Continuous-variable quantum key distribution (CV-QKD) enables the efficient distribution of unconditionally secure keys for both communicating parties. Despite significant advancements in optical-based CV-QKD, a persistent challenge is the limited resilience of optical quantum signals to atmospheric noise in free-space channels. This susceptibility leads to considerable degradation in the performance of optical CV-QKD systems, particularly under adverse climatic conditions. Meanwhile, recent advancements in superconducting quantum computing have driven the demand for quantum communication between spatially separated superconducting processors operating at microwave frequencies. To solve the problems, we propose a Gaussian-modulated coherent-state protocol operating at microwave frequencies, utilizing traveling-wave parametric amplifiers to generate entangled states. Our numerical analysis demonstrates that CV-QKD at microwave frequencies enables secure communication in a free-space channel at room temperature. Furthermore, we show that this protocol outperforms the THz frequency protocol over a distance of 20 m and exhibits stronger immunity to interference. Finally, we consider the impact of microwave antenna gain on the protocol's performance, highlighting the decisive role of antenna gain in determining the maximum transmission distance in practical application scenarios.

The typical bound on parameter estimation, known as the standard quantum limit (SQL), can be surpassed by exploiting quantum resources such as entanglement. To estimate the magnetic probe field, we propose a quantum sensor based on a variable-range many-body quantum spin chain with a moderate transverse magnetic field. We report the threefold benefits of employing a long-range system as a quantum sensor. First, sensors with quasi-long-range interactions can always beat the SQL for all values of the coordination number, while a sensor with long-range interactions does not have this ubiquitous quantum advantage. Second, a long-range Hamiltonian outperforms a nearest-neighbor (NN) Hamiltonian in terms of both estimating precision and system-size scaling. Finally, we observe that the system with long-range interactions can go below the SQL in the presence of a high temperature of the initial state, while sensors having NN interactions cannot. Furthermore, a sensor based on the long-range Ising Hamiltonian proves to be robust against impurities in the magnetic field and when the time-inhomogeneous dephasing noise acts during interaction of the probe with the system.

For optical phase estimation via homodyne measurement, we generalize the theory from detector's linear to nonlinear response regime, which accounts for the presence of saturation effect. For optical coherent light, we carry out analytic expressions for detector's current and estimate precision. Using specific device parameters, we illustrate the improved estimation after accounting for the saturation effect.

The enhancement of electron-ion radiative-recombination rates at low energies remains an open issue, particularly following the proposal of field-induced recombination (FIR) as a potential mechanism and the corresponding calculations by Hörndl et al. [Phys. Rev. A 74, 052712 (2006) ]. We present a refined model for FIR and radiative stabilization, incorporating a two-step cascade process. Our simulations determine the bound-state distribution $P(n,l)$, revealing a turning point at low principal quantum numbers that underscores the limitations of previous extrapolations. Furthermore, we show that in regions with high $n$ and low $l$, $P(n,l)$ exceeds the statistical approximation, $P(n,l)=P\left(n\right)\frac{2l+1}{n^2}$, due to the influence of a magnetic field. With an expanded sample size of at least $1\times {10}^{11}$ trajectories, our model accurately estimates recombination rates for bare ions, including ${\mathrm{He}}^{2+}$, ${\mathrm{N}}^{7+}$, ${\mathrm{C}}^{6+}$, ${\mathrm{Ne}}^{10+}$, ${\mathrm{Si}}^{14+}$, and ${\mathrm{Cl}}^{17+}$, achieving excellent agreement with experimental data.

Photon subtraction (PS) can enhance entanglement, which, for pure states, induces a decrease in the purity of reduced states. In contrast, by analyzing the purities of Gaussian states before and after subtracting or adding a single photon, we prove that the purity of a Gaussian state can be increased by less than 20%. On the one hand, it reveals that PS and photon addition (PA) can reduce entanglement, and on the other hand, it shows that PS or PA can achieve a limited amount of Gaussian state purification. Through the analysis of several examples, we demonstrate the inherent mechanism and applicable scope of photon-subtraction-based purification. We further compare the performance between PS and PA for purifying single-mode Gaussian states, and analyze their capabilities in generating Wigner negativity within purification regimes. In a multimode system, we find that PS can simultaneously increase entanglement and purify some of the reduced states. These findings demonstrate the new application potential of PS and PA, specifically their capability to directly suppress Gaussian noise.

Quantum continuous measurement strategies are an essential element in many modern sensing technologies, leading to potentially enhanced estimation of unknown physical parameters. In such schemes, continuous monitoring of the quantum system, which encodes the parameters of interest, gives rise to different quantum trajectories conditioned on the measurement outcomes which carry information on the parameters themselves. Importantly, different trajectories carry different amounts of information, i.e., they are more or less sensitive to the unknown parameters of interest. In this work, we propose an approach on how to systematically engineer the quantum open-system dynamics in order to increase the probability of obtaining trajectories of high sensitivity. We focus on the simplest case scenario of a single two-level system interacting with ancillas, which are in turn measured using the discretized version of continuous monitoring. We analyze the performance of our protocol and demonstrate that it may lead to a substantial enhancement of sensitivity, as quantified by the classical Fisher information, even when applied to such small system sizes, holding the promise of direct implementation in state-of-the-art experimental platforms and large, complex many-body systems.

In hybrid quantum systems, the interface between quantum and classical domains is essential for the generation, control, and measurement of quantum states. Quantum-classical interfaces (QCIs) are ubiquitous in devices such as optical modulators, quantum sensors, and signal processors, where classical signals influence quantum dynamics. In this paper, we employ the Lindblad master equation to simulate the evolution of a quantum system interacting with a classical control system. Our model captures both linear and nonlinear interactions by incorporating first- and second-order susceptibilities, and it quantifies the influence of externally applied control parameters on decoherence and state evolution. As an illustrative example, we analyze an optical modulator and demonstrate how variations in material response and drive conditions affect photon statistics, coherence, and phase-space distributions. The findings offer a path to an all-encompassing model for understanding and optimizing QCIs, with wide-ranging implications for the performance, design, and robustness of next-generation quantum devices.

The study of measurements in quantum mechanics exposes many of the ways in which the quantum world is different. For example, one of the hallmarks of quantum mechanics is that observables may be incompatible, implying among other things that it is not always possible to find joint probability distributions which fully capture the joint statistics of multiple measurements. Instead, one must employ more general tools such as the Kirkwood-Dirac quasiprobability (KDQ) distribution, which may exhibit negative or nonreal values heralding nonclassicality. In this paper, we consider the KDQ distributions describing arbitrary collections of measurements on disjoint components of some generic multipartite system. We show that the system dynamics ensures that these distributions are classical if and only if the Hamiltonian supports quantum Darwinism. Thus, we demonstrate a fundamental relationship between these two notions of classicality and their emergence in the quantum world.

Quantum processors operated through long-range interaction mediated by a microwave resonator have been envisioned to allow for high connectivity. The ability to selectively operate qubits rely on the possibility to dynamically suppress the coupling between each qubit and the resonator, however, there always remains a residual coupling. In this article, we investigate the effect of high processor connectivity on average two-qubit gate fidelity in a cavity-based architecture with tunable coupling. Via a perturbative approach, we quantify the crosstalk errors from transverse residual couplings and show that they scale as $n{m}^2$ where $n$ is the number of idle qubits and $m$ is the ratio between the transverse residual and active couplings. Setting an error threshold $E_{\mathrm{thr}}$, we demonstrate that crosstalks restrict the hardware topology and prevent the full use of all-to-all connectivity. We predict that the maximum number of qubits allowed by $E_{\mathrm{thr}}$ scales as $n\propto E_{\mathrm{thr}}/m^2$.

Adiabatic quantum computing has demonstrated how the quantum Zeno effect can be used to construct quantum optimizers. However, much less work has been done to understand how more general Zeno effects could be used in a similar setting. We use a construction based on three-state systems rather than directly in qubits, so that a qubit can remain after projecting out one of the states. We find that our model of computing is able to recover the dynamics of a transverse field Ising model. Several generalizations are possible, but our methods allow for constraints to be implemented nonperturbatively and do not need tunable couplers, unlike simple transverse field implementations. We further discuss how to implement the protocol physically using methods building on stimulated Raman adiabatic passage (STIRAP) protocols for state transfer. We find a substantial challenge that settings defined exclusively by measurement or dissipative Zeno effects do not allow for frustration, and in these settings pathological spectral features arise leading to unfavorable runtime scaling. We discuss methods to overcome this challenge, for example including gain as well as loss as is often done in an optical setting.

Efficient and systematic numerical methods for robust control design are crucial in quantum systems due to inevitable uncertainties or disturbances. We propose an approach that models uncertainties as random variables and quantifies robustness using the expectation of infidelity by reformulating it as a weighted tensor product quadrature. We employ the Smolyak algorithm to develop a parametric robust quantum control scheme, which balances the reduction of computational cost with the enhancement of estimation accuracy. We demonstrate the effectiveness of our proposed algorithm by incorporating the Smolyak sparse grids into conventional gradient-based quantum optimal control methods such as grape and goat. In robust control problems concerning quantum gate realization, low infidelity and strong robustness can be achieved. These results contribute to improving the reliability and security of quantum computing and communication systems in the presence of real-world imperfections.

We explore the utilization of higher-order discretization techniques in optimizing the gate count needed for quantum computer based solutions of partial differential equations. To accomplish this, we present an efficient approach for decomposing $d$-band diagonal matrices into Pauli strings that are grouped into mutually commuting sets. Using numerical simulations of the one-dimensional wave equation, we show that higher-order methods can reduce the number of qubits necessary for discretization, similar to the classical case, although they do not decrease the number of Trotter steps needed to preserve solution accuracy. This result has important consequences for the practical application of quantum algorithms based on Hamiltonian evolution.

Classical trajectory Monte Carlo calculations for fast positron and electron impact ionization of argon are used to provide information about how the pre- and postcollision kinematics alter the triply differential spectra. It is shown that in addition to the scattering direction expected for a positive or negative projectile passing near a partially screened nuclear charge, scattering in the opposite direction also occurs. Independent of energy transfer, when the impact parameter is less than the mean radius of the $3p$ electron cloud, the expected direction of scattering always occurs. In contrast, for scattering in the opposite direction, the impact parameter is larger than the mean $3p$ radius for low energy transfer but decreases to values near the $3p$ radius for large energy transfer. The change in impact parameter and direction of scattering are shown to influence the recoil to binary triply differential cross-section ratios and explains the unexpectantly large ratios that have been observed at higher emission energies and may explain the larger ratio observed for electron impact.

The complex physics of inner-shell ionization of target atoms by heavy-ion impact has remained only partially solved for decades. Recently, agreement between theory and experiment has been achieved by considering inner-shell ionization of target atoms due to projectile electron capture in addition to direct Coulomb ionization including multiple ionization effects. A thorough investigation exhibits such a picture only if the atomic parameters of the target atoms are correct. In fact, the theoretical approach is found to be right, but the problem arises with the faulty atomic parameters. Furthermore, we show that fluorescence yields play a major role among the atomic parameters. We explore such a powerful method that enables us to measure the correct and accurate fluorescence yields for almost every element in the periodic table. As per our present knowledge, this, in turn, not only solves the said complex issue fully but also makes the particle/proton-induced x-ray emission analysis more reliable and accurate using both light and heavy ions.

We have formulated a threshold intensity $I_{\mathrm{th}}$ for the light excitation required to amplify the photoluminescent (PL) intensity confined in organic whispering-gallery-mode (WGM) resonators. The parameters included in the formula comprise the propagation loss, the resonator thickness, the energy of the light, the light confinement factor, the stimulated emission cross section, the PL quantum yield, and the fraction of the exciton energy that can be afforded to generate the stimulated emission for each PL peak. To compare the theoretical $I_{\mathrm{th}}$ with the experimental $I_{\mathrm{th}}$, we fabricated 300-nm-thick disk-shaped WGM resonators from the highly efficient blue laser molecule 2,7-bis[9,9-di(4-methylphenyl)-fluoren-2-yl]-9,9-di(4-methylphenyl)fluorene (terfluorene) on 1-µm-high glass microdisks with diameters ranging from 3 to 10 µm. The theoretical $I_{\mathrm{th}}$ replicated the experimentally obtained values, thus demonstrating the validity of the proposed formula. The $I_{\mathrm{th}}$ value in the WGM resonators was three orders of magnitude higher than the corresponding value reported for a simple slab-type waveguide that was fabricated from terfluorene. We assessed the mechanism that causes the high $I_{\mathrm{th}}$ using the proposed formula and found that the high $I_{\mathrm{th}}$ value mainly originated from attenuation of the confined PL emission that occurred during orbiting within the WGM resonators. This result suggests that the formula will be beneficial in clarifying the dominant factor that governs $I_{\mathrm{th}}$.

In this work, we utilize a two-level atom and a $\mathrm{\Lambda }$-type atom to link two identical waveguides, subsequently extending the model to a giant-atom configuration. Our analytical solutions and numerical simulations demonstrate that this setup can achieve single-photon isolation and nonreciprocal frequency conversion by tuning the atom-waveguide coupling strengths $g_i$, respectively. We also examine single-photon scattering in the giant-atom model within both the Markovian and non-Markovian regimes. The results reveal that ultranarrow scattering windows are induced by the phases ${\varphi }_1$ and ${\varphi }_2$ under specific conditions, making them well suited for precise frequency conversion and sensing. Additionally, in the non-Markovian regime, the spectra exhibit irregular polygonal shapes, offering enhanced opportunities for exploring nonreciprocal frequency conversion in the off-resonant regime. Our work provides an alternative perspective on achieving optical nonreciprocity at the single-photon level in atom-waveguide systems.

Entanglement and squeezing are two useful resources in quantum technologies. Recent studies have increasingly focused on generating these nonclassical features in macroscopic objects. However, establishing entanglement and squeezing in the degrees of freedom of mechanical motion of macroscopic objects is generally hindered by thermal fluctuations of their environments. Here, we propose a scheme to generate steady-state squeezing and entanglement between two rotating mirrors of a Laguerre-Gaussian cavity by quantum squeezed-reservoir engineering. It is revealed that, by injecting a broadband squeezed light into the cavity, stable squeezing in the relative momentum and entanglement between the two rotating mirrors are created. Compared to the previous work where only the entanglement between the two rotating mirrors appears under the condition of different angular frequencies, our result highlights the stable entanglement of the two mirrors with identical frequency and the simultaneous presence of the entanglement and squeezing of the two mirrors. Our finding deepens our understanding of macroscopic quantum phenomena of Laguerre-Gaussian cavity optomechanical system.