High-order harmonic generation (HHG) plays a key role in producing coherent extreme ultraviolet (EUV) and x-ray radiation, which is crucial for applications in attosecond science and high-energy density physics. The intensity and order of high-order harmonics are crucial for advancing their applications. In this paper, a scheme is proposed to generate harmonics by the relativistic oscillating mirror (ROM) based on the energy exchange between electron and excited plasma wave by Landau damping, which employs dual laser pulses to sequentially excite the plasma surface electron oscillation to enhance the intensity and order of harmonics. Through theory and numerical analysis, we discover that our scheme significantly enhances the intensity of higher-order harmonics by an order of magnitude compared to typical harmonic generation methods of the same energy, and observe the higher-order harmonics. Furthermore, we investigate the essential factors affecting energy exchange in Landau damping and find that there exists an optimal pulse interval and energy ratio to maximize the energy exchange. The scheme based on Landau damping for generating high-order harmonics offers an alternative perspective for advancing HHG research and applications.

Surface-enhanced vibrational spectroscopy is a powerful tool widely used in scientific research and industry for label-free and sensitive identification of molecular species. Two main enhancement mechanisms, namely, electromagnetic enhancement and chemical enhancement, have been accepted to be collectively responsible for striking modifications in the enhanced spectra of molecules absorbed on metal plasmonic substrates. However, the two effects and their respective contribution in various metal- or dielectric-molecule systems remain imperfectly understood. Here we exemplify a hybrid molecule-metal system and decouple the two contributions to surface-enhanced Raman-scattering (SERS) intensity. Our findings demonstrate the nanoscale near-field enhancement can be estimated with only the measured far-field SERS intensity. Notably, the chemical effect on the SERS signal exhibits an essentially different response against the enhanced near field between molecular vibrational lines. Hence, this work may be helpful in better understanding the origin of the anomalously changed Raman patterns when adsorbates upon metal surfaces, but also offers heuristic insights into the underlying interactions between various enhancement effects in surface-enhanced vibrational spectroscopy.

Generation of quantum entanglement between a pair of qubits is studied in a cavity-QED platform. The qubit pair is placed inside a common cavity environment. We show that the relative strength of qubit-photon couplings is crucial for establishing interqubit entanglement. Resonance as well as off resonance between the qubits and photon are considered. For the off-resonant case we detect a threshold value of the coupling ratio, beyond which the maximally entangled state is always available. The resonant case displays interesting nonmonotonic behavior, where the maximum entanglement peaks at an intermediate coupling ratio. The driven-dissipative dynamics of our model exhibits nontrivial dependence of steady-state entanglement on the drive strength.

We present an entangling gate scheme that achieves high-fidelity operations with excited motional states, despite multiple error sources. While we demonstrate this approach for trapped-ion chains, the underlying principles are applicable to a wide range of quantum computing platforms. Our scheme incorporates all relevant motional modes and exhibits enhanced robustness against both motional heating effects and detuning errors, critical features for building robust and scalable quantum computers.

Quantum key distribution (QKD) offers an unprecedented level of security, which, unlike its classical analog, is not based on computational assumptions. But tight lower and upper security bounds are still not provided for some QKD protocols. Here, we propose an effective attack strategy for coherent one-way (COW) QKD protocol. It is a generalized photon-number splitting (PNS) attack combined with sequential unambiguous state discrimination (USD) attack. We discuss the possibility of a generalized PNS attack because this attack demonstrates efficiency and does not introduce error, while previously the COW protocol was considered secure against PNS attack. To our knowledge, it is the most efficient attack on the COW protocol to date, hence we sharpen the upper security bound for this protocol.

Optimal control theory is applied to analyze the time-optimal solution with a single scalar control knob in a two-level quantum system without quantum decoherence. Emphasis is placed on the dependence on the maximum control strength $u_{\text{max}}$. General constraints on the optimal protocol are derived and used to rigorously parametrize the time-optimal solution. Two concrete problems are investigated. For generic state preparation problems, both multiple bang-bang and bang-singular-bang are legitimate and should be considered. Generally, the optimal is bang-bang for small $u_{\text{max}}$, and there exists a state-dependent critical amplitude above which singular control emerges. For the X-gate operation of a qubit, the optimal protocol is exclusively multiple bang-bang. The minimum gate time is about 80% of that based on the resonant Rabi $\pi$-pulse over a wide range of control strength; in the $u_{\text{max}}\to 0$ limit, this ratio is derived to be $\pi /4$. To develop practically feasible protocols, we present methods to smooth the abrupt changes in the bang-bang control while preserving perfect gate fidelity. The presence of bang-bang segments in the time-optimal protocol indicates that the high-frequency components and a full calculation (instead of the commonly adopted rotating wave approximation) are essential for the ultimate quantum speed limit.

Tunneling is an iconic concept that captures the peculiarity of quantum dynamics but, despite its ubiquity, questions remain. We focus on strong-field tunneling, which is vital to all attosecond science. We find an unexpected optical tunneling event that happens when the instantaneous electric field vanishes and there is no barrier. This event arises from a color switchover in a strongly polychromatic field. The tunneling without a barrier reveals the disconnect between the standard intuition built on the picture of a quasistatic barrier and the nonadiabatic nature of the process.

We consider a SU(3) spin-orbit-coupled Bose gas with nonlocal soft-core interactions confined in a harmonic trapping potential. The presence of SU(3) spin-orbit coupling induces three minima of the single-particle spectrum. In the absence of spin-exchange interaction, our results show that regardless of the sign of the soft-core interaction, the ground state occupies one single minimum in the momentum space and corresponds to a threefold-degenerate magnetized phase. For the ferromagnetic condensate, the ground state shows an amplitude-modulated plane-wave state, where its modulated amplitude and frequency are related to the strength of the soft-core interaction and SU(3) spin-orbit coupling, respectively. In particular, due to the subtle balance of the soft-core interaction and SU(3) spin-orbit coupling, a stable higher-order spin vortex lattice can be formed in the lattice phase of an antiferromagnetic condensate, where the density distributions of each component and the total system show spatial configurations with threefold and sixfold symmetries, respectively. We also give an experimental protocol to observe such states within current experimental capacity.

We describe a gyroscope that measures rotation based on the effects of the rotation on the polarization of light. Rotation induces a differential phase shift in the propagation of left- and right-circularly polarized light and this phase shift can be measured in suitably designed interferometric setups. The signal in this setup is independent of the frequency of light, unlike various sources of noise such as vibrations, which cause phase shifts that depend on the frequency. Such vibrations are the practical limit on the sensitivity of conventional Sagnac-style optical interferometers that are typically used as gyroscopes. In the proposed setup, one can potentially mitigate this source of noise by simultaneously using two (or more) sources of light that have different frequencies. The signal in this setup scales with the total storage time of the light. Due to its frequency independence, it is thus most optimal to measure the signal using superconducting radio-frequency systems where the high finesse of the available cavities enables considerably longer storage times than is possible in an optical setup.

We present a theoretical framework for investigating the nonclassical collective relaxation in arrays of $\mathrm{\Lambda }$-type three-level atoms, where the two optical transitions are coupled to a pair of orthogonal chiral or achiral waveguide modes. We demonstrate that the atomic entanglement emerges in the course of relaxation and persists in the final steady state of the system. We also reveal the entanglement among the emitted photons as well as between the atoms and the photons. The presence and the degree of the entanglement depend crucially on the interatomic distance and the chirality of the waveguide. Our findings open a new way to engineer dissipation-induced entanglement.

We propose schemes for steady-state emission of quadrature-squeezed light from a single-atom cavity quantum electrodynamics system. Two effective atom-cavity interaction models are suggested, motivated by correlated photon-pair emissions and multiphoton coherences that manifest themselves as squeezing of light emitted from the cavity. We consider idealized models to examine the mechanisms and optimal regimes for generating squeezed light. We also demonstrate realizations of these models based on both single ${}^{87}\mathrm{Rb}$ and ${}^{133}\mathrm{Cs}$ atoms, using combinations of laser- and laser-plus-cavity-driven Raman transitions between magnetic sublevels in a single ground hyperfine state. We show that these systems can in principle produce a much stronger level of squeezing than has previously been possible from a single-atom source.

Noisy unsharp measurements incorporated in quantum information protocols may hinder performance, reducing the quantum advantage. However, we show that, unlike projective measurements which completely destroy quantum correlations between nodes in quantum networks, sequential applications of noisy measurements can mitigate the adverse impact of noise in the measurement device on quantum information processing tasks. We demonstrate this in the case of concentrating entanglement on chosen nodes in quantum networks via noisy measurements performed by assisting qubits. In the case of networks with a cluster of three or more qubits, we exhibit that sequentially performing optimal unsharp measurements on the assisting qubits yields localizable entanglement between two nodes akin to that obtained by optimal projective measurements on the same assisting qubits. Furthermore, we find that the proposed approach using consecutive noisy measurements can potentially be used to prepare desired states that are resource for specific quantum schemes. We also argue that assisting qubits have greater control over the qubits on which entanglement is concentrated via unsharp measurements, in contrast to sharp-measurement-based protocols, which may have implications for secure quantum communication.

Entanglement is one of the most prominent features of quantum mechanics and serves as an essential resource in quantum information science. Therefore, the certification of entanglement is crucial for quantum information processing tasks. While entanglement witnesses are the most frequently used method for entanglement certification in experiments, recent research shows that even tiny errors in measurements may significantly undermine the effectiveness of a witness. In this work, we propose a randomized-measurement-based method to solve this problem. Through this method, the errors in measurement results can be substantially suppressed, thereby restoring the certification capability of entanglement witnesses. Our method is not only applicable to general types of witnesses, including multiparty entanglement and high-dimensional entanglement witnesses, but also experimentally friendly in the sense that only slight modifications are needed to the original measurement settings.

In this work we apply a procedure based on the quantum imaginary time evolution method to solve the unit-disk maximum independent set problem. Numerical simulations are performed for instances of six-, eight-, and ten-qubit graphs. We find that the failure probability of the procedure is relatively small and rapidly decreases with the number of shots. In addition, a theoretical upper bound for the failure probability of the procedure is obtained.

A charged test particle moving through a plasma suffers the effects of collisions with the plasma particles. The effects of these interactions may be described in terms of the two main energy-loss parameters, namely, the stopping power and the energy-loss straggling; these are the main parameters that characterize the interaction of external particles with the plasma. Here we perform a comparative study of the ionic and electronic contributions to the stopping and the straggling for the hydrogen isotopes, alpha particles, positrons, and muons, traversing different types of plasmas: tokamak plasmas, the dense helium region of a giant star, and the solar interior. To calculate these parameters we apply a quantum approach to describe the interaction of charged particles with plasmas, based on one side on the quantum dielectric plasma model, which includes a full description of statistical effects on energy exchange processes together with a quantum treatment of electron excitations. In addition, we incorporate the effect of the plasma ions through a semiclassical dielectric function that combines a quantum description of plasma electrons with a classical dielectric model for plasma ions. Through various examples we analyze the partial (electronic and ionic) and total contributions to the stopping power and energy straggling for protons, positrons, and muons. The comparison between these cases provides insights into the effects of statistical fluctuations and ionic contributions to the energy-loss processes for each of these particles and different plasma media.

Dielectronic recombination resonance strengths, energy-differential cross sections, and recombination rate coefficients are calculated fully relativistically for ${\mathrm{Fe}}^{2+}$ ions. The ground-state and resonance energies are determined using the multiconfiguration Dirac-Hartree-Fock method. Radiative and autoionization rates are computed with a relativistic configuration-interaction method. For the calculation of Auger widths and resonance strengths, the continuum electron is treated within the framework of the relativistic distorted-wave model. Notably, the calculated level energies for ${\mathrm{Fe}}^{2+}$ not only align well with experimental results but also show improvements compared to earlier theoretical studies. These fully relativistic calculations provide a more accurate and comprehensive understanding of the recombination process. This is particularly important in astrophysics and plasma physics, especially for studying phenomena such as kilonova events.

We demonstrate cooling of calcium atoms using a two-photon transition from the ${}^1{S}_0$ ground state to the upper $4s5s{}^1{S}_0$ state via the ${}^1{P}_1$ intermediate state. We achieve temperatures as low as $260\mu \mathrm{K}$ in a magnetooptical trap (MOT), well below the Doppler limit ($T_{\text{D}}=0.8\mathrm{m}\mathrm{K}$) of the ${}^1{P}_1$ state. We characterize the temperature, lifetime, and confinement of the MOT over a range of experimental parameters, observing no reduction in lifetime due to coupling to the higher state. We perform theoretical simulations of the cooling scheme and observe good agreement with the experimental results. The two-photon cooling scheme presented in this work provides an alternative to the standard Doppler cooling applied to alkaline-earth atoms, based on a sequence of two magnetooptical traps. The advantages of our scheme are the possibility of varying the effective linewidth of the ${}^1{P}_1$ state, a higher transfer efficiency (close to $100\%$), and a more straightforward experimental implementation.

We investigate the phase-space dynamics of the Kramers-Henneberger (KH) atom solving the time-dependent Schrödinger equation for reduced-dimensionality models and using Wigner quasiprobability distributions. We find that, for the time-averaged KH potential, coherent superpositions of eigenstates perform a cyclic motion confined in momentum space, whose frequency is proportional to the energy difference between the two KH eigenstates. This cyclic motion is also present if the full time-dependent dynamics are taken into consideration. However, there are time delays regarding the time-averaged potential, and some tail-shaped spilling of the quasiprobability flow towards higher momentum regions. These tails are signatures of ionization, indicating that, for the potential studied in this work, a small momentum spread is associated with stabilization. A comparison of the quasiprobability flow with classical phase-space constraints shows that, for the KH atom, the momentum must be bounded from above. This is a major difference from a molecule, for which the quasiprobability flow is confined in position space for small internuclear separation. Furthermore, we assess the stability of different propagation strategies and find that the most stable scenario for the full dynamics is obtained if the system is initially prepared in the KH ground state.