This work addresses the “speed-fidelity tradeoff” bottleneck in Mølmer-Sørensen (MS) entangling gates for ion-trap quantum computing by proposing a quantum entangling gate design based on the symmetry-induced transparency (SIT) in Floquet theory. By elucidating the physical essence of the SIT effect, the research reveals that SIT can serve as an effective mechanism for quantum state-selective control. Based on this theoretical mechanism, we designed a scheme to successfully suppress the off-resonant carrier excitation of the entanglement gate, and thereby provided intrinsic protection for entanglement gate operations. Theoretical analysis and numerical simulations show that, for a laser-ion coupling strength $\mathrm{\Omega }=0.9\left(2\pi \right)\mathrm{MHz}$, when the modulation parameters are set to the SIT point values [${\omega }_{\text{m}}=1.08\left(2\pi \right)\mathrm{MHz}$ and $2{\epsilon }_{\text{m}}/{\omega }_{\text{m}}=5.422$], our scheme achieves a gate fidelity of 99.74%. This surpasses the 82.30% fidelity of traditional MS gates under identical duration and coupling strength. Furthermore, the Floquet entangling gate achieves 99.9% fidelity fastest at $24.7\mu \mathrm{s}$, which is superior to the conventional MS gate's $34.6\mu \mathrm{s}$. This study presents a paradigm for the design of high-fidelity rapid quantum gates with Floquet engineering.

Parity mixing in photoionization, i.e., when emitted electrons have different parities but the same energy, causes interference observable only in angle-resolved measurements. The interference typically manifests as a symmetry violation in the photoelectron angular distributions. The traditional, based on high-order harmonic generation, reconstruction of attosecond beating by interference of two-photon transitions (RABBITT) scheme with high-order harmonics separated by twice the seed field energy precludes parity mixing. In contrast, a free-electron laser provides a possibility to generate even harmonics. Using triple the fundamental frequency as a seed, one obtains a comb of alternating even and odd harmonics, separated by three times the initial frequency [two-sideband (2-SB) RABBITT; see  P. K. Maroju, Nature (London) 578, 386 (2020)]. In this setup, there are two sidebands between the main photoelectron lines, versus one in the traditional scheme. In the paper, we examine the general properties of a two-sideband scheme and analyze the symmetry breakdown of photoelectron angular distributions for various polarization geometries of the incident pulse. We find a crucial difference in symmetries between 2-SB RABBITT and other photoionization schemes with parity mixing. Illustrative calculations are carried out for neon with pulse parameters typical for modern facilities. The possibility to reconstruct the temporal profile of the pulse from the angle-resolved measurements is discussed.

We study a one-dimensional topological model featuring a Su-Schrieffer-Heeger type pattern of nearest-neighbor couplings in combination with the longer-range interactions exponentially decaying with the distance. We demonstrate that even relatively weak long-range couplings can trigger the topological transition if their range is large enough. This provides an additional facet in the control of topological phases.

Self-testing is the strongest certification procedure that uniquely characterizes the physical system based on the observed statistics, without any knowledge of the inner workings of the devices. The optimal quantum violation of a Bell inequality enables such a device-independent (DI) self-testing of the source and the measurement devices. In this work, we demonstrate the DI self-testing based on the arbitrary-input chained Bell inequality. We devise a systematic and elegant sum-of-squares technique enabling dimension-independent optimization of the quantum violation. Our approach enables the derivation of the state along with the relationship between the local observables directly from the optimization condition. One significant aspect is the robustness of such self-testing in real experimental situations involving noise and imperfection, leading to deviation from the optimal quantum violation. We provide an analytical technique for robust self-testing in the presence of noise. As an application of our scheme, we demonstrate the generation of two-bit DI randomness and analyze the robustness of such randomness. Our optimization method is both simple and elegant, making it suitable for deriving the optimal quantum violation of various arbitrary-input Bell inequalities.

Quantum walks represent a paradigmatic framework to explore and manipulate quantum behaviors. In this paper, we investigate the genuinely quantum features of continuous-time quantum walks by combining a single-time and a multitime quantifier of nonclassicality. On the one hand, we consider the quantum-classical dynamical distance $D_{\mathrm{QC}}\left(t\right)$, which measures the departure of the time-evolved quantum state of a continuous-time quantum walk from the classical state of a random walk on the same graph. On the other hand, we analyze the joint probability distributions associated with sequential measurements of the walker's position, assessing their violation of the classical Kolmogorov consistency conditions via a dedicated quantifier $\overline{K}\left(t\right)$. We demonstrate a quadratic short-time scaling of $\overline{K}\left(t\right)$, which differs from the known linear scaling of $D_{\mathrm{QC}}\left(t\right)$, but, as the latter, is fully determined by the degree of the initially occupied node and is independent of the global graph topology. At longer times, instead, $\overline{K}\left(t\right)$ exhibits a pronounced topology-driven behavior: it is strongly suppressed on complete graphs while remaining finite and oscillatory on cycles, in contrast with the almost topology-independent asymptotics of $D_{\mathrm{QC}}\left(t\right)$. We then extend the analysis to Markovian open-system dynamics, focusing on dephasing in the position basis (Haken-Strobl model) and in the energy basis (intrinsic decoherence). Site dephasing drives both quantifiers to zero, with the decay of $\overline{K}\left(t\right)$ controlled by the spectral gap of the corresponding Lindblad generator. By contrast, energy-basis dephasing preserves a finite asymptotic value of $\overline{K}\left(t\right)$, depending on the overlap structure of the Laplacian eigenspaces with the site basis. Our results demonstrate that distinct notions of nonclassicality lead to qualitatively and quantitatively different assessments of how quantum a given walk is, pointing at the intricate nature of temporal quantum correlations in networked quantum systems under realistic decoherence.

Trapped-electron systems present a promising qubit platform for implementing quantum information protocols. Here we propose an efficient scheme to generate the stationary entanglement between floating-electron-based qubits. We consider a two-electron circuit QED hybrid system, where the motional states of two spatially separated electrons are coupled to a shared double-stripline superconducting resonator via electric dipole interaction. By applying a bichromatic periodic modulation of resonator frequency to simultaneously induce red and blue sidebands, the high-fidelity stationary entanglement between the electron charge qubits can be generated via the interfering sidebands to mimic the dissipative effects of a squeezed reservoir. To further leverage the superior coherence properties of spin states over charge states, we introduce a local magnetic-field gradient to establish spin-orbit coupling for each electron. This allows the stabilized charge-qubit entanglement to be transferred to the spin degrees of freedom via a state-replication mechanism. The present work may advance the realization of robust quantum information processing with trapped-electron systems.

We find a process satisfying the axioms of hyperdecoherence, which produces standard quantum theory from the theory of quantum boxes (higher-order quantum theory with the nonsignaling tensor product). This hyperdecoherence map evades the no-go theorem of Lee and Selby [Proc. R. Soc. A 474, 20170732 (2018)] by relaxing constraints on signaling to the past and the uniqueness of purifications. We discuss natural opposing conclusions, namely, that the existence of this map might be evidence of a genuine hyperdecoherence process producing causal quantum theory from its causally indefinite higher-order theory or that it serves as an indication that the axioms of hyperdecoherence might need careful reconsideration, especially regarding the subtle albeit central role that purity plays.

We designed a scheme for a neutral atom Rydberg blockade controlled-Z $\left(\mathrm{CZ}\right)$ gate based on the double sequence of adiabatic pulses applied symmetrically to both atoms and using counterdiabatic driving for Rydberg excitation. This provides a substantial reduction in the quantum gate operation time compared with previously proposed double adiabatic schemes, and makes our scheme competitive with modern time-optimal protocols for high-fidelity entangling gates with neutral atoms. Our approach creates a bridge between fully adiabatic and time-optimal gate schemes. The use of adiabatic passage reduces the sensitivity of gate fidelity to variations in laser intensity, while counterdiabatic driving provides short gate times. The intensity and phase profiles of the laser pulse acting on the atoms are described analytically depending only on the gate duration. We demonstrated the applicability of this scheme for single-photon and two-photon schemes of Rydberg excitation in rubidium and cesium atoms, and, for the first time, discussed the implementation of a $\mathrm{CZ}$ gate using three-photon excitation of rubidium atoms. In contrast with many modern $\mathrm{CZ}$ gate protocols, our scheme does not generate intrinsic single-qubit phase shifts, although they still appear in two-photon configuration. We also designed a numerically optimized amplitude-robust gate with an analytically defined phase profile of the laser pulse and compared its performance with the counterdiabatic gate scheme.

We investigate localization transitions in interacting Bose-Einstein condensates (BECs) confined in tilted optical lattices, focusing on both the continuum limit accessed via shallow lattice depths and the tight-binding limit realized in the deep lattice regime. Utilizing the Gross-Pitaevskii equation (GPE) and the many-body Bose-Hubbard model, we analyze the scaling behavior of localization indicators, such as the root-mean-square width and fidelity susceptibility, as a function of the applied tilt. Our results reveal clear signatures of a localization-delocalization transition driven by the linear potential, with scaling properties that characterize criticality even in the presence of interactions within the GPE description. Despite the single-mode nature of the condensate wave function, we demonstrate that it can effectively probe quantum criticality. Building on this, we propose the use of interacting BECs in tilted lattices as a platform for quantum critical sensing, where the condensate wave function serves as both a sensitive probe of localization and a practical resource for quantum-enhanced metrology. This approach opens avenues for precision gradient sensing based on localization phenomena in bosonic systems.

Achieving both large energy storage capacity and high charging speed is a core objective in the drive to advance battery performance. In this work, we discuss the concept of multimode and multilevel quantum batteries. Our in-depth investigations reveal that as the number of energy levels increases, the energy storage capacity and ergotropy of the quantum battery exhibit linear growth. Notably, its charging power and energy extraction efficiency undergo rapid, superlinear enhancement, effectively demonstrating the outstanding advantages of multilevel quantum batteries. These systems offer a viable and promising solution for boosting battery performance, highlighting the enormous potential and bright application prospects of quantum technology in the field of energy storage.

Finite nuclear mass ${\left(Z\alpha \right)}^{2}m/M{E}_F$ corrections to the hyperfine splitting in hydrogenic systems are calculated using a combined relativistic heavy particle and nonrelativistic quantum electrodynamics. The obtained results are in disagreement with previous calculations by Bodwin and Yennie [Phys. Rev. D 37, 498 (1988)]. The comparison of improved theoretical predictions with the corresponding measurements in hydrogen reveals $2\sigma$ discrepancy, which may indicate problems with the proton structure corrections.

Enantiospecific state transfer is a special quantum control problem whose importance is emphasized due to its potential applications in enantiodetection and enantioseparation. However, crucial discussions about obtaining highly efficient enantiospecific state transfer working on the tight lower bound of energy-time costs were missing. Here, we establish a frame to systematically study this issue by mapping the control protocol to the curve in the parameter space that is determined by the evolution of control fields and the efficiency of enantiospecific state transfer. We derive a tight lower bound on the conjoint costs, considering factors such as time, energy input from control fields, and the efficiency of enantiospecific state transfer. This leads to the formulation of a different criterion for evaluating and improving enantiospecific state transfer techniques. Furthermore, we present an optimal approach for implementing highly efficient control protocols with minimal time-energy expenditure.

Quantum droplets (QDs), formed by ultradilute quantum fluids under the action of the Lee-Huang-Yang (LHY) effect, provide a unique platform for investigating a wide range of macroscopic quantum effects. Recent studies of QDs' breathing modes and collisional dynamics have revealed their compressibility and extensibility, which suggests that their elasticity parameters can be identified. In this work we derive the elastic bulk modulus (BM) of QDs by means of theoretical analysis and numerical simulations and establish a relation between the BM and the eigenfrequency of the QD's intrinsic vibrations. The analysis reveals the dependence of the QD's elasticity on the particle number and the strength of interparticle interactions. We additionally provide a realistic estimate of the bulk modulus for the system, yielding a concrete physical value that may serve as a reference for future experimental measurements. Taken together, these results also point to possibilities for realizing elastic media governed by the LHY effect.

Four-wave parametric amplification can be extended to the ${\mathrm{TW}/\mathrm{cm}}^2$ regime using femtosecond pump pulses to amplify nearly octave spanning pulses achieving gain greater than 20 ${\mathrm{mm}}^{-1}$, which we call Kerr instability amplification. Cross-polarized wave generation exploits Kerr anisotropy to induce a transient intensity-dependent polarization evolution. In this work we combine Kerr instability amplification with cross-polarized wave generation to simultaneously amplify and rotate the output polarization of a signal beam, and we explore laser and crystal parameters to control the resulting polarization. In 1 mm MgO(100), we amplify linearly polarized light by $2000\times$ orthogonal to the pump and seed polarization. The parametric amplification and polarization rotation offers excellent pulse contrast enhancement for future high-power laser systems. Furthermore, the polarization provides an additional observable to study the nonlinear dynamics occurring in this extreme ultrafast light-matter interaction.

We study theoretically the spatial evolution of optical beams inside a graded-index fiber exhibiting saturable nonlinearity. Utilizing an approach based on the variational principle, we identify the existence of bistable spatial solitons inside such a nonlinear medium, whose stability, analyzed through a linear stability analysis, is due to the saturating nature of the nonlinearity. Spatial solitons adhere to a specific amplitude-width relationship. Any deviation from this relationship leads to oscillating-type solutions with a period that increases with the saturation level of the nonlinearity. Theoretically calculated values of this period agree well with numerical findings.

Both numerical and experimental results show that precise control of the density-gradient scale length can switch flexibly between the coherent-wake-emission (CWE) and relativistic-oscillating-mirror (ROM) regimes of high-order harmonic generation when a solid surface is irradiated by subrelativistic midinfrared laser pulses. The use of longer-wavelength laser sources enables the achievement of steeper wavelength-normalized plasma gradients compared to near-infrared systems. Furthermore, wavelength scaling reduces the peak intensity for a given value of normalized vector potential and makes the laser-plasma interaction more tolerant to possible prepulses and picosecond pedestals. Detailed scans of the dependence of high-order harmonic energy on the delay between the ionizing prepulse and the main pulse demonstrate a pronounced maximum in harmonic conversion efficiency in the CWE regime at delays of $1<\mathrm{\Delta }\tau <2$ ps. The density-gradient expansion velocity is derived by matching the delay-traced CWE harmonic yield to its calculated dependence on the gradient scale length. The minimum density-gradient scale length achieved is $L<\lambda /100$. For longer delays, 12 $\le \mathrm{\Delta }\tau \le$ 65 ps, which correspond to smooth plasma-density gradients $0.2\lambda \lesssim L\lesssim \lambda$, remarkable persistence of the ROM harmonics is observed.

We propose a Floquet-engineered framework for the coherent control of the light-matter interaction in a two-level system (TLS) located in a time-modulated cavity. Strictly phase-preserving operation of the TLS-cavity interaction is demonstrated, allowing the interruption and retrieval of coherent Rabi oscillations without the loss of quantum information. By introducing a phonon reservoir, it is proved that the frequency instability induced by non-Markovian processes does not produce significant phase decoherence during Floquet modulation. Our results provide insights into the fundamental physics of a driven quantum system and establish Floquet engineering as a powerful tool for coherent quantum information processing.

We propose and analyze a robust population-transfer protocol in a driven two-level system based on a sudden sign change of the detuning at the maximum of a smooth coupling pulse. Away from the jump the dynamics is adiabatic, while the sign flip produces a single nonadiabatic “kick” in the adiabatic basis. Within a simple stepwise adiabatic–sudden approximation we obtain a compact analytic expression for the final transition probability, identify the parameter regimes in which the transfer efficiency becomes high, and show that the result depends only on the change of the mixing angle across the detuning jump, i.e., solely on the ratio of the peak Rabi frequency to the detuning. Numerical simulations of the full time-dependent Schrödinger equation confirm the validity and robustness of this description over a broad parameter range. We then use the Majorana decomposition to extend the scheme to an SU(2)-symmetric three-state chain driven by the same coupling and detuning functions. In this setting the three-state propagator is expressed in closed form through the two-level Cayley-Klein parameters, which allows us to derive explicit transition probabilities for all three initial states. In particular, we show that for strong coupling the protocol yields almost complete population transfer between the two outer states, with only a small final population of the middle state, while retaining the same intrinsic robustness as in the underlying two-level model.

Generally, the precision of microwave measurements is limited by the natural linewidth of the employed spectroscopic features. To overcome this constraint, we exploit collective radiative effects in many-body atomic ensembles as a means of linewidth engineering. We introduce a measurement strategy in which superradiance gives rise to an ultranarrow transmission feature that can be directly accessed via spectroscopic readout, with a linewidth scaling as $f/\mathrm{\Gamma }\propto 1/N$. Additionally, we establish that an incoherent pumping scheme can prepare a stable subradiant state. This state offers an alternative pathway for enhancing sensitivity, which originates from its intrinsically narrow linewidth. Our proposal can be readily implemented on various platforms, such as Rydberg atoms, quantum dots, and waveguide materials, offering an approach to achieving quantum-enhanced measurement precision.