We investigate the dynamics of polar systems coupled to classical external beams in the ultrastrong coupling regime. The permanent dipole moments (PDMs) sustained by polar systems can couple to the electromagnetic field, giving rise to a variety of processes such as difference frequency and harmonic generation, reflected in the complicated dynamics of the atomic system. Here, we demonstrate that the atomic evolution can be described simply in a dynamic reference frame. We derive a Jaynes-Cummings-like framework with effective parameters describing the frequency shift, coupling strength with the driving field, and a rescaled relaxation rate. The familiar linear scaling of the interaction strength with the field amplitude is replaced with a nonlinear dependence, suggesting potential applications for improving the coherence of quantum system ensembles with permanent dipoles.

Symmetry plays a crucial role in quantum physics, dictating the behavior and dynamics of physical systems. In this paper, we develop a hypothesis-testing framework for quantum dynamics symmetry using a limited number of queries to the unknown unitary operation and establish the quantum max-relative entropy lower bound for the type-II error. We construct optimal ancilla-free protocols that achieve optimal type-II error probability for testing time-reversal symmetry ($T$ symmetry) and diagonal symmetry ($Z$ symmetry) with limited queries. Contrasting with the advantages of indefinite causal-order strategies in various quantum information processing tasks, we show that parallel, adaptive, and indefinite causal-order strategies have equal power for our tasks. We establish optimal protocols for $T$ symmetry testing and $Z$ symmetry testing for six and five queries, respectively, from which we infer that the type-II error exhibits a decay rate of $O\left(m^{-2}\right)$ with respect to the number of queries $m$. This represents a significant improvement over the basic repetition protocols without using global entanglement, where the error decays at a slower rate of $O\left(m^{-1}\right)$.

Characterizing noise is key to the optimal control of the quantum system it affects. Using a single-qubit probe and appropriate sequences of $\pi$ and non-$\pi$ pulses, we show how one can characterize the noise a quantum bath generates across a wide range of frequencies—including frequencies below the limit set by the probe's ${\mathbb{T}}_2$ time. To do so we leverage an exact expression for the dynamics of the probe in the presence of non-$\pi$ pulses and a general inequality between the symmetric (classical) and antisymmetric (quantum) components of the noise spectrum generated by a Gaussian bath. We analyze the robustness of our method on state-preparation and measurement errors and pulse imperfection. Simulation demonstrates the effectiveness of our method.

We introduce a family of high-rate amplitude-damping (AD) Shor codes, designed to effectively correct AD errors while maintaining immunity to collective coherent (CC) errors. The proposed $\left[\right[(w+1)(w+K),K\left]\right]$ AD codes can approximately correct up to $w$ AD errors, with flexible parameters $(w,K)$, and we provide a rigorous proof that these codes satisfy the approximate quantum error-correction conditions. These codes leverage structured stabilizer measurements, enabling efficient detection of AD errors using only local operations and ancillary qubits. We further construct a family of CC-AD Shor codes by concatenating these AD codes with the dual-rail code.

Atomic excitation and ionization in antiproton collisions with the helium atom initially in the singlet and triplet states are considered. This is done by employing an implementation of the convergent close-coupling approach to ion-atom collisions using the Coulomb-Sturmian functions. We demonstrate that the unified treatment of excitation and ionization proposed in our previous work [Phys. Rev. A 110, 032810 (2024)] can be applied to targets with more than one electron. This allows us to obtain state-resolved excitation cross sections up to arbitrarily high bound energy levels, including the high-lying Rydberg states, and the singly differential ionization cross section in the entire electron energy range including at and arbitrarily close to the threshold. A comprehensive set of results for the elastic, ionization, and excitation cross sections in both initial configurations of the target is presented. The cross-section data for antiproton collisions with triplet-state helium, previously unavailable, are now determined.

Nonlinear spin dynamics are essential in exploring nonequilibrium quantum phenomena and have broad applications in precision measurement. Among these systems, the combination of a bias magnetic field and feedback mechanisms can induce self-sustained oscillations at the base Larmor frequency due to nonlinearity. These features have driven the development of single-species and multiple-species spin masers. The latter, with multiple discrete Larmor frequencies, provides significant advantages for precision measurement by mitigating uncertainties in precession frequencies due to long-term drifts in experimental conditions. The self-sustained oscillations of single-species and multiple-species spin masers correspond to limit cycles and quasi-periodic orbits of the stable nonlinear dynamics of the systems, respectively; the correspondence is elucidated in a recent study on a related spin system featuring two discrete intrinsic Larmor frequencies under dual bias magnetic fields [T. Wang et al., Commun. Phys. 8, 41 (2025)]. Here, we extend the study to the case where the intrinsic Larmor frequencies of individual spins of the system, given rise to by an inhomogeneous bias magnetic field, form a continuum. We show that generically the stable dynamics of the system includes limit cycles, quasi-periodic orbits, and chaos. We establish the relation between the synchronization frequency of limit cycles and the field inhomogeneity and derive an equation determining the stability of limit cycles. Furthermore, detailed characteristics of different dynamical phases, especially the robustness of limit cycles and quasi-periodic orbits against experimental fluctuations, are discussed. Our findings not only encompass the case of discrete Larmor frequencies, but also provide crucial insights for precision measurement and the exploration of continuous time crystals and quasi-crystals.

Ultrafast nuclear dynamics in laser-induced dissociative double ionization of ${\mathrm{H}}_2$ is challenging to investigate with a multicycle elliptically polarized femtosecond laser pulse due to the difficulty of distinguishing electrons released in neighboring optical cycles. In this study we experimentally reveal these dynamics by employing the same laser technique and overcoming this limitation through comparison with the isotopic counterpart of ${\mathrm{D}}_2$. By analyzing the internuclear-distance-dependent sum momentum of the ionic fragments, we reconstruct the time-dependent nuclear motions with subcycle temporal resolution. Furthermore, we observe a decrease in the momentum magnitude of the second released electron with increasing internuclear distance, attributed to the combined effects of the instantaneous laser field strength and the Coulomb attraction of the molecular ion at the moment of ionization. Our findings highlight the critical role of isotope effects in elucidating ultrafast nuclear dynamics in laser-induced molecular fragmentation.

Dynamical backaction cooling has been demonstrated to be a successful method for achieving the motional quantum ground state of a mechanical oscillator in the resolved-sideband regime, where the mechanical frequency is significantly larger than the cavity decay rate. Nevertheless, as mechanical systems increase in size, their frequencies naturally decrease, thus bringing them into the unresolved-sideband regime, where the effectiveness of the sideband cooling approach decreases. Here we demonstrate, however, that this cooling technique in the unresolved-sideband regime can be significantly enhanced by utilizing a nonlinear cavity as shown in the experimental work of Zoepfl et al. [Phys. Rev. Lett. 130, 033601 (2023)]. The above arises due to the increased asymmetry between the cooling and heating processes, thereby improving the cooling efficiency. In addition, we show that injecting a squeezed vacuum into the nonlinear cavity paves the way to ground-state cooling of the mechanical mode. Notably, the required squeezing parameters are far less stringent than in the linear case, simplifying experimental implementation.

Metallic nanoparticles, frequently applied to improve the efficiency or sensitivity of various antenna-reactor systems, utilize the plasmonic effect due to the unusual ability of plasmons to transmit energy at the nanoscale. The absorption and emission of light by plasmons are especially high in metallic nanoscale clusters with ${10}^{7-10}$ electrons, but not in small clusters with a lower number of electrons. We demonstrate that the exceptional properties of plasmons at the nanoscale are conditioned by a large anharmonicity of plasmon oscillations induced by strong Lorentz friction, which significantly deviates local charge oscillations from the harmonic regime in this scale of metal spatial confinement. Due to this anharmonicity, plasmons can maintain their oscillations despite strong damping associated with efficient energy transfer, even when such large damping would cause a harmonic oscillator to become overdamped and stop. We also identify a sum-rule-type formula for plasmon polaritons in metallic nanochains arising from the anharmonicity of plasmons.

We describe the theory of focusing waves to a predefined focal point deep inside a disordered three-dimensional (3D) opaque medium by the shaping of $N$ different field sources outside the medium, also known as wavefront shaping. Our study is motivated by recent experiments where wavefront shaping, mediated by the controlled interference of scattered waves, coexists with a classical diffusion picture where wave interference appears irrelevant. We derive the energy density of the wave field both near the focal point and anywhere else inside the medium, averaged over realizations after optimizing the focus. To this end, we conceive of a point source at the focal point inside that emits waves to an external detector array that—by time reversal—becomes a source array that emits the desired shaped fields like a spatial light modulator. In this picture, the average energy density including the focusing is described by the $C_1, C_2, C_3$, and even $C_0$ intensity correlations known from mesoscopic transport theory. We draw the remarkable conclusion that by averaging over optimized wavefronts of the array, associated with different realizations of the disorder, a background energy density is obtained that obeys a diffusion equation in which $C_2$ correlations create an energy source inside. Hence a classical property (diffusion) coexists with interference ($C_2$ correlations). We derive the peak and background energy density distributions for $C_0, C_1$, and $C_2$ correlations, including for a slab. We find that the relative enhancement of the energy density at the focal point is proportional to the angular opening of the array, which de facto replaces the role of the inverse dimensionless conductance $1/g$ as the universal parameter in the transmission statistics, and it generalizes the number of segments known from experiments. We exploit the concept of a source for “optimized background energy” inside the medium to compare several different energy density profiles proposed in the literature, which are associated with optimized transmission by a slab using wavefront shaping. We explain why the best matching models have the first diffusion mode dominating, as was found in earlier work. Our approach avoids random-matrix theory restricted to waveguide geometries, confirms the role non-Gaussian correlations in the wavefront shaping for focusing deep inside 3D random media, and highlights the notion of an energy source created in this process. Our results are relevant for applications where the internal energy density in opaque media is crucial, varying from white-light illumination, projection optics, semiconductor metrology, (bio)sensing, to photovoltaics.

In precision force sensing of multimechanical mode optomechanical systems, coherent interference can decouple certain degenerate vibrational modes from the cavity field, leading to incomplete information regarding the measured signal. In this paper, we propose a scheme to enhance and control the detection bandwidth in optomechanical force sensing by exploiting synthetic magnetism achieved through tuning phonon-hopping interactions. By toggling between broken and unbroken dark mode, this approach effectively manages the response bandwidth and exhibits intriguing additional noise characteristics. Specifically, when the dark mode remains unbroken, the thermal noise is robust and reduced to half of that of a standard device. In contrast, when the dark mode is broken, thermal noise increases substantially at mechanical resonance but remains the same as when the dark mode is unbroken at effective detection frequencies. Moreover, our scheme offers the dual benefit of amplifying the mechanical response while suppressing additional noise, with the potential to surpass the standard quantum limit.

The lack of information obtained from informationally incomplete quantum measurements can prevent the detection of quantum resources, such as optical nonclassicality. We develop a technique that overcomes this limitation for single-mode quantum states conditionally prepared through measurements on another mode of a two-mode state. This task is performed by testing steering of latent nonclassicality—a class of correlations beyond quantum entanglement and quantum discord—for which we introduce a rigorous description.

Beam splitters are key elements in optical and photonic systems and are therefore employed in both classical and quantum technologies. Depending on the intended application, these devices can split incident light according to its power, polarization state, or wavelength. In this work, we theoretically present a type of beam splitter capable of separating a light beam into its two-mode bright and dark components. We propose a prototype based on an optical cross-cavity system resonantly coupled to a $\mathrm{\Lambda }$-type three-level atom. The dark component of the incoming light is transmitted because the antisymmetric collective mode of the cavity setup is decoupled from the atom. Meanwhile, the bright component is reflected due to a quantum interference, which arises from the high cooperativity between the atom and the symmetric collective mode of the cavity setup. Although the device requires only a two-level atom to operate, using a three-level atom allows the device to be turned on or off by controlling the atomic ground states. Our results pave the way for applications of beam splitters that leverage the collective properties of light. Manipulating and exploiting this additional degree of freedom can advance the field of quantum optics and contribute to the development of quantum technologies.

We show that the average trajectories of relativistic quantum particles in Schwarzschild spacetime, obtained via quantum mechanical weak measurements of momentum and energy, are equivalent to the predicted flow lines of probability current in curved spacetime quantum theory. We subsequently demonstrate that these trajectories correspond exactly to classical null geodesics in a hybrid Schwarzschild-Alcubierre spacetime. This threefold equivalence demonstrates how quantum theory in curved spacetime can be formulated via operationally defined measurements, and that such a theory may be interpreted deterministically, in the spirit of hidden-variable models such as Bohmian mechanics, through the novel connection to an underlying “guiding metric.”

We characterize a class of superclassical non-Markovian open quantum system dynamics that are defined by their lack of measurement invasiveness when the corresponding observable commutates with the premeasurement state. This diagonal noninvasiveness guarantees that joint probabilities for measurement outcomes fulfill classical Kolmogorov consistency conditions. These features are fulfilled regardless of the previous (measurement) system history and are valid at arbitrary later times after an arbitrary system initialization. It is shown that a subclass of depolarizing dynamics, which are based on a (time-irreversible) nonunitary system-environment coupling, satisfy the required properties. The relationship with other operational [S. Milz et al., Phys. Rev. X 10, 041049 (2020)] and nonoperational [M. Banacki et al., Phys. Rev. A 107, 032202 (2023)] notions of classicality in non-Markovian open quantum systems is studied in detail and exemplified through different examples.

In this work we develop an open quantum system view of the parametric approximation, which allows us to obtain systematic perturbative corrections to it. We consider the Jaynes-Cummings model with dissipation, assuming that the field is in the regime close to the parametric approximation with depletion. We obtain non-unitary corrections to the parametric approximation and additional dynamical Lamb-shift contributions to it. For high detuning, these nonunitary corrections appear to be non-Markovian before depletion. We also show that even after depletion, initial non-Markovian behavior contributes to the dynamics via laser-induced polishing of the density matrix.

Recent results show that Kochen-Specker (KS) sets of observables are fundamental to quantum information, computation, and foundations beyond previous expectations. Among KS sets, those that are unique up to unitary transformations (i.e., “rigid”) are especially important. The problem is that we do not know any rigid KS set in ${\mathbb{C}}^3$, the smallest quantum system that allows for KS sets. Moreover, none of the existing methods for constructing KS sets leads to rigid KS sets in ${\mathbb{C}}^3$. Here, we show that two fundamental structures of quantum theory define two rigid KS sets. One of these structures is the supersymmetric informationally complete positive-operator-valued measure. The other is the minimal state-independent contextuality set. The second construction provides a clue to solve the minimal KS problem, the most important open problem in this field. We prove that there is no KS set of 30 elements that can be obtained from the minimal state-independent contextuality set by completing bases and adding elements that are orthogonal to two previous elements. We conjecture that 31 is the solution to the minimal KS set problem.