A two-dimensional honeycomb lattice composed of gyrotropic rods is studied. Beginning with Maxwell's equations, a perturbed Wannier method is used to derive a tight-binding model with nearest and next-nearest neighbors. The resulting discrete model leads to a generalized (photonic) Haldane model that supports topologically protected modes with nonzero Chern numbers. Varying the radii of the rods breaks inversion symmetry and can change the topology of the system. This model analytically describes experimental results associated with topological waves in magneto-optical honeycomb lattices. This method can also be applied to more general Chern insulator lattices. When on-site Kerr-type nonlinear effects are considered, coherent soliton-like modes are found to propagate robustly through boundary defects.

Exact formulas are derived for systems involving Landau-Zener transition rates and for absorption spectra in quantum dots. These rectify previous inaccurate approximations utilized in experimental studies. The exact formulas give an explicit expression for the maxima and minima of the transition rate at any oscillating period and reveal a number of striking physical consequences, such as the suppression of oscillations for half-integer values of the detuning parameter and that the periodic dependence on the detuning parameter changes at special values of the driving field amplitude. The fluorescence spectra of quantum dots exhibit similar properties.

Two theoretical models of semiclassical matrix method and quantum Green's function are extended to the system of x-ray thin-film planar cavity with inner-shell electronic resonances. The semiclassical model is based on the matrix formalism by treating each layer as the propagating matrix. The crucial idea is to expand the propagating matrix of the resonant atomic layer under ultrathin-film approximation, and then derive the analytical expression of the spectral observation, i.e., the cavity reflectance. Typical cavity-enhanced decay rate and cavity-induced energy shift as well as the Fano interference which were observed in recent experiments could be phenomenologically interpreted. The quantum model employs analytical Green's function to calculate the cavity system. The system Hamiltonian and the effective energy-level are derived. The effective energy-level scheme indicates that the cavity effect can control intermediate core-hole state. To test the validity of the semiclassical matrix and quantum Green's function models, the classical Parratt's formalism and the dispersion correction of the atomic refractive index are also recalled. Very good agreements in the reflectivity spectra between the semiclassical and quantum models with Parratt's results are observed. The equivalence between the matrix and Green's function models is discussed analytically and numerically. Based on these two theoretical models, the similarities and particularities of inner-shell electronic system are discussed and compared with the nuclear system. Several examples, including the weaker dipole moment, distorted Fano lineshape, and negligible collective effect, are given. The present semiclassical matrix and quantum Green's function models will be useful to predict new phenomena, to optimize cavity structure for future experiments, and to promote the emerging of x-ray quantum optical effects with modern x-ray spectroscopy techniques.

Steganography is an alternative to cryptography, where information is protected by secrecy (being disguised as innocent communication or noise) rather than being scrambled. In this work we develop schemes for steganographic communication using Fock and coherent states in optical channels based on disguising the communications as thermal noise, with a fidelity approaching 1. We derive bounds on their efficiency in terms of the communication rate and the required keyrate in the case of an all-powerful eavesdropper and provide explicit methods of encoding and error correction for the noiseless channel case.

We demonstrate an experimental technique for the quantum-state tomography of the collective qutrit states of a room-temperature alkali-metal vapor. It is based on the measurements of the polarization of light traversing the vapor subjected to a magnetic field. To assess the technique's robustness against errors, experimental investigations are supported with numerical simulations. This not only allows us to determine the fidelity of the reconstructed states, but also to analyze the quality of the reconstruction for specific experimental parameters, such as light tuning and the number of measurements. By utilizing the conditional number, we demonstrate that the reconstruction robustness can be optimized by a proper adjustment of experimental parameters, and further improvement is possible with the repetition of specific measurements. Our results demonstrate the potential of this high-fidelity reconstruction method of quantum states of room-temperature atomic vapors.

Stabilizer entropy (SE) quantifies the spread of a state in the basis of Pauli operators. It is a computationally tractable measure of nonstabilizerness and thus a useful resource for quantum computation. SE can be moved around a quantum system, effectively purifying a subsystem from its complex features. We show that there is a phase transition in the residual subsystem SE as a function of the density of non-Clifford resources. This phase transition has important operational consequences: it marks the onset of a subsystem purity estimation protocol that requires $poly\left(n\right)exp\left(t\right)$ many queries to a circuit containing $t$ non-Clifford gates that prepares the state from a stabilizer state. Then, for $t=O\left({log}_{2}n\right)$, it estimates the purity with polynomial resources, and, for highly entangled states, attains an exponential speed-up over the known state-of-the-art algorithms.

We study a hybrid structure of a ferromagnetic insulator and a superconductor connected by a weak link, which accommodates Andreev bound states whose spin degeneracy is lifted due to the exchange interaction with the ferromagnet. The resultant spin-resolved energy levels realize a two-state quantum system, provided that a single electron is trapped in the bound state, i.e., an Andreev spin qubit. The qubit state can be manipulated by controlling the magnetization dynamics of the ferromagnet, which mediates the coupling between external fields and the qubit. In particular, our hybrid structure provides a simple platform to manipulate and control the spin qubit using spintronic techniques. By employing a modified Hahn spin echo protocol for the magnetization dynamics, we show that our Andreev spin qubit can realize a nonadiabatic geometric gate.

Rydberg-atom synthetic dimensions in the form of a lattice of $n{}^{3}S_1$ levels, $58\le n\le 63$, coupled through two-photon microwave excitation are used to examine dynamics within the single-particle Su-Schrieffer-Heeger Hamiltonian. This paradigmatic model of topological matter describes a particle hopping on a one-dimensional lattice with staggered hopping rates. Tunneling rates between lattice sites and on-site potentials are set by the microwave amplitudes and detuning, respectively. An atom is first excited to a Rydberg state that lies within the lattice and then subject to the microwave dressing fields. After some time, the dressing fields are turned off and the evolution of the population distribution in the different final lattice sites monitored using field ionization. The measurements show the existence of long-lived symmetry-protected edge states and reveal the existence of direct long-distance tunneling between the edge states. The results are in good agreement with model calculations and further demonstrate the potential of Rydberg-atom synthetic dimensions to simulate and faithfully reproduce complex Hamiltonians.

On the basis of the variational method we study the energy levels of pionic helium $\left(\pi \text{−}e\text{−}\text{He}\right)$ and kaonic helium $\left(K\text{−}e\text{−}\text{He}\right)$ with an electron in the ground state and a meson in the excited state with principal and orbital quantum numbers $n\sim l+1\sim 20$. Variational wave functions are taken in the Gaussian form. Matrix elements of the basic Hamiltonian and corrections to vacuum polarization and relativism are calculated analytically in a closed form. We calculate some bound-state energies and transition frequencies which can be studied in the experiment.

The energy transfer processes in ${\mathrm{Cs}}^{+}-{\mathrm{N}}_2$ collisions were experimentally studied at laboratory (lab) collision energies $E_{\mathrm{lab}}=50-250\mathrm{eV}$ and lab scattering angles $\theta >2^{\circ }$ via differential scattering spectroscopy. A classical trajectory calculation method was used to analyze the experimental results. We observed specific electronic excitations to the $\mathrm{C}{\mathrm{s}}^{+}+{\mathrm{N}}_2\left(A{}^3\mathrm{\Sigma }\right)$ and $\mathrm{C}{\mathrm{s}}^{+}+{\mathrm{N}}_2\left(a{}^1\mathrm{\Pi }\right)$ states in the energy-transfer spectra with significant excitation probabilities in backward scatterings at lower center-of-mass (CM) energies $E=13.6-18.3\mathrm{eV}$ $(E_{\mathrm{lab}}=78-105\mathrm{eV})$ and a CM angle $\mathrm{\Theta }\sim {165}^{\circ }$ $(\theta =5^{\circ })$. The results of the simulations of energy transfer spectra through classical trajectory calculations indicate that electronic transitions occur around the molecular orientation $\gamma =\pi /2$ and at vibrotational excitation energies $\mathrm{\Delta }E/E<0.12$. The potential crossings for the ${\mathrm{N}}_2\left(A{}^3\mathrm{\Sigma }\right)$ and ${\mathrm{N}}_2\left(a{}^1\mathrm{\Pi }\right)$ excitations are located at the intermolecular potentials of $V_{\mathrm{IM}}\left(R,\gamma =\pi /2\right)=13.4$ and 16.4 eV, respectively. ${\mathrm{N}}_2\left(A{}^3\mathrm{\Sigma }\right)$ excitation is the dominant electronic excitation process at lower energies of $E<17.5\mathrm{eV}$ $(E_{\mathrm{lab}}<100\mathrm{eV})$. Evaluations using an empirical charge-overlap model reasonably reproduce intermolecular potential crossings. Moreover the proposed model can provide effective information on the spin-changing ${\mathrm{N}}_2\left(A{}^3\mathrm{\Sigma }\right)$ excitation.

We achieve the complete characterization of the transition phase in the resonant two-photon ionization (TPI) of hydrogen at the high laser intensity where Rabi oscillation occurs. Different from previous studies which focused on the transition phase of resonant TPI in the perturbation region, in our work, we reveal the effect of Rabi oscillations on the transition phase in the high-intensity region. With bichromatic extreme-ultraviolet light, the transition phases of the $S$ and $D$ partial waves are decoded. By solving the time-dependent Schrodinger equation, we show that the transition phase depends on not only the pulse duration but also the laser intensity. This dependence is traced back to the modification of the atomic state structure, rather than the competition between resonant and nonresonant ionization paths at the lower laser intensity. Our work provides a way to manipulate the two-photon transition phase through laser intensity.

Transverse radiation forces acting on the silicon nanoparticles illuminated by a tightly focused laser beam are investigated by using the Maxwell stress-tensor method combined with generalized Lorenz-Mie theory. It is found that the radiation force can be tuned flexibly by electric–magnetic multipolar resonances and the hybridization of the both kinds of resonances, and the nanoparticle receives a negative or positive transverse radiation force. We also analyze theoretically the far-field scattering patterns and the contributions of each typical electric–magnetic resonance and hybridization of electric and magnetic modes to the radiation forces on silicon nanoparticles with the multipolar expansion method. The simulation research results show that the negative (positive) transverse radiation forces mainly originate from the asymmetric far-field scattering when the particles are positioned at different positions in the focus plane. Our findings cannot only provide an effective strategy to enhance the efficiency of optical trapping of nanoparticles with the excitation of electric–magnetic resonances but also facilitate the optical sorting, selective trapping, and assembling of nanoparticles with a single tightly focused laser beam.

Temporal dissipative Kerr solitons (DKSs) and their frequency counterparts, Kerr frequency combs (KFCs), have been extensively explored in the anomalous dispersion regime for both continuous wave (CW) and pulse-pumped microresonators. Compared to the CW pumping, the pulsed pump scheme has been shown to produce DKSs with higher robustness. Recently DKS states have also been explored in the normal dispersion regime as well. In this work, we demonstrate a stable single DKS state originating from a normal dispersion regime of a silicon nitride $\left({\text{Si}}_3{\text{N}}_4\right)$ racetrack microring resonator with a built-in spectral filter for a chirped super-Gaussian pulsed pump. Numerical simulations aided with analytically derived formulations indicate that a stable square-shaped chirped DKS with a broad KFC can be generated. The proposed scheme also enhances the stability of the single-chirped DKS state. The interplay of input chirp along with the spectral filtering effect causes the stable single DKS to be trapped conveniently to the center of the driving pulse background. We believe that the findings along with derived analytical formulations will be useful and might find applications in communication, spectroscopy, optical tweezers, and more general all-optical manipulation of light pulses.

We investigate the generation of the frequency components at the second-order sidebands based on a spinning resonator containing a degenerate optical parametric amplifier (OPA). We show that an OPA driven by different pumping frequencies inside a cavity can enhance and modulate the amplitude of the second-order sideband with different influences. We find that both the second-order sideband amplitude and its associated group delay sensitively depend on the nonlinear gain of the OPA, the phase of the field driving the OPA, the rotation speed of the resonator, and the incident direction of the input fields. Tuning the pumping frequency of the OPA can remain the localization of the maximum value of the sideband efficiency and nonreciprocal behavior due to the optical Sagnac effect, which also can adjust the linewidth of the suppressive window of the second-order sideband. Furthermore, we extend the study of the second-order sideband to the non-Markovian bath, which consists of a collection of infinite oscillators (bosonic photonic modes). We illustrate that the second-order sidebands in a spinning resonator exhibit a transition from the non-Markovian to the Markovian regime by controlling the environmental spectral width. We also study the influences of the decay from the non-Markovian environment coupling to an external reservoir on the efficiency of second-order upper sidebands. This indicates a promising way to enhance or steer optomechanically induced transparency devices in nonlinear optical cavities and provides potential applications for precision measurement, optical communications, and quantum sensing.

Non-Markovian effects in open quantum system dynamics usually manifest the backflow of information from the environment to the system, indicating complete-positive divisibility breaking of the dynamics. We provide a criterion for witnessing such non-Markovian dynamics exhibiting information backflow, based on partial moments of Choi matrices. The moment condition determined by the positive semi-definiteness of a matrix does not hold for a Choi state describing non-Markovian dynamics. We then present some explicit examples in support of our proposed non-Markovianity detection scheme. Finally, a moment-based measure of non-Markovianity for unital dynamics is formulated.

Quantum computing offers unprecedented speed and power for solving complex problems, but its susceptibility to errors makes error-correction codes crucial. An important metric for evaluating the performance of error-correction codes is the code distance, which measures the number of errors a code can correct. Current qubit-plane architectures are implemented using the same code distance. However, a larger code distance reduces the logical error rate but increases resources. In fact, distance-5 code uses more resources but offers higher resilience to logical errors compared to distance-3 code. We propose a qubit-plane architecture for rotated and unrotated surface codes called Qentaurus that combines various code distances of logical qubits for robust error correction while maintaining qubit efficiency. Qentaurus is designed to be highly scalable regarding computation, layout, and code distance. We performed a case study using Qentaurus with distance-3 and -7 codes to evaluate the performance. Moreover, we additionally developed a qubit placement scheme that considers the critical path of the quantum circuit to reduce the logical error rate and gate count. Our main experimental results demonstrated that Qentaurus considerably reduces the overall logical error rate (by 15.76% on average and up to 25.25%) than that of the Distance-3 Checkerboard architecture. After additionally implementing the qubit placement scheme, the average reduction increases to 13.44% (up to 39.07%). Notably, the Qentaurus-3&7 maintains logical error rates comparable to those of the Distance-5 Checkerboard, achieving considerable qubit efficiency. The Distance-5 Checkerboard demands 227.7% more physical qubits than that required by the Distance-3 Checkerboard, while Qentaurus-3&7 utilizes just 131.2% of the physical qubits required by Distance-3 the Checkerboard, marking a substantial reduction of 96.5% in the resource gap. The overall gate count is reduced by an average of 1.74% with Qentaurus alone and by 3.33% on average with the qubit placement scheme. We believe that the proposed Qentaurus and qubit placement scheme have the potential to significantly improve the error-correction performance of large-scale quantum computers in practical applications.

We propose a protocol for robust quantum state engineering using composite pulses (CPs) in four-level systems. The analytical expression of the propagator is derived for the implementation of universal single-qubit gates and the maximum superposition state. By carefully designing the relative phases between pulses, the CP sequences can compensate for the pulse area error to any desired order. We present two classes of CP sequences, one generating robust population inversion and the other generating robust superposition states. As applications, we employ the well-designed CP sequences to achieve the conversion of the W and Greenberger-Horne-Zeilinger states with high fidelity in a Rydberg atomic system. It is shown that the CP sequences yield excellent robustness with respect to the pulse area errors, and they possess a short evolution time.

Coherent coupling between a ferromagnetic magnon and a superconducting qubit demonstrates a special fascination in the quantum information field. Here, we construct a hybrid system to realize tunable strong coupling between a transmon and a magnon. In a hybrid system, the transmon couples to a cavity while the magnon couples to the other cavity. Meanwhile, the two cavities are coupled via a hopping interaction. In addition, we demonstrate some applications of this system to quantum information. Specifically, a high-fidelity $i$swap${}^{†}$ gate and a maximally entangled state between the transmon and the magnon with this coupling system can be realized even if the dissipation is considered. Moreover, by employing more cavities and magnons, the $W$-type entangled state can be created with multiple transmons. This hybrid system provides an alternative platform for scalable quantum information processing.

Stimulated Raman adiabatic passage is a prominent and widely applied technique that transfers population between quantum states in three-level quantum systems. In this paper, we propose an optimized version of stimulated Raman adiabatic passage, which can be implemented faster than the conventional version. In order to achieve quantum adiabaticity in shorter times, we use the geometric optimization method based on the dynamical quantum geometric tensor to regularize nonadiabatic transitions and to eliminate nonadiabatic effects during the evolution process. Optimal stimulated Raman adiabatic passage can shorten the operation time required for adiabatic population transfer appreciably, thereby avoiding decoherence resulting from long-time evolution. Moreover, optimal stimulated Raman adiabatic passage is robust against systematic and random errors. Our results provide an optimal route for the realization of fast and accurate population transfer in three-level quantum systems.

Qubit technologies using nitrogen-vacancy color centers in diamond require precise knowledge of the centers, including the number of emitters within a diffraction-limited region and their orientations. However, it is challenging to determine the number of emitters when there is a finite background, which affects the precision of resulting quantum protocols. Here we show the photoluminescence (PL) intensity and quantum correlation (Hanbury Brown and Twiss) measurements as a function of polarization for one- and two-emitter systems. The sample was produced by implanting low concentrations of adenine $\left({\mathrm{C}}_5{\mathrm{H}}_5{\mathrm{N}}_5\right)$ into a low nitrogen chemical vapor deposition diamond. This approach yielded well-spaced regions with few nitrogen-vacancy centers. We were able to differentiate between two emitter systems and single emitters with background by mapping the PL intensity and quantum correlation as a function of polarization. This method allows us to quantify the background signal at implanted sites, which may vary from off-site background levels. This approach also provides a valuable all-optical mechanism for the determination of one or two emitter systems useful for quantum sensing, communication, and computation tasks.