Although the frequency shift and odd-even intensity modulation in high-order harmonic generation (HHG) have both been observed for asymmetric laser-target systems, they are typically studied as two separate effects. In this Letter we provide a comprehensive picture of these two nonlinear optical phenomena, unifying them through a common origin: asymmetry of the laser-target system. By tuning asymmetric laser-target systems, we discover a transition from the harmonic frequency shift to the odd-even intensity modulation upon increasing the duration of the driving laser pulse. Specifically, these phenomena are observed simultaneously for laser pulses with intermediate pulse duration. For numerical evidence, we solve the time-dependent Schrödinger equation, while insight into the underlying physics is obtained from a simplified analytically tractable model. Understanding the asymmetric characteristics reflected in the HHG as provided is crucial for retrieving laser-target information, sampling external fields, and probing molecular dynamics.

The return of the information from the environment to the system is a phenomenon that can be related to the existence of non-Markovian mechanisms in the environment, and such a transformation of resources can be useful for quantum information applications. Thus, understanding the details of the system-environment information dynamics, i.e., the transference of quantum resources, is of key importance to design noise-resilient quantum technologies. In this Letter, we show how a quantum resource propagates from the main system to an environment, using as a model a single qubit coupled to two linear chains of qubits, and also the information dynamics among the environment qubits. In this way, we characterize the propagation of information leaving the main qubit and going through the environment. Finally, we connect the conditions for the emergence of this dynamics to the existence of quantum Darwinism.

A critical challenge for density-functional theory (DFT) in practice is its limited ability to treat static electron correlation, leading to errors in its prediction of charges, multiradicals, and reaction barriers. Recently, we combined one- and two-electron reduced density-matrix theories with DFT to obtain a universal $O\left(N^3\right)$ generalization of DFT for static correlation. In this Letter, we enhance the theory's treatment of large molecules by renormalizing the trace of the two-electron identity matrix in the correction using Cauchy-Schwarz inequalities of the electron-electron repulsion matrix. We apply the resulting functional theory to linear hydrogen chains as well as the prediction of the singlet-triplet gap and equilibrium geometries of a series of acenes. This renormalization of the generalized DFT retains the $O\left(N^3\right)$ computational scaling of DFT while enabling the accurate treatment of static correlation for a broad range of molecules and materials.

We study the role of material nonlocality (spatial dispersion) in dynamical Casimir effects in time-varying frequency-dispersive nanophotonic systems. We first show that local models may lead to nonphysical predictions, such as diverging emission rates of entangled polariton pairs. We then theoretically demonstrate that nonlocality regularizes this behavior by correcting the asymptotic response of the system for large wave vectors and leads to physical effects missed by local models, including a significant broadening of the emission rate distribution, which are relevant for future experimental observations. Our work sheds light on the importance of nonlocal effects in this new frontier of nanophotonics.

This work presents our theoretical study of the dissociative recombination (DR) of the closed-shell diatomic system ${\mathrm{CF}}^{+}$ based on an approach recently applied to the ${\mathrm{CH}}^{+}$ molecule. Our extended treatment uses the UK $R$-matrix theory and the multichannel quantum defect theory (MQDT) procedure to uniformly resolve the direct and indirect mechanisms of DR while bypassing explicit dissociative state and electronic coupling calculations. The theoretical results exhibit good overall agreement with previous experimental measurements. At lower scattering energies, good agreement is found only if the rotational structure of the ion is included and the theoretical cross sections are averaged over initial rotational levels corresponding to the temperature at which the experimental measurements were made. At higher scattering energies, our rotationally resolved results are very similar to those obtained without including the ion's rotational structure.

The ability to create and harness entanglement is crucial to the fields of quantum sensing and simulation, and ultracold-atom–cavity systems offer pristine platforms for this undertaking. Here, we present a method for creating and controlling entanglement between solely the motional states of atoms in a cavity without the need for electronic interactions. We show this interaction arises from a general atom-cavity model and discuss the role of the cavity frequency shift in response to atomic motion. This cavity response leads to many different squeezing interactions between the atomic momentum states. Furthermore, we show that when the atoms form a density grating, the collective motion leads to one-axis twisting, a many-body energy gap, and metrologically useful entanglement even in the presence of noise. Notably, an experiment recently demonstrated this regime leads to an effective momentum-exchange interaction between atoms in a common cavity mode [Luo et al., Science 384, 551 (2024)]. This system offers a highly tunable, many-body quantum sensor and simulator.

Photon bound states have been identified as particular solutions to the scattering of two photons from a single emitter, but from these results the full nature of these states remains elusive. We study a clear and unambiguous signature that these bound states are truly bound. To this end we consider an alternative configuration of close-by waveguides, each chirally coupled to two-level emitters. We show that in this system the photon bound states behave as rigid molecules, where photons do not tunnel individually but rather collectively, such that there is rarely a single photon in each waveguide. We further identify new classes of bound states in this system.

We showcase the impact of almost-periodicity on the parametric amplification associated with the first-order momentum gap in photonic time-crystals with time-varying permittivity. Utilizing a vectorial coupled-wave theory approach, we rigorously analyze the scattering by a temporal slab of the considered medium. We pinpoint a critical regime wherein flaws in material tuning paradoxically enhance amplification due to the coupling of fewer, broader modes, resulting in a higher and broader pulselike amplification envelope. Additionally, we demonstrate that the intensity reflectances of time-reversed waves corresponding to secondary “Bragg” resonances achieve remarkably high levels of subharmonic parametric amplification, with the epsilon-near-zero regime serving as a preferred candidate for experimental implementation. Our counterintuitive findings highlight the potential of intentionally leveraging modulation desynchronization and impurities in the temporal unit cell of photonic time-crystals to enhance both the level and the bandwidth of amplification.

The YbOH molecule is one of the most sensitive systems for the electron electric dipole moment ($e\mathrm{EDM}$) searches. Zeeman splittings of the $e\mathrm{EDM}$ sensitive levels have significant implications to control and suppress important systematic effects due to stray magnetic fields in experiments for $e\mathrm{EDM}$ searches. The electric-field-dependent $g$ factors of the lowest rotational level of the first excited bending vibrational mode of ${}^{174}\mathrm{YbOH}$ are calculated. $l$-doublet levels with a small $g$-factor difference are found and main contributions to the difference are determined.

We explore the process of orbital angular momentum (OAM) transfer from a twisted light beam to an electron in atomic ionization within the first Born approximation. The characteristics of the ejected electron are studied regardless of the detection scheme. We find that the outgoing electron possesses a definite projection of OAM when a single atom is located on the propagation axis of the photon, whereas the size of the electron wave packet is determined solely by the energy of the photon rather than by its transverse coherence length. Shifting the position of the atom yields a finite dispersion of the electron OAM. We also study a more experimentally feasible scenario, a localized finite-size atomic target, and develop representative approaches to describing coherent and incoherent regimes of photoionization.

Cold-atom analog experiments are a promising new tool for studying relativistic vacuum decay, enabling one to empirically probe early-Universe theories in the laboratory. However, existing proposals place stringent requirements on the atomic scattering lengths that are challenging to realize experimentally. Here we eliminate these restrictions and show that any stable mixture between two states of a bosonic isotope can be used as a faithful relativistic analog. This greatly expands the landscape of suitable experiments, and will expedite efforts to study vacuum decay with cold atoms.

We analyze the dynamics of the Rabi model describing interactions of a qubit with a single soft-mode oscillator. We show that the model with a slightly violated parity generates symmetric Schrödinger cat states of the oscillator, which suddenly perish in a spontaneous unitary process similar to the measurement-induced wave-function collapse. The effect is sufficiently robust to be tested experimentally, e.g., with trapped ions, macroscopic mechanical oscillators, or superconducting circuits.

Single-photon sources (SPSs) are an essential resource for many quantum information technologies. We demonstrate how the inclusion of time-delayed coherent feedback in a scalable waveguide system can significantly improve the two key SPS figures of merit: coherence and indistinguishability. Our feedback protocol is simulated using a quantum trajectory discretized waveguide model which can be used to directly model Hanbury Brown and Twiss (HBT) and Hong-Ou-Mandel (HOM) interferometers. With the proper choice of the round trip phase, the non-Markovian dynamics from the time-delayed feedback improves the indistinguishability of the SPS by up to $57\%$. We also show how this mechanism suppresses the detrimental effects of off-chip decay and pure dephasing.

We analytically solve a model for light scattering in Ising dynamics of metastable atomic qubits, generalizing the approach of Foss-Feig et al. [Phys. Rev. A 87, 042101 (2013)] to include leakage outside the qubit manifold. We analyze the influence of these fundamental errors in simulations of proposed experiments with metastable levels of ${}^{40}\mathrm{Ca}^{+}$ ions in a Penning trap. We find that “effective magnetic fields” generated by leaked qubits have significant impacts on spin-spin correlation functions for Greenberger-Horne-Zeilinger state preparation or for quantum simulations with strong coupling, while spin squeezing uses a much weaker coupling and is largely insensitive to the simulated leakage errors, even with a few hundred ions. Our theory and results are expected to be useful in modeling a variety of metastable qubit experiments in the future.

Transfer of conserved quantities between two remote regions is generally assumed to be a rather trivial process: a flux of particles carrying the conserved quantities propagates from one region to another. However, we demonstrate a flow of angular momentum from one region to another across a region of space in which there is a vanishingly small probability of any particles (or fields) being present. This shows that the usual view of how conservation laws work needs to be revisited.

We report a precise measurement of the isotope shifts in the $4{}^{2}S_{1/2}\to 3{}^{2}D_{3/2}$ electric quadrupole transition at 732 nm in ${}^{42,44,48}\mathrm{Ca}^{+}$ relative to ${}^{40}\mathrm{Ca}^{+}$ via high-resolution laser spectroscopy of cotrapped ions, finding measured shifts of $2775392374.8\left(6.0\right), 5347679835.1\left(5.9\right)$, and $10003129115.1\left(5.7\right)$ Hz between ${}^{42,44,48}\mathrm{Ca}^{+}$ and ${}^{40}\mathrm{Ca}^{+}$, respectively. When combined with prior measurements on the $4{}^{2}S_{1/2}\to 3{}^2{D}_{5/2}$ transition [F. W. Knollmann et al., Phys. Rev. A 100, 022514 (2019)], a King plot analysis shows the data to be consistent with linearity below the level of parts per billion. This observed linearity, which is free of nuclear systematics, improves the previous isotope-shift-based limits of ${\mathrm{Ca}}^{+}$ for couplings of a beyond the standard model scalar boson to electrons and neutrons by a factor of 3. Our new limit excludes part of the coupling range remaining for a new physics interpretation after accounting for one higher-order nuclear term in the nonlinear King plot of Yb/${\mathrm{Yb}}^{+}$.

The transition energies of the two $1s$ core-excited soft x-ray lines (dubbed $q$ and $r$) from $1s^{2}2s{}^1{S}_{1/2}$ to the respective upper levels $1s{(}^{2}S)2s2p{(}^{3}P){}^2{P}_{3/2}$ and ${}^2{P}_{1/2}$ of Li-like oxygen, fluorine, and neon were measured and calibrated using several nearby transitions of He-like ions. The major remaining source of energy uncertainties in monochromators, the periodic fluctuations produced by imperfect angular encoder calibration, is addressed by a simultaneously running photoelectron spectroscopy measurement. This leads to an improved energy determination of 5 parts per million, showing fair agreement with previous theories as well as with our own, involving a complete treatment of the autoionizing states studied here. Our experimental results translate to an uncertainty of only 1.6 km/s for the oxygen line $qr$ blend used to determine the outflow velocities of active galactic nuclei, ten times smaller than previously possible.

We report an experimental observation of nontrivial intensity correlation of a continuous-wave fiber ring laser. The laser beam consists of approximately 450 000 independent longitudinal cavity modes, each in a coherent state representing a group of identical photons. Based on Glauber's second-order coherence theory, we conclude that the nontrivial intensity correlation is the result of a randomly created and randomly paired distinguishable group of identical photons interfering with the pair itself. This result is fundamentally interesting and practically useful.

Combining quantum and Bayesian principles leads to optimality in metrology, but the optimization equations involved are often hard to solve. This Letter mitigates this problem with a class of measurement strategies for quantities isomorphic to location parameters, which are shown to admit a closed-form optimization. The resulting framework admits any parameter range, prior information, or state, and the associated estimators apply to finite samples. As an example, the metrology of relative weights is formulated from first principles and shown to require hyperbolic errors. The primary advantage of this approach lies in its simplifying power: it reduces the search for good strategies to identifying which symmetry leaves a state of maximum ignorance invariant. This will facilitate the application of quantum metrology to fundamental physics, where symmetries play a key role.

We propose a technique to determine the absolute number density ${\displaystyle N}$ of an alkali-metal vapor confined within a nanocell. This method is based on near-resonant driving of the vapor's constituent atoms and determining the mean interparticle distance (or equivalently the temperature) from the power spectrum associated with the offset photon intensity-intensity correlation function ${\displaystyle g^{\left(2\right)}\left(\tau \right)-1}$. In our investigation, we treat the atoms as an average of interacting and radiating dipole pairs randomly positioned within the nanocell. We observe that the power spectrum of the emitted light has a central dip for interparticle distances corresponding to $\sim \lambda /5$ and that this result is robust to variations in the driving Rabi frequency and the average detuning. With our proposed method, we can overcome the limitations in defining the absolute number density ${\displaystyle N}$, which is currently typically deduced from the temperature of heating elements applied externally to the cell.