In quantum information and computation, the generation of entanglement through unitary gates remains a significant and active area of research. However, there are states termed as absolutely separable, from which entanglement cannot be created through any nonlocal unitary action. Thus, from a resource-theoretic perspective, nonabsolutely separable states are useful as they can be turned into entangled states using some appropriate unitary gates. In this work, we propose an efficient method to detect nonabsolutely separable states. Our approach relies on evaluating moments that can bypass the need for full state tomography, thereby enhancing its practical applicability. We then present several examples in support of our detection scheme. We also address a closely related problem concerning states whose partial transpose remains positive under any arbitrary nonlocal unitary action. Furthermore, we examine the effectiveness of our moment-based approach in the detection of quantum channels that are not absolutely separating, which entails the detection of resource-preserving channels. Finally, we demonstrate the operational significance of nonabsolutely separable states by proving that every such state can provide an advantage in a quantum-channel discrimination task.

We present the quantum-mechanical calculations of the dimensionless sensitivity coefficients $Q$ for small changes in the fundamental physical constant $\mu =m_e/m_p$ (the electron-to-proton mass ratio) for a number of low-frequency (1–50 GHz) transitions of the acetaldehyde (${\mathrm{CH}}_3\mathrm{CHO}$) molecule. The calculations show that $Q$ varies in the range from 0.62 to 3.61. An example of the practical use of the ${\mathrm{CH}}_3\mathrm{CHO}$ and ${\mathrm{CH}}_3\mathrm{OH}$ lines tracing the same regions in three molecular clouds, located at large galactocentric distances ($D_{\mathrm{GC}}\sim 8$ kpc), is considered. This results in a limit on the $\mu$ variations of $\mathrm{\Delta }\mu /\mu =(0.1\pm 0.4)\times {10}^{-7}$, which is in line with the most stringent upper limits on changes in $\mu$ previously obtained based on other molecules and methods. The limit obtained restricts hypothetical violations of the Einstein principle of the local position invariance at the level of $4\times {10}^{-8}$ in the Galactic disk at large galactocentric distances.

We investigate the fragmentation dynamics of the diiodoacetylene (${\mathrm{C}}_2{\mathrm{I}}_2$) molecule under the impact of 10-keV/u ${\mathrm{Ar}}^{8+}$ ions, focusing on complete three-body ionic breakup channels. The experimental results, analyzed using Newton and Dalitz plots, are compared with classical trajectory simulations based on a pure Coulomb-repulsion model. We identify the presence of concerted and sequential fragmentation mechanisms of ${\mathrm{C}}_2{\mathrm{I}}_2{}^{3+}$. Concerted breakup is found to be the dominant pathway across all channels, where near-perpendicular emission of the lightest fragment is observed relative to its heavier counterparts. Sequential fragmentation proceeds predominantly via the intermediate ${\mathrm{C}}_2{\mathrm{I}}^{2+}$, which dissociates into either ${\mathrm{C}}_2{}^{+}+{\mathrm{I}}^{+}$ or ${\mathrm{C}}^{+}+{\mathrm{CI}}^{+}$ fragments. An alternative sequential route through ${\mathrm{CI}}^{2+}$ leading to ${\mathrm{C}}^{+}+{\mathrm{I}}^{+}$ is also possible. These sequential pathways reveal clear signatures of intramolecular scattering, which are governed by the lifetime of the intermediate dication. Our analysis shows that the internal energy of ${\mathrm{C}}_2{\mathrm{I}}_2{}^{3+}$ plays a critical role in determining the preferred dissociation pathways.

Controllable single-photon scattering of a periodically modulated two-level giant atom indirectly coupled to a one-dimensional waveguide at two separate points via two single-mode cavities is investigated in both the Markovian and non-Markovian regimes. By using an effective Floquet Hamiltonian approach, we demonstrate how this architecture is capable of transcending various limitations of conventional static giant-atom systems. The periodic modulation serves as a dynamic controller that enables in situ reconfiguration of quantum interference, allowing a single device to be realized for multiple functions, including programmable Fano resonance lineshapes and dynamic switching between transmission and reflection at different frequencies. Notably, the cavity-mediated coupling facilitates the emergence of tunable multidip spectra even in the Markovian regime, a feature previously unattainable in giant atom-waveguide-only configurations without non-Markovian retardation. The underlying Floquet engineering generates dressed-state sidebands that effectively transform the giant atom-cavity structure into a dynamically controllable multilevel system, creating new pathways for photon control through interference among these synthetic states. In the non-Markovian regime, this modulation capability combines with retardation effects can be further used to produce dynamic controllable spectra with multiple peaks and dips. Perfect transmission or reflection with tunable ultranarrow or ultrawide scattering windows can be achieved. This work establishes a paradigm for dynamically reconfigurable quantum devices with promising applications in quantum routing, spectral shaping, and information processing.

Macroscopic quantum superpositions, such as mechanical Schrödinger-cat states, are central to emerging quantum technologies in sensing and bosonic error-correcting codes. We propose a scheme to generate such states by coupling a nanomechanical resonator to a coherently driven two-level system via both transverse and longitudinal interactions. Driving the qubit at twice the oscillator frequency activates resonant two-phonon exchange processes, enabling coherent conversion of drive energy into phonon pairs and their dissipative stabilization. Starting from the full time-dependent Hamiltonian, we derive an effective master equation for the mechanical mode by perturbative elimination of the lossy qubit. The reduced dynamics feature engineered two-phonon loss and a coherent squeezing term, which together drive the resonator into a deterministic Schrödinger-cat state. Our approach requires only a single driven qubit and no auxiliary cavity, offering a scalable and experimentally accessible route to macroscopic quantum superpositions in circuit-QED and related platforms.

Quantum capacity quantifies the amount of quantum information that can be transmitted by a quantum channel with an arbitrary small probability of error. Mathematically, the quantum capacity is given by an asymptotic formula involving the one-shot quantum capacity of the associated channel, which, due to purely quantum effects such as superadditivity of one-shot quantum capacity, is rarely computable. The one-shot quantum capacity is mathematically characterized in terms of optimizing an entropic quantity referred to as coherent information over all possible input states of a channel, the computation of which also tends to be intractable due to the difficulty of optimizing the coherent information. In this work, we develop perturbative methods for analyzing the behavior of first- and second-order derivatives of coherent information of a quantum channel with respect to small perturbations of the input state. By doing so, we are able to derive three general criteria for determining whether an input state yields suboptimal coherent information. We then show how our criteria yield sufficient conditions for superadditivity of one-shot quantum capacity, and also for detecting a positive gap between one-shot private capacity and one-shot quantum capacity. The utility of our criteria is illustrated through examples which yield several results regarding the one-shot quantum capacity of qubit depolarizing channels, Pauli channels, and dephrasure channels.

A major difficulty in quantum computation is the ability to implement fault-tolerant computations, protecting information against undesired interactions with the environment. Stabilizer codes were introduced as a means to protect information when storing or applying computations in Hilbert spaces where the local dimension is fixed, i.e., in Hilbert spaces of the form ${\left({\mathbb{C}}^D\right)}^{\otimes n}$. If $D$ is a prime power then one can consider stabiliser codes over finite fields [A. Ketkar et al., IEEE Trans. Inf. Theory 52, 4892 (2006)], which allows a deeper mathematical structure to be used to develop stabilizer codes. However, there is no practical reason that the subsystems should have the same local dimension and in this article we introduce a stabilizer formalism for mixed-dimensional Hilbert spaces, i.e., of the form ${\mathbb{C}}^{D_1}\otimes \cdots \otimes {\mathbb{C}}^{D_n}$. More generally, we define and prove a Singleton bound for quantum error-correcting codes of mixed-dimensional Hilbert spaces. We redefine entanglement measures for these Hilbert spaces and follow [F. Huber et al., J. Phys. A: Math. Theor. 51, 175301 (2018)] and define absolutely maximally entangled states as states which maximize this entanglement measure. We provide examples of absolutely maximally entangled states in spaces of dimensions not previously known to have absolutely maximally entangled states.

Quantum simulation plays a pivotal role in understanding complex quantum systems. Yet, its implementation on noisy intermediate-scale quantum devices remains limited by hardware constraints. Variational fast forwarding (VFF), a hybrid quantum-classical algorithm, aims to address this challenge by approximating the diagonalization of short-time evolution operators. However, the original design of VFF relies on parametrized quantum circuits $W\left(\mathit{\theta }\right)$ and $D(\mathit{\gamma },\mathrm{\Delta }t)$, which often incur excessive gate complexity, impeding practical deployment on near-term quantum hardware. Meanwhile, current quantum circuit synthesis or compilation algorithms are insufficient for VFF circuit design, as this task requires the simultaneous construction of circuits $W\left(\mathit{\theta }\right)$ and $D(\mathit{\gamma },\mathrm{\Delta }t)$, while ensuring that $W\left(\mathit{\theta }\right)$ approximates the eigenvectors and $D(\mathit{\gamma },\mathrm{\Delta }t)$ approximates the corresponding eigenvalues and remains diagonal. To overcome this limitation, we propose an approach that jointly designs shallow $W\left(\mathit{\theta }\right)$ and $D(\mathit{\gamma },\mathrm{\Delta }t)$ circuits using multiagent reinforcement learning (MARL). In this fully cooperative framework, two agents collaboratively learn to construct the circuits while sharing rewards to optimize overall circuit performance. Our results demonstrate that the MARL-generated circuits generally achieve lower gate counts and tend to enable more reliable and faster convergence to the specified accuracy target compared to those produced by the standard VFF. Furthermore, our results highlight that our method typically outperforms qfast in terms of quantum gate counts, although occasional exceptions are observed. Simulated experiments on benchmark Hamiltonians confirm that our approach can reduce resource consumption and be used in situations where hardware capabilities limit the depth of quantum circuits.

The NOON states play a critical role as physical resources in quantum information processing and quantum metrology, yet their preparation efficiency and applicability are often constrained by complicated operational procedures or the requirement for nonlinear interactions. In this paper we propose an efficient protocol to generate the NOON states within two microwave cavities embedded in a superconducting system, assisted by an auxiliary five-level qudit. The state preparation is accomplished in three steps for an arbitrary photon number $N$ by adjusting only external classical fields while keeping the qudit-cavity coupling strengths and the qudit level spacings fixed. Based on parameters accessible in superconducting systems, numerical simulations show that the protocol achieves relatively high fidelity for the NOON-state preparation even in the presence of parameter fluctuations and decoherence effects. Thus, this protocol may provide a practical approach for preparing the NOON states with current technology. Notably, since nonlinear interactions are not required, the protocol is flexible and has the potential to be applied across various physical systems.

The dispersion coefficients $C_6$, $C_8$, and $C_{10}$ for hydrogen molecules (${\mathrm{H}}_2$, ${\mathrm{D}}_2$, and HD) in the three lowest pure vibrational states are computed nonadiabatically using a variational approach in Hylleraas coordinates. We also report the scalar static quadrupole polarizability ${\alpha }_2$ and octupole polarizability ${\alpha }_3$ for hydrogen molecules in their ground vibrational states. Comparisons with existing values obtained under the Born-Oppenheimer approximation show that the contribution of nuclear rotational transitions becomes non-negligible for $C_8$ and $C_{10}$ at the percentage accuracy level. This effect is particularly pronounced for ${\alpha }_2$, where nuclear rotational transitions provide the dominant contribution. Specifically, for ${\alpha }_2$ of ${\mathrm{H}}_2$ in the ground state, the contribution from electronic transitions amounts to 17.03, while that from nuclear rotational transitions reaches 58.33, resulting in a total static quadrupole polarizability of 75.36(2).

We experimentally investigate four-wave mixing (FWM) of matter waves in two geometric configurations in ${}^{39}\mathrm{K}$ Bose-Einstein condensates with the atomic interaction tuned via Feshbach resonances. For one configuration with the single-spin component, the FWM yield increases with a larger scattering length. For the two-spin component configuration, we specifically investigate FWM in both the droplet and gas parameter regimes. We find that the FWM yield reaches its maximum near the critical parameter region between the gas and droplet phases. Our research can help to optimize the FWM yield for matter-wave amplification and entangled atom pair generation, making it conducive to applications in quantum information processing and precision measurement.

We present an experimental study of the Rayleigh scattering of highly linearly polarized hard x rays. For this study, photons produced by a synchrotron source with an energy of $175\mathrm{k}\mathrm{e}\mathrm{V}$ are scattered by a thin gold foil target. While the linear polarization in terms of degree of polarization and its orientation of the incident photon beam are well known, the outgoing scattered photons are analyzed by means of Compton polarimetry. For this, a polarimeter detector is positioned at different observation angles with respect to the incident photon beam. For the first time the polarization of elastically scattered photons outside of the polarization plane of the incident radiation is investigated, enabling novel access to a so-far untested component of the polarization transfer. The experimental findings are compared to predictions of two theoretical models. It is found that, when leaving the polarization plane of the incident photon beam, a strong deviation from the form factor approximation is evident, while calculations of the Rayleigh scattering in the $S$-matrix approach agree with the experimental results. Furthermore, this study is a foundation for future experiments dedicated to a polarization-resolved analysis of Delbrück scattering.

Quantum contextuality, where measurement outcomes depend on the measurement context, implies a failure of classical realism in quantum systems. As recently shown, the transition between measurement contexts can be mapped onto the path that a quantum particle takes through an interferometer. Here, we investigate the relation between contextuality and the counterfactual gain observed in the output ports of such an interferometer when one of the paths is blocked. It is shown that experimental evidence of contextuality can only be obtained when counterfactual gain is observed for a specific combination of blocked path and output port. Using a silicon photonic integrated circuit, we experimentally observe the counterfactual gain for a selection of input states and evaluate the associated evidence for contextuality. The results confirm that contextuality can only be observed in the presence of counterfactual gain.

We consider non-Hermitian tight-binding one-dimensional Hamiltonians and show that imposing a certain symmetry causes all eigenvalues to pair up and the corresponding eigenstates to coalesce in pairs. This pairwise coalescence (PC) is an enhanced version of an exceptional point—the complete spectrum pairs up, not just one pair of eigenstates. The symmetry is that of reflection excluding the central two sites, and allowing flipping of nonreciprocal hoppings (“generalized off-center reflection symmetry”). Two simple examples of PC exist in the literature—our construction encompasses these examples and extends them to a vast class of Hamiltonians. We study several families of such Hamiltonians, extend to cases of full-spectrum higher-order coalescences, and show how the PC point corresponds to amplified nonorthogonality of the eigenstates and enhanced loss of norm in time evolution.

We demonstrate that the dynamically assisted Schwinger mechanism, which typically requires the superposition of two distinct electromagnetic fields, can be effectively realized using a single Sauter-type electric field with a vibrating position. We compare it with a conventional oscillating field of the same strength and frequency, showing that the vibrating field is more efficient. Its stable electric force suppresses both virtual particle annihilations and Pauli blocking. Furthermore, we verify that the underlying mechanism in the vibrating field is analogous to that of a combined field. Our study proposes a new approach for testing the Schwinger process without requiring the precise alignment of multiple beams.

Kohn-Sham density functional theory (KS-DFT) provides the exact ground-state energy and electron density of a molecule, contingent on the as-yet-unknown universal exchange-correlation (XC) functional. Recent research has demonstrated that neural networks can efficiently learn to represent approximations to that functional, offering accurate generalizations to molecules not present during the training process. With the latest advancements in quantum-enhanced machine learning (ML), evidence is growing that quantum neural network (QNN) models may offer advantages in ML applications. In this work, we explore the use of QNNs for representing XC functionals, enhancing and comparing them to classical ML techniques. We present QNNs based on differentiable quantum circuits as quantum (hybrid) models for XC in KS-DFT, implemented across various architectures. We assess their performance on one-dimensional (1D) and 3D systems. To that end, we expand existing differentiable KS-DFT frameworks and propose strategies for efficient training of such functionals, highlighting the importance of fractional orbital occupation for accurate results. We prove that, under specific assumptions, noise-induced errors do not accumulate across SCF iterations, and we confirm this result through numerical experiments. Our best QNN-based XC functional yields energy profiles of the ${\mathrm{H}}_2$ and planar ${\mathrm{H}}_4$ molecules that deviate by no more than 1 mHa from the reference DMRG and FCI/6-31G results, respectively. Moreover, they reach chemical precision on a system, ${\mathrm{H}}_2{\mathrm{H}}_2$, not present in the training dataset, using only a few variational parameters. This work lays the foundation for the integration of quantum models in KS-DFT, thereby opening new avenues for expressing XC functionals in a differentiable way and facilitating computations of various properties.

Traditionally, the characterization of quantum resources has focused on individual quantum states. Recent literature, however, has increasingly explored the characterization of resources in multistates—ordered collections of states indexed by a varying parameter. In this work, we provide a unitary-invariant framework to pinpoint imaginarity and coherence in sets of qubit states: We prove that Bloch vectors must be coplanar to be imaginarity free and collinear to be incoherent, yielding exact rank-based tests of coherence and imaginarity and closed-form bounds for existing robustness quantifiers, all based on two-state overlaps only. We also show that the set of imaginarity-free multistates is not convex, and that third-order invariants completely characterize multistate imaginarity of single-qubits but not of higher-dimensional systems. As our main technical result, we show that every Bargmann invariant of single-qubit states is determined (up to conjugation) by two-state overlaps. Beyond qubits, we give purity and system-agnostic coherence witnesses from equality constraints on higher-order invariants and connect our results to practical protocols—characterization of partial distinguishability, spin-chirality detection, and subchannel discrimination.

Quantum reservoir computing employs fixed quantum dynamics as a feature map for machine learning. Integrating multiple quantum reservoirs, however, raises a key question: how few intermodule connections are sufficient to match the performance of a single reservoir? To address this, we explicitly separate intramodule dynamics from intermodule couplings and systematically examine different connectivity schemes. We find that even a small number of well-placed connections between two modules can match single-reservoir accuracy, with simple one-to-one connections proving highly effective. Performance generally improves with increasing intermodule entanglement, and these correlations persist for both $ZZ$-coupled and random modular reservoirs. Extensions to three modules and evaluations across multiple datasets (MNIST, Fashion-MNIST, CIFAR-10) suggest that the modular architecture can be applied to diverse reservoir types and image-classification datasets. These results motivate modular quantum reservoir designs that align naturally with realistic hardware, such as two-dimensional quantum-chip layouts or networks of small integrated quantum systems.