We propose a coherent-control scheme for engineering quantum correlations in a cavity optomechanical (COM) system consisting of a driven optical cavity with an embedded nonlinear medium and a membrane, assisted by a coherent feedback loop. The nonlinear medium and the membrane are pumped to implement optical and mechanical parametric amplifications with controllable modulation frequencies and pump amplitudes. Within the stable regime identified by Floquet analysis, the feedback-induced enhancement of optomechanical entanglement persists when the period-averaged mean-field intracavity photon number is matched, showing that the enhancement cannot be attributed simply to an increase in this quantity. A systematic comparison of the four configurations further reveals a nonadditive interplay between dual parametric amplification and coherent feedback. Varying the amplitude reflectivity of the beam splitter and the feedback-loop phase enables effective control of optomechanical entanglement and directional Einstein-Podolsky-Rosen steering, including a transition from one-way to two-way steering. The combined scheme also improves the robustness of both correlations against thermal noise, providing a potential route toward robust quantum-state engineering in COM systems.

We show that a driven-dissipative cavity-magnonic dimer supports multistability with coexisting symmetric and symmetry-broken steady states. The interplay between magnon Kerr nonlinearity and photon tunneling induces magnon self-trapping, leading to a persistent population imbalance between the two resonators. In the vicinity of saddle-node bifurcations, the system exhibits critical slowing down, with relaxation times far exceeding the intrinsic dissipation scale and critical exponents close to the universal saddle-node value $\alpha =1/2$. Beyond the semiclassical dynamics, we probe fluctuation signatures through the quantum fidelity and mutual information between the two magnon modes. We find that both the infidelity and the mutual information increase sharply near the phase boundaries, providing signatures of the multistable and symmetry-broken phases. Our results establish cavity magnonic dimers as a versatile platform for exploring nonlinear nonequilibrium physics in hybrid quantum systems.

We present the results of an experimental study of the anomalous anisotropy of alignment signals in cesium vapors under strong spin-exchange conditions near zero magnetic field with linearly polarized optical pumping. We show that the anisotropy of the Hanle resonances in the plane perpendicular to the pump beam increases with Cs vapor number density: In one direction the widths remain broadened by spin exchange, whereas in the other they approach the spin-exchange relaxation-free limit. With a further increase in number density, additional nonlinear effects arise such as signal amplification, bistability, hysteresis, and memory. To explain these effects we construct an illustrative theoretical model incorporating spontaneous polarization effects under strong-spin-exchange conditions. The model qualitatively shows that the ultranarrow alignment resonances may originate from quadrupole anisotropy arising from the projection of spontaneous transverse orientation onto the detection axis. The unique properties of these resonances, such as their extremely small width and magnetic-field-controlled bistability with a long-term memory effect, make them promising for use in quantum sensing and information.

Waveguide quantum electrodynamics provides a powerful platform for exploring quantum optical phenomena by enhancing atom-photon interactions through photon confinement in a waveguide. Here we investigate the photon-scattering dynamics of a weak coherent pulse incident from the left on a giant atom coupled to a bidirectional waveguide, focusing on effects absent in the small-atom approximation. Using an extended input-output formalism, we calculate the relevant correlation functions and show that the competition between two scattering processes is governed by the ratio of the pulse width to the atomic lifetime, leading to time-dependent switching between bunching and antibunching. In addition, tuning the phase accumulated between the two coupling points of the giant atom allows the photon statistics to be switched among three distinct regimes, each with a finite phase bandwidth. We also discuss the experimental feasibility in superconducting circuits. Our results provide a route toward giant-atom-based control of photon pulses and potential applications in quantum control.

We formulate Zeno and anti-Zeno dynamics in a finite-memory reservoir as a time-domain measurement-filter problem. A two-level system exchanges an excitation with a finite chain of damped memory modes and is subjected to repeated partial, nonselective measurements. In the weak-coupling limit, the population decay rate is written as an overlap between a finite reservoir memory kernel and a measurement-dependent filter. For any finite chain, this separation gives a compact expression that connects pulsed and continuous monitoring while retaining correlations across successive measurement intervals. When all memory-mode frequencies are tuned to the two-level transition, a single lossy mode gives monotonic Zeno suppression, whereas two modes form the smallest chain that can produce anti-Zeno-to-Zeno switching without an externally imposed onsite detuning. Exact stroboscopic Liouvillian calculations verify the analytic filter prediction and quantify finite-coupling corrections. For longer chains, the response is controlled by the frequencies, lifetimes, and boundary weights of the memory poles rather than by chain length alone.

Non-Gaussian operations are essential for most bosonic quantum technologies. Yet, realizable non-Gaussian gates are rather limited in type and generally suffer from accuracy-duration trade-offs. In this work, we propose to use quantum signal processing (QSP) techniques to engineer non-Gaussian gates on hybrid qumode-qubit systems. For systems with dispersive coupling, our scheme can generate a non-Gaussian gate that produces a phase shift depending on the modulus of the boson number. This gate reproduces the selective number-dependent arbitrary phase gates under certain parameter choices, but with higher accuracy within a short, fixed, and excitation-independent interaction time. The gate unlocks different applications, for example, in entangling logical qudits and deterministically generating multicomponent cat states. Additionally, our versatile QSP formalism can be extended to systems with other interactions and also engineer nonunitary operations, such as noiseless linear amplification and generalized-parity measurement.

The simulation of strongly correlated fermionic systems remains a significant challenge in computational physics due to the exponential growth of the Hilbert space and the fermionic sign problem. In this work, we report a quantum computing demonstration exploring the unified physics of the Fermi polaron and the Bose-Einstein condensate to Bardeen-Cooper-Schrieffer crossover. We develop an effective Hamiltonian formalism that bridges pairing superfluidity and impurity physics, mapping the system onto a gate-based quantum processor via the Jordan-Wigner transformation. By utilizing a first-order Trotter-Suzuki decomposition, we implement an ancilla-controlled Ramsey interferometry protocol to resolve the system's spectral response. Our implementation captures the smooth transition from a dressed quasiparticle (polaron) regime to a stable molecular bound state, characterized by a linear energy renormalization in the strong-coupling limit. We benchmark the quantum protocol against exact diagonalization and demonstrate its execution on the Barcelona Supercomputing Center quantum hardware. To ensure reproducibility, we provide comprehensive device calibration metrics, including qubit coherence times and gate fidelities at the time of execution. Despite inherent hardware noise, the hybrid variational approach qualitatively observes the bifurcation of the spectral density.

Due to the recent availability of intense, short, XUV laser pulses, the investigation of the Autler-Townes (AT) doublet in resonant ionization phenomena is experiencing a renaissance. In particular, the asymmetry of the AT doublet was found to be sensitive to numerous physical parameters, providing a promising tool to probe and control the bound-state electron dynamics. Recently, the exact analytic evaluation of the AT doublet under rectangular pulse driving [A. Csehi, Phys. Rev. A 113, 033121 (2026)] provided a unified picture, revealing that the asymmetry is attributed to three main factors: (i) the interference of different ionization pathways, (ii) the distinct decay rates of the emerging dressed states, and (iii) the energy dependence of the bound-to-continuum couplings. Here, going beyond the constant envelope approximation, we identify the finite nature of the rising and falling edges of the laser pulse envelope function as an additional source of the asymmetry. Smoothly varying the envelope from a rectangular to a realistic pulsed profile (e.g., Gaussian), we show that the relative phases of the delayed photoelectrons, which are released at the temporal slits opened up by the Rabi cycling atom, can be efficiently modulated. This allows for the interference control of the electron yields at the main AT peaks. The found general asymmetry effect primarily hinges upon the asymmetry of the dressed-state energies in the continuum, caused by detunings or ac Stark shifts, and thus efficiently modified by the parameters of the laser, like intensity, photon frequency, and duration.

We investigate the quantum optical properties of high-harmonic generation in semiconductors, focusing on the nonclassicality and entanglement of harmonic modes. By conditioning the emitted field on harmonic excitation, we obtain conditional harmonic states that exhibit pronounced nonclassical features in the plateau region. For harmonic photon energies above the minimum energy gap, the conditional states associated with interband transitions exhibit significantly stronger entanglement and nonclassicality than those associated with intraband processes. These results reveal how microscopic interband and intraband electron dynamics determine the quantum properties of conditional harmonic states, providing insight into the microscopic origin of nonclassical features in high-harmonic generation and highlighting the potential for generating nonclassical extreme-ultraviolet light sources.

We investigate the dynamics of Bose-Einstein-condensate droplets composed of ${}^{164}\mathrm{Dy}$ atoms formed in a double-well potential following removal of the interwell barrier. By solving the dipolar Gross-Pitaevskii equation, we determine phase diagrams of ground-state configurations as functions of the number of atoms confined in the double-well potential. For an interwell separation $2d=3\mu \mathrm{m}$, we consider the relative dipolar-to-contact interaction strengths ${\varepsilon }_{dd}=1.4$, 1.45, and 1.5. Symmetry-broken lowest-energy configurations are found for ${\varepsilon }_{dd}=1.5$ and, over a reduced range of numbers of atoms, for ${\varepsilon }_{dd}=1.45$, whereas no symmetry-broken states are found for ${\varepsilon }_{dd}=1.4$ up to $N=5\times {10}^4$. We analyze the subsequent time evolution after removal of the central barrier, revealing both droplet oscillations and merger events leading to the formation of larger droplets. The oscillations are driven by the external potential and by the repulsive tails of the in-plane component of the dipolar interaction. For the two-droplet states at ${\varepsilon }_{dd}=1.5$ and $2d=3\mu \mathrm{m}$, merging is found for $N=8000$ and 8800, whereas the droplets remain separated for the investigated cases with $N\ge 9000$. Increasing the initial separation to $2d=5\mu \mathrm{m}$ shifts the approximate upper number of atoms for merging to $N\simeq 1.65\times {10}^4$, demonstrating that the crossover is controlled by the competition between the postquench excess energy and the repulsive interdroplet barrier.

We propose to measure the dispersions of Fermi polaron and molecule by engineering spin-orbit coupling (SOC) on the impurity, which induces spin flip with finite momentum transfer. The polaron dispersion can be probed at small SOC momentum from the linear response of impurity spin. For molecule, we show that it can be prepared through an adiabatic steady-state evolution when setting SOC momentum as the Fermi momentum of majority bath. By gradually reducing SOC strength to zero, the steady state smoothly evolves to a molecular state with directional symmetry breaking. The corresponding dispersion can then be probed experimentally through the pair momentum distribution of molecules at finite density. Our scheme reveals a fundamental momentum difference between Fermi polaron and molecule, thereby offering a clear physical picture for their first-order transition in single-impurity system.

We propose a mechanism for coherent densification in a tilted optical lattice. The starting point is a well-known property of Wannier-Stark ladders: a homogeneous resonant coupling between neighboring Wannier-Stark states restores coherent translation. When the coupling amplitude is position dependent and has a node, the local transport velocity vanishes at the node. A single wave packet is thereby transported toward this point, while a cloud distributed around the node undergoes a reduction of its spatial extent. The aim is to provide a reversible density-redistribution tool in a lattice of fixed period, complementary to matter-wave lenses, accordion lattices, box traps, and coherent-transport protocols. We introduce a nearest-neighbor effective model in which the coupling varies linearly with the site index. This model admits an exact analytic solution via a generating function and the method of characteristics; in the continuum limit it predicts exponential contraction of distances to the node. We discuss a primary implementation based on a spatially shaped Raman or Bragg coupling, and an alternative based on temporal modulation of the lattice depth. Estimates for cesium indicate that densification times ranging from a few tens to a few hundreds of milliseconds are achievable with realistic parameters. The mechanism is not dissipative cooling, but a coherent compression step that can be halted, reversed, or followed by a controlled expansion stage.

We report the observation of light storage in a dual memory associated with Zeeman coherence belonging to different hyperfine ground-state levels of cold cesium atoms. This double memory is based on the phenomenon of spontaneous Zeeman coherence transfer as predicted by the tensorial density-matrix formalism describing light interaction with a degenerate two-level system. We employ the two degenerate two-level systems $6S_{1/2},F=4\to 6P_{3/2},F^{\prime }=4$ and $6S_{1/2},F=3\to 6P_{3/2},F^{\prime }=3$, where a coupling beam and a probe beam orthogonally polarized interact resonantly with the first system, inducing Zeeman coherence in both the ground and excited hyperfine levels. The spontaneous Zeeman coherence transfer to the hyperfine ground state of the second system allows for storing the optical information of the probe beam simultaneously in the two hyperfine ground-state levels. Delayed four-wave mixing is employed to demonstrate the observation of the dual memory. Moreover, we also demonstrate the direct optical pumping of Zeeman coherence between the two hyperfine ground states induced by the incident coupling beam, which allows us to increase the storage time of the memory. A theoretical model using a simplified two degenerate two-level system is developed and accounts qualitatively well for the main observed results.

In quantum position verification (QPV), a prover certifies her location to a set of verifiers by performing a quantum computation. One of the first QPV protocols was proposed by Kent, Munro, and Spiller in 2011: The prover receives a qubit $Q$ from one direction, receives an orthogonal basis $\{v,v^{\perp }\}$ from the opposite direction, and then measures $Q$ in $\{v,v^{\perp }\}$ and broadcasts the result. Several variants of this protocol have been proposed and analyzed, but the question of whether the original protocol is secure has never been fully resolved. In this work we show that there is no perfect finite-dimensional cheating strategy for the single-qubit measurement protocol. Our proof uses tools from real algebraic geometry.

Building on hybrid quantum lattice gas algorithm, we revisit the possibilities of this quantum lattice model. By deriving a correction to the predicted viscosity, we provide analytical and numerical results that refine original formulation. We introduce a minimal two-dimensional generalization of the algorithm, which allows us to simulate anisotropic Burger-like equations while retaining only two lattice velocities. This approach opens a promising route toward embedding momentum conservation and advancing toward Navier-Stokestwo dynamics in two dimensions, going beyond Frisch, Hasslacher, and Pomeau and lattice Boltzmann method with a quantum native model. We highlight how the presented algorithm is more efficient for full state evolution than other quantum nonlinear solvers, nonetheless we clarify its advantages compared to classical lattice gas methods. Being that this model is between classical and quantum computation, it gives a unique perspective on simulating nonlinearities with quantum computers, confirming quantum lattice gas models as a crucial playground for quantum simulations of nonlinearities.

Superconducting circuits exhibit remarkable potential for constructing large-scale quantum simulation and computation systems, featuring numerous qubits, extended coherence time, and precise control. Nevertheless, the growing number of signal cables poses a challenge in dilution refrigerators due to space and heat-load constraints. To overcome this issue, we experimentally implement an optically assisted transmission line as an alternative to coaxial cables. By modulating microwave signals on laser intensities at room temperature and regenerating the signals at a cryogenic plate within the dilution refrigerator, we demonstrate full control of superconducting qubits using photocurrent. We demonstrate and benchmark both single-qubit and two-qubit gates on frequency-tunable transmon qubits, achieving fidelities of 99.915% $\pm$ 0.005% and 99.676% $\pm$ 0.041%, respectively, reaching the requirement of the surface code.

Precise environmental-noise characterization in open quantum systems is a key step toward high-fidelity quantum control and targeted decoherence suppression in computing and sensing applications. Nonparametric quantum noise spectroscopy (QNS) provides a general-purpose, model-agnostic framework for estimating the spectral properties of an environment. The ability to perform such protocols under realistic constraints is key to their practical applicability. Notably, it is important to account for control constraints and understand how they limit the ability to learn about noise correlations as experiment-agnostic objects. We show how adopting a control-centric point of view allows one to recast the noise spectroscopy problem in such a way that (i) the central objects are now the time-ordered polyspectra and (ii) control filter functions are no longer encumbered by time ordering. In particular, we show that this approach enables the generalization of frequency-comb QNS protocols to arbitrary control scenarios without introducing additional control symmetries that remove time ordering from filter functions, improving estimation in typically pathological scenarios. We demonstrate the targeted reconstruction of the time-ordered polyspectra across classical Gaussian and quantum non-Gaussian environments via simulations.

Quantum effect enables enhanced estimation precision in metrology, with the Heisenberg limit (HL) representing the ultimate limit allowed by quantum mechanics. Although the HL is generally unattainable in the presence of noise, quantum error correction (QEC) can recover the HL in various scenarios. A notable example is estimating a Pauli $Z$ signal under bit-flip noise using the repetition code, which is both optimal for metrology and robust against noise. However, previous protocols often assume noise affects only the signal accumulation step, while the QEC operations, including state preparation and measurement, are noiseless. To overcome this limitation, we study fault-tolerant quantum metrology where all qubit operations are subject to noise. We focus on estimating a Pauli $Z$ signal under bit-flip noise, together with state preparation and measurement errors in all QEC operations. We propose a fault-tolerant metrological protocol where a repetition code is prepared via repeated syndrome measurements, followed by a fault-tolerant logical measurement. We demonstrate the existence of an error threshold, below which errors are effectively suppressed and the HL is attained.