We consider an array of $N_e$ noninteracting qubits or emitters that are coupled to a one-dimensional cavity array with tunneling energy $J$ and nonlinearity of strength $U$. The number of cavities is assumed to be larger than the number of qubits. Working in the two-excitation manifold, we focus on the band-gap regime where the energy of two excited qubits is off-resonant with the two-photon bound state band. A two-step adiabatic elimination of the photonic degrees of freedom gives rise to a one-dimensional spin Hamiltonian with effective interactions; specifically, the Hamiltonian features constrained single-qubit hopping and pair hopping interactions not only between nearest neighbors but also between next-to-nearest and next-to-next-to-nearest spins. For a regularly arranged qubit array, we identify parameter combinations for which the system supports droplet-like bound states whose characteristics depend critically on the pair hopping. The droplet-like states can be probed dynamically. The bound states identified in our work for off-resonance conditions are distinct from localized hybridized states that emerge for on-resonance conditions.

The Hamiltonian for the one-dimensional Su-Schrieffer-Heeger (SSH) chain is one of the simplest Hamiltonians that supports topological states. This work considers between one and three finite SSH chains with open boundary conditions that either share a lattice site (or cavity), which—in turn—is coupled to a two-level emitter, or are coupled to the same two-level emitter. We investigate the system properties as functions of the emitter-cavity coupling strength $g$ and the detuning between the emitter energy and the center of the band gap. It is found that the energy scale introduced by the edge states that are supported by the uncoupled finite SSH chains leads to a $g$-dependent hybridization of the emitter and edge states that is unique to finite-chain systems. A highly accurate analytical three-state model that captures the band-gap physics of $k$-chain $(k\ge 1)$ systems is developed. To quantify the robustness of the topological system characteristics, the inverse participation ratio for the cavity-shared and emitter-shared systems consisting of $k$ chains is analyzed as a function of the on-site disorder strength. The $g$-dependent hybridization of the emitter and uncoupled edge states can be probed dynamically.

The minimum weight perfect matching (MWPM) decoder is the standard decoding strategy for quantum surface codes. However, it suffers a harsh decrease in performance when subjected to biased or nonidentical quantum noise. In this work, we modify the conventional MWPM decoder so that it considers the biases, the nonuniformities, and the relationship between $X$, $Y$, and $Z$ errors of the constituent qubits of a given surface code. Our modified approach, which we refer to as the recursive MWPM decoder, obtains an $18\%$ improvement in the probability threshold $p_{\text{th}}$ under depolarizing noise. We also obtain significant performance improvements when considering biased noise and independent nonidentically distributed (i.ni.d.) error models derived from measurements performed on state-of-the-art quantum processors. In fact, when subjected to i.ni.d. noise, the recursive MWPM decoder yields a performance improvement of $105.5\%$ over the conventional MWPM strategy, and in some cases, it even surpasses the performance obtained over the well-known depolarizing channel.

We propose a method of individual single-qubit addressing of linear trapped-ion chains utilizing two ultrastable femtosecond frequency combs. For that, we suggest implementing the single-qubit gates with two counterpropagating frequency combs overlapping on the target ion and causing the ac Stark shift between the qubit levels. With analytical calculations and numerical modeling, we show that the arbitrary single-qubit rotations can be indeed realized using only laser fields propagating along the ion chain. We analyze the error sources for the proposed addressing method and prove that it allows implementing the single-qubit gates with high fidelity.

Noise and errors are inevitable parts of any practical implementation of a quantum computer. As a result, large-scale quantum computation will require ways to detect and correct errors in quantum information. Here we present such a quantum error-correcting scheme for correcting the dominant phase and decay errors in superconducting qubit architectures using a hybrid approach combining autonomous correction based on engineered dissipation with traditional measurement-based quantum error correction. Using numerical simulations with realistic device parameters for superconducting circuits, we show that this scheme can achieve a five- to tenfold increase in storage time while using only six qubits for the encoding and two ancillary qubits for the operation of the autonomous correction, providing a potentially large reduction of qubit overhead compared to typical measurement-based error-correction schemes. Furthermore, the scheme relies on standard interactions and qubit driving available in most major quantum computing platforms, making it implementable in a wide range of architectures.

We have theoretically studied the field-strength-dependent high-order harmonic generation (HHG) from doped systems like nanosize or bulk materials. Our results show that when the amplitude of the vector potential $A_{\mathrm{peak}}$ of the driving laser reaches the half-width of the Brillouin zone ($\pi /a_0$), the harmonic yield of the undoped systems is larger than that of the doped systems. The band-climbing process enhances the interband transition of HHG for higher conduction bands. We find that the donor-doped states and valence-band states from the doped system both contribute to the HHG. The destructive interference between the above two channels results in the harmonic yield from the undoped system overtaking that of the doped system when $A_{\mathrm{peak}}$ is around or higher than $\pi /a_0$, especially when the phase of HHG contributed by the above two channels is close to $\pi$ rad. When $A_{\mathrm{peak}}$ is below $\pi /a_0$, the harmonic yield of the doped systems is stronger than that of the undoped systems. The atomic doping density also influences the field-strength-dependent spectra.

Finding a feasible protocol for probing the quantum nature of gravity has been attracting an increasing amount of attention. We propose a protocol to enhance the detection of gravitationally induced entanglement by exploiting the two-phonon drive in a hybrid quantum setup. We consider the setup consisting of a test particle in a double-well potential, a qubit, and a quantum mediator. There is gravitational interaction between the test particle (TP) and the mediator, and a spin-phonon coupling between the mediator and the qubit. By introducing a two-phonon drive, the entanglement between the TP and the qubit are significantly enhanced and the entanglement generation rate is remarkably increased compared with the case without the two-phonon drive. Moreover, the entanglement between the TP and the qubit can be partially preserved in the presence of dephasing by the proposed strategy. This work would open a different avenue for experimental detection of the quantum nature of gravity, which could find applications in quantum information science.

The merged-beam rate coefficients of dielectronic and trielectronic recombinations (DR and TR) within $\mathrm{\Delta }N=0$ channels for Be-like ${\mathrm{Ar}}^{14+}$ were measured by Huang $etal.$ [Astrophys. J. Supp. Ser. 235, 2 (2018)] at the cooler storage ring in Lanzhou, China. Meanwhile, theoretical data were also calculated with autostructure (AS) code for comparison with the measured resonance spectrum. However, the AS calculations significantly underestimated TR resonance strengths in most cases. In the present work, we find that the electron correlation between DR and TR resonance states with different captured electron principal quantum numbers ($n$) for the captured electrons (multi-$n$ CI) can significantly increase the strength of TR resonances with $n=6$ and 7, which is confirmed through the relativistic distorted-wave (RDW) approximation in the flexible atomic code (FAC) and the semirelativistic distorted-wave (SRDW) approximation in the AS. The corresponding plasma rate coefficients show a significant increase and are in better agreement with the experiment, with differences from the experiment reduced to $\sim 10\%$ in both photoionized plasmas (PP) and collisionally ionized plasmas (CP) temperature ranges, as opposed to the previous theoretical results that exhibited differences of up to $\sim 30\%$ over the same temperature ranges. Taking into account the multi-$n$ CI could account for most of the discrepancy between existing theoretical calculations and experimental results. Understanding electron correlation and its consequences is important for obtaining accurate TR rate coefficients that will be useful for modelers assessing its role in describing non-local thermodynamic equilibrium astrophysical plasma.

We investigate multiphoton double ionization of helium by ultrashort XUV pulses involving up to 5 XUV photons by numerically solving the time-dependent Schrödinger equation for helium in its full dimensionality. We explore the crucial role of dynamical electron correlations by systematically varying the pulse duration and the number of absorbed photons. With increasing photon number, a multitude of ionization pathways opens up. The relative importance of correlations varies with pulse duration. Even in the spectrally sequential regime, correlation effects become increasingly important as the pulse duration becomes shorter and as the number of absorbed photons is increased. We identify a pathway to double ionization (DI) involving correlated excitation ionization as an intermediate step which can provide a contribution to the total DI yield comparable to the conventional direct sequential double ionization. Supported by concurrent classical trajectory Monte Carlo simulations, we identify strong electron-electron correlations in double ionization even at large distances from the nucleus.

We report on a computational study of resonant inelastic x-ray scattering (RIXS), at different fluorine $K$-edge resonances of the ${\mathrm{SF}}_6$ molecule, and corresponding nonresonant x-ray emission. Previously measured polarization dependence in RIXS is reproduced and traced back to the local $\sigma$ and $\pi$ symmetry of the molecular orbitals and corresponding states involved in the RIXS process. Also electron-hole coupling energies are calculated and related to experimentally observed spectator shifts. The role of dissociative S-F bond dynamics is explored to model detuning of RIXS spectra at the $|\mathrm{F}1s^{-1}6a_{1g}^1\rangle$ resonance, which shows challenges to accurately reproduce the required steepness for core-excited potential energy surface. We show that the RIXS spectra can only be properly described by considering breaking of the global inversion symmetry of the electronic wave function and core-hole localization, induced by vibronic coupling. Due to the core-hole localization we have symmetry forbidden transitions, which lead to additional resonances and changing width of the RIXS profile.

We examine theoretically a continuity between atomic and molecular Fermi superfluids in a Bose-Fermi mixture near the Feshbach resonance. Considering a two-channel model with Fermi atoms $f$, Bose atoms $b$, and the closed-channel molecules $F$, we construct a mean-field framework based on the perturbative expansion of the $b\text{−}f\text{−}F$ Feshbach coupling. The resulting effective Hamiltonian not only exhibits the continuity between atom-atom to molecule-molecule Cooper pairings but also becomes equivalent to the two-band-superconductor model with Suhl-Matthias-Walker–type pair-exchange coupling. We demonstrate how these atomic and molecular Fermi superfluids coexist within the two-band-like superfluid theory. The pair-exchange coupling $ff\leftrightarrow FF$ and resulting superfluid gaps $\langle ff\rangle$ and $\langle FF\rangle$ are found to be strongly enhanced near the Feshbach resonance due to the interplay between the infrared singularity of Bogoliubov phonons and their Landau damping arising from the coupling with fermions. The pair-exchange coupling can be probed via the observation of the intrinsic Josephson effect between atomic and molecular superfluids.

Highly excited Rydberg atoms are a powerful platform for quantum simulation and information processing. Here, we propose atomic ring networks to study chiral currents of Rydberg excitations. The currents are controlled by a phase pattern imprinted via a Raman scheme and can persist even in the presence of dephasing. Depending on the interplay between the Rabi coupling of Rydberg states and the dipole-dipole atom interaction, the current shows markedly different features. The excitations propagate with a velocity displaying a characteristic peak in time, reflecting the chiral nature of the current. We find that the time-averaged current in a quench behaves similarly to the ground-state current. This analysis paves the way for the development of new methods to transport information in atomic networks.

A wave packet localized within a binding potential feeds an outgoing probability current as it tunnels through a finite-width potential barrier, imprinting its localization-related information on the outgoing probability-current waveform. The product of the wave number and the gradient of the outgoing probability current provides a readout for this information, connecting to the wave-packet localization, Fisher information, and related information entropy production, thus revealing the information-flow underpinning of tunneling dynamics.

A chirped isolated attosecond pulse (IAP) will be usually produced if several chirped supercontinuum high-order harmonics are coherently superposed. Thus, to generate a broad bandwidth quasi-chirp-free IAP or even short isolated subattosecond pulse, one needs to reduce the attosecond chirp (attochirp) of high-order harmonic pulses. In this paper, we theoretically demonstrate the possibility of generating the quasi-chirp-free IAP from neon atom driven by the optimized four-color laser field, which is synthesized by two fundamental near-infrared lasers and their second-harmonic fields. By increasing the colors in synthesized field from two or three to four, the attochirp of broadband high-order harmonics over 100 eV is significantly reduced, and the generated IAP is shortened from more than 50 attosecond (as) to 37 as, approaching the Fourier-transform-limited pulse duration of 36 as. The classical analysis reveals that this mainly stems from the simultaneous return of electrons ionized at different times to the parent ion, as has been shown from other investigations regarding the necessity of producing attochirp-free high harmonics with multicolor laser fields. Most importantly, such short quasi-chirp-free isolated IAP survives after the macroscopic propagation effects are carried out, under proper gas pressure and laser beam waist. This work provides an efficient method for experiments to generate the quasi-chirp-free IAPs by using the synthesized and optimized four-color laser field.

We investigate the dynamical spreading of spatial correlations after a quantum quench starting from a magnetically disordered state in the transverse-field Ising model at one (1D) and two spatial dimensions (2D). We analyze specifically the longitudinal and transverse spin-spin correlation functions at equal time with use of several methods. From the comparison of the results in 1D obtained by the linear spin-wave approximation (LSWA) and those obtained by the rigorous analytical approach, we show that the LSWA can asymptotically reproduce the exact group velocity in the limit of strong transverse fields while it fails to capture the detailed time dependence of the correlation functions. By applying the LSWA to the 2D case, in which the rigorous analytical approach is unavailable, we estimate the propagation velocity to be $Ja/\left(2\hslash \right)$ at the strong-field limit, where $J$ is the Ising interaction and $a$ is the lattice spacing. We also utilize the tensor-network method based on the projected-entangled pair states for 2D and quantitatively compute the time evolution of the correlation functions for a relatively short time. Our findings provide useful benchmarks for quantum simulation experiments of correlation spreading and theoretical refinement of the Lieb-Robinson bound in the future.

We theoretically investigate a supersolid structure taking the form of stacked, disk-shaped superfluid droplets connected via a dilute superfluid, in an antidipolar condensate. A phase diagram is determined for varying the particle number and scattering length, identifying the regions of a regular dipolar superfluid, supersolid stacks, and isolated stacked disk-shaped droplets in an experimentally realizable trapping potential. The collective Bogoliubov excitation spectrum across the superfluid-supersolid phase transition is studied, and the transition point is found to be associated with the breaking of the degeneracy of the two lowest-lying modes. The dynamical generation of the supersolid stacks is also investigated by ramping down the scattering length across the phase transition. Moreover, we have studied the impact of vortex-line penetration on the phase transition. We have found that the presence of a vortex line causes the supersolid region to move towards weaker contact interactions. Our detailed numerical simulations highlight that an antidipolar condensate can create such supersolid stacks within an experimentally reachable parameter regime.

We numerically study the dynamics of a one-dimensional matter-wave soliton colliding with a vibrating atomic mirror. After colliding, the soliton splits into several wave packets with discrete momentum, and the kinetic energy of internal atoms is quantized. The reflected wave packets are originally diffusing but can form soliton states by quenching the nonlinear strength. The number and total kinetic energy of wave packets are obviously dependent on the vibrating amplitude and frequency of the mirror. We extend the physical system to an atomic mirror with double-frequency vibration, where the property of energy quantization also occurs and is related to the difference frequency. Our results may provide an effective way to prepare and manipulate matter-wave solitons, and are expected to contribute to the development of soliton interferometers.

We show how to prepare macroscopic entanglement between an atomic ensemble and a large number of magnons in a ferrimagnetic yttrium-iron-garnet crystal. Specifically, we adopt an optomagnomechanical configuration where the magnetostriction-induced magnomechanical displacement couples to an optical cavity via radiation pressure, and the latter further couples to an ensemble of two-level atoms that are placed inside the cavity. We show that, by properly driving the cavity and magnon modes, optomechanical entanglement is created which is further distributed to the atomic and magnonic systems, yielding stationary entanglement between atoms and magnons. The atom-magnon entanglement is a result of the combined effect of opto- and magnomechanical cooling and optomechanical parametric down-conversion interactions. A competition mechanism between two mechanical cooling channels is revealed. We further show that genuine tripartite entanglement of three massive subsystems, i.e., atoms, magnons, and phonons, can also be achieved in the same system. Our results indicate that the hybrid optomagnomechanical system may become a promising system for preparing macroscopic quantum states involving magnons, photons, phonons, and atoms.

We demonstrate the generation of degenerate photon pairs from spontaneous parametric down-conversion in titanium-indiffused waveguides in lithium niobate at cryogenic temperatures. Since the phase matching cannot be temperature tuned inside a cryostat, we rely on a precise empirical model of the refractive indices when fabricating a fixed poling period. We design the phase-matching properties of our periodic poling to enable signal and idler photons at $1559.3\left(6\right)$ nm and characterize the indistinguishability of our photons by performing a Hong-Ou-Mandel interference measurement. Despite the effects of photorefraction and pyroelectricity, which can locally alter the phase matching, we achieve cryogenic indistinguishable photons within 1.5 nm of our design wavelength. Our results verify sufficient understanding and control of the cryogenic nonlinear process, which has wider implications when combining quasi-phase-matched nonlinear optical processes with other cryogenic photonic quantum technologies, such as superconducting detectors.

We exactly solve a quantum Fermi accelerator model consisting of a time-independent non-Hermitian Hamiltonian with time-dependent Dirichlet boundary conditions. A Hilbert space for such systems can be defined in two equivalent ways, either by first constructing a time-independent Dyson map and subsequently unitarily mapping to fixed boundary conditions or by first unitarily mapping to fixed boundary conditions followed by the construction of a time-dependent Dyson map. In turn, this allows to construct time-dependent metric operators from a time-independent metric and two time-dependent unitary maps that freeze the moving boundaries. From the time-dependent energy spectrum, we find the known possibility of oscillatory behavior in the average energy in the $\mathcal{P}\mathcal{T}$ regime, whereas in the spontaneously broken $\mathcal{P}\mathcal{T}$ regime we observe the feature of a one-time depletion of the energy. We show that the $\mathcal{P}\mathcal{T}$ broken regime is mended with a moving boundary, equivalently to mending it with a time-dependent Dyson map.