potential applications in designing sensitive on-chip devices.

Multipartite nonlocality is an important measure of multipartite quantum correlations. In this paper, we show that the nonlocal $n$-site Mermin-Klyshko operator ${\widehat{M}}_n$ can be exactly expressed as a matrix product operator with a bond dimension $D=2$, and then the calculation of nonlocality measure $\mathcal{S}$ can be simplified into standard one-dimensional (1D) tensor networks. With the help of this technique, we analyze finite-temperature multipartite nonlocality in several typical 1D spin chains, including an $XX$ model, an $XXZ$ model, and a Kitaev model. For the $XX$ model and the $XXZ$ model, in a finite-temperature region, the logarithm measure of nonlocality (${log}_2\mathcal{S}$) is a linear function of the temperature $T$, i.e., ${log}_2\mathcal{S}\sim -aT+b$. It provides us with an intuitive picture about how thermodynamic fluctuations destroy multipartite nonlocality in 1D quantum chains. Moreover, in the $XX$ model $\mathcal{S}$ presents a magnetic-field-induced oscillation at low temperatures. This behavior has a nonlocal nature and cannot be captured by local properties such as the magnetization. Finally, for the Kitaev model, we find that in the limit $T\to 0$ and $N\to \infty$ the nonlocality measure may be used as an alternative order parameter for the topological-type quantum phase transition in the model.

We study the multimagnon blockade and magnon-induced tunneling in the hybrid ferromagnet-superconductor system, where a magnon mode representing the collective motion of spins in the yttrium iron garnet (YIG) sphere is coupled with a three-level $\mathrm{\Delta }$-type fluxonium qubit via the virtual-photon excitation from the microwave cavity. By appropriately choosing the parameters, the single magnon blockade, multimagnon blockade including two- and three-magnon blockades, and magnon-induced tunneling could be achieved. Fortunately, the switch from magnon blockade to magnon-induced tunneling could be obtained via controlling the overall phase of the loop transition determined by the driving fields. Furthermore, the single-magnon blockade could be transformed into a multimagnon blockade via the phase modulation. The scheme we present may provide an alternative method to manipulate the few-magnon states and have potential applications in quantum communication and quantum information processing.

Essentially all known quantum gates rely on a weak-coupling approximation resulting in linear dynamics. With the explicit example of trapped ions, we show how high-fidelity quantum gates can be achieved outside such an approximation, and we derive readily implementable driving fields to realize gates with extremely high fidelities for ions well outside the Lamb-Dicke regime with motional temperatures achievable by only Doppler cooling.

The electronic $2P\text{−}2S$ Lamb shift and the $2S\text{−}1S$ energy interval in muon-electron ions of lithium, beryllium, and boron with the muon in its ground state are calculated within the framework of perturbation theory in the fine-structure constant and the electron-muon mass ratio. The corrections of first and second orders of perturbation theory, which include the effects of vacuum polarization, nuclear structure, and recoil, are taken into account. The analytical results obtained in perturbation theory are compared with the results of calculations within the variational approach. The values obtained for the $2P\text{−}2S$ Lamb shift and the $2S\text{−}1S$ interval can be used for comparison with future experimental data and verification of quantum electrodynamics.

We use time-delayed, counter-rotating, circularly polarized few-cycle attosecond nonoverlapping pulses to study the temporal coherent control of the resonant process of two-photon double ionization (TPDI) of hydrogen molecule via doubly excited states for pulse propagation direction along $\widehat{\mathbf{k}}$ either parallel or perpendicular to the molecular axis $\widehat{\mathbf{R}}$. For $\widehat{\mathbf{k}}\parallel \widehat{\mathbf{R}}$ and a pulse carrier frequency of 36 eV resonantly populating the $Q_2{}^1\mathrm{\Pi }_u^{+}\left(1\right)$ doubly excited state as well as other ${}^1\mathrm{\Pi }_u^{+}$ doubly excited states, we find that the indirect ionization pathway through these doubly excited states changes the character of the kinematical vortex-shaped momentum distribution produced by the two direct ionization pathways from fourfold to twofold rotational symmetry. This result is similar to what found in TPDI of the He atom involving ${}^{1}P_{\pm 1}^o$ doubly excited states [Ngoko Djiokap and Starace, J. Opt. 19, 124003 (2017)]; however, angular distributions exhibiting a quantum beat effect between the ground state and a doubly excited state seen for the He atom are observed here for its molecular counterpart with an anomaly in shape and magnitude, not in frequency. The sixfold differential probability integrated over the azimuthal angle of the photoelectron pair shows that this anomaly is due to autoionization decays and quantum beats between doubly excited states. For $\widehat{\mathbf{k}}\perp \widehat{\mathbf{R}}$ and a broadband pulse carrier frequency of 30 eV populating the $Q_1{}^1\mathrm{\Pi }_u^{+}\left(1\right)$, $Q_1{}^1\mathrm{\Sigma }_u^{+}\left(1\right)$, $Q_2{}^1\mathrm{\Pi }_u^{+}\left(1\right)$, and $Q_1{}^1\mathrm{\Sigma }_u^{+}\left(2\right)$ doubly excited states, the momentum distribution is shown to exhibit dynamical electron vortices with four spiral arms, which originates from the interplay between the ${}^1\mathrm{\Delta }_g^{+}$, ${}^1\mathrm{\Pi }_g^{+}$, and ${}^1\mathrm{\Sigma }_g^{+}$ dynamical ionization amplitudes. Our treatment within either the adiabatic-nuclei approximation or fixed-nuclei approximation shows that the latter provides a very good account for this correlated process.

We report an unusual type of Talbot effect. Unlike the traditional Talbot effect taking place in the lateral direction, this ring-shaped Talbot effect takes place for beams with certain topological charges. A rigorous theoretical derivation for this phenomenon is provided based on the Fresnel diffraction theory. Experimental demonstration is conducted by periodically modulating a spatial light modulator to create beams with the ring-shaped periodical phase distribution. With a properly adjusted phase pattern, the beam will exhibit equally spaced ring intensity distribution at the fractional Talbot length.

We theoretically introduce a type of topological dipole soliton propagating in a Floquet topological insulator based on a kagome array of helical waveguides. Such solitons bifurcate from two edge states belonging to different topological gaps and have bright envelopes of different symmetries: fundamental for one component, and dipole for the other. The formation of dipole solitons is enabled by unique spectral features of the kagome array which allow the simultaneous coexistence of two topological edge states from different gaps at the same boundary. Notably, these states have equal and nearly vanishing group velocities as well as the same sign of the effective dispersion coefficients. We derive envelope equations describing the components of dipole solitons and demonstrate in full continuous simulations that such states indeed can survive over hundreds of helix periods without any noticeable radiation into the bulk.

Nonlinear stimulated Raman adiabatic passage (STIRAP) is a fascinating physical process that dynamically explores chaotic and nonchaotic phases. In a recent paper [A. Dey and M. Kulkarni, Phys. Rev. Res. 2, 042004(R) (2020)], such a phenomenon is realized in a cavity-QED platform. There, the emergence of chaos and its impact on STIRAP efficiency are mainly demonstrated in the semiclassical limit. In the present paper we treat the problem in a fully quantum many-body framework. With the aim of extracting quantum signatures of a classically chaotic system, it is shown that an out-of-time-ordered correlator (OTOC) measure precisely captures chaotic and nonchaotic features of the system. The prediction by OTOC is in precise matching with classical chaos quantified by Lyapunov exponent analysis. Furthermore, it is shown that the quantum route corresponding to the semiclassical followed state encounters a dip in single-particle purity within the chaotic phase, depicting a consequence of chaos. A dynamics through the chaotic phase is associated with spreading of the many-body quantum state and an irreversible increase in the number of participating adiabatic eigenstates.

In this paper, optical simulation of double rapid adiabatic passage (RAP) is investigated in three evanescently coupled waveguides with longitudinally varying detunings of the propagation constants. Unlike the time-dependent detuning which is kept linearly increased or decreased when the RAP is applied in two-state atomic systems, here all possible situations of the corresponding space-dependent detunings with or without sign flips are considered. At the maximum of the couplings in the waveguide structure, the detunings tend to become zero and the sign flips are supposed to occur. Theoretical analysis shows that different light evolutions such as light splitting, complete light transfer, or complete light return can be realized adiabatically, depending on the choices of the detunings. Moreover, two of the above three phenomena can be observed simultaneously in the waveguide structure under some circumstances. The theoretical analysis is confirmed by numerical calculations. Due to the robustness of the double RAP, the technique can be applied to design optical devices such as achromatic beam splitters or beam couplers.

Topological error correction provides an effective method to correct errors in quantum computation. It allows quantum computation to be implemented with a higher error threshold and high tolerating loss rates. We present a topological error correction scheme with continuous variables based on an eight-partite Gaussian cluster state. We show that topological quantum correlation between two modes can be protected against a single quadrature phase displacement error occurring on any mode and some of two errors occurring on two modes. More interestingly, some cases of errors occurring on three modes can also be recognized and corrected, which is different from the topological error correction with discrete variables. We show that the final error rate after correction can be further reduced if the modes are subjected to identical errors occurring on all modes with equal probability. The presented results provide a feasible scheme for topological error correction with continuous variables and it can be experimentally demonstrated with a Gaussian cluster state.

We estimate the resources required to prepare the ground state of a quantum many-body system on a quantum computer of intermediate size. This estimate is made possible using a combination of quantum many-body methods and analytic upper bounds. Our routine can also be used to optimize certain design parameters for specific problem instances. Lastly, we propose and benchmark an improved quantum state preparation procedure. We find that it reduces the circuit $T$-depth by a factor as large as ${10}^6$ for intermediate-size lattices.

We propose a fast and robust quantum state transfer protocol employing a Su-Schrieffer-Heeger chain, where the interchain couplings vary in time. Based on simple considerations around the terms involved in the definition of the adiabatic invariant, we construct an exponential time-driving function that successfully takes advantage of resonant effects to speed up the transfer process. Using optimal control theory, we confirm that the proposed time-driving function is close to optimal. To unravel the crucial aspects of our construction, we proceed to a comparison with two other approaches: one where the underlying Su-Schrieffer-Heeger chain is adiabatically time-driven and another where the underlying chain is topologically trivial and resonant effects are at work. By numerically investigating the resilience of each protocol to static noise, we highlight the robustness of the exponential driving.

Retrieving classical information encoded in optical modes is at the heart of many quantum information processing tasks, especially in the field of quantum communication and sensing. Yet, despite its importance, the fundamental limits of optical mode discrimination have been studied only in a few specific examples. Here we present a toolbox to find the optimal discrimination of any set of optical modes. The toolbox uses linear and semidefinite programming techniques, which provide rigorous (not heuristic) bounds, and which can be efficiently solved on standard computers. We study both probabilistic and unambiguous single-shot discrimination in two scenarios: the channel-discrimination scenario, typical of metrology, in which the verifier holds the light source and can set up a reference frame for the phase; and the source-discrimination scenario, more frequent in cryptography, in which the verifier only sees states that are diagonal in the photon-number basis. Our techniques are illustrated with several examples. Among the results, we find that, for many sets of modes, the optimal state for mode discrimination is a superposition or mixture of at most two number states; but this is not general, and we also exhibit counterexamples.

We study high-order harmonic generation (HHG) from aligned molecules in strong elliptically polarized laser fields numerically and analytically. Our simulations show that the spectra and polarization of HHG depend strongly on the molecular alignment and the laser ellipticity. In particular, for small laser ellipticity, large ellipticity of harmonics with high intensity is observed for parallel alignment, along with forming a striking ellipticity hump around the threshold. We show that the interplay of the molecular structure and two-dimensional electron motion plays an important role here. This phenomenon can be used to generate bright elliptically polarized extreme ultraviolet pulses.

We propose a way for sympathetic cooling of ions in an electromagnetic Paul trap: it implies the use for this purpose of cold buffer atoms in the region of atom-ion confinement-induced resonance (CIR). The problem is that the unavoidable micromotion of the ion and the long-range nature of its interaction with the environment of colder atoms in a hybrid atomic-ion trap prevent its sympathetic cooling. We show that the destructive effect of ion micromotion on its sympathetic cooling can, however, be suppressed in the vicinity of the atom-ion CIR. Here, the resonant blocking of the complete approach of an atom with an ion during a collision also blocks the enhancement of its micromotion. We investigate the effect of sympathetic cooling around CIRs in atom-ion and atom-atom confined collisions within the quasiclassical-quantum approach using the $\mathrm{Li}-{\mathrm{Yb}}^{+}$ and Li-Yb confined systems as an example. In this approach, the Schrödinger equation for a cold light atom is integrated simultaneously with the classical Hamilton equations for a hotter heavy ion or atom during collision. We have found the region near the atom-ion CIR where the sympathetic cooling of the ion by cold atoms is possible in a hybrid atom-ion trap. We also show that it is possible to improve the efficiency of sympathetic cooling in atomic traps by using atomic CIRs.

We theoretically investigate dynamic structure factors of a strongly interacting Fermi superfluid near an orbital Feshbach resonance with random phase approximation approach at zero temperature, and find their dynamical characters across the phase transition from a balanced conventional Bardeen-Cooper-Schrieffer (BCS) superfluid to a polarized Sarma superfluid by continuously varying the chemical potential difference of two spin components. In a Bose-Einstein-condensate-like regime of the Fermi superfluid, dynamic structure factors can help distinguish the in-phase ground state from the out-of-phase metastable state by the relative location of molecular excitation and Leggett mode, or the minimum energy to break a Cooper pair. In the phase transition from BCS to Sarma superfluid, we find the dynamic structure factor of Sarma superfluid has its own specific gapless excitation at a small transferred momentum where the collective phonon excitation acquires a finite width, and also a relatively strong atomic excitation at a large transferred momentum, because of the existence of unpaired Fermi atoms. Our results can be used to differentiate Sarma superfluid from BCS superfluid.

We consider the two-body problem in a periodic potential, and study the bound-state dispersion of a spin-$\uparrow$ fermion that is interacting with a spin-$\downarrow$ fermion through a short-range attractive interaction. Based on a variational approach, we obtain the exact solution of the dispersion in the form of a set of self-consistency equations, and apply it to tight-binding Hamiltonians with on-site interactions. We pay special attention to the bipartite lattices with a two-point basis that exhibit time-reversal symmetry, and show that the lowest-energy bound states disperse quadratically with momentum, whose effective-mass tensor is partially controlled by the quantum metric tensor of the underlying Bloch states. In particular, we apply our theory to the Mielke checkerboard lattice, and study the special role played by the interband processes in producing a finite effective mass for the bound states in a nonisolated flat band.

In the development of microwave-to-optical (MO) quantum transducers, suppressing added noise induced by the optical excitation remains a major challenge. Here we report an integrated superconducting cavity electro-optic circuit based on single crystalline thin-film aluminum nitride of ultralow microwave and optical losses. We demonstrate efficient bi-directional MO conversion at milli-Kelvin temperatures, with near-ground state microwave thermal excitation $({\overline{n}}_{\mathrm{e}}=0.09\pm 0.06)$, despite the peak power of the optical drive exceeding the cooling power of the dilution refrigerator mixing chamber. Our dynamical study further reveals different light-induced noise generation mechanisms and provides crucial guidelines for optimizing electro-optic circuits in future hybrid microwave-optical quantum links.

In the present work, we consider the existence and spectral stability of standing-wave solutions to a model for light propagation in a twisted multicore fiber with no gain or loss of energy. Numerical parameter continuation experiments demonstrate the existence of standing-wave solutions for sufficiently small values of the coupling parameter. Furthermore, standing waves exhibiting optical Aharonov-Bohm suppression, where there is a single waveguide which remains unexcited, exist when the twist parameter $\varphi$ and the number of waveguides $N$ is related by $\varphi =\pi /N$. Spectral computations and numerical evolution simulations suggest that standing-wave solutions where the energy is concentrated in a single site are neutrally stable. Solutions with asymmetric coupling and multipulse solutions are also briefly explored.