We propose In-like ${\mathrm{Pr}}^{10+}$ as a candidate for the development of a high-accuracy optical clock with high sensitivity to a time variation of the fine-structure constant, $\dot{\alpha }/\alpha$, as well as favorable experimental systematics. We calculate its low-lying energy levels by combining the configuration interaction and the coupled cluster method, achieving uncertainties as low as $0.1\%$, and improving previous work. We benchmark these results by comparing our calculations for the $|5s^{2}5p{}^{2}P_{1/2}\rangle \to |5s^{2}5p{}^{2}P_{3/2}\rangle$ transition in ${\mathrm{Pr}}^{10+}$ with a dedicated measurement and for ${\mathrm{Pr}}^{9+}$ with a recent experiment, respectively. In addition, we report calculated hyperfine-structure constants for the clock and logic states in ${\mathrm{Pr}}^{10+}$.

We integrate full core-hole density-functional theory with Franck-Condon calculations, considering Duschinsky rotation, to simulate vibrationally resolved C $1s$ x-ray photoelectron spectra of eight linear alkanes, from methane to octane (${\mathrm{C}}_n{\mathrm{H}}_{2n+2}, n=1–8$). Results align excellently with experimental absolute binding energies and profiles. The spectrum of ethane serves as a “spectral seed,” with each longer alkane's atom-specific spectrum displaying similar characteristics, albeit with shifts and slight intensity adjustments. Detailed assignments clarify the distinct spectra in short alkanes ($n=1–4$) and their stabilization in long alkanes ($n=5–8$). Carbons are classified as central or distal (${\mathrm{C}}_1$ and ${\mathrm{C}}_2$), with central carbons contributing nearly identically to the lowest-energy feature A, while distal carbons contribute to the second-lowest-energy feature B (both are 0-0 transitions). Increasing molecule size adds more central carbons, enhancing feature A and weakening feature B. Our analysis identifies two dominant Franck-Condon active vibrations: a localized ${\mathrm{C}}^{*}$-H stretching (approximately 3400–3500 ${\mathrm{cm}}^{-1}$) and a delocalized C-H bending (approximately 1400–1500 ${\mathrm{cm}}^{-1}$) modes. Structural analysis shows that core ionization minimally affects alkane geometry, less than in ring compounds from previous studies. This work extends our protocol from rigid ring compounds to flexible molecules, contributing to building a high-resolution theoretical x-ray photoelectron spectroscopy library and enhancing the understanding of vibronic coupling.

We theoretically study the electron dynamics of temporal double-slit interference in strong-field tunneling ionization of ${\mathrm{H}}_2^{+}$ by numerically solving the time-dependent Schrödinger equation. As the internuclear distance varies, we observe a notable shift of this interference pattern appearing in photoelectron momentum distributions. Quantum-orbit analysis reveals that the shift is induced by the initial longitudinal displacement of electrons immediately after tunneling ionization, which is closely related to the tunnel site. When the molecule is stretched to a critical internuclear distance, another ionization channel of electrons tunneling ionized from the uphill site is opened. By analyzing the shift of the temporal double-slit interference, we demonstrate that the electrons from this channel have substantial effects on the interference fringes. This work shows the important role of tunneling sites and reveals the electron interference dynamics in strong-field tunneling ionization of molecules. It suggests that the temporal double-slit interferogram can serve as a tool to retrieve complex laser-driven electron dynamics.

A comprehensive theoretical approach to describe the electron-ion radiative recombination in the presence of intense, short laser pulses, which accounts for $1/c$ corrections, is presented. Its essential step is based on expanding the relativistic Coulomb-Volkov solution describing an electron in a combined Coulomb potential and a laser field in powers of $1/c$. In this way, we can trace the origin of nondipole effects observed in the spectrum of emitted radiation. Hence, as we demonstrate for high-frequency pulses assisting the process, a significant extension of the cutoff and asymmetry in angular distributions of the emitted radiation can be attributed to the electron recoil off the laser pulse. In addition, we investigate the possibility of enhancing the efficiency of the generated high-energy radiation by chirping the pulse.

We numerically study solitary-wave solutions for the nonlinear Schrödinger equation perturbed by the effects of third-order and negative fourth-order dispersion. At a single wave number, an analytical expression for a localized solution with nonzero velocity, here referred to as Kruglov and Harvey's solitary-wave solution, is known to exist. To obtain solitary waves for general wave numbers and velocities, we employ a custom spectral renormalization method. For a selected set of system parameters and a range of wave numbers, we characterize the resulting pulses via a fit model, allowing us to formulate empirical relations between the pulse parameters. Deeper insight into the interaction dynamics of these solitary waves can be obtained through collisions. These collisions are typically inelastic and allow for the formation of short-lived two-pulse bound states with very particular dynamics. Finally, we detail the properties of Kruglov and Harvey's soliton solution under weak loss. For short propagation distances our numerical results verify earlier predictions of perturbation theory and show that the pulse shape is altered upon propagation. For long distances we observe a crossover to linear pulse broadening.

We provide a scheme by utilizing a two-cavity setup to generate useful quantum mechanically entangled states of two cavity fields, which themselves are prepared in Schrödinger cat states. The underlying atom-field interaction is considered off-resonant and three atoms are successively sent through the cavities, initially fed with coherent fields. Analytical solution of the protocol, followed by conditional measurements on the atoms, produce a family of eight such entangled states. Entanglement properties of the obtained states are characterized by the von Neumann entropy. We reveal the parameter domain for tuning the entanglement, the prime tuning parameters being the atom-field interaction time and the field amplitudes. The parameter domains for both quasi-Bell and non-quasi-Bell states are discussed. We also present a Wigner phase-space representation of the reduced state of the cavity, showing negative values and interference patterns similar to those of a compass state, used in quantum precision measurements, and despite its large entropy.

Generalizations of Bell's theorem, particularly within quantum networks, are now being analyzed through the lens of causal inference. However, the exploration of interventions, a central concept in causality theory, remains significantly unexplored. In this work, we take an initial step in this direction, by analyzing the instrumental scenario and proposing hybrid Bell inequalities integrating observational and interventional data. Focusing on binary outcomes with any number of inputs, we obtain the complete characterization of the observational-interventional polytope, equivalent to a Hardy-like Bell inequality, albeit describing a distinct quantum experiment. To illustrate its applications, we show a significant enhancement regarding threshold detection efficiencies for quantum violations also showing the use of this hybrid approach in quantum steering scenarios.

A pair of interferometers can be coupled by allowing one path from each to overlap such that if the particles meet in this overlap region, they annihilate. It was shown by one of us over 30 years ago that such annihilation-coupled interferometers can exhibit apparently paradoxical behavior. More recently, Bose et al. [Phys. Rev. Lett. 119, 240401 (2017)] and Marletto et al. [Phys. Rev. Lett. 119, 240402 (2017)] have considered a pair of interferometers that are phase coupled (where the coupling is through gravitational interaction). In this case one path from each interferometer undergoes a phase-coupling interaction. We show that these phase-coupled interferometers exhibit the same apparent paradox as the annihilation-coupled interferometers, though in a curiously dual manner.

Quantum speed limits (QSLs) impose fundamental constraints on the evolution speed of quantum systems. For systems with time-independent Hamiltonians, the QSLs are intricately linked to the energy spectra of the initial state, which signify the generalization of the time-energy uncertainty relation. Based on arbitrary-order norms of the Hamiltonian, dual energies from inverted spectra, and the standard deviation of energy, different types of QSLs have been proposed, revealing three dynamical parameter regimes of quantum state evolution. Despite the theoretical advancements, a comprehensive experimental examination remains elusive. In this research, by introducing unified bounds suitable for all evolution times, we present the experimental verification to the QSLs in various scenarios with a superconducting circuit. We explore the dynamical evolution with regard to each parameter regime as well as the critical point by carefully engineering the initial-state population of the superconducting resonator. We test our unified bounds over finite-level states and infinite-level classical and nonclassical Gaussian states, which validates the criteria for the QSLs. Our work substantially deepens our comprehension of unitary quantum evolution.

We study the steady-state, internal current circulation of local thermal currents in a bosonic trimer with a geometric phase inside it. Driven by two external reservoirs at different temperatures, the system exhibits a nonreciprocal current between two of its sites in thermal equilibrium, without violating the zeroth law of thermodynamics. Notably, our study explores both the limits where the internal hopping amplitude falls within weak- and strong-coupling regimes, relative to the system's dissipation rates. While nonreciprocity persists in the strong-coupling regime, we observe a novel phase reversal in the nonequilibrium current. We discuss its experimental feasibility and propose exciting new possibilities for implementing quantum-controlled heat devices.

Counterintuitive to classical notions, quantum conditional entropy can be negative, playing a pivotal role in information-processing tasks. This article delves deeply into quantum channels, emphasizing negative conditional entropy breaking (NCEB) channels and introducing negative conditional entropy annihilating (NCEA) channels. We characterize these channels from topological and information-theoretic perspectives, examining their properties when combined serially. We also examine NCEB channels in parallel. Our exploration extends to complimentary channels associated with NCEB, leading to the introduction of information-leaking channels. Utilizing the parameters of the standard depolarizing channel, we provide tangible examples and further characterization. We demonstrate the relationship of NCEB and NCEA with newly introduced channels like coherent information (CIB) breaking and mutual information (MIB) breaking, along with standard channels like zero capacity channels. The preservation of quantum resources is an integral constituent of quantum information theory. Recognizing this, we lay prescriptions to detect channels that do not break the negativity of conditional entropy, ensuring the conservation of this quantum resource.

Based on the conviction of switching information from locally accessible to locally hidden environs, the concept of hidden nonlocality activation has recently been highlighted by Bandyopadhyay et al. [Phys. Rev. A 104, L050201 (2021)]. They have demonstrated that a certain locally distinguishable set of pure quantum states can be transformed into a locally indistinguishable set with certainty through orthogonality preserving local measurements. This transformation makes the set locally inaccessible, despite being locally accessible before. This phenomenon is defined as the activation of hidden nonlocality. In this paper, we present two classes of locally distinguishable sets within $(2m+1)\otimes 2\otimes (2m+1)$ systems. One class reveals nonlocality through local operations, whereas the other requires joint measurements for it. As the latter class depends on nonlocal operations to exhibit nonlocality, it arguably has a lower degree of nonlocality, and accordingly, can be considered as more local compared to the first class. This analysis exhibits a stronger manifestation of locality by elucidating the nuanced interplay between these distinct local phenomena within the framework of quantum state discrimination. Furthermore, we also explore their significant applications in the context of data hiding. Additionally, we introduce the concept of strong local set and compare it with various activable sets, highlighting differences in terms of locality.

We present a hybrid quantum-classical Green's function Monte Carlo (GFMC) algorithm for estimating the excited states of the nuclear shell model. The conventional GFMC method, widely used to find the ground state of a quantum many-body system, is plagued by the sign problem, which leads to an exponentially increasing variance with the growth of system size and evolution time. This issue is typically mitigated by applying classical constraints but at the cost of introducing bias. Our approach uses quantum subspace diagonalization (QSD) on a quantum computer to prepare a quantum trial state, replacing the classical trial state in the GFMC process. We also incorporated a modified classical shadow technique in the implementation of QSD to optimize quantum resource utilization. Besides, we extend our hybrid GFMC algorithm to find the excited states of a given quantum system. Numerical results suggest our method largely enhances accuracy in determining excited-state energies, offering an improvement over the conventional method.

The study of muonium-muonium binding and scattering is crucial for precision experiments involving muonium. In this paper, we investigate the bound-state energies and structures of the ${\left(\mathrm{muonium}\right)}_2$ molecule for total orbital angular momentum $L=0$ using the non-Born-Oppenheimer variational method combined with explicitly correlated Gaussian basis functions. Additionally, we employ the confined variational method to explore muonium-muonium scattering at scattering momenta below $0.11a_0^{-1}$, where $a_0$ is the Bohr radius. Our approach provides accurate $S$-wave phase shifts, as well as the $S$-wave scattering length of $3.71a_0$ using the effective range formula. In this study, the two positive muons and two electrons are considered in a spin singlet state.

We realize textbook experiments on Bose-Einstein condensate tunneling through thin repulsive potential barriers. In particular, we demonstrate atom tunneling though a single optical barrier in the quantum scattering regime where the de Broglie wavelength of the atoms is larger than the barrier width. Such a beam splitter can be used for atom interferometry and we study the case of two barriers creating an atomic Fabry-Pérot cavity. Technically, the velocity spread of the atoms is reduced due to the use of a ${}^{39}\mathrm{K}$ Bose-Einstein condensate with no interactions. The potential barriers are created optically and their width is tunable due to the use of a digital micromirror device. In addition, our scattering experiments enable in situ characterization of the optical aberrations of the barrier optical system.

Quantum error correction codes, such as noise-biased squeezed-cat qubits, hold promise for preserving quantum coherence and advancing various quantum information technologies. In this paper we demonstrate the stabilization of squeezed-cat qubits through the cooling of a mechanical oscillator. By investigating the bit-flip rate related to two-photon heating processes originating from the mechanical oscillator, we reveal the oscillator's potential as a probe for detecting environmental temperature. Moreover, we observe that the extended bit-flip lifetime of the squeezed-cat qubit significantly enhances temperature detection sensitivity. Finally, we analyze the impacts of single-photon loss and dephasing channels on thermometry.

Nonreciprocal photon blockade is of particular interest due to its potential applications in chiral quantum technologies and topological photonics. In the regular cases, nonreciprocal transmission (classical nonreciprocity) and nonreciprocal photon blockade (quantum nonreciprocity) often appear simultaneously. Nevertheless, how to achieve purely quantum nonreciprocity (no classical nonreciprocity) remains largely unexplored. Here, we propose a spatially separated transmission scheme: that the photons transport in different directions take different paths, in an optical system consisting of two spinning cavities coupled indirectly by two common drop-filter waveguides. Based on the spatially separated transmission scheme, we demonstrate a purely quantum nonreciprocity (nonreciprocal photon blockade) that cannot be observed in the classical transmission spectra. In addition, we find that the nonreciprocal photon blockade is enhanced nonreciprocally, i.e., the nonreciprocal photon blockade is enhanced when the photons transport in one direction but suppressed in the reverse direction. We identify that the nonreciprocal enhancement of nonreciprocal photon blockade is induced by the destructive or constructive interference between two paths for two photons passing through the whole system. The spatially separated transmission scheme proposed in the work provides a different approach to observe purely quantum nonreciprocal effects.

Zero-noise extrapolation (ZNE) stands as the most widespread quantum error mitigation technique used in the recovery of noise-free expectation values of observables of interest by means of noisy intermediate-scale quantum (NISQ) machines. Recently, Otten and Gray proposed a multidimensional generalization of polynomial ZNE for systems where there is no tunable global noise source [M. Otten et al., Phys. Rev. A 99, 012338 (2019)]. Specifically, the authors refer to multiqubit systems where each of the qubits experiences several noise processes with different rates, i.e., a nonidentically distributed noise model. The authors proposed a hypersurface method for mitigating such noise, which is technically correct in the sense that if the required samples are obtained, the observable can be mitigated. However, the proposed method presents an excessive experiment repetition overhead, making it impractical, at least from the perspective of quantum computing. Specifically, we discuss that in order to perform a simple mitigation task to multinomial order 3 considering 100 qubits, there, 2.5 years or over a million quantum processors running in parallel would be required. In this Comment, we show that the traditional extrapolation techniques can be applied for such a nonidentically distributed noise setting consisting of many different noise sources, implying that the measurement overhead is practical. In fact, we discuss that the previous mitigation task can be resolved in the order of minutes using a single quantum processor by means of standard ZNE. To do so, we clarify what it is meant by a tunable global noise source in the context of ZNE, a concept that we consider important to be clarified for a correct understanding of how and why these methods work.