Informationally complete measurements form the foundation of universal quantum state reconstruction, while quantum parameter estimation is based on the local structure of the manifold of quantum states. In this work, we establish a link between these two aspects within the context of a single informationally complete measurement by employing a suitably adapted operator frame theory. In particular, we derive bounds on the ratio between classical and quantum Fisher information in terms of the spectral properties of the associated frame operator, and relate these bounds to the optimal and least informative directions for parameter encoding. We also demonstrate the connection between our results and Fisher-symmetric informationally complete measurements.

We determine the optimal information-disturbance tradeoff for estimating an unknown spatial direction encoded in two antiparallel spins. Rotational covariance reduces the optimization over all instruments to a finite-dimensional Choi problem: a positive seed operator obeys one trace constraint for each irreducible sector of the input representation, while both the directional score and the operation fidelity are linear functionals of this seed. For two antiparallel spin-$1/2$ particles, whose physical representation decomposes as $0\oplus 1$, we derive the two-multiplier dual problem and characterize the optimal instrument from the kernel vectors of the dual slack operator. The optimal operation is a covariant filter with scalar-vector coherence and is generally not a convex interpolation between the identity channel and a measure-and-reprepare strategy. At maximum information, we recover the Gisin-Popescu score, but the least disturbing output state is optimized independently, giving a smaller disturbance than both the parallel-spin benchmark and antiparallel measure-and-reprepare. We also formulate the parallel benchmark and, as a central extension of the method, treat antiparallel spin-coherent states of arbitrary spin $j$. In this case the signal coherently occupies all sectors $\ell =0,...,2j$ of $j\otimes j$, the endpoint information is governed by nearest-neighbor sector coherences, and the endpoint disturbance is obtained from an explicit finite block-diagonal eigenvalue problem.

As a fundamental phenomenon in quantum systems, spontaneous emission constitutes an inevitable source of error, which ultimately degrades the fidelity of quantum logic gates. A successful quantum logic gate needs to operate on the condition that no decay event, such as spontaneous emission, occurs. Such successes can be ensured by postselection based on syndrome extraction according to the theory of quantum error correction or quantum error mitigation. In this case, the wave function of qubits remains a pure state but is subject to additional influences from spontaneous emission, even without actual decay events. Therefore, such a process must be appropriately described by a modified version of Schrödinger equation for the dynamics conditioned on no-decay. Calculations reveal that this effect must be seriously taken into consideration for the design of high-fidelity quantum logic gates. With respect to realistic experimental conditions, even if the coherence is well preserved, improving the fidelity of manipulating physical qubits requires careful consideration of the subtle influences of decay processes such as spontaneous emission. Specifically, the gate and readout processes in the atomic qubit platform are discussed.

We investigate the impact of acoustic phonons on biexciton preparation in an InGaAs/GaAs semiconductor quantum dot driven by laser pulses designed using shortcuts to adiabaticity (STA). The electronic dynamics is calculated with the time-evolving matrix product operator (TEMPO) algorithm, which provides a nonperturbative treatment of the memory effects generated by the acoustic phonon bath. The calculations use a spectral-density parameter set corresponding to very strong coupling to longitudinal acoustic phonons and a high cutoff associated with strongly confined carriers in a small quantum dot. For subpicosecond control, the STA pulses produce near-complete ground-to-biexciton transfer over a broad range of temperatures and biexciton binding energies. For longer pulses, the final biexciton population decreases because the phonon bath has more time to react to the driven charge dynamics, with stronger degradation at higher temperatures and larger binding energies. We compare the STA protocol with reference schemes including constant pulses, on-off-on pulse sequences, rapid adiabatic passage, and a hybrid acousto-optical swing-up method. We also quantify the sensitivity of the preparation to two-photon detuning and multiplicative amplitude noise. Finally, we benchmark the TEMPO simulations against a second-order perturbative evaluation of the Nakajima-Zwanzig memory kernel, finding close agreement for ultrafast pulses and visible deviations for longer durations and higher temperatures. The results identify the regimes in which STA pulses remain efficient for biexciton preparation in quantum dots subject to acoustic phonon decoherence.

High-precision variational calculations for helium in Hylleraas coordinates are used to obtain a combination of quantum defect expansions for the nonrelativistic energy and $1/n$ expansions for the relativistic and quantum electrodynamic corrections. The extrapolations based on direct calculations for the singlet and triplet $P$ states up to principal quantum number $n=35$ provide ionization energies of the $1snp{}^{1}P_1$ and ${}^{3}P_c$ (centroid) states up to $n=102$ with accuracies better than $\pm 1$ kHz. The calculated ionization energies are combined with 28 measured transition frequencies to obtain values for the ionization energy of the $1s2s{}^{3}S_1$ state. The final result of 1 152 842 742.705(16) MHz differs from theory by $0.474\pm 0.052$ MHz, and provides a strong confirmation of the $9\sigma$ disagreement between theory and experiment obtained previously by quantum defect extrapolation of experimental data to the series limit. An analysis of the quantum defect method is presented, and second-order mass polarization (recoil) terms are identified that vary as $1/n^2$ in lowest order. The nonrelativistic part provides a theoretical justification for the effective reduced-mass Rydberg $R_M^{(+)}$ based on the phenomenological model of a Rydberg electron scattering from a ${\mathrm{He}}^{+}$ core. The Ritz expansion for the nonrelativistic energy is verified to 20-figure accuracy.

In the traditional scheme of extreme ultraviolet (XUV) attosecond pulse pump and infrared (IR) femtosecond laser probe, modifications of spectral signatures in the attosecond transient absorption (ATA) spectra are usually achieved by variations in the attosecond pulse waveform, time delay between the two fields, and IR-field intensity. However, the effect of the carrier-envelope phase (CEP) of the IR laser is neglected. Here, we theoretically demonstrate that the CEP modulation of the laser-dressed ATA spectra in a singly excited helium atom depends on the IR pulse area. Using a full multilevel model and a simplified three-level model, we find that the absorption line shape of the $2p$ state exhibits a pronounced periodic modulation as a function of the CEP exclusively when the pulse area reaches $\pi$. In contrast, this modulation becomes negligible at other pulse areas. Analytical derivations based on the $\delta$-pulse approximation reveal that this effect originates from the complete population transfer between the bright $2p$ state and the dark $3d$ state at specific IR pulse areas. We further confirm the robustness of this pulse-area-dependent feature against variations in IR laser duration, central wavelength detuning, and attochirp of the XUV pulse. Finally, we show that the CEP modulation remains robustly observable when considering macroscopic propagation effects in a gas medium. We expect that such CEP sensitivity of the ATA spectra could be used as an all-optical diagnostic tool for absolute CEP measurement.

The study of highly charged ions offers a unique platform for probing the breakdown of nonrelativistic theory under the influence of extreme electromagnetic environments. Here we investigate the photoabsorption of highly charged ions within the dipole approximation using both the time-dependent Schrödinger equation (TDSE) and the time-dependent Dirac equation (TDDE), modeling the external field as an instantaneous broadband excitation. Nonrelativistic scaling relations with respect to the nuclear charge are utilized as a diagnostic tool to systematically identify and quantify relativistic contributions. Within the purely nonrelativistic TDSE framework, these scaling relations hold exactly, allowing the absorption spectra of arbitrary highly charged ions to be inferred directly from a neutral hydrogenic reference. However, as the nuclear charge increases, relativistic effects become dominant through a sizable blueshift in the absorption cross section, due to the relativistic enhancement of the binding energy. We further evaluate semirelativistic TDSE approximations by direct comparison with full TDDE simulations, assessing their predictive power and establishing the regimes where a full Dirac treatment is indispensable for quantitative accuracy. These results establish a baseline for subattosecond relativistic quantum coherence and transient polarization, providing the theoretical groundwork for impulse spectroscopy at next-generation x-ray free-electron laser facilities.

Multipole plasmon resonances in metallic nanostructures are investigated with an emphasis on their excitation and control via surface nonlinear polarization. Surface-induced second- and third-order nonlinear processes enable the activation of multipole plasmon modes that are absent in the linear optical response. The excitation and enhancement of different multipole plasmons are governed by the combined tuning of geometric ellipticity and excitation configuration, allowing selective mode activation and suppression. Without applying electron beams and structured light, the surface-nonlinearity-based excitation can be a viable scheme for engineering the multipole plasmons under far-field illumination.

Spin-glass theory serves as a quintessential framework for studying complex systems in statistical physics. Central to this theory is replica symmetry breaking, a distinctive phase transition between paramagnetic and spin-glass states, which has emerged as a pivotal concept for identifying nontrivial correlated states. To date, optical analogies of replica symmetry breaking have predominantly relied on intensity fluctuations, largely overlooking polarization as the inherent mapping of photonic spin. Here we propose an effective statistical method by mapping the optical polarization states onto a continuous classical spin model. We experimentally identify the emergence of a polarization replica symmetry breaking during the buildup of mode locking. Our findings, supported by numerical simulations, suggest that polarization replica symmetry breaking dynamics may represent a broader feature of ultrafast photonic systems and offer a route to studying their statistical mechanical behavior. The discovery offers profound insights into photonic polarization replica symmetry breaking states and the intricate polarization dynamics governing mode-locking processes.

We investigate the short-term temporal dynamics of superradiance in closely spaced quantum emitters. Building on Dicke's 1954 framework, we analyze the sequential emergence of coherence, superradiance, and entanglement, revealing a distinct temporal hierarchy in their extremal values: relative coherence develops first, followed by the peak of correlated emission, then minimal entanglement, and subsequently minimal spin-spin correlation is reached. These findings suggest that enhanced relative coherence precedes correlated emission, and when spin correlations are negligible, entanglement and correlated emission become linked in time.

In developing quantum science and technologies, it is essential to demonstrate so-called quantum advantages, which are performances that can be achieved only with the assistance of quantum resources. Most of the time, different quantum features lead to different advantages. Interestingly, there are certain classes of tasks where quantum advantages are achievable by general quantum resources. This work reports such a class of tasks in thermodynamics—we provide a work-extraction task that certifies general quantum resources of both states and channels, suggesting general quantum effects can provide nonclassical advantages in work extraction. We also show that such work-extraction tasks can be applied to certify quantum entanglement in a one-sided device-independent way. As an application, we report a novel type of anomalous energy flow—a type of locally extractable energy that is attributed to the globally distributed entanglement. Finally, we show that the existence of this novel anomalous energy flow is equivalent to measurement incompatibility.

The experimental access to ultrastrong couplings in light-matter interactions has made the quantum phase transition (QPT) in the quantum Rabi model practically relevant, while the physics of the QPT has not yet been fully explored. The polaron picture is a method capable of analyzing in the entire coupling regime and extracting the essential physics behind the QPT. However, the asymmetric deformation of polarons is missing in the current polaron picture. In this work we propose an improved variational method in asymmetric polaron picture (APP). Our APP not only increases the method accuracy but also reveals more underlying physics concerning the QPT. We find that in the ground state both the polarons and antipolarons are asymmetrically deformed to a large extent, which leads to a richer variation diagram. We also analyze the first excited state in which we unveil an asymmetry direction reversal for the polarons and an effective attraction or repulsion changeover differently from the ground state. Finally, we apply the APP in quantum Fisher information analysis and critical coupling extraction; the improvements indicate that the polaron asymmetry makes a considerable contribution to the quantum resource in quantum metrology and plays an unnegligible role in the QPT. Our results and mechanism clarifications expose more subtle energy competitions and abundant physics, and the method potentially might have broader applications in light-matter interactions.

We define and study the properties of squeezed quantum multiplets, which are sets of $D$-orthonormal quantum states formed by superpositions of states squeezed along $D$ equally spaced directions in quadrature space. We present analytical expressions for phase-space distributions (Wigner and characteristic functions) representing squeezed multiplets. We use these expressions to show that some squeezed multiplets are highly sensitive to perturbations in all phase-space directions, suggesting potential applications in metrology. We also compare ordinary squeezed multiplets with superpositions of higher-order squeezed states, including trisqueezed and quadsqueezed states, highlighting both similarities and important differences between these families of states.

Motivated by device-independent quantum information protocols, we study a class of marginal-free asymmetric Clauser-Horne-Shimony-Holt (CHSH) expressions in which Alice's two measurement settings carry different weights. For $0<\alpha \le 1$, within the two-qubit projective-measurement model, we derive a tight analytic lower bound on the concurrence of arbitrary two-qubit states compatible with an observed asymmetric CHSH value. We then combine this entanglement-certification result with established entropic-uncertainty, convexification, and Gauss-Radau–Navascués-Pironio-Acín semidefinite-programming techniques to obtain finite-level lower-bound estimates on the conditional von Neumann entropy $H\left(A_0\right|E)$. Finally, within the same finite-level semidefinite-programming setting, we compare the entropy estimates obtained from asymmetric and standard CHSH expressions under white-noise and finite-detection-efficiency models. The results indicate that mildly asymmetric choices of $\alpha$ can give more favorable finite-level estimates of the visibility and detection-efficiency thresholds in selected regimes of the models considered here.

We have measured momentum analyzed electrons and singly charged recoil ions ejected in slow collisions of ${\mathrm{D}}^{-}$ ions with He and Ar in triple coincidence with neutralized projectiles. From the data, we extracted relative triple differential cross sections for detachment and detachment with simultaneous target ionization as a function of projectile energy. The results were compared with our perturbative calculations. At large projectile energies, we observed signatures of a nonsequential process, in which the electrons are ejected from the projectile and the target by a single mutual interaction. We find that contributions from sequential processes are important as well and, at small energies, even dominant.

We present a comparative study of optical zero-quantum two-dimensional coherent spectroscopy (2DCS) in alkali-metal vapors of potassium (K) and rubidium (Rb). Both systems exhibit pronounced sideband structures in their respective zero-quantum spectra. In K vapor, these sidebands are consistent with electronic fine-structure coherence between the spin-orbit split sublevels of a single atom. In contrast, Rb vapor exhibits anomalous sidebands that cannot be explained within a single-atom picture. Numerical simulations based on a correlated two-atom model attribute these anomalous features to nonradiative coherences involving collective doubly excited states. By monitoring the temporal evolution of these sidebands, we find that the dependence of the decay time on vapor density is consistent with an interaction-induced dephasing picture associated with interatomic dipole-dipole interactions. These findings illustrate the crucial role of collective effects in contributing to the zero-quantum spectra and establish zero-quantum 2DCS as a sensitive probe for interatomic correlations.

Optical two-dimensional coherent spectroscopy (2DCS) provides sensitive detection of collective resonances of two or more atoms, usually by using double- or multiquantum excitations. Here, we report the observation of two-atom collective resonances by using zero-quantum 2DCS. We perform one-quantum (1Q) and zero-quantum (0Q) 2DCS on rubidium atoms in a three-level ladder configuration involving the $5S_{1/2}, 5P_{3/2}$, and $5D$ states. The 1Q 2D spectra exhibit the expected diagonal and cross peaks corresponding to the ladder transitions and their couplings. In contrast, the 0Q 2D spectra obtained at elevated excitation power reveal additional peaks at nonzero mixing frequencies that cannot be explained by an isolated single-atom ladder scheme. Density-matrix simulations confirm that the single-atom picture produces only zero-frequency features in the 0Q 2D spectrum. We show that the observed nonzero-frequency peaks originate from collective resonances of two atoms, enabled by higher-order nonlinear excitation pathways that create coherences between two doubly excited collective states. The power dependence of each peak confirms the order of the corresponding nonlinear processes. A theoretical model incorporating two-atom collective states reproduces the experimental 0Q 2D spectrum. These results show that zero-quantum 2DCS can provide a complementary higher-order pathway for observing collective resonances and many-body correlations in atomic and other systems with ladder-type energy-level structures.

We investigate the role of the electronic structure of the projectile in collisional ionization and fragmentation of ${\mathrm{CO}}_2$ via multielectron capture at 0.31 a.u. impact velocity. Focusing on the ${\text{CO}}_2^{3+}\to {\text{O}}^{+}:{\text{C}}^{+}:{\text{O}}^{+}$ break-up channel as a representative channel, we report kinetic energy release distributions (KERDs) for collisions with equivelocity ${\mathrm{N}}^{q+}$ and ${\mathrm{O}}^{q+}$ projectiles. We consider two complementary categories of measurements. In the first category, in which different projectiles of the same charge are considered, we find that KERDs obtained with ${\mathrm{N}}^{q+}$ and ${\mathrm{O}}^{q+}$ impact ($q=4,6$) are broadly similar, but they differ significantly from the earlier reported KERD with ${\mathrm{Ar}}^{q+}$ impact. In the second category, pronounced differences are observed between the KERDs obtained with isoelectronic ${\mathrm{N}}^{q+}$ and ${\mathrm{O}}^{(q+1)+}$ ($q=3,5,7$) projectiles. These results provide clear evidence that projectile electronic structure plays a critical role in multielectron capture collisions.

We develop a general theory of spin-dependent density-density correlations, that is valid for any temperature, interactions, dimensions, and mass or population status of Fermi gases with two internal states. We use gauge invariance and the Pauli principle to establish constraints on the spin-dependent density-density correlations that are consistent with the fluctuation-dissipation and Wick's theorem. As an example, we study the spin-dependent density-density correlations from the BCS to the Bose regime in two dimensions at zero temperature, inspired by recent continuum quantum gas microscope experiments in ${}^6\mathrm{Li}$. We show that two-particle irreducible contributions, involving collective excitations, many-particle scattering, and vertex corrections, are essential to describe the experimental data. In particular, the two-particle irreducible terms are responsible for the emergence of an experimentally observed minimum in the opposite-spin density-density correlations.