We consider a quantum sensor network of qubit sensors coupled to a field $f(\mathit{x};\mathit{\theta })$ analytically parameterized by the vector of parameters $\mathit{\theta }$. The qubit sensors are fixed at positions ${\mathit{x}}_1,\cdots ,{\mathit{x}}_d$. While the functional form of $f(\mathit{x};\mathit{\theta })$ is known, the parameters $\mathit{\theta }$ are not. We derive saturable bounds on the precision of measuring an arbitrary analytic function $q\left(\mathit{\theta }\right)$ of these parameters and construct the optimal protocols that achieve these bounds. Our results are obtained from a combination of techniques from quantum information theory and duality theorems for linear programming. They can be applied to many problems, including optimal placement of quantum sensors, field interpolation, and the measurement of functionals of parametrized fields.

We measure polarization-resolved fundamental, second, and third harmonic nonlinear Thomson scattering out the side of a laser focus with ${10}^{18}\mathrm{W}/{\mathrm{cm}}^2$. The separate measured polarization components are each associated with a distinct dimension of predicted electron figure-8 motion. Taken together, the measured angular emission patterns for the two polarizations unambiguously confirm the figure-8 motion. Electrons are donated from low-density helium (${10}^{-3}$ to 1 Torr) ionized early during the laser pulse. Time-resolved single-photon detection is used to distinguish signal from noise.

Polygonal microcavities supporting whispering-gallery modes (WGMs) are widely investigated for optoelectronic applications. Recently, polygonal microcavities with reduced symmetry of various types have been realized in a large number of materials, while the corresponding theoretical analysis of WGMs was either absent or done only for individual cavity shapes. We perform a systematic analysis of irregular polygonal microcavities in general, allowing us to obtain deterministic conclusions on WGMs for all possible parameters of all types of shape irregularities from a set of analytical equations and inequalities. We test the theory with several typical irregular hexagonal microcavities that were experimentally reported, and we discuss the loss mechanism verified by wave optics simulations. The results provide a powerful analyzing tool for experimentalists working on irregular polygonal microcavities, and they constitute a significant extension to the scope of WGM theory.

We present a simple and efficient method for computing single and double ionization cross sections in ion-atom and ion-molecule collisions using $L^2$ Gaussian basis sets. Gaussian functions are widely employed to compute bound states of ions, atoms, and molecules. However, the description of continuum states, and therefore ionization phenomena, remains a theoretical challenge. Our approach is tested on the benchmark system antiproton-helium collisions in the so-called intermediate energy range. A good agreement with numerically exact calculations is observed. The proposed method is general and can thus be employed in any collisional systems in the challenging nonperturbative regime. Our work opens the way to investigate multiple ionization processes by ion impact in multicenter polyelectronic systems.

This Letter presents a nonclassical mechanism of two-atom interferometry. The two-atom interferometer introduces two different yet indistinguishable alternatives for randomly paired atoms to create a joint atom-detection event. The superposition of two-atom amplitudes yields two-atom interference with peculiar features which outshine the classic atom interferometer: (1) two-atom interference is still observable when the time delay of the interferometer is greater than the coherence time of the atom beam and (2) two-atom interference may eliminate phase noises, including background, variation, turbulence, and Raman laser induced phase noises, thereby allowing for higher sensitivity and stability sensing than is achievable by classic atom interferometers. These features are crucial in high sensitivity-accuracy acceleration and rotation measurements. The presented concept and mechanism of two-atom interference can be adapted to other matter-wave interferometers, such as two-neutron interferometer and two-electron interferometer.

It was recently argued that the geometric optics mean path length of rays inside a refractive object under Lambertian illumination is independent of the scattering strength of the medium [Savo et al., Science 358, 765 (2017)]. We here show that it is, in fact, different in the case of zero scattering. We uncover and explain the role of trapped ray trajectories in creating this unexpected discontinuity from zero to low scattering. This allows us to derive alternative analytic results for the zero scattering mean path length of simple refractive shapes. We believe this work provides a fresh perspective on the study of path length inside refractive objects, with possible applications in, for example, the study of scattering by large particles or the design of optical systems.

A critical set of two-loop quantum electrodynamics corrections to the $g$ factor of hydrogenlike ions is calculated in the Furry picture. These corrections are due to the polarization of the external magnetic field by the quantum vacuum, which is dressed by the binding field. The result obtained for the self-energy–magnetic-loop diagrams is compared with the current state-of-the-art result, derived through a perturbative expansion in the binding strength parameter $Z\alpha$, with $Z$ the atomic number and $\alpha$ the fine-structure constant. Agreement is found in the $Z\to 0$ limit. However, even for very light ions, the perturbative result fails to approximate the magnitude of the corresponding correction to the $g$ factor. The total correction to the $g$ factor coming from all diagrams considered in this work is found to be highly relevant for upcoming experimental tests of fundamental physics with highly charged ions.

We perform atomic relativistic many-body calculations of the field isotope shifts and calculations of corresponding nuclear parameters for all stable even-even isotopes of the ${\mathrm{Yb}}^{+}$ ion. We demonstrate that if we take nuclear parameters of the Yb isotopes from a range of the state of the art nuclear models, which all predict strong quadrupole nuclear deformation, and then calculate nonlinearity of the King plot caused by the difference in the deformation in different isotopes, the result is consistent with the nonlinearity observed in the experiment [I. Counts et al., Phys. Rev. Lett. 125, 123002 (2020)]. The changes of nuclear rms radius between isotopes extracted from experiment are consistent with those obtained in the nuclear calculations.

We investigate multicomponent fermions in a flat band and predict experimental signatures of non-Fermi-liquid behavior. We use dynamical mean-field theory to obtain the density, double occupancy and entropy in a Lieb lattice for $\mathcal{N}=2$ and $\mathcal{N}=4$ components. We derive a mean-field scaling relation between the results for different values of $\mathcal{N}$, and study its breakdown due to beyond mean-field effects. The predicted signatures occur at temperatures above the Néel temperature and persists even in the presence of a trapping potential, thus they are observable in current ultracold gas experiments.

Studying the conversion between transverse light modes via four-wave mixing in a heated rubidium vapor, we demonstrate and explain a transfer between azimuthal and radial mode numbers. They relate to orthogonal modal dimensions, which one would not normally expect to interact. While angular momentum conservation in this nonlinear process dictates the selection rules for the angular mode number, the role of the radial mode number is more esoteric. We demonstrate systematically that the Gouy phase is the key to understanding this conversion, leading to strikingly different conversion behavior in the thick- and thin-medium regimes. Our experimental investigation of the transition between these regimes bridges the gap between previous experiments in atomic thick media and work in nonlinear crystals. Our work sets a clear starting point to explore the thick-medium regime, allowing efficient radial-to-azimuthal and radial-to-radial mode conversion.

Brun, Devetak, and Hsieh [Science 314, 436 (2006)] demonstrated that preshared entanglement between the sender and receiver enables quantum communication protocols that have better parameters than schemes without the assistance of entanglement. Subsequently, the same authors derived a version of the so-called quantum Singleton bound that relates the parameters of the entanglement-assisted quantum-error-correcting codes proposed by them. We present an entanglement-assisted quantum communication scheme with parameters violating this bound in certain ranges. For a fixed transmission rate, our scheme allows one to correct a larger fraction of errors.

Cluster expansions for the exponential of local operators are constructed using tensor networks. In contrast to other approaches, the cluster expansion does not break any spatial or internal symmetries and exhibits a very favorable prefactor to the error scaling versus bond dimension. This is illustrated by time evolving a matrix product state using very large time steps, and by constructing a robust algorithm for finding ground states of two-dimensional Hamiltonians using projected entangled pair states as fixed points of two-dimensional transfer matrices.

In $3\text{D}+t$ numerical simulations, we propose an experiment wherein a mixture of gases is used for the continuous transition from $\mathsf{X}$- to $\mathsf{O}$-shaped angle-wavelength spectrum of a femtosecond infrared filament. The mixture consists of transparent nitrogen at 40 bar pressure and $85{}^{\circ }\mathrm{C}$ temperature as well as absorbing water vapor at 0–0.36 bar, with the rovibrational resonance at a wavelength slightly longer than the $1.3\mu \mathrm{m}$ center of the pump pulse driving the filament. The continuous increase in the water vapor density results in the gas mixture dispersion changing from normal to anomalous and transformation of the short-wavelength conical emission onto on-axis propagating radiation.

Floquet-Magnus (FM) expansion theory is a powerful tool in periodically driven (Floquet) systems under high-frequency drives. In closed systems, it dictates that their stroboscopic dynamics under a time-periodic Hamiltonian is well captured by the FM expansion, which gives a static effective Hamiltonian. On the other hand, in dissipative systems driven by a time-periodic Liouvillian, it remains an important and nontrivial problem whether the FM expansion gives a static Liouvillian describing continuous-time Markovian dynamics, which we refer to as the “Liouvillianity” of the FM expansion. We answer this question for generic systems with local interactions. We find that while noninteracting systems can either break or preserve Liouvillianity of the FM expansion, generic few-body and many-body interacting systems break it under any finite drive, which is essentially caused by propagation of interactions via higher-order terms of the FM expansion. Liouvillianity breaking implies that Markovian dissipative Floquet systems in the high-frequency regimes do not have static (Markovian) counterparts, giving a signature of emergent non-Markovianity. Our theory provides a useful insight in the quest for unique phenomena in dissipative Floquet systems.

The low-energy electron-impact dissociation of molecular hydrogen has been a source of disagreement between various calculations and measurements for decades. Excitation of the ground state of ${\mathrm{H}}_2$ into the dissociative $b{}^3{\mathrm{\Sigma }}_u^{+}$ state is now well understood, with the most recent measurements being in excellent agreement with the molecular convergent close-coupling (MCCC) calculations of both integral and differential cross sections [Zawadzki et al., Phys. Rev. A 98, 062704 (2018)]. However, in the absence of similar measurements for vibrationally excited or isotopically substituted ${\mathrm{H}}_2$, cross sections for dissociation of these species must be determined by theory alone. We have identified large discrepancies between MCCC calculations and the recommended $R$-matrix cross sections for dissociation of vibrationally excited ${\mathrm{H}}_2$, ${\mathrm{D}}_2$, ${\mathrm{T}}_2$, HD, HT, and DT [Trevisan et al., Plasma Phys. Contr. Fusion 44 1263 (2002); Plasma Phys. Contr. Fusion 44, 2217 (2002)], with disagreement in both the isotope effect and dependence on initial vibrational level. Here we investigate the source of the discrepancies, and discuss the consequences for plasma models, which have incorporated the previously recommended data.

Parity-time ($\mathcal{PT}$)-symmetric systems, featuring real eigenvalues despite their non-Hermitian nature, undergo spontaneous $\mathcal{PT}$ symmetry breaking at the exceptional point (EP). The phase transition from $\mathcal{PT}$ symmetry to $\mathcal{PT}$ symmetry breaking has shown exotic functionalities in the classical realm, such as loss-induced transparency or lasing revival. This transition is also expected to produce a more unconventional effect in pure quantum $\mathcal{PT}$ devices. Here, we report experimental evidences of spontaneous $\mathcal{PT}$ symmetry breaking in a single cold ${}^{40}\mathrm{Ca}^{+}$ ion and, more importantly, a counterintuitive effect of perfect quantum coherence occurring at the EP. Excellent agreement between experimental results and theoretical predictions is identified. In view of the versatile role of cold ions in building a quantum memory or processor, our experiment provides an alternative platform to explore $\mathcal{PT}$-symmetric physics and utilize pure quantum EP effects.