We investigate collective emission from an ensemble of $N_T$ two-level target atoms (TAs) coupled to a photonic waveguide, and show that the associated superradiant scaling can be flexibly engineered by introducing an auxiliary ensemble of control atoms (CAs). By tuning the spatial configuration and coupling strengths of the CAs, the radiative intensity emitted by the TAs can be enhanced beyond, suppressed below, or restored to the standard Dicke $N_T^2$ scaling, enabled by controllable interatomic coherence. We further demonstrate interference-induced chiral superradiance, whose directionality can be markedly amplified by employing giant CAs. Using the discrete truncated Wigner approximation, we capture beyond-mean-field effects in finite-sized ensembles and reveal correlation-driven features of the superradiant dynamics. Our results provide an experimentally accessible route to manipulate superradiance and chirality in waveguide QED, opening new possibilities for engineered quantum light sources.

Cold atomic clouds are promising platforms for generating correlated photons, but multiple scattering and associated Doppler broadening limit their temporal coherence. Here we demonstrate that cloud geometry provides a powerful means to extend the coherence time of scattered light. In the experiment, intensity-correlation measurements show that an elongated (quasi-one-dimensional) cloud exhibits systematically longer coherence times than a spherical (three-dimensional) cloud of the same on-axis optical thickness, as a direct consequence of the suppression of multiple scattering in the elongated geometry. Random-walk simulations reproduce this trend and further show that elongation drives the coherence time toward the single-scattering limit. The combined results establish cloud geometry as a robust control parameter for temporal coherence in cold atom ensembles, with potential applications in quantum optics and communication.

Parrondo's paradox, where the alternation of two losing strategies can produce a winning outcome, has recently been demonstrated in continuous-time quantum walks (CTQWs) through periodic defect modulation. We extend this phenomenon to aperiodic protocols. We show that deterministic, nonrepetitive defect switching can enhance quantum spreading in CTQWs compared to the defect-free case. Furthermore, we establish that the degree of this enhancement is strongly influenced by the autocorrelation and persistence characteristics of the applied aperiodic sequence. Our findings indicate that aperiodic defect modulation reliably maintains Parrondo's effect and provides new ways to control wavepacket properties in CTQWs.

Building on the recently developed quantum state texture resource theory, we demonstrate that the difference between maximum and minimum textures is a valid purity monotone in any dimension and provide a lower bound for existing purity measures. We introduce a texture-based resource monotone applicable across general convex resource theories, encompassing quantum coherence, nonstabilizerness, and entanglement. In particular, we propose the notion of nonlocal texture, which corresponds to the geometric measure of bipartite and multipartite entanglement in pure states. Furthermore, we demonstrate that the texture of the entire ground state or its subsystems can effectively signal quantum phase transitions in the Ising chain under both transverse and longitudinal magnetic fields, offering a powerful tool for characterizing quantum criticality.

Sensitivity in metrology without entanglement is limited by the standard quantum limit (SQL). Recent studies have found that the Heisenberg-limited scaling, the ultimate sensitivity in quantum metrology, can be achieved by generalized cat states, which are characterized by an index that indicates coherence among macroscopically distinct states and are associated with additive observables. Although generalized cat states include diverse states, encompassing classical mixtures of exponentially large numbers of states, the preparation of large generalized cat states has not been demonstrated yet. Here we characterize SQL-beating catlike states using the index $q$ indicating macroscopic coherence and prove that any state with $q>1.5$ has a potential to surpass the SQL when used as a sensor. We propose a protocol to generate them through repetitive measurements on a quantum spin system of $N$ spins, which we call a spin ensemble. Starting from a thermal equilibrium state of the spin ensemble, we demonstrate that we can increase the coherence among the spin ensemble via repetitive weak measurements of its total magnetization, which is indirectly measured through an ancillary qubit collectively coupled to the ensemble. Notably, our method for creating the SQL-beating catlike states requires no dynamical control over the spin ensemble. As a potential experimental realization, we discuss a hybrid system composed of a superconducting flux qubit and donor spins in silicon. Our results pave the way for the realization of entanglement-enhanced quantum metrology in state-of-the-art technology.

We introduce bosonic error correction codes for particle loss and dephasing errors, constructed from states generated by particle-number measurements on two-mode Gaussian states. We analyze these states for their suitability in correcting such errors and identify the features most conducive to error mitigation.

Estimating quantum states from experimental data, typically conducted through quantum state tomography (QST), plays a crucial role in verifying the correctness and evaluating the performance of quantum devices. Nonetheless, executing QST for generic unstructured or low-rank quantum states demands an overwhelming number of state copies, escalating exponentially with the quantity of individual quanta within the system, even with the most optimized measurement configurations. Recent work has shown that for one-dimensional quantum states that can be effectively approximated by matrix product operators (MPOs), a polynomial number of copies of the state suffices for reconstruction. Compared to MPOs in one dimension, projected entangled-pair states (PEPSs) and projected entangled-pair operators (PEPOs), which represent typical low-dimensional structures in two dimensions, are more prevalent as a looped tensor network. However, a formal analysis of the sample complexity required for estimating PEPSs or PEPOs has yet to be established. In this paper, we aim to address this gap by providing theoretical guarantees for the stable recovery of PEPSs and PEPOs. Our analysis primarily focuses on two quantum measurement schemes: (1) informationally complete positive operator valued measures (IC-POVMs), specifically the spherical $t$ designs $(t\ge 3)$, and (2) projective rank-one measurements—in particular, Haar random projective measurements. We first establish stable embeddings for PEPSs (or PEPOs) to ensure that the information contained in the states can be preserved under these two measurement schemes. We then show that a constrained least-squares estimator achieves stable recovery for PEPSs (or PEPOs), with the recovery error bounded when the number of state copies scales linearly under spherical $t$ designs and polynomially under Haar-random projective measurements with respect to the number of qudits. These results provide theoretical support for the reliable use of PEPSs and PEPOs in practical quantum information processing.

We develop a method for entanglement detection in bipartite quantum states by using the violation of the rank-1-generated property of matrices. The positive-semidefinite matrices form a convex cone that has extremal elements of rank 1. However, convex conic subsets resulting from the presence of linear constraints allow extremal elements of rank greater than or equal to 2. The problem of deciding when a matrix is rank-1 generated, i.e., a sum of rank-1 positive-semidefinite matrices, has been studied extensively in optimization theory. This rank-1-generated property acts as an entanglement criterion, and we use this property to find different classes of positive under partial transposition (PPT) entangled states. We do this by mapping some faces of PPT density matrices to convex cones that are not rank-1 generated. We show that all separable states map to a rank-1-generated matrix. In general, the same is not true for the corresponding mapping of PPT entangled states. We also extend this approach to construct PPT entangled edge states. Finally, we provide witnesses that detect violations of the rank-1-generated property.

The existence of a maximally entangled pure state is a cornerstone result of entanglement theory that has paramount consequences in quantum information theory. A natural generalization of this property is to consider whether a notion of maximal entanglement is possible among all states with the same spectrum (where the aforementioned case of pure states corresponds to the particular choice in which the spectrum is a delta distribution, i.e., rank-1 states). Despite positive evidence in the past that such a notion might exist at least in the case of two-qubit states, it was recently shown by de Vicente [Phys. Rev. Lett. 133, 050202 (2024)] that the answer to the above question is negative. This reference proved this for particular choices of the spectrum in the case of rank-2 two-qubit density matrices. While this settles the problem in general, it still leaves open whether there are other choices of the spectrum outside the case of pure states where a maximally entangled state for a fixed spectrum might exist. In this work, we extend this impossibility result to all rank-2 and rank-3 two-qubit states as well as for a large class of eigenvalue distributions in the case where the rank equals four.

Noncyclic geometric gates are designed to circumvent the stringent constraints imposed by conventional cyclic conditions, thereby offering enhanced flexibility in the choice of evolution trajectories. Theoretically, such gates can mitigate errors stemming from deviations from cyclicity, highlighting their potential for improving the fault tolerance of quantum gates. However, current research on noncyclic geometric gates remains limited, as the flexibility in their evolution choice and their resilience against multiple error sources prevalent in realistic quantum systems have not been comprehensively validated. In this work, we systematically evaluate all possible noncyclic evolution conditions, clarify their corresponding geometric trajectories, and identify the optimal one for enhancing gate robustness. This is achieved by quantitatively assessing the resilience of the constructed noncyclic geometric gates against universal systematic errors and residual crosstalk. The optimized gates demonstrate a significant improvement in gate robustness compared to representative dynamical Rabi gates and conventional cyclic geometric gates. Moreover, we verify the physical feasibility of high-fidelity noncyclic geometric gates in superconducting quantum circuits, with particular attention to the impacts of intrinsic leakage errors and decoherence effects. Therefore, this work contributes a viable pathway toward robust gate implementation in practical fault-tolerant quantum processors.

We investigate theoretically the high-order harmonic generation (HHG) from the ZnO crystal by solving the semiconductor Bloch equations. It is shown that the harmonic intensities are enhanced and the individual harmonics are broadened by introducing a time delay between the two-color laser pulses. We find that a higher conduction band population does not lead to a higher emission efficiency of HHG, and we illustrate that the number of electron tunneling windows is the dominant mechanism causing the enhancement of HHG in the ZnO crystal. By analyzing crystal-momentum-resolved contributions, we demonstrate that ionization from a much wider range of initial momenta contributes to the enhancement of the harmonic spectrum. Moreover, we investigate the influence of the dynamical Bloch oscillations on HHG. Our results reveal the connection between the tunneling window and HHG, which is crucial for generating bright attosecond sources from solids.

Pure-quartic solitons (PQSs) supported by negative fourth-order dispersion have recently attracted considerable interest. In this work we study both adiabatic and nonadiabatic compression of PQSs in nonlinear optical fibers with pure-quartic dispersion in the presence of distributed gain and loss. Within a variational framework, we show that, for weak constant gain, the adiabatic compression dynamics can be mapped onto the motion of an effective particle in a slowly deformed potential, providing an intuitive physical picture. To overcome the long propagation distance required by conventional adiabatic condition, we exploit shortcuts to adiabaticity (STA) based on inverse engineering and derive analytical gain-loss profiles, with appropriate boundary conditions that realize a prescribed fast compression over a shorter propagation distance. Numerical simulations confirm the theoretical predictions and indicate a minimum propagation distance below which noticeable waveform distortion emerges. Compared with standard adiabatic references, the STA design significantly reduces the required compression distance while maintaining high-fidelity PQS evolution.

The excitation of surface plasmon-polariton (SPP) at the interface between a metal and a dielectric layer with Raman gain in the Kretschmann configuration has been considered for two cases, depending on the direction of the pump wave. Nonlinear effects imposing limitations on the Raman gain have been taken into account for both cases. Conditions where gain compensates for losses and SPP amplification are possible have been formulated. The tendencies in the change of SPP intensity are described. For case A, the pump wave propagates parallel to the SPP wave and gain is limited by pump depletion. In case B, the pump wave propagates normal to the SPP wave and gain is limited by saturation effect.

Dual- and multiwavelength lasers, i.e., lasers with the ability to emit at two or more wavelengths in a controlled fashion, represent an exciting new twist in laser physics. Harnessing the mode competition to control them better remains, however, a challenge. In this work we numerically explore the effect of optical feedback on the emission properties of a dual-wavelength laser using a rate-equation model. We focus on switching capability and investigate the impact of key laser and feedback parameters. We connect the emergence of simultaneous emission to a lower cross saturation between modes and demonstrate that robust switching can be achieved using a short feedback cavity and a sufficiently strong feedback. In particular and in contrast to previous publications, we highlight that the feedback phase difference between the two modes is not a critical parameter. Our results are consistent with recent experimental observations, supporting the relevance of feedback-based control techniques for multiwavelength lasers.

We study the free-space propagation of products of displaced Laguerre-Gaussian beams. Each displaced factor admits an exact representation as a superposition of standard Laguerre-Gaussian beam modes through the optical analog of displaced number states. We reduce the resulting product to a single modal expansion with closed-form weight coefficients and explicit azimuthal selection rules. Working in the reference frame defined by the centroid of the transverse displacements, we evaluate the net orbital angular momentum directly from the modal weights, which provides a criterion to predict transverse rotation. We identify three propagation regimes: no transverse rotation for zero net orbital angular momentum, rigid rotation for products of identical factors with zero radial index, and nonrigid rotation with intensity redistribution otherwise. Our framework enables engineering structured light beams whose transverse rotation encodes propagation information, relevant to depth-sensitive optical microscopy, imaging, and tracking.

We propose a scheme for enhanced sensing of weak anharmonicities based on a three-mode anti-parity-time (anti-PT) symmetric cavity-magnon-waveguide system. By tuning the optical gain to the active cavity mode, the linewidth suppression point for the anti-PT symmetric Hamiltonian can be flexibly controlled even when the two dissipative magnonic modes experience strong intrinsic decay. This essential characteristic is utilized for detecting weak nonlinearities in both the cavity and magnonic modes, with both demonstrating similar high levels of sensitivity. Moreover, the sensitivity can be greatly improved with a detuned laser drive. Based on the integrated passive-active three-mode anti-PT symmetric system, the sensing scheme can be generalized to various physical systems with anharmonicities.

Interferometry is a powerful method for studying many fundamental phenomena ranging from gravitational waves to anyon excitations. Aside from loss of coherence in the interferometer paths, quantum mechanics sets an incompressible bound for linear interferometry arising from the quantum noise of the light beam, called the shot-noise limit. Here, we implement a nonlinear quantum interferometer that uses a microwave cavity and a magnetic field resilient anharmonic superconducting quantum circuit made of granular aluminum. The circuit enables interferometry beyond the shot-noise limit. A direct application of our findings is the detection of dark matter (axions or dark photons), high-frequency gravitational waves, or astronomical masers.

We study the protocol of qutrit teleportation using three-level V-type atoms as qutrits. The impact of noise arising from the spontaneous emission of the atoms is taken into account in the teleportation process via Kraus-operator formalism. Assuming that the V-type atomic system has parallel dipole moments of the decaying transitions, the effects of quantum interference between spontaneous emission channels are considered. We show that the quantum interference between decay channels can protect the fidelity of quantum teleportation against spontaneous-emission noise, thereby assisting teleportation with nonclassical fidelity.