Neutral-atom systems are promising platforms for quantum simulation and computation, owing to their long coherence times. However, their intrinsically weak ground-state interactions pose a major limitation to the advancement of scalable quantum simulation and computation. To address this challenge, we propose an approach to enhancing the ground-state interaction strength of neutral atoms via Floquet modulation of a Rydberg atomic ensemble. Each Floquet period consists of ground-state coupling followed by a pulse driving the transition from the ground state to the Rydberg state. Theoretical analysis and numerical simulations demonstrate that after a defined evolution time, neutral atoms within Rydberg ensembles can collectively form a $W$ state in the ground-state manifold. Even when the Rydberg interaction strength is far below the blockade regime, the fidelity remains remarkably high. Finally, we analyze the application of this scheme in the preparation of single-photon sources. In general, our proposed mechanism offers an efficient and highly controllable method for quantum state preparation within the Rydberg atomic ensembles, significantly enhancing the accuracy and stability of quantum state engineering while providing a well-controlled quantum environment for single-photon generation.

Cold atoms are promising platforms for metrology and quantum computation, yet their many-body dynamics remains largely unexplored. We here investigate Rabi oscillations from optically thick cold clouds, driven by high-intensity coherent light. A dynamical displacement from the atomic resonance is predicted, which can be detected through the collective Rabi oscillations of the atomic ensemble. Different from linear-optics shifts, this dynamical displacement grows quadratically with the optical depth, yet it reduces with increasing pump power as dipole-dipole interactions are less effective.

Compared with Bessel beams, frozen waves maintain nondiffracting propagation while allowing customizable longitudinal intensity profiles, particularly enabling controllable orbital angular momentum variation along the propagation direction. Such characteristics provide a powerful toolkit for diverse applications. However, most existing studies remain within the realm of linear optics, and it is still unclear whether the structural features of frozen waves can be preserved during nonlinear parametric processes. Here, by imagining a frozen wave carried by one fundamental beam onto the nonlinear crystal, we equivalently mimic a nonlinear frozen-wave phase plate that enables coherent transfer of a twisted frozen wave from one beam to another. When another fundamental beam is incident on the crystal, the sum-frequency signal generated through the nonlinear parametric process inherits the frozen-wave structure. More importantly, by leveraging nonlinear vortex filtering, we further experimentally demonstrate the coherent-transfer property. Our scheme can be extended to other nonlinear optical processes, enriching the frozen-wave control toolkit and holding promise for optical micromanipulation and large alphabet optical communications.

By analytically solving the quantum Rabi model, we investigate the photonic properties of its ground eigenstate. In particular, we find that in the deep-strong-coupling regime, where the coupling strength $g$ exceeds the mode frequency $\omega$ and the photonic state is effectively squeezed in one of its quadratures. The squeezing reaches its maximum at the curve corresponding to the quantum phase transition of the quantum Rabi system and decreases rapidly on both sides of the phase transition. Notably, for $g/\omega \approx 3$, which is experimentally testable in existing trapped-ion platforms, the achievable squeezing parameter can reach approximately $r\approx 0.8$. Intriguingly, the photonic state is squeezed while its number distribution follows a super-Poissonian distribution, with the largest deviation from Poissonian behavior occurring at the phase transition between the normal and superradiant phases. In other words, the ground state of the quantum Rabi model contains super-Poissonian quantum squeezed photons.

Quantum gases of atoms and molecules in optical cavities offer a formidable laboratory for studying the out-of-equilibrium dynamics of open quantum systems with long-range interactions. Long-range interactions are here mediated by multiple scattering of cavity photons and can induce the formation of quantum structures in space and time. Control of these dynamics requires a detailed understanding of all relevant mechanisms at play. Due to the strong correlations induced by light, however, perturbative theoretical models, which reduce the number of degrees of freedom, do not correctly capture the regime where the interplay of photon-mediated long-range forces and quantum fluctuations of light and matter become significant, such as across the transition to self-organization. In this work, we present the derivation of an effective Lindblad master equation for the dynamics of the sole motional variables of polarizable particles, such as atoms or molecules, that dispersively couple to cavity fields. The master equation is valid even for relatively large intracavity photon numbers and is apt to study both the steady-state regime and the out-of-equilibrium dynamics where quantum fluctuations of the field seed the onset of macroscopic coherences. We validate the theoretical description by showing that it captures the dynamics across a wide temperature interval, from Doppler cooling down to the ultracold regime, and from weak to strong cavity-mediated interactions. Our theory provides a powerful framework for the description of cavity-induced dynamics of quantum matter. In doing so, it permits to connect models of statistical mechanics with cavity-QED experimental platforms, thus enabling quantum simulation of long-range-interacting matter.

Universal photon blockade, characterized by a strong photon antibunching effect over a broad range of nonlinear conditions, has been proposed recently. In this work we present a scheme to achieve universal nonreciprocal photon blockade. By coupling an asymmetric Fabry-Pérot cavity to a $\mathrm{\Lambda }$-type three-level atom and embedding a degenerate optical parametric amplifier inside the cavity to generate squeezed light, we demonstrate a direction-dependent photon blockade effect. Moreover, due to the energy-level anharmonicity induced by the $\mathrm{\Lambda }$-type three-level atom and the destructive quantum interference between different transition pathways, this phenomenon exists across a broad regime of nonlinear conditions, thereby demonstrating universal nonreciprocal photon blockade. In addition, we show that quantum squeezing is essential for realizing high-quality nonreciprocity within experimentally feasible parameter regimes. Our scheme provides a feasible method toward controllable and robust nonreciprocal single-photon sources across a wide parameter regime.

Non-Hermitian systems exhibit unique phenomena beyond conventional Hermitian physics, such as the non-Hermitian skin effect (NHSE) and exceptional points (EPs). In this work we introduce a one-dimensional double-chain model with nonreciprocal coupling to investigate the interplay between the NHSE and Anderson localization induced by quasiperiodic modulation. In the clean limit, the system exhibits a complex spectrum featuring EPs, tunable parity-time-symmetry breaking, and band splitting. When a quasiperiodic potential is added, a rich phase diagram emerges, including extended, localized, and critical phases, as well as mobility edges. Remarkably, we observe reentrant localization nonmonotonic transitions between extended and localized states driven by the competition between the NHSE and disorder scattering. Furthermore, we propose a concrete circuit implementation using negative-impedance converters, offering an experimentally accessible platform to simulate these non-Hermitian topological and localization effects at the classical level. Our results deepen the understanding of phase transitions in non-Hermitian quasicrystals and provide a direct route toward controllable simulators for observing and manipulating such phenomena in classical and future quantum settings.

Dirac particles can undergo perfect transmission through a sufficiently high potential barrier in the Klein zone. Although the perfect Klein tunneling (often referred to as the Klein paradox) is similar to the nonrelativistic resonant transmission which occurs only when the kinetic energy exceeds the barrier, the underlying mechanism is believed to be fundamentally distinct. In this work, we show that for the relativistic double-barrier model the perfect-transmission curve can pass continuously from the above-barrier zone to the Klein zone. Additionally, in the Klein zone, perfect transmission occurs even for subcritical barrier heights, supported by both bound-state analysis and wave-packet dynamics. These findings suggest a connection between perfect Klein tunneling and resonant transmission, and provide additional insights into the physical nature of the Klein paradox.

We provide experimental validation of tight entropic uncertainty relations for the Shannon entropies of observables with mutually unbiased eigenstates in high dimensions. In particular, we address the cases of dimensions $d=3$, 4, and 5 and consider from 2 to $d+1$ mutually unbiased bases. The experiment is based on pulsed frequency bins measured with a multioutput quantum pulse gate, which can perform projective measurements on a complete high-dimensional basis in the time-frequency domain. Our results fit the theoretical predictions: the bound on the sum of the entropies is never violated and is saturated by the states that minimize the uncertainty relations.

Twin-field quantum key distribution (TF-QKD) has become a leading protocol to bring quantum communications to the national scale. The protocol requires the establishment of a shared phase and frequency reference between distant parties, which is commonly achieved by using an external reference laser in an optical injection locking (OIL) architecture. In this work, we analyze the side channels in OIL-based TF-QKD that may arise from adversarial manipulation of the various degrees of freedom of this untrusted reference beam. We experimentally demonstrate two realistic attack scenarios: fast intensity modulation of the reference laser, and additional signals embedded in the reference light exploiting wavelengths undetectable by conventional monitoring techniques. These attacks can allow a potential eavesdropper to deterministically increase the mean photon number of the sources, or circumvent the decoy-state technique, respectively. To counter these vulnerabilities, we propose practical and highly effective countermeasures that reinforce the security of TF-QKD systems without significant additional complexity or performance degradation.

Radiation during strong-field tunneling ionization is investigated. The spontaneous as well as the coherent components of the radiation are calculated describing the ionization dynamics via the strong-field approximation, and the role of the quantum dynamics at tunneling is analyzed. The competition between different mechanisms in different spectral regions is examined. Transitionlike radiation (Brunel radiation) is dominant at low frequencies, Thomson scattering at the laser frequency, and radiative recombination via the three-step process at high-order harmonics. To distinguish the role of the quantum dynamics, simple man Drude models are developed for coherent as well as for spontaneous radiation; the models are based on the electron trajectory out of the tunneling barrier. The quantum dynamics is shown to modify the coherent Brunel radiation for near-zero frequencies in asymmetric laser pulses. The significant role of free-free transitions is demonstrated for spontaneous radiation in the low-frequency region.

We numerically study a gas of two-component fermions coupled to a transversely pumped optical cavity and confined to a two-dimensional static square optical lattice. In the dispersive regime, the steady state of the system is described by an extended Hubbard Hamiltonian with cavity-mediated long-range interactions. Using real-space dynamical mean-field theory (RDMFT), we investigate the formation of the (superradiant) checkerboard density-wave phase. Our analysis focuses on paramagnetic phases both at quarter and half filling. At quarter filling, we find a reentrant homogeneous Fermi liquid to density wave phase transition with increasing temperature, which is due to the higher entropy of the ordered phase. At half filling, in addition to the Fermi liquid to Mott insulator phase transition, marked by a vanishing quasiparticle residue at the Fermi level, we identify the transition into a density-wave phase. Due to perfect Fermi surface nesting at half filling, we find that arbitrarily small long-range interactions destabilize the system towards the density-wave phase in the absence of short-range interactions. By varying short- and long-range interactions at a fixed low temperature, we obtain the full phase diagram and identify a region of coexistence between the homogeneous Fermi liquid and Mott insulating phase with the density-wave phase. In this region we determine the thermodynamic phase transition by comparing the energies of the different RDMFT solutions.

We investigate the geometric quantum complexity of Bose-Einstein condensates (BECs) at finite temperature. Specifically, we use the Bures and Sjöqvist metrics, generalizations of the Fubini-Study metric for mixed quantum states, as well as the Nielsen geometric complexity approach based on purification of mixed states. Starting from the Bogoliubov Hamiltonian of BEC, which exhibits an $\text{SU}(1,1)$ symmetry, we explicitly derive and compare the complexities arising from these three distinct measures. For the Bures and Sjöqvist metrics, analytical and numerical evaluations of the corresponding geodesics are provided, revealing characteristic scaling behaviors with respect to temperature. In the Nielsen complexity approach, we rigorously handle the gauge freedoms associated with mixed-state purification and nonuniqueness unitary operations, demonstrating that the resulting complexity aligns precisely with the Bures metric. Our work provides a comparative study of the geometric complexity of finite-temperature BECs, revealing its intimate connections to symmetry structures and temperature effects in BEC systems.

We introduce a fundamental mechanism for generating unipolar light at the atomic scale—nonrecollision unipolar emission—where tunneling ionization from a rectangular quantum well produces half-cycle attosecond pulses. Unlike all known microscopic strong-field processes in atoms (confirmed by our 1D and full 3D time-dependent Schrödinger equation simulations), which rely on recollision dynamics and yield oscillatory radiation, the flat potential of a rectangular quantum well suppresses electron return. This enables a clean ionization-stop sequence, resulting in unidirectional charge displacement and perfect unipolar waveforms. Numerical solutions of the time-dependent Schrödinger equation demonstrate that optimally shaped two-cycle laser pulses trigger this process, generating subfemtosecond unipolar pulses without secondary oscillations. This work establishes solid-state nanostructures as elementary quantum sources of unipolar radiation, solving the long-standing problem of identifying their fundamental origin and opening a research avenue for attosecond science and petahertz optoelectronics.

The thermodynamic uncertainty relation quantifies a trade-off between the relative fluctuations of trajectory currents and the thermodynamic cost, indicating that the current precision is fundamentally constrained by entropy production. In classical bipartite systems, it has been shown that information flow between subsystems can enhance the current precision alongside thermodynamic dissipation. In this study we investigate how information flow, local dissipation, and quantum effects jointly constrain current fluctuations within a subsystem of interacting quantum systems. Unlike classical bipartite systems, quantum subsystems can exhibit simultaneous state changes and maintain quantum coherence, which fundamentally alters the precision-dissipation trade-off. For this general setting, we derive a quantum thermokinetic uncertainty relation for interacting multipartite systems, establishing a thermodynamic trade-off between current fluctuations, information flow, local dissipation, and quantum effects. Our analysis shows that, in addition to local dissipation, both information exchange and quantum coherence play essential roles in suppressing current fluctuations. These results have important implications for the performance of quantum thermal machines, such as information-thermodynamic engines and quantum clocks. We validate our theoretical findings through numerical simulations on two representative models: an autonomous quantum Maxwell's demon and a quantum clock. These results extend uncertainty relations to multipartite open quantum systems and elucidate the functional role of information flow in fluctuation suppression.