The driven Kerr parametric oscillator, of interest to fundamental physics and quantum technologies, exhibits an excited state quantum phase transition (ESQPT) originating in an unstable classical periodic orbit. The main signature of this type of ESQPT is a singularity in the level density in the vicinity of the energy of the classical separatrix that divides the phase space into two distinct regions. The quantum states with energies below the separatrix are useful for quantum technologies, because they show a catlike structure that protects them against local decoherence processes. In this work, we show how chaos arising from the interplay between the external drive and the nonlinearities of the system destroys the ESQPT and eventually eliminates the cat states. Our results demonstrate the importance of the analysis of theoretical models for the design of new parametric oscillators with ever larger nonlinearities.

We study non-Markovianity in the exact dynamics of two two-level atoms in a resonant cavity. We find a critical behavior in the form of a Markovian to non-Markovian transition at a finite interatomic distance and a discontinuity in the limit of infinitely close atoms. Actually, in this highly correlated regime, we find that non-Markovianity always arises if the system size exceeds a critical threshold. To this end, a nonsingular, additive non-Markovian measure with appropriate asymptotic behavior is formulated. These results show how inter-subsystem distance and system size can act as precise controls for modulating memory effects in multipartite open quantum systems.

Cavity-assisted atom interferometry has been proposed for gravity measurement in the past decade due to its promising advantages such as low laser power consumption, high beam quality, and strong atom-photon interactions. However, the previous cavity-related works have difficulties measuring the absolute gravitational acceleration $g$ due to their limited cavity design and locking methods. We experimentally demonstrate a Raman-type absolute atom gravimeter assisted by a newly designed degenerate optical cavity. The gravimeter achieves a decent sensitivity of about $28.5\left(9\right)\times {10}^{-8}g{\mathrm{Hz}}^{-1/2}$ under our noisy laboratory conditions and without magnetic shielding of the atom interference area. The accuracy reaches 20 $\mu \mathrm{Gal}$ within half an hour of continuous measurement. We also systematically measure and analyze the main noises affecting the $g$-measure sensitivity, indicating no physical obstacles to achieving higher sensitivity. Our cavity scheme is compatible and can be easily integrated with current free-space atom interferometers. By fully utilizing the cavity's advantages, it will benefit many areas in future fundamental research and fieldable applications.

We demonstrate a matter-wave velocimeter which probes the free-induction decay of an atomic lattice formed by interaction with an optical standing wave in the reference frame of the laser-cooled sample. During the drop time of the atoms in a gravitational field, we ensure lattice formation by changing the relative detuning of counterpropagating components of the standing wave. The contrast of the lattice, which is sensitive to the first-order Doppler shift, is measured by backscattering a near-resonant traveling-wave electric field. We infer the center-of-mass velocity by determining the line center of the backscattered spectrum. By varying the drop time, we also measure the gravitational acceleration of the falling lattice. We find that the spectrum can be considerably narrowed by imprinting a Ramsey fringe using two time-separated excitation pulses. We demonstrate these ideas using a sample of rubidium atoms with a temperature of $\sim 10\mu \mathrm{K}$, achieving a precision of $600\mu \mathrm{m}/\mathrm{s}$ for measurements of velocity and $2\mathrm{mm}/{\mathrm{s}}^2$ for determinations of gravity. Since the primary limitation in precision relates to the thermal coherence length of the sample, we show that the most precise atomic velocimeter can be realized by applying these techniques to a typical Bose gas.

We study spontaneous wave-function collapse models, which modify the Schrödinger equation with nonlinear and stochastic terms offering a solution to the quantum measurement problem. We find that constraints on the phenomenological parameters of the continuous spontaneous localization (CSL) model can be tightened using the recent analysis of the Laser Interferometer Space Antenna Pathfinder mission's angular motion data. We derived a stronger constraint on the CSL model than previously achieved with translational motion. Additionally, we identify general conditions under which rotational measurements provide an advantage over translational ones in testing collapse models. These results enhance the experimental strategies for probing fundamental modifications of quantum mechanics.

Coherent storage of quantum information is crucial to many quantum technologies. Long coherence times have been demonstrated in trapped-ion qubits, typically using the hyperfine levels within the ground state of a single ion. However, recent research suggests qubits encoded in metastable states could provide architectural benefits for quantum information processing, such as the possibility of effective dual-species operation in a single-species system and erasure-error conversion for fault-tolerant quantum computing. Here, we demonstrate long-lived encoding of a quantum state in the metastable states of a trapped ion. By sympathetically cooling with another ion of the same species, employing a tuned CPMG dynamical-decoupling sequence, and constantly monitoring for leakage errors, we demonstrate a coherence time of 136(42) s with a qubit encoded in the metastable $5D_{5/2}$ state of a single ${}^{137}{\mathrm{Ba}}^{+}$ ion. In agreement with a model based on empirical results from dynamical-decoupling-based noise spectroscopy, we find that dephasing of the metastable levels is the dominant source of error once leakage errors are removed.

The dissociation of an acidic molecule in water clusters is a captivating and important subject. This paper employs hybrid density-functional theory to optimize $\mathrm{HCl}{\left({\mathrm{H}}_2\mathrm{O}\right)}_{n=1–8}$ structures under electric fields $(E_F=0-50\mathrm{MV}/\mathrm{cm})$. It finds that strong $E_F$ greatly enhances the average dipole moments (ADMs) of dissociated clusters $(n\ge 4)$, while undissociated clusters, including $n\le 3$, remain undissociated and exhibit negligible ADM changes. This means that the correlation between HCl dissociation and ADMs can be realized by increasing the $E_F$ strength, without changing the requirement of four water molecules for one HCl dissociation. A physical model elucidates this correlation, and beam deflection method is shown to operate effectively under significant $E_{Fs}$ (∼40 MV/cm), challenging the conventional assumption of lower field strengths.

We study tests of quantum steering in which the trusted party does not have perfect control of their measurements. We report on steering inequalities that remain unaffected when introducing up to a threshold magnitude of measurement imprecision. This phenomenon, which we call an imprecision plateau, thus permits a departure from the standard assumption of idealized measurements without incurring any cost to the detection power of steering experiments. We provide an explanation for why imprecision plateaus are possible, a simple criterion for their existence, and tools for analyzing their properties. We also demonstrate that these plateaus have natural applications when the assumption of perfect measurements is relaxed: They allow for maintaining both the noise robustness and loss robustness of standard steering tests and the performance rate of idealized one-sided device-independent random number generators.

We show that the $\mathrm{SU}\left(N\right)$ Fermi-Hubbard model (FHM) on two sites, where $N$ is the number of flavors of each fermion, corresponds to an exactly solvable two-level many-boson model that Richardson [J. Math. Phys. 9, 1327 (1968)] analytically solved long ago. We express the Bethe ansatz solutions as a function of the physical parameters of the $\mathrm{SU}\left(N\right)$ FHM, and recast its eigenvalues and eigenstates in terms of the Richardson pair energies and creation operators. In this context, the connection with the well-studied Lipkin-Meshkov-Glick (LMG) model, known as equivalent to the Richardson model, is established and serves as a guideline to the prediction of some $N$-body physics phenomena in the two-site $\mathrm{SU}\left(N\right)$ FHM with $N$ particles. In particular, the LMG second-order quantum phase transition (QPT) is shown to occur in the $\mathrm{SU}\left(N\right)$ FHM for an attractive density-density interaction $U$ equal to $U_c=-1/\left(2N\right)$, in units of the (absolute value of the) tunneling amplitude between the two sites. We show the finite-size energies, the gap, and the kinetic energy, which all reveal the transition, as a function of $U$ for values of $N$ from $N=3$ to $N=36$, suggesting that the QPT could be experimentally achieved with current technologies involving $\mathrm{SU}\left(N\right)$ ultracold atoms or molecules. Finally, we show the entanglement entropy of the first site with respect to the second, and it scales like $N$ at the transition, in contrast with several two-mode models.

According to Bell's theorem, any physical theory based on local variables cannot fully reproduce quantum correlations. A critical question arises whether it is possible to develop an alternative framework based on nonlocal variables that can reproduce quantum correlations. Leggett proposed a nonlocal model, termed crypto-nonlocality, grounded in considerations of the reality of photon polarization. However, this model is inherently restricted to two dimensions. In this Letter, we extend the framework to higher dimensions and propose a method for constructing experimentally testable Leggett-type inequalities in arbitrary dimensions. From an information-theoretic perspective, our exploration of models that predict outcomes exceeding the scope of quantum mechanics is particularly intriguing and expected to deepen our understanding of quantum correlations.

A central task in quantum metrology is to exploit quantum correlations to outperform classical sensitivity limits. Metrologically useful entanglement is identified when the quantum Fisher information (QFI) exceeds a separability bound for a given parameter-encoding Hamiltonian. However, so far, only results for linear Hamiltonians are well established. Here, we characterize metrologically useful entanglement for nonlinear Hamiltonians, presenting separability bounds for collective angular momenta. Also, we provide a general expression for entangled states maximizing the QFI, which can be written as the superposition between the Greenberger-Horne-Zeilinger-like and singlet states. Finally, we compare the metrological usefulness of linear and nonlinear cases, in terms of entanglement detection and random symmetric states.

Assuming Markovian time evolution of a quantum sensing system, we study the general characterization of the optimal sensitivity scalings with time, under most general quantum control protocols. We allow the estimated parameter to influence both the Hamiltonian as well as the dissipative part of the quantum master equation and focus on the asymptotic-time along with the short-time sensitivity scalings. We find that via simple algebraic conditions (in terms of the Hamiltonian, the jump operators as well as their parameter derivatives), one can characterize the four classes of metrological models that represent: quadratic-linear, quadratic-quadratic, linear-linear, and linear-quadratic time scalings. We also investigate the relevant time scales on which the transition between the two regimes appears. Additionally, we provide universal numerical methods to obtain quantitative bounds on sensitivity that are the tightest that exist in the literature. Simplicity and universality of our results make it suitable for diverse applications in quantum metrology.

We have studied core-valence double photoionization (DPI) of Ne leading to the $1s^{-1}2s^{-1}{}^{1,3}S$ and $1s^{-1}2p^{-1}{}^{1,3}P$ states of ${\mathrm{Ne}}^{2+}$ over a range of photon energy extending up to approximately 200 eV above the thresholds using a multielectron coincidence technique. Each DPI process is distinctly identified in the energy correlation between two photoelectrons. The ratios of singlet-to-triplet cross sections exhibit variation with photon energy, indicating a spin dependence in the DPI dynamics. The state-resolved DPI probabilities have been investigated by an incoherent sum of the knock-out (KO) and shake-off (SO) contributions, revealing a significant disparity in the dynamics of the KO mechanism between the DPI into the ${}^{1}P$ and ${}^{3}P$ states.

The Leggett-Garg inequality (LGI) serves as a criterion to determine the adherence of macroscopic system dynamics to macrorealism, as introduced by Leggett and Garg. A violation of this inequality implies either the absence of a realistic description of the system or the impossibility of noninvasive measurement. In this Letter, we investigate the violation of the LGI for the system of bosons in a double-well potential. Specifically, we explore the violation of the LGI in the dynamics of bosons in a double-well potential in the Bose-Einstein-condensation (BEC) regime, where the system can be considered as two weakly coupled Bose condensates, and in the single-particle regime to establish the conditions under which the violation of the LGI occurs. Our analysis reveals that the LGI is violated due to Josephson oscillations, while it remains unviolated in the strong-coupling regime, attributed to the self-trapping phenomena. Notably, we observe that the violation of the LGI becomes increasingly significant as the particle number increases. These findings provide valuable insights into the macrorealistic behavior of Bose condensates and highlight the effect of measurements on the dynamics of a macroscopic system.

We investigate the fundamental and practical precision limits of thermometry in bosonic and fermionic environments by coupling an $N$-level probe to them and continuously monitoring it. Our findings show that the ultimate precision limit, quantified by the Fisher information, scales linearly with $N$, offering an exponential improvement over equilibrium thermometry, where the scaling is only ${log}^{2}N$. For a fixed Hamiltonian structure, we develop a maximum-likelihood estimation strategy that maps the observed continuously monitored trajectories of the probe into temperature estimates with minimal error. By optimizing over all possible Hamiltonian structures, we discover that the optimal configuration is an effective two-level system, with both levels exhibiting degeneracy that increases with $N$—a stark contrast to equilibrium thermometry, where the ground state remains nondegenerate. Our results have practical implications. First, continuous monitoring is experimentally feasible on several platforms and accounts for the preparation time of the probe, which is often overlooked in other approaches such as prepare and reset. Second, the linear scaling is robust against deviations from the effective two-level structure of the optimal Hamiltonian. Additionally, this robustness extends to cases of initial ignorance about the temperature. Thus, in global estimation problems, the linear scaling remains intact even without adaptive strategies.

We theoretically study the effects of Rabi oscillations on resonant x-ray scattering in ${\mathrm{Ne}}^{+}$ using intense attosecond and few-femtosecond pulses. The total photon yield strongly depends on the pulse area and has an upper bound at high intensities. Resonant scattering yields with attosecond pulses can be an order of magnitude higher than those far from resonances. Interference between resonance fluorescence and elastic scattering channels depends on the pulse area and initial state. Our results suggest that resonant x-ray scattering can be exploited for high-resolution, site-specific imaging.

We present a quantitative description of the thermalization dynamics of two-dimensional (2D) Bose superfluids far from equilibrium. Our analysis leverages a quantum kinetic formalism and allows us to identify two successive regimes of relaxation: an initial damping of quasiparticles due to Landau scattering processes, followed by the slower establishment of equilibrium at long time. For an initial state far from equilibrium, we find that Landau damping differs from the conventional picture of exponentially relaxing quasiparticles. Moreover, our results showcase a pronounced mechanism of slow algebraic transport at late times, rooted in energy conservation. Using theoretical and numerical arguments, we construct a detailed dynamical portrait of thermal equilibration in 2D superfluids.

Rydberg tweezer arrays provide a versatile platform to explore quantum magnets with dipolar $\mathit{XY}$ or van der Waals Ising $\mathit{ZZ}$ interactions. Here, we propose a scheme combining dipolar and van der Waals interactions between two Rydberg states, where the amplitude of the latter can be greater than that of the former, realizing an extended Hubbard model with long-range tunnelings in optical tweezer arrays. On the triangular lattice with repulsive interactions, we predict the existence of a robust supersolid phase with a critical entropy per particle $S/N\approx 0.19$ accessible in current Rydberg tweezer experiments supported by large-scale quantum Monte Carlo simulations. We further demonstrate the experimental feasibility by identifying pairs of Rydberg states in ${}^{87}\mathrm{Rb}$ realizing the required interactions. Such a lattice supersolid is long lived, found over a wide parameter range in an isotropic and flat two-dimensional geometry, and can be realized for hundreds of particles allowing one to directly probe the defect-induced picture of supersolids. Its thermodynamical and dynamical properties can hence be studied at a far larger scale than hitherto possible.

We show that applying a few-femtosecond midinfrared laser pulse parallel to the backbone of a halogenated, conjugated organic molecule induces localized ionization followed by long-lasting field-free charge migration (CM). Through time-dependent density-functional theory calculations, we find that this CM is robust with respect to the parameters of the applied laser pulse. We document the spatial and temporal origin of the localized hole, which is initiated via strong-field tunnel ionization in the halogen end of the molecule and near the peak of the laser electric field. In bromobutadiyne, we find a range of wavelengths around $1500\text{nm}$ that consistently leads to the initiation of high-contrast CM. In addition, we show that the inclusion of Ehrenfest nuclear dynamics does not disrupt the creation of the localized hole, nor the subsequent CM motion, in bromobutadiyne or parabromoaniline.

Recent measurements of the ionization energies of the Rydberg ${}^{1}P$ states of helium for principal quantum number $n=24$ and higher present a new challenge to theoretical atomic physics. A long-standing obstacle to high-precision atomic theory for three-body systems is a rapid loss of accuracy for variational calculations with increasing principal quantum number $n$. We show that this problem can be overcome with the use of a “triple” basis set in Hylleraas coordinates. Nonrelativistic energies accurate to 23 significant figures are obtained with basis sets of relatively modest size (6744 terms). Relativistic and quantum electrodynamic effects are calculated, including an estimate of terms of order $m{\alpha }^6$ from a $1/n^3$ extrapolation, resulting in an estimated accuracy of $\pm 1$ kHz. The calculated ionization energy of 5 704 980.348(1) MHz is in excellent agreement with the experimental value 5 704 980.312(95) MHz. These results establish the ionization energy of the $1s24p{}^{1}P_1$ state as an absolute point of reference for transitions to lower-lying states, and they confirm an $11\sigma$ disagreement between theory and experiment in the triplet spectrum of helium. Results are also given for the $1s24p{}^{3}P_J$ states, in agreement with a recent experiment on the triplet Rydberg series, thereby confirming the discrepancy. Results are also given for the $1s24p{}^3{P}_J$ states which are in agreement with a recent experiment on the triplet-$P$ Rydberg series, thereby confirming a discrepancy of $0.468\pm 0.055$ MHz for the ionization energy of the $1s2s{}^3{S}_1$ state.