Collisions of atomic ions in optical clocks with background gas molecules result in systematic frequency shifts. We evaluate the frequency shift in ${\mathrm{Sr}}^{+}$ clocks due to collisions with helium atoms and hydrogen molecules, using ab initio spin-orbit-coupled potential energy surfaces together with a master equation for the collisional shift. Our results lead to improved uncertainty estimates for collisional frequency shifts in ${\mathrm{Sr}}^{+}$ optical clocks.

The present contribution explores phase transitions that occur in the ground state (GS) of spin-1 Bose-Einstein condensates (BECs) with spin-orbit coupling (SOC) under the action of gradient magnetic fields. By solving the corresponding linearized system in an exact fashion, we identify the conditions under which the GS phase transitions occur, thus transforming excited states into GSs. The study of the full nonlinear system, including both density-density and spin-spin interactions, is numerically analyzed. For the case of repulsive spin-spin interactions, the results resemble the linear case, while attractive spin-spin interactions lead to the formation of mixed states near the GS phase-transition points. Additionally, higher-order vortex states are found to be stable even in the nonlinear regime. These findings demonstrate that arbitrary winding numbers corresponding to stable GSs can be achieved, thus contributing to the understanding of topological properties in SOC BECs.

We investigate the effective potential and scattering length of ultracold polar molecules under different shielding techniques. First, we derive the effective potential for two polar molecules in the presence of (i) an elliptical polarization field alone, (ii) combined elliptical and linear polarization fields, and (iii) combined elliptical polarization and static fields. The effective potential is then expressed as a sum of a zero-range contact interaction and a long-range dipolar interaction under the Born approximation. We find that the methods (i) and (ii) only partially suppress attractive interactions, while the method (ii) allows for the construction of bound states with different polarization shapes. The last shielding method (iii) can achieve complete cancellation of residual attractive forces [Micheli et al., Phys. Rev. A 76, 043604 (2007); Gorshkov et al., Phys. Rev. Lett. 101, 073201 (2008)], which is particularly significant for maintaining quantum degeneracy in ultracold dipolar systems. Our results provide a comprehensive understanding of the effective potential and scattering length of shielding polar molecules, which is crucial for studying many-body physics in ultracold dipolar systems.

Non-Abelian braiding, as a promising approach for topological quantum computation, is typically based on topologically protected quantum gates. However, existing braiding schemes have primarily focused on arrays constructed from periodic lattices, which provide limited types of quantum gates and lack flexibility in braiding path design. In this work, we propose two structurally distinct X junctions based on a one-dimensional single-particle tight-binding model, and present a braiding protocol that yields a topologically protected $X$ gate by adiabatically moving defects. The two proposed $\mathrm{X}$ junctions are introduced into the Stampfli-type quasicrystal, and each is successfully used to demonstrate non-Abelian braiding. Our braiding scheme allows for multimode braiding and flexible path design, providing a novel approach for designing non-Abelian devices in optical and acoustic platforms.

In this article, we derive a theoretical formalism that unifies the rigorous coupled wave analysis and the dynamical diffraction theory. Based on this formalism, we design a computational approach for the diffraction calculation for the nanoscale lamellar gratings with an arbitrary line profile shape. In this approach, the gratings line profile is approximated as a polygon. This proves to be convenient since such an approach does not rely on the geometry model of the grating. We test the proposed approach against other computational theories and a synchrotron scattering experiment.

We study an optomechanical system, where a mechanical oscillator interacts with a Gaussian input optical field. In the linearized picture, we analytically prove that if the input light field is the vacuum state, or is frequency-independently squeezed, the stationary entanglement between the oscillator and the output optical field is independent of the coherent coupling between them, which we refer to as the universality of entanglement. Furthermore, we demonstrate that entanglement cannot be generated by performing arbitrary frequency-dependent squeezing on the input optical field. Our results hold in the presence of general, Gaussian environmental noise sources, including non-Markovian noise.

While two perfect phase-matching mechanisms exist in homogeneous single crystals, birefringent phase-matching (BPM) dominates for nonlinear optical applications. The anomalous-dispersion phase matching (ADPM), though demonstrated in gases and liquids, has not been established in crystals due to the parasitic material absorption that induces the anomalous dispersion in the first place. This absorption bottleneck can be overcome by a crystal possessing an additional spectral gap in its absorption spectrum, beyond the regular band gap, which we term the “hypergap”. Here, we theoretically propose and experimentally demonstrate such a hypergap and the ADPM-enabled third harmonic generation in potassium chromate ${\mathrm{K}}_{\text{2}}{\mathrm{CrO}}_{\text{4}}$. Our results introduce a class of nonlinear optical materials that do not rely on lattice anisotropy or periodic poling.

Energy extraction is one of the fundamental challenges in realizing quantum batteries (QBs). Here we propose a wireless transfer scheme with parity-time symmetry to efficiently extract the energy stored in non-Hermitian QBs to consumption hubs. For linear cases, the transfer energy oscillates periodically in the unbroken symmetry region and grows hyperbolically in the broken region. For nonlinear cases, the transfer energy eventually reaches and remains steady-state values arising from the feedback mechanism of the nonlinear saturable gain. The non-Hermitian QBs with gains can persistently absorb energy and store it in batteries while transfering it to consumption hubs, thus exhibiting more considerable robustness in the large detuning regime and significant enhancement in the weak coupling regime than those in partial deep-strong coupling regime. Furthermore, we also introduce a classical simulator and demonstrate some counterintuitive advantages of the non-Hermitian QBs. Our work overcomes energy bottlenecks of wireless transfer schemes in QBs and may provide inspirations for practical applications of QBs.

The use of graphical models to represent causal hypotheses has enabled revolutionary progress in the study of the foundations of quantum theory. Here we consider directed acyclic graphs each of which contains both nodes representing observed variables as well as nodes representing latent or hidden variables. When comparing distinct causal structures, a natural question to ask is if they can explain distinct sets of observable distributions or not. Statisticians have developed a great variety of tools for resolving such questions under the assumption that latent nodes are interpreted classically. Here we highlight how the change to a quantum interpretation of the latent nodes induces distinctions between causal scenarios that would be classically indistinguishable. We especially concentrate on quantum scenarios containing latent nodes with at least one latent parent, a.k.a. possessing intermediate latents. This initial survey demonstrates that many such quantum processes can be operationally distinguished by considerations related to monogamy of nonlocality, especially when computationally aided by a hierarchy of semidefinite relaxations which we tailor for the study of such scenarios. We conclude by clarifying the challenges that prevent the generalization of this work, calling attention to open problems regarding observational (in)equivalence of quantum causal structures with intermediate latents.

Simultaneous quantum-classical communications (SQCC) protocols are a family of continuous-variable quantum key distribution (CV-QKD) protocols that allow for quantum and classical symbols to be integrated concurrently on the same optical pulse and mode. In this work, we present a revised analysis of simultaneous quantum-classical communications in Gaussian-modulated coherent-state CV-QKD protocols. We address security concerns inherently associated with SQCC schemes and provide an updated model of the coupling between the classical and quantum channels. We provide evidence for our model via Monte Carlo simulation. We compute the performance of our revised SQCC protocol in terms of the secret-key generation rate optimized over free parameters and demonstrate improved quantum efficiency for a given classical bit-error rate. Lastly, we extend our analysis into the finite-key regime, where we propose a scheme for composably secure SQCC under realistic operating conditions and demonstrate that our scheme retains the advantage in quantum performance over previous models.

The Rydberg blockade effect plays an important role in realizing two-qubit gates in atomic arrays. Meanwhile, such mechanics will increase the crosstalk between atoms and enhance the decoherence. In this paper, we propose a new scheme to realize the controlled-phase gate without Rydberg blockade. The scheme works effectively with large atomic spacings and is insensitive to the thermal motions of atoms. The proposal is robust against random noises due to the geometric characteristic and operates fast based on the nonadiabatic evolution. The proposed gate is actually a geometric gate that consolidates the nonadiabatic holonomic control and the unconventional geometric control simultaneously. The interference between two different types of geometric phases can be investigated in the proposal. Furthermore, we show that the actuating quantity (the product of interaction strength and the evolution time) of the proposed scheme is 90 times smaller than the present Rydberg blockade scheme. Therefore, our proposal provides a fast and robust way to realize geometric quantum control, and it may trigger the discoveries of new geometric gates in high-dimensional Hilbert space.

Radio frequency antennas based on highly excited Rydberg atom vapors can in principle reach sensitivities beyond those of any conventional wire antenna, especially at lower frequencies where very long wires are needed to accommodate the growing wavelength. Conventional Rydberg sensors are based on individual atom response, with increased signal resolution relying on the $O\left({10}^3\right)$ electric dipole moment enhancement, scaling as the square of the Rydberg-state principal quantum number $N\sim 50$. However, despite more than 30 years of steady advances, beyond-classical signal sensitivity has yet to be demonstrated. More recently, the optical bistability effect, a many-body nonequilibrium phase transition occurring at somewhat higher vapor densities, has been exploited for an order of magnitude or more increased sensitivity for some setups through tuning into the critical region. However, the results fall significantly short of those using more advanced “conventional” Rydberg sensor setups—which achieve even greater enhancement by exploiting resonant interaction between a pair of nearby Rydberg levels. This paper seeks to combine the many-body and resonant enhancement effects by extending the bistable phase analysis to include a pair of resonant Rydberg levels, supported by an exact treatment of the atom velocity thermal average. A dynamic linear response formalism is developed as well to explore the trade-off between measurement sensitivity and finite-bandwidth signals. We demonstrate regions of the phase diagram which could be exploited for record breaking, beyond-classical sensitivity. Of course, only a limited number of vapor parameters are under full experimental control, and experiments will be needed to quantitatively constrain the effective mean-field interaction parameters appearing in the theory and thereby define the accessible regions of the phase diagram.

We present the ab initio calculation of leading-order relativistic corrections to the pure vibrational energies of the muonium-muonium molecule (${\mathrm{Mu}}_2$), an exotic four-lepton system composed of two positive muons ${\mu }^{+}$ and two electrons. These corrections are evaluated perturbatively using variational wave functions constructed from an explicitly correlated Gaussian basis, within a fully non-Born-Oppenheimer framework. We accurately determine the rotationless vibrational spectrum, dissociation energy, and key spectroscopic constants, including the harmonic constant ${\omega }_e=13322.11{\text{cm}}^{-1}$ and the anharmonicity constant ${\omega }_e{x}_e=1097.37{\text{cm}}^{-1}$. Furthermore, we present a range of expectation values for various structural quantities that provide insight into the intrinsic properties of this system. Our results represent a meaningful step toward a more accurate theoretical description of ${\mathrm{Mu}}_2$.

By integrating molecular optomechanics with molecular switches, we propose a scheme for a molecular quantum heat engine. If the molecular switch is effectively classical, the engine operates autonomously through hysteretic feedback, without external driving or modulation. However, this description needs to be revisited and modified in the case of a fully quantum molecular switch. Through a comparative analysis conducted within both semiclassical and fully quantum frameworks, we reveal the central role of quantum tunneling and quantum correlations in the operation of this advanced molecular machine in that regime.

The formation of the supercontinuum spectrum during filamentation in compressed nitrogen with chirped femtosecond optical pulses in the sharp focusing regime has been investigated experimentally and theoretically. We show that chirped pulses exhibit a decrease in the spectral broadening of the pulse with increasing gas pressure. At the same time, pulses with a negative chirp demonstrate a wider supercontinuum than pulses equal in modulation depth but positive in chirp sign. A theoretical analysis and numerical modeling of the propagation of high-power chirped femtosecond optical pulses under conditions corresponding to the experiment are carried out, and an adequate analytical model explaining the main features of the formation of spectral characteristics of chirped optical radiation in a compressed gas is elaborated.

We present a quantum theoretical framework that describes the simultaneous emergence of magnon- and photon lasing in a tripartite hybrid solid-state platform. The system integrates optically pumped nitrogen-vacancy (NV) centers, a magnetic yttrium-iron-garnet (YIG) microsphere, and a microwave (MW) cavity. Through a comprehensive analysis of the system's dynamics, we reveal the cooperative interactions that drive coherent-stimulated emission in both the magnonic (spin-wave) and photonic (electromagnetic) domains. The model outlines a scalable configuration in which NV centers, excited via optical pumping, couple efficiently to the magnon modes of the YIG sphere placed inside the microwave cavity. Numerical simulations based on experimentally accessible parameters confirm the feasibility of dual-domain lasing. This work advances the integration of photonic and magnonic platforms, opening new opportunities for quantum-enhanced sensing, low-power spintronics, and on-chip quantum information processing.

We consider the problem of two-photon cooperative emission in systems of two-level atoms. Two physically distinct regimes are analyzed. First, we investigate the case of a small number of atoms. We study the evolution of two-photon super- and subradiant states and the associated two-photon spectra. Second, we investigate the problem of atoms at constant density confined to a spherical volume. We analyze separately the cases in which the radius is small or large compared to the resonant wavelength.