We present an x-ray based stimulated Raman approach to control the preparation of populations and coherent superpositions of (valence) electronic states in generic molecular systems, including optically dark states. Leveraging on the unique properties of core-level excitations as intermediates of the Raman process, we demonstrate that optically forbidden singlet-singlet and singlet-triplet transitions are made accessible. We explore the role of both intrinsic molecular properties (transition dipole moments and involved molecular orbitals) and external field parameters to explain the efficiency and selectivity of the process towards specific classes of electronic states, and to control it towards the amplification of desired populations or superpositions. While the reported mechanism naturally applies to virtually all organic and bio-organic molecules and can straightforwardly be extended to a broader class of molecules, atoms and materials, detailed results for two molecular systems are here reported as a testbed of the proposed approach, for which experimental feasibility is eventually discussed.

Loss spectroscopy is a key tool for investigating systems where important system parameters are linked to intrinsic resonant loss processes. We investigate loss processes of impurity atoms embedded in a medium of a Bose-Einstein condensate close to a Feshbach resonance. In this case, three-body loss processes occur faster than the measurement duration, impeding a direct time-resolved measurement. Here, we discuss how an even faster two-body loss process can be used to probe the system. The time-dependent number of atoms in the medium is reconstructed from such measurements, allowing for the extraction of the three-body loss rate coefficient $L_3$ and its scaling with scattering length. Moreover, the medium atom number is reconstructed from spectroscopic loss measurements. This allows for a comparison of the medium densities based on both the extracted loss rates and the spectroscopically reconstructed atom number. Finally, the number of lost medium atoms per loss event is evaluated and found to exceed 2 at strong interactions, which is attributed to secondary collisions in the medium. These investigations establish the use of a fast loss mechanism as a new tool in the field and provide quantitative measurements of three-body losses at large interaction strengths.

The concepts of geometric phase and wave-particle duality are interlinked to several fundamental phenomena in quantum physics, but their mutual relationship has remained a largely uncharted topic. Here we address this subject by studying the geometric phase of a photon in double-slit interference. In particular, we discover a general complementarity relation for the photon that connects the geometric phase it exhibits in the observation plane and the which-path information it encases at the two slits. The relation can be seen as quantifying wave-particle duality of the photon via the geometric phase, thus corroborating an important link between two ubiquitous notions in quantum physics.

In a recent article [X. Lai et al., Phys. Rev. Lett. 133, 033601 (2024)], the coherence time of degenerate entangled photon pairs (biphotons) generated via backward spontaneous four-wave mixing in a cold atomic ensemble was shown to be immune to optical loss and dephasing. This finding is crucial for practical applications in quantum information processing, quantum communication, and networking, where loss is inevitable. However, in studying the underlying mechanism for this loss- and dephasing-insensitive biphoton coherence time, the previous article did not take quantum noise into account. In this work, we employ the Heisenberg-Langevin approach to study this effect and provide a rigorous theoretical proof of the symmetry-protected biphoton coherence by taking quantum noise into consideration, as compared to the perturbation theory in the interaction picture.

Wave-particle duality, a fundamental principle of quantum mechanics, encapsulates the complementary relationship between the wave and particle behaviors of quantum systems. In this paper, we treat quantum coherence and classical distinguishability as complementary resources and uncover a novel duality relation, which is explored through quantum state discrimination under incoherent operations, extending beyond typical interference scenarios. We prove that in an ensemble of mutually orthogonal pure states, the sum of “co-bits,” quantifying the coherence preserved under incoherent free operations, and classical bits, representing the distinguishability extracted via quantum state discrimination, is bounded. This coherence-distinguishability duality relation exposes an inherent tradeoff between the simultaneous preservation of a system's quantum coherence (wavelike property) and the extraction of its classical distinguishability (particlelike property). Our findings provide a fresh perspective on wave-particle duality through quantum resource theories, offering complementary insights into manipulating quantum and classical resources, with implications for quantum foundations and quantum technologies.

We propose a spin-motion state for high-precision quantum metrology with super-Heisenberg scaling of the parameter estimation uncertainty using a trapped ion system. Such a highly entangled state can be created using the Tavis-Cummings Hamiltonian, which describes the interaction between a collective spin system and a single vibrational mode. Our method relies on an adiabatic evolution in which the initial motional squeezing is adiabatically transferred into collective spin squeezing. In the weak squeezing regime, we show that the adiabatic evolution creates a spin-squeezed state, which reduces the quantum projective noise to a sub-shot-noise limit. For strong bosonic squeezing we find that the quantum Fisher information follows a super-Heisenberg scaling law $\propto N^{5/2}$ in terms of the number of ions $N$. Furthermore, we discuss the spin-squeezing parameter which quantifies the phase sensitivity enhancement in Ramsey spectroscopic measurements and show that it also exhibits a super-Heisenberg scaling with $N$. Our work enables the development of high-precision quantum metrology based on entangled spin-boson states that lead to faster scaling of the parameter estimation uncertainty with the number of spins.

The variational quantum eigensolver (VQE) algorithm is particularly well suited for estimating the ground-state energies of molecular systems in the noisy intermediate-scale quantum era. However, the most popular unitary coupled-cluster ansatz requires a large number of qubits and gates that are constrained by short coherence time and hardware noise. In this study, we present schemes for estimating the ground-state energies of the molecules ${\mathrm{H}}_2$ and LiH using the VQE algorithm in a trapped-ion system with a high-dimensional method. It is shown that significantly fewer quantum gates are required to achieve the same result as the qubit-based model. High-dimensional quantum simulation can make more use of quantum resources. Utilizing the recently developed fixed-input-state method, the depth of the circuits can be further reduced. Only one and three single-qubit gates are required for the simulation of the ${\mathrm{H}}_2$ and LiH molecules, respectively. Finally, combined with the zero-noise extrapolation technique to mitigate the effects of noise, the accuracy of the results is significantly improved. These findings indicate that using a fixed input state in high-dimensional quantum simulations can enhance the performance of near-term quantum computing hardware.

We consider the Zeeman shift in Rydberg atoms induced by room-temperature blackbody radiation (BBR). BBR shifts to the Rydberg levels are dominated by the familiar BBR Stark shift. However, the BBR Stark shift and the BBR Zeeman shift exhibit different behaviors with respect to the principal quantum number of the Rydberg electron. Namely, the BBR Stark shift asymptotically approaches a constant value given by a universal expression, whereas the BBR Zeeman shift grows steeply with principal quantum number due to the diamagnetic contribution. We show that for transitions between Rydberg states, where only the differential shift between levels is of concern, the BBR Zeeman shift can surpass the BBR Stark shift. We exemplify this in the context of a proposed experiment targeting a precise determination of the Rydberg constant.

Quantum fluctuations are fundamental in many-body physics, often altering the nature of quantum phases beyond mean-field predictions. In this work, we explore their impact on a spatial-temporal ordered phase in a cavity-boson system, a phase previously predicted within the mean-field framework. By incorporating higher-order quantum fluctuations of the cavity field, we derive a set of quantum kinetic equations and numerically analyze the system's dynamics. Our results show that while mean-field spatial-temporal order vanishes over time, oscillatory behavior persists at the level of quantum fluctuations, demonstrating that the phase survives in a modified form. This study underscores the importance of quantum fluctuations in nonequilibrium quantum systems and reveals the limitations of the mean-field approach in predicting the stability of complex quantum phases.

This paper investigates the distribution and dynamics of extreme pulses generated by interband cascade lasers at different temperatures, incorporating the effect of ac amplitude. As the temperature increases, both the total number and the percentage of extreme pulses rise and become more regular. The number of generated extreme pulses shows a trend of increase at first and then decrease with the increase of ac amplitude. When the ac amplitude is sufficiently large, the laser is fully modulated and exhibits periodic oscillations. Elevated temperature also alters the locking region, leading to the vanishing of the forbidden band in the extreme pulse distribution. By varying the temperature, the number of extreme pulses changes, and the laser dynamics transition among quasiperiodic, periodic, and chaotic states.

High-index dielectric materials are promising candidates for photonic technologies that rely on enhanced light-matter interactions. Nanophotonic applications have been hindered by a lack of high-index transparent dielectrics in the visible regime. Despite its low index of refraction at visible wavelengths, water exhibits an index approaching 9 at 2.5 GHz, making it a promising model material for studying morphology-dependent resonances in centimeter-scale objects. Such Mie scattering resonances are well characterized in spheres but are much less understood in spheroids because of the mathematical complexity of multipolar expansions in spheroidal geometries. In this work we combine analytical, numerical, and finite-element methods to study microwave resonances in aqueous spheroids. We obtain three-dimensional standing-wave solutions in aqueous spheroids and show that this conceptually simple description of electromagnetic resonances maps surprisingly well onto the complex resonant landscape of certain spheroidal modes, as elucidated by finite-element simulations. In the process, we identify some size and shape regimes that yield uniquely intense spheroidal resonances.

We investigated an orbital-angular-momentum (OAM) pointer within the framework of von Neumann measurement and discovered its significant impact on optimizing superpositions of Gaussian and Laguerre-Gaussian (LG) states. Calculations of quadrature squeezing, correlation functions, Wigner function, and the signal-to-noise ratio (SNR) support our findings. Specifically, by carefully selecting the anomalous weak value (WV) and the coupling strength between the measured system and the pointer, we demonstrated that the initial Gaussian state transforms into a non-Gaussian state after postselection measurement. We also noticed that in the weak coupling regime it is possible to get perfect quadrature squeezing of our output state with anomalous WVs. This transition highlights the potential of OAM pointer states to enhance the performance of quantum systems by tailoring state properties for specific applications.

In this work, we consider a composite atom-cavity system interacting with a ring resonator. In such a structure, the spontaneous breaking of time-translation symmetry can be observed. We show that a quadratic observation time dependence of the system's sensitivity to perturbations takes place in the broken time-translation symmetry regime and also in the transition area to the normal state. This dependence is due to the system's ability to retain the memory of the initial state of the atom. Outside these areas, the system does not retain the memory of the initial state of the atom, and the sensitivity scales linearly with the observation time. Our results open up a way for the implementation of systems with broken time-translation symmetry in sensing and metrology.

No-cloning and no-broadcasting of arbitrary quantum states are intrinsic features differentiating quantum mechanics from the classical realm. Recent studies have explored the broadcasting of quantum features such as asymmetry, magic, and imaginarity within the framework of resource theories. In this work, we study the quantitative aspects of coherence from the perspective of no-broadcasting. Although when the coherence is quantified by a superadditive measure, it can be readily proved that the amount of coherence cannot be broadcast, such an approach becomes invalid for nonsuperadditive measures of coherence. Thus, it is desirable to find methods to study the no-broadcasting of coherence effective for both superadditive and nonsuperadditive coherence measures. By employing a novel characterization of bipartite quantum states with vanishing basis-dependent correlations, we establish a no-broadcasting theorem of coherence in terms of Wigner-Yanase skew information, which is an important version of quantum Fisher information and a nonsuperadditive measure of coherence. The result extends the conventional no-broadcasting from quantum states to information-theoretic contents of quantum states and can be regarded as an instance of no-broadcasting of quantum Fisher information. We also establish no-broadcasting of the relative entropy of coherence, without relying on superadditivity.

Solid-state color centers embedded in diamond, silicon carbide, and other host matrices offer a promising platform for nanoscale quantum sensing. Sometimes a relatively strong transverse zero-field splitting is introduced in a color center due to its local strain and the low symmetry of the host structure. While clock transitions induced by transverse zero-field splitting serve as a method to prolong the coherence time, the Zeeman sublevels of solid-state color centers become insensitive to first-order magnetic-field signals, thereby limiting their utility as magnetometers. In this work, we address this challenge and achieve wide-band ac field detection spanning from hundreds of kilohertz to hundreds of megahertz by utilizing a combination of orthogonal microwaves and phase modulation at different frequencies. Our control method effectively suppresses the system noise and the amplitude fluctuation of the driving field, which extends the coherence time of the quantum system.

In 1935, Einstein, Podolsky, and Rosen argued that quantum mechanics is incomplete based on the assumption that local actions cannot influence elements of reality at a distant location (local realism). In this work, using a recently defined quantum reality quantifier, we show that Alice's local quantum operations can be correlated with the erasure of the reality of observables in Bob's causally disconnected laboratory. To this end, we implement a modified optical quantum eraser experiment, ensuring that Alice's and Bob's measurements remain causally disconnected. Using an entangled pair of photons and quantum state tomography, we experimentally verify that, even with the total absence of any form of classical communication, the choice of quantum operation applied by Alice on her photon is correlated with the erasure of a spatial element of reality of Bob's photon. Our results reveal that Bob's photon can entangle two extra noninteracting degrees of freedom, thus confirming that Bob's photon path is not an element of physical reality.

In quantum chemistry, there is a debate about the existence of density functionals beyond the ground electronic state. Here, we apply the tool kit of density-functional theory (DFT) to analyze the characteristics of the eigenvalues of an arbitrary symmetric matrix. The latter can represent a model Hamiltonian, which is not necessarily obtained from a basis-set representation of the exact physical Hamiltonian. Based on the analogy with an exactly solvable physical model, we introduce a density functional for each eigenvalue of the matrix and show that, similar to an exactly solvable model, only the lowest and highest eigenvalues of an arbitrary symmetric matrix have well-defined single-valued functionals of density with unique convexity or concavity characteristics. All other eigenvalues lack such density functionals, as the respective quantities are multivalued density functions and lack unique convexity or concavity. The problem of the nonexistence of functionals for the excited states can be overcome by forming weighted sums of eigenvalues in analogy with the Gross-Oliveira-Kohn ensemble DFT, which results in well-defined density functionals with admissible characteristics.

Recent studies have highlighted the frequent applications of structured light modes in optically pumped atomic magnetometers. In this work, we theoretically explore how a Poincaré beam probes an optically polarized atomic medium. Specifically, we consider atoms polarized by a plane wave with linear polarization, immersed in a constant external magnetic field. We analyze how the polarization of the pump and probe light fields, along with the external magnetic field, impact the absorption profile. To this end, we employ a density-matrix approach based on the Liouville–von Neumann equation. Our results reveal that the absorption profile exhibits an asymmetric pattern that depends on the magnetic-field strength and the mutual orientation of the pump and probe light propagation directions relative to the quantization axis. For illustration, we assume the incoming radiation drives an electric dipole transition, $5s{}^2{S}_{1/2}$ ($F=1$) $\to 5p{}^2{P}_{3/2}$ ($F=0$), in rubidium atoms subjected to a magnetic field. These findings may aid in designing future experiments on optically pumped atomic magnetometers utilizing structured light modes.