We study partial coherence and its connection with entanglement. First, we provide a sufficient and necessary condition for bipartite pure-state transformation under partially incoherent operations: A bipartite pure state can be transformed to another one if and only if a majorization relationship holds between their partial coherence vectors. As a consequence, we introduce the concept of maximal partial coherent states in the sense that they can be used to construct any bipartite state of the same system via partial incoherent operations. Second, we provide a strategy to construct measures of partial coherence by use of symmetric concave functions. Third, we establish the relationship between partial coherence and entanglement. We show that the minimal partial coherence under local unitary transformations is also a measure of entanglement for bipartite pure states, which can be extended to all mixed states by convex-roof extension. Conversely, every entanglement measure can be constructed as a partial coherence measure, and we further show that partial coherence measures are induced through maximal entanglement under partially incoherent operations for bipartite pure states.

Catalysts used in quantum resource theories need not be in isolation and, therefore, are possibly correlated with external systems, which the agent does not have access to. Do such correlations help or hinder catalysis, and does the classicality or quantumness of such correlations matter? To answer this question, we first focus on the existence of a noninvasively measurable observable that yields the same outcomes for repeated measurements since this signifies macrorealism, a key property distinguishing classical systems from quantum systems. We show that a system quantumly correlated with an external system so that the joint state is necessarily perturbed by any repeatable quantum measurement also has the same property against general quantum channels. Our full characterization of such systems called totally quantum systems, solves the open problem of characterizing tomographically sensitive systems raised in [Phys. Rev. Lett. 130, 020802 (2023)]. An immediate consequence is that a totally quantum system cannot catalyze any quantum process, even when a measure of correlation with its environment is arbitrarily low. It generalizes to a stronger result, that the mutual information of totally quantum systems cannot be used as a catalyst either. These results culminate in the conclusion that, out of the correlations, that a generic quantum catalyst has with its environment, only classical correlations allow for catalysis, and, therefore, using a correlated catalyst is equivalent to using an ensemble of uncorrelated catalysts.

A squeezed-displaced state can be used as an optical signal for efficient binary quantum optical communication. Here, we consider quantum optical communication where an atomic indirect measurement is used to decide the message encoded in a binary phase-shift-keying signal composed of squeezed-displaced states, and show that the incorrect decision probability of an atomic indirect measurement can be less than that of a homodyne measurement. We further show that an atomic indirect measurement can lower the incorrect decision probability even when the quantum channel is exposed to weak phase-diffusion noise.

We analyze the generation of entanglement in a multipartite optical network. We generalize the twin-field strategy to the multipartite case and show that our protocol has advantageous rate-loss scalings of distributing $W$ states and Dicke states over the star networks. We provide precise theoretical formulas and quantitative performance analyses. Also, analysis of the same protocol using Gaussian states as resources, which is a typical setup in many experimental tests, is provided. We numerically study the feasibility of the protocol in realistic experimental conditions with imperfect detectors. We observe that the advantage depends on the chosen fidelity and is affected by the dark counts of the detector.

We study near-threshold atomic photoionization in the presence of an external uniform static electric field, where the two-dimensional flux of the produced slow (meV) photoelectrons is projected on a position sensitive detector (PSD). The ionized highly excited Stark states depend sensitively on the static field and, consequently, its presence is an essential ingredient for properly interpreting the recorded images, particularly the influence of laser polarization. We illustrate this by employing a two-photon ionization scheme and by obtaining images for different angles between the linear ionizing-laser polarization and the field direction, the latter adopted as the quantization axis. It is shown that, given the $m=0$ magnetic quantum number of the initial (ground) state, such a scheme allows for the population of $\left|m\right|=0,1$, and 2 final Stark states with $\left|m\right|$-excitation weights depending on the aforementioned angles. By referencing dipole excitation selection rules to the chosen quantization axis, hydrogenic calculations accurately reproduce this multiple-$\left|m\right|$ presence and $\left|m\right|$-beating effects witnessed in the experimental threshold photoelectron images of the Mg atom. It is, furthermore, shown that a set of total ionization cross sections recorded at selected angles between light polarization and field direction can lead to the decomposition into their $\left|m\right|$ contributions. These observations demonstrate the ability to manipulate the $\left|m\right|$ composition and consequently the shape of outgoing slow electron Stark wave packets by means of laser polarization rotation. On a more practical level, emphasis is given on their relevance to tomographic techniques realized on a quite similar apparatus and frequently used for the reconstruction of three-dimensional momentum distributions using a set of two-dimensional projections on the PSD. We show that, since these techniques rely on the assumption that the sole role of the static electric field is to merely project photoelectrons on the PSD, they are simply not applicable under conditions where the excited state is influenced by this field.

We describe a unified numerical model which allows fast and accurate simulation of nonlinear light propagation in nanoparticle composites, including various effects such as group velocity dispersion, second- and third-order nonlinearity, quasi-free-carrier formation and plasma contributions, exciton dynamics, scattering, and so on. A developed software package, Simulator of Light Propagation in Composites (solpic), is made available for the community. Using this model, we analyze and optimize efficient generation of terahertz (THz) radiation by two-color pulses in ZnO–fused-silica composite, predicting an efficiency of 3%. We compare the role of various nonlinear effects contributing to the frequency conversion and show that the optimum conditions of THz generation differ from those expected intuitively.

We study the Markovian and non-Markovian dynamics in a giant atom system which couples to a coupled resonator waveguide (CRW) via two distant sites. Under certain conditions, we find that the giant atom population can exhibit an oscillating behavior and the photon can be trapped in the giant atom regime. These phenomena are induced by the interference effect among the bound states both in and outside the continuum, which is peculiar for two-site atom-CRW coupling. As an application of the photon trapping, we theoretically propose a magic cavity model where the giant atom serves as either a perfect or leaky cavity, depending on the distance between the coupling sites. The controllability of the magic cavity from a perfect to a leaky one cannot be realized in the traditional cavity or circuit QED setup. The predicted effects can be probed in state-of-the-art waveguide QED experiments and provide a striking example of how the different kinds of bound states modify the dynamics of a quantum open system in a structured environment.

Efficiently detecting entanglement based on measurable quantities is a basic problem for quantum information processing. Recently, the measurable quantities called partial-transpose (PT) moments have been proposed to detect and characterize entanglement. Recently [L. Zhang et al., Ann. Phys. (Berlin) 534, 2200289 (2022)], we have already identified the two-dimensional region, comprised of the second and third PT moments, corresponding to two-qubit entangled states, and described the whole region for all two-qubit states. In the present paper, we visualize the three-dimensional (3D) region corresponding to all two-qubit states by further involving the fourth PT moment (the last one for two-qubit states). The characterization of this 3D region can finally be achieved by optimizing some polynomials. Furthermore, we identify the dividing surface which separates the two parts of the whole 3D region corresponding to entangled and separable states, respectively. Due to the measurability of PT moments, we obtain a complete and operational criterion for the detection of two-qubit entanglement.

The Ising model is a widely studied class of models in quantum computation. In this paper, we investigate the computational characteristics of the random-field Ising model (RFIM) with long-range interactions that decays as an inverse polynomial of distance, which can be achieved in the current ion trap system. We prove that for an RFIM with long-range interaction embedded on a two-dimensional plane, solving its ground state is NP-complete for all diminishing exponents, and prove that the one-dimensional RFIM with long-range interaction can be efficiently approximated when the interaction decays fast enough.

A simple expansion of the scattering length in terms of $S$-matrix poles is derived for the angular momenta $l>0$. This expression shows that the dominant role in the low-energy collisions is played by the poles lying close to the origin of the complex momentum plane. Among these poles a peculiar class of virtual states is found to respond very weakly to the scaled perturbation potentials and thus resistant to producing trajectories in the complex momentum plane. Properties and an impact on low-energy collisions of these resistant virtual states are discussed.

In this study, we investigate the nuclear dynamics in nitrogen monoxide after valence ionization by a pump pulse and subsequent probing with a time-delayed x-ray pulse. We calculate the development of the resulting vibrational wave packet, taking into account three different ionization mechanisms: one-photon, multiphoton, and tunneling ionization. Using a two-time propagation method, we solve the nonstationary nuclear Schrödinger equation to obtain time-resolved x-ray absorption spectra (TRXAS), considering the finite duration of the probe pulse. Our simulations show that the TRXAS profile accurately reflects the vibrational wave packets' trajectory in the cationic ground state. Additionally, we find that the TRXAS evolution is highly sensitive to small changes in the probed potential energy curve, making it a useful tool for reconstructing molecular potentials and determining anharmonicity and equilibrium bond length. This method can be applied to other polyatomic molecules and pump mechanisms.

Atomic clocks provide a reproducible basis for our understanding of time and frequency. Recent demonstrations of compact optical clocks, employing thermal atomic beams, have achieved short-term fractional frequency instabilities of order ${10}^{-16}$, competitive with the best international frequency standards available. However, a serious challenge inherent in compact clocks is the necessarily smaller optical beams, which results in rapid variation in interrogating wave fronts. This can cause inhomogeneous excitation of the thermal beam leading to long-term drifts in the output frequency. Here we develop a model for Ramsey-Bordé interferometry using optical fields with curved wave fronts and simulate the ${}^{40}\mathrm{Ca}$ beam clock experiment described by Olson et al. [Phys. Rev. Lett. 123, 073202 (2019)]. The results of Olson et al. showed surprising and unexplained behavior in the response of the atoms in the interrogation. Our model predicts signals consistent with experimental data and can account for the significant sensitivity to laser geometry that was reported. We find the signal-to-noise ratio is maximized when the laser is uncollimated at the interrogation zones to minimize inhomogeneity and also identify an optimal waist size determined by both laser inhomogeneity and the velocity distribution of the atomic beam. We investigate the shifts and stability of the clock frequency, showing that the Gouy phase is the primary source of frequency variations arising from laser geometry.

The quantization axis defines the local interaction between the electromagnetic fields and the atoms. We studied both theoretically and experimentally the hyperfine Raman transition with different quantization axis and found a four-photon Raman transition which happened when the quantization axis is shifting from being parallel to perpendicular to the Raman beams' direction. It keeps the magnetic-field-insensitive feature like the two-photon Raman transition but can be used straightforwardly in multidimensional applications, like inertial sensors or even higher-dimensional quantum walk systems. In addition to inertial sensors, we also discuss the potential quantum sensing applications in magnetometers when the quantization axis is away from perpendicular due to magnetic-field perturbations.

Using the single-site Gutzwiller method, we theoretically study the ground state and the interspecies entanglement properties of interexchange symmetric multicomponent (two- and three-component) bosonic mixtures in an optical lattice, and the results are generalized to an $n$-component $(n=2,3,4,...)$ system. We compute the mean-field phase diagram, the interspecies entanglement entropy, and the ground-state spectral decomposition. Three phases—namely, the $n$-component superfluid state (nSF), the $n$-component Mott insulator state (nMI), and the supercounterfluid state (SCF)—are observed. Interestingly, we find that there are $n-1$ SCF lobes to separate every two neighboring nMI lobes in the phase diagram. More importantly, we derive the exact general expression of the interspecies entanglement entropy for the SCF phase. In addition, we also investigate the demixing effect of an $n$-component mixture and demonstrate that the mixing-demixing critical point is independent of $n$.

Recently, thulium has been condensed to Bose-Einstein condensate. While the condensate of Thulium has a lot of applications in quantum simulations and other areas of physics, it can also serve as a unique diagnostic tool for many atomic experiments. In the present study, the Bose-Einstein condensate of Thulium was successfully loaded into an optical lattice and utilized to diagnose the lattice as well as detect unwanted reflections in the experiments with the 1064 nm optical lattice, which will further be used in a quantum gas microscope experiment.

We experimentally observe longer than long trajectory influence in high-order harmonic generation (HHG) by varying the peak intensity of the driving laser field either through direct attenuation or by chirping the laser pulse. Using a theoretical Gaussian beam model to simulate spatial interference resulting from quantum path interference, we show that the measured interference patterns cannot be solely explained by the well-established short and long trajectories. The structure change is most prominent for the more divergent, off-axis components of the lower plateau harmonic region, affecting the direction and amplitude of the extreme ultraviolet light emitted, and is thus of importance for understanding and controlling the fundamentals of the HHG process.

Characterizing the most efficient evolution, the quantum speed limit (QSL) plays a significant role in quantum technology. How to generalize the well-established QSL from closed systems to open systems has attracted much attention. In contrast to the previous schemes to derive the QSL from the reduced dynamics of open systems, we propose a QSL bound from the point of view of the total system consisting of the open system and its environment using a quantum-state-diffusion method. The application of our scheme to a two-level system reveals that the system possesses an infinite speedup capacity in the noiseless case, which is destroyed by the environment under the Born-Markovian approximation. It is interesting to find that the capacity in the noiseless case is recovered in the non-Markovian dynamics as long as a bound state is formed in the energy spectrum of the total system. Enriching the characterization schemes of the QSL, our result provides an efficient way to control the QSL of open systems.

The performance of a quantum information processing protocol is ultimately judged by distinguishability measures that quantify how distinguishable the actual result of the protocol is from the ideal case. The most prominent distinguishability measures are those based on the fidelity and trace distance, due to their physical interpretations. In this paper we propose and review several algorithms for estimating distinguishability measures based on trace distance and fidelity. The algorithms can be used for distinguishing quantum states, channels, and strategies (the last also known in the literature as “quantum combs”). The fidelity-based algorithms offer physical interpretations of these distinguishability measures in terms of the maximum probability with which a single prover (or competing provers) can convince a verifier to accept the outcome of an associated computation. We simulate many of these algorithms by using a variational approach with parameterized quantum circuits. We find that the simulations converge well in both the noiseless and noisy scenarios for all examples considered. Furthermore, the noisy simulations exhibit a parameter noise resilience. Finally, we establish a strong relationship between various quantum computational complexity classes and distance estimation problems.