Non-Hermitian many-body systems can be spectrally unstable, so small perturbations may induce large eigenvalue shifts. The pseudospectrum quantifies this instability and provides a perturbation-robust diagnostic. For inverse-polynomially small $\epsilon$, we show that deciding whether a point $z\in \mathbb{C}$ is $\epsilon$-close to the spectrum is PSPACE-hard for 5-local operators, whereas deciding whether $z$ lies in the $\epsilon$-pseudospectrum is quantum-Merlin-Arthur-complete (QMA-complete) for 4-local operators. This identifies pseudospectrum membership as a natural computational target. We then present a concrete end-to-end quantum framework for deciding pseudospectrum membership, which combines a singular-value estimation step with a dissipative state preparation algorithm. Our quantum singular-value Gaussian-filtered search (QSIGS) combines quantum singular value transformation (QSVT) with classical postprocessing to achieve Heisenberg-limited query scaling for singular-value estimation. To prepare suitable input states, we introduce an algorithmic Lindbladian protocol for approximate ground right singular vectors and prove its effectiveness for the Hatano-Nelson model. Finally, we demonstrate the full pipeline on a trapped-ion quantum computer and distinguish points inside and outside the target pseudospectrum near the exceptional point of a minimal non-Hermitian qubit model.

We reanalyze a recent experiment by Sharoglazova et al. [Nature (London) 643, 67 (2025)] highlighting the role of the transient regime. We prove that in the evanescent state of the stationary regime their experimental data can be interpreted in terms of Bohmian quantum mechanics. At the same time, Bohm's quantum potential can be reinterpreted as a kinetic-energy term in the framework of Nelson's stochastic quantum mechanics, with a hidden-variable, nonclassical speed fitting the experimental data as well. The experiment can be interpreted as well within orthodox quantum mechanics and is therefore not conclusive in selecting or challenging any framework.

Tests of gravity-induced entanglement have been proposed as a route to probing the quantum nature of gravity, but existing schemes rely on free-fall interferometry of massive spatial superpositions, imposing severe experimental constraints. We show that systems exhibiting effectively inertial dynamics in the short-time regime reproduce the same gravitational phase accumulation responsible for entanglement generation. Deviations from the free-fall phase enter at order ${(\tau /T)}^2$, where $\tau$ is the interaction timescale and $T$ is the characteristic period of the constrained motion. We analyze a representative mechanically constrained implementation using carbon nanotube pendula and show that the resulting correction to the entangling phase remains small in experimentally relevant regimes, leading to a negligible modification of the interference visibility used to certify entanglement. These results demonstrate that gravity-induced entanglement protocols extend beyond free-fall implementations to a broader class of constrained dynamical systems, complementing existing proposals for experimental realisations of the Bose-Marletto-Vedral protocol.

We identify localizable entanglement (LE) as an order parameter for measurement-induced phase transitions (MIPTs). LE exhibits universal finite-size scaling with critical exponents that match previous MIPT results and gives a nice operational interpretation connecting MIPTs to classical percolation. Remarkably, we find that LE decays exponentially with distance in the area-law phase as opposed to being essentially constant for the volume-law phase and, thereby, discover an intrinsic length scale ${\xi }_E$ that diverges at the critical measurement probability $p_c$. While classical percolation transition captures successful transport across a network, MIPT as characterized by LE can be interpreted as quantifying the amount of quantum teleportation between two given nodes in a quantum circuit. Building on this insight, we propose a two-ancilla protocol that provides an experimentally accessible readout of entanglement redistribution across the transition.

Recent work has identified a dynamical squeezing phase transition in power-law interacting bilayer XXZ spin models, separating a fully collective phase with Heisenberg-limited squeezing from a partially collective phase with universal critical scaling. Here we test and establish the universality of this transition along two qualitatively different microscopic axes: lattice geometry, by studying square, triangular, and honeycomb two-dimensional $\left(2\mathrm{D}\right)$ bilayers as well as $1\mathrm{D}$ ladders, and a symmetry-preserving rescaling $\lambda$ of the interlayer couplings relative to the intralayer ones. Combining a Bogoliubov instability analysis with discrete truncated Wigner simulations, we find that the transition persists across all four lattice geometries and over a wide range of $\lambda$ with critical exponents consistent within error, providing strong evidence for a genuine nonequilibrium universality class. The Bogoliubov theory recovers the previously identified scaling $a_Z^{*}\propto L$ in the long-range interacting regime $\alpha <d+2$, and yields an analytical scaling $a_Z^{*}\propto L^{2/(\alpha -d)}$ for the critical aspect ratio with system size for $\alpha >d+2$, with $\alpha$ the power-law exponent in dimension $d$. This uncovers a previously unrecognized sublinear regime for short-range interactions. By tuning $\lambda$, we vary the interlayer coupling strength at fixed layer spacing, demonstrating that the dynamical transition can be driven purely through interaction engineering without modifying the underlying geometry. These findings provide a versatile route toward controlling entanglement generation in Rydberg-array, polar molecule, and trapped-ion platforms with applications in quantum sensing and simulation.

We propose a method for generating Schrödinger-cat states—defined as equal superpositions of arbitrary coherent spin states—using a concise sequence of rapid, alternating one-axis twisting and rotation pulses. We demonstrate that the required shearing strength for the protocol, which scales linearly with time, decreases with increasing number of atoms $\left(N\right)$ in proportion to $1/\sqrt{N}$. The resulting states achieve optimal quantum Fisher information among superpositions of two coherent spin states with a specified angular separation. This makes them well suited to exceed the classical phase sensitivity limit in quantum metrology applications. Notably, our protocol is compatible with a time-reversal strategy for quantum metrology, ensuring its practical viability. We also demonstrate that the Heisenberg-limit scaling remains intact when reducing the twisting alongside the number of atoms, mitigating losses from photon scattering. Although the states are then prepared only approximately, they retain enough catlike structure to sustain this scaling.

Kerr parametric oscillators can encode Kerr-cat qubits, whose biased-noise nature can reduce the hardware overhead required for fault-tolerant quantum computation. In a recently proposed $R_{ZZ}$-gate scheme for Kerr-cat qubits, a tunable coupler must be displaced while avoiding nonadiabatic excitation. To address this challenge, we propose and theoretically analyze a protocol that applies fast-forward scaling to the coupler displacement. The protocol suppresses nonadiabatic excitation of the coupler and enables high-speed $R_{ZZ}$ gates.

The coherent interaction between free electrons and optical near-fields enables the active modulation of electron wave packets, which is a mechanism central to photon-induced near-field electron microscopy (PINEM). While existing theories effectively describe these interactions at high kinetic energies, the growing interest in low-energy ultrafast electron microscopy may demand frameworks that explicitly account for finite wave packet geometries and recoil effects. In this paper, we develop a rigorous three-dimensional (3D) quantum scattering theory for electron wave packets interacting with time-periodic short-range potentials, with optical near-field modulation serving as the primary motivation. By mapping the time-dependent dynamics into an extended Floquet space, we formally connect the modulation process to time-independent multichannel scattering. We evaluate the resulting scattering amplitudes using both an exact $R$-matrix approach and a multichannel eikonal approximation. In the present 3D formulation, the eikonal approximation recovers PINEM-like probabilities, but now weighted by the incoming wave packet's transverse profile. For the parameters considered, the comparison between the two methodologies verifies the accuracy of the latter. As a benchmark application, we consider a spherically symmetric oscillating potential. This model demonstrates the generation of distinct energy sidebands and reveals that the modulation strength is sensitive to the transverse focusing of the incident electron pulse.

The tight focusing of ultrashort laser pulses by an off-axis parabolic (OAP) mirror beyond the slowly varying temporal envelope (SVTE) approximation is studied. Nonmonochromatic effects are found to generate appreciable spatiotemporal coupling even near the focus, and the SVTE approximation leads to an overestimate of the pulse powers in the focal spot by up to 25%. To describe this regime, we develop a model based on Stratton-Chu integrals and use it to quantify the spatiotemporal coupling effects depending on the laser pulse duration, its position, and mirror off-axis angle. A comparison with the SVTE description yields an explicit criterion for the SVTE approximation. The fields, predicted by the nonmonochromatic model, are validated in three-dimensional smilei simulations, providing a compact framework for modeling few-femtosecond OAP focusing in high-intensity laser-plasma experiments.

We demonstrate a mechanism driving complex critical phenomena in open light-atom interacting systems by investigating a parametrically amplified quantum Rabi model (QRM) subject to both single- and two-photon decay. In the classical oscillator limit, four composite phases emerge, arising from the possible normal or superradiant regimes across the upper and lower spin branches. A mean-field analysis reveals that the two-photon decay activates the intrinsic nonlinearity of the QRM. The synergy of the coherent and dissipative two-photon processes, together with the spin-boson coupling, constitutes an “inverted” regime where superradiance emerges exclusively at weak coupling. This regime features first- and second-order superradiant dissipative phase transitions (DPTs) separated by a tricritical point. Utilizing an adiabatic approach and the semi-classical Langevin formalism, we further study the steady-state structure beyond the mean-field level. The universality classes of the DPTs are identified, with the corresponding critical and finite-size scaling exponents derived and a scaling ansatz proposed to describe the critical behavior.

We demonstrate unconventional photon blockade in a symmetric Kerr dimer driven with equal-amplitude fields at a ${90}^{\circ }$ phase difference. The minimum intercavity coupling is $J_{min}=\gamma /4$ at a Kerr nonlinearity $U\ll \gamma$ achievable in standard photonic molecules. The quadrature-driven site emits strongly antibunched light with a smooth, oscillation-free second-order correlator directly resolvable with standard detectors. The scheme operates under continuous-wave and pulsed excitation, and fabrication disorder can be fully compensated by retuning the drive phase and amplitude ratio, removing the need for postfabrication cavity trimming.

The optimum conditions for unidirectional coupling in quasi-phase-matched non-Hermitian systems are analyzed for both autonomous and externally driven configurations. The quasi-phase-matched coupling mechanism is due to periodic modulation of the real and imaginary parts of the coupling interface, which results in not equal coupling coefficients between interacting waves. The conventional $\mathcal{PT}$ symmetry theory suggests that the strongest unidirectionality should occur exactly at the exceptional point (EP). We show that this expectation is in general incorrect, as the optimum is shifted away from the EP depending on the detuning from exact quasi-phase-matching resonance. The shift is signed: reversing the detuning moves the optimum to the opposite side of the nominal EP. We formulate a unified two-mode description for autonomous and driven systems, and derive the normal forms governing the displaced observable singularities near the nominal EPs.

Contextuality and measurement incompatibility are two fundamental aspects of nonclassicality, and their manifestations in observed quantum correlations are often deeply interconnected. Recently, measurement incompatibility has been studied in connection with nonlocality, particularly in terms of their robustness under various quantum channels. This line of investigation helps to establish a connection between the channels that break nonlocality and those that break incompatibility. In this study, we focus on an asymmetric bipartite Bell scenario involving three and four inputs on Alice's and Bob's sides, respectively, with each of these inputs having dichotomous outcomes. Under the assumption of locality, the observed statistics in this asymmetric scenario obeys the elegant Bell inequality (EBI). Here, we use a different version of the EBI that relies on the assumption of the preparation noncontextuality. By taking the violation of this noncontextual version of EBI as a witness of preparation contextuality we establish a connection between the channels that break contextuality and the channels that break triplewise measurement incompatibility. Our results suggest that any channel which breaks EBI contextuality will also break Clauser-Horne-Shimony-Holt contextuality; however, the reverse does not hold. We also show that a depolarizing channel that breaks $N$-wise incompatibility can also break a certain form of contextuality, witnessed by a generalized inequality involving $N$ measurements on one wing of a bipartite Bell scenario.

We demonstrate that machine learning provides a powerful tool for discovering approximate quantum error-correcting (AQEC) codes beyond conventional algebraic frameworks. Building upon observations from hybrid quantum-classical learning, we introduce families of noise-strength-adapted (NSA) codes whose codewords vary with noise strength. In particular, through machine learning, we first identify four-qubit NSA self-complementary and pair-complementary codes that outperform standard Leung–Nielsen–Chuang–Yamamoto amplitude damping (AD) code in both worst-case fidelity and Knill–Laflamme condition violation. We then derive analytical generalizations of both forms into families of NSA AD codes for arbitrary numbers of qubits, as well as an NSA variant of the 0-2-4 binomial code for single-photon loss. We note that the pair-complementary codes, which have no known non-NSA analogs, achieve even better performance, with higher-order loss suppression and larger fidelity, than the self-complementary ones. Our results demonstrate that adaptation to noise strength can systematically lead to significant improvements in error correction capability and also showcase how machine learning can help discover valuable code formalisms that may not emerge from traditional design approaches.

Quantum networks require interfaces that translate memory photons to telecom wavelengths while controlling spatial modes; these tasks are typically performed by separate components. We present a programmable alternative: A structured pump writes a virtual Bragg grating that enables simultaneous spatiospectral conversion and real-time control of emission. Using a ${\mathrm{LiNbO}}_3$ whispering-gallery resonator, we predict and numerically validate 93% spatial coupling and bidirectional conversion between 736 and 1347 nm. This reconfigurable interface eliminates cascaded losses and the need for hardware modifications.

Decoding stabilizer codes such as the surface and toric codes involves evaluating free-energy differences in a disordered statistical mechanics model, in which the randomness comes from the observed pattern of error syndromes. We study the statistical distribution of logical failure rates across observed syndromes in the toric code, and show that, within the coding phase, logical failures are predominantly caused by exponentially unlikely syndromes. Therefore, postselecting on not seeing these exponentially unlikely syndrome patterns offers a scalable accuracy gain. In general, the logical error rate can be suppressed from $p_f$ to $p_f^b$, where $b\ge 2$ in general; in the specific case of the toric code with perfect syndrome measurements, we find numerically that $b=3.1\left(1\right)$. Our analysis extends directly to code families for which the relevant free-energy or decoder-confidence variable obeys a large-deviation principle.

In continuous-variable systems, quantifying the structural complexity of quantum states is crucial for distinguishing trivial classical-like configurations from genuinely quantum features. Here we propose a family of phase-space complexity measures, termed Rényi-Wehrl-Fisher complexities, for single-mode continuous-variable quantum states. This family integrates global delocalization from the Rényi-Wehrl entropy and local sharpness from the Fisher information. Notably, we establish a universal lower bound saturated if and only if the state is displaced thermal. The measure is invariant under phase-space displacements, rotations, and uniform scalings and recovers the previously introduced Wehrl-Fisher complexity when the Rényi parameter approaches unity. The tunable Rényi parameter acts as a phase-space selector. On the one hand, it amplifies low-probability tails where quantum interference resides or highlights the high-probability core where the bulk of the distribution concentrates. This tunability reveals a fundamental dichotomy hidden in the fixed Shannon-entropy framework. Moreover, the complexity is strictly parameterindependent for all Gaussian states but acquires strong parameter dependence for non-Gaussian states. This dichotomy enables an experimental diagnostic. By tuning the parameter in heterodyne measurements, one certifies non-Gaussianity in a self-calibrating manner. We derive closed-form Fock-state expressions. The complexity distinguishes squeezed states with unbounded range, detects cat-state coherence where the fixed measure is blind, and captures entropy-Fisher competition in mixed non-Gaussian states. These results establish the Rényi-Wehrl-Fisher complexity as a rigorous, experimentally accessible, and versatile tool for characterizing structural complexity and quantum nonclassicality in continuous-variable systems.

We introduce a framework for scalable and broadband free-space phase-matched four-wave mixing in ring resonators. This method for four-wave mixing reduces the complexity of coupling an emitter to a quantum network by combining the spatial and spectral interfaces between them into one nonlinear optical process. The device is compliant with current heterogeneous integration capabilities and supports frequency phase matching across approximately 165 nm. We outline a fabrication-ready diamond-on-insulator pathway towards modular unit cells that natively bridge visible color centers to the infrared spectrum for scalable quantum networks. We also present and analyze an end-to-end framework for considering single-photon coupling efficiency from a color center to a quantum network. This framework represents a step forwards in analyzing and reducing system-scale losses in a spin-photon interface.