We investigate the entanglement in the ground state of systems comprising two and three qubits with random interactions. Since the Hamiltonians also contain deterministic one-body terms, by varying the interaction strength, one can continuously interpolate between deterministic separable eigenstates and fully random entangled eigenstates, with nontrivial intermediate behavior. Entanglement strongly depends on the underlying topology of the interaction among the qubits. Since W states correspond to a zero-measure set as compared to the set of Greenberger-Horne-Zeilinger (GHZ) states, in all investigated cases the ground states are of the latter type. However, for a certain class of interactions (nonseparable collective potential) high GHZ entanglement is produced, while for fully separable pairwise interactions the marginal GHZ ground states concentrate in the vicinity of W states.

Entanglement is a fundamental feature of quantum physics and a key resource for quantum communication, computing, and sensing. Entangled states are fragile and maintaining coherence is a central challenge in quantum information processing. Nevertheless, entanglement can be generated and stabilized through dissipative processes. In fact, entanglement has been shown to exist in the steady state of certain interacting quantum systems subject solely to incoherent coupling to thermal baths. This has been demonstrated in a range of bi- and multipartite settings using systems of finite dimension. Here we focus on the steady state of infinite-dimensional bosonic systems. Specifically, we consider any set of bosonic modes undergoing excitation-number-preserving interactions of arbitrary strength and divided between an arbitrary number of parties that each couple weakly to thermal baths at different temperatures. We show that a unique steady state is always separable.

Ordering and classifying multipartite quantum states by their entanglement content remains an open problem. One class of highly entangled states, useful in quantum information protocols, the absolutely maximally entangled (AME) ones, are especially hard to compare as all their subsystems are maximally random. While it is well known that there is no AME state of four qubits, many analytical examples and numerically generated ensembles of four-qutrit AME states are known. However, we prove the surprising result that there is truly only one AME state of four qutrits up to local unitary equivalence. In contrast, for larger local dimensions, the number of local unitary classes of AME states is shown to be infinite. Of special interest is the case of local dimension 6, where it was established recently that a four-party AME state does exist, providing a quantum solution to the classically impossible Euler problem of 36 officers. Based on this, an infinity of quantum solutions are constructed, and we prove that these are not equivalent. The methods developed can be usefully generalized to multipartite states of any number of particles.

Pauli-based computation (PBC) is a universal model for quantum computation with qubits where the input state is a magic (resource) state and the computation is driven by a sequence of adaptively chosen and compatible multiqubit Pauli measurements. Here we generalize PBC for odd-prime-dimensional systems and demonstrate its universality. Additionally, we discuss how any qudit-based PBC can be implemented on actual, circuit-based quantum hardware. Our results show that we can translate a PBC on $np$-dimensional qudits to adaptive circuits on $n+1$ qudits with $O\left(p{n}^2/2\right)$ sum gates and depth. Alternatively, we can carry out the same computation with $O\left(pn/2\right)$ depth at the expense of an increased circuit width. Finally, we show that the sampling complexity associated with simulating a number $k$ of virtual qudits is related to the robustness of magic of the input states. Computation of this magic monotone for qutrit and ququint states leads to sampling complexity upper bounds of, respectively, $O\left(3^{1.0848k}{\epsilon }^{-2}\right)$ and $O\left(5^{1.4022k}{\epsilon }^{-2}\right)$, for a desired precision $\epsilon$. We further establish lower bounds to this sampling complexity for qubits, qutrits, and ququints: $\mathrm{\Omega }\left(2^{0.5431k}{\epsilon }^{-2}\right)$, $\mathrm{\Omega }\left(3^{0.7236k}{\epsilon }^{-2}\right)$, and $\mathrm{\Omega }\left(5^{0.8544k}{\epsilon }^{-2}\right)$, respectively.

We present a comprehensive model describing the Kerr nonlinear dynamics of an electric field in a cylindrical microresonator with a small effective radius variation, known as a surface nanoscale axial photonics (SNAP) device. The proposed system of equations for coupled azimuthal modes takes into account full azimuthal dispersion as well as the impact of the radiation source on the microresonator parameters. The model comprises coupling coefficients determined experimentally and appears to be a powerful tool for studying nonlinear effects, including the generation of axial-azimuthal modes and optical frequency comb generation in SNAP devices. We highlight the features of nonlinear dynamics that are specific to the SNAP platform and illustrate the power of the proposed model with optimization of the coupling point of the light source, getting two-orders-of-magnitude improvement for the nonlinear threshold.

We show that the nonlinear stage of the universal phenomenon of modulational instability, and in particular its recurrent behavior, is deeply affected by arbitrarily weak losses. Indeed linear damping leads to spontaneous breaking of the recurrence symmetry through separatrix crossing occurring at multiple critical values of the attenuation, across which the dynamics switches between two types of recurrence exhibiting dramatic slowing down. We provide experimental evidence for this phenomenon in a fiber optics experiment designed in such a way that the effective losses can be carefully tailored by techniques based on Raman amplification.

The quasienergy spectrum recently measured in experiments with a squeeze-driven superconducting Kerr oscillator showed good agreement with the energy spectrum of its corresponding static effective Hamiltonian. The experiments also demonstrated that the dynamics of low-energy states can be explained with the same emergent static effective model. The spectrum exhibits real (avoided) level crossings for specific values of the Hamiltonian parameters, which can then be chosen to suppress (enhance) quantum tunneling. Here we analyze the spectrum and the dynamics of the effective model up to high energies, which should soon be within experimental reach. We show that the parameter values for the crossings, which can be obtained from a semiclassical approach, can also be identified directly from the dynamics. Our analysis of quantum tunneling is done with the effective flux of the Husimi volume of the evolved states between different regions of the phase space. Both initial coherent states and quench dynamics are considered. We argue that the level crossings and their consequences on the dynamics are typical to any quantum system with one degree of freedom, whose density of states presents a local logarithmic divergence and a local step discontinuity.

In the following paper, we discuss a possible detection of nonlocality in two-mode light states in the Bell protocol, where the local observables are constructed using displacement operators, implemented by Mach-Zehnder interferometers fed by strong coherent states. We report numerical results showing that maximizing the Braunstein-Caves chained Bell inequalities requires equal phases of displacements. On the other hand, we prove that nonlocality cannot be detected if the phases of displacements are unknown. Hence, the Bell experiment has to be equipped with a synchronization mechanism. We discuss such a mechanism and its consequences.

Noiseless linear amplification (NLA) is useful for a wide variety of quantum protocols. Here we propose a fully scalable amplifier which, for asymptotically large sizes, can perform perfect fidelity NLA on any quantum state. Given finite resources, however, it is designed to perform perfect fidelity NLA on coherent states and their arbitrary superpositions. Our scheme is a generalization of the multiphoton quantum scissor teleamplifier, which we implement using a coherent-state superposition resource state. Furthermore, we prove our NLA is also a loss-tolerant relay for multiary phase-shift keyed coherent states. Finally, we demonstrate that our NLA is also useful for continuous-variable entanglement distillation, even with realistic experimental imperfections.

Quantum channel capacity is a fundamental quantity in order to understand how well quantum information can be transmitted or corrected when subjected to noise. However, it is generally not known how to compute such quantities since the quantum channel coherent information is not additive for all channels, implying that it must be maximized over an unbounded number of channel uses. This leads to the phenomenon known as superadditivity, which refers to the fact that the regularized coherent information of $n$ channel uses exceeds one-shot coherent information. In this article, we study how the gain in quantum capacity of qudit depolarizing channels relates to the dimension of the considered systems. We make use of an argument based on the no-cloning bound in order to prove that the possible superadditive effects decrease as a function of the dimension for such family of channels. In addition, we prove that the capacity of the qudit depolarizing channel coincides with the coherent information when $d\to \infty$. We also discuss the private classical capacity and obtain similar results. We conclude that when high-dimensional qudits experiencing depolarizing noise are considered, the coherent information of the channel is not only an achievable rate, but essentially the maximum possible rate for any quantum block code.

We study a recently introduced simple method [S. Jin, N. Liu, and Y. Yu, Quantum simulation of partial differential equations via Schrödingerisation, arXiv:2212.13969] for solving general linear partial differential equations with quantum simulation. This method converts linear partial differential equations into a Hamiltonian system, using a simple transformation called the warped phase transformation. Here we provide a more-in-depth technical discussion and expand on this approach in a more detailed and pedagogical way. We apply this to examples of partial differential equations, including heat, convection, Fokker-Planck, linear Boltzmann, and Black-Scholes equations. This approach can also be extended to general linear partial differential equations, including the Vlasov-Fokker-Planck equation and the Liouville representation equation for nonlinear ordinary differential equations. Extension to higher-order time derivatives is also possible.

For twin-field quantum key distribution (TF-QKD), it is commonly known that erroneous time synchronization leads to decoherence of the twin fields, compromising the performance of the TF-QKD. However, more quantitative information about such an effect still needs further revelation in order to understand the detailed requirement of time synchronization in TF-QKD and to further optimize the TF-QKD system for better performance. Here, we provide an analytical model describing TF-QKD with time-synchronization errors, choosing phase-matching quantum key distribution for our simulation. The model is constructed by a relation between the fringe visibility of the interferometry and the time-synchronization error described by the time difference between the arrival of twin-field signals from both users. By a numerical simulation, we demonstrate that our model can dramatically improve the tolerance of the system against the time-synchronization error, achieving a much higher secure key rate and communication distance with inaccurate time synchronization. Through this work, we intend to start a further discussion on the synchronization demand of TF-QKD.

Shell effects in the differential cross section of fast charged particle scattering on an atom are studied as deviations of the Hartree-Fock mean atomic potential predictions from those for the Thomas-Fermi atom. Significant deviations of that kind are found at moderate momentum transfers, where they are caused by intermediate atomic shells and regularly oscillate with $Z$, and also for hard-scattering observables, where they are caused by inner shells (relativistic at high $Z$) and depend on $Z$ monotonically. In application to multiple Coulomb scattering in thick targets, shell corrections to the Molière screening angle are calculated. They are found to pick up commensurable contributions from inner and outer shells, leading to superposed monotonic and oscillatory deviations from the Thomas-Fermi prediction.

The accelerated optical lattice has emerged as a valuable technique for the investigation of quantum transport physics and has found widespread application in quantum sensing, including atomic gravimeters and atomic gyroscopes. In our study, we focus on the adiabatic evolution of ultracold atoms within an accelerated optical lattice. Specifically, we derive a time bound that delimits the duration of atomic adiabatic evolution in the oscillating system under consideration. To experimentally substantiate the theoretical predictions, precise measurements to instantaneous band populations were conducted within a one-dimensional accelerated optical lattice, encompassing systematic variations in both lattices' depths and accelerations. The obtained experimental results demonstrate a quantitatively consistent correspondence with the anticipated theoretical expressions. Afterwards, the atomic velocity distributions are also measured to compare with the time bound. This research offers a quantitative framework for the selection of parameters that ensure atom trapped throughout the acceleration process. Moreover, it contributes an experimental criterion by which to assess the adequacy of adiabatic conditions in an oscillating system, thereby augmenting the current understanding of these systems from a theoretical perspective.

The speed-dependent spectral line profiles collapse into a simple Lorentz profile in the regime dominated by the velocity-changing collisions. We derive general formulas for the effective width and shift of the Lorentzian for arbitrary speed-dependent collisional broadening and shift and velocity-changing collision operators. For a quadratic speed dependence of collisional broadening and shift, and the billiard ball model of velocity-changing collisions, we provide simple analytical expressions for the effective Lorentzian width and shift. We show that the effective Lorentzian width and shift split into components originating from the well-known Dicke-narrowed Doppler width, speed-averaged collisional broadening and shift, their speed dependencies, and a product term that mixes the contributions of the broadening and shift speed dependencies. We show how the components depend on rates of speed-changing and velocity-changing collisions related to the perturber-absorber mass ratio. We validate analytical formulas numerically on the example of ${\mathrm{H}}_2$ transition perturbed by He.

We report here a portable and robust home-built atom gravimeter (USTC-AG11) continuously working in a seismic station in Zhaotong, Yunnan, for over five months. Based on the principle of matter-wave interference, the atom gravimeter is very sensitive to the local gravity and reaches the precision of micro-Gal ($1\times {10}^{-8}\mathrm{m}/{\mathrm{s}}^2$) level. With the technique of vibrational compensation, the sensitivity of the atom gravimeter reaches $38\mu \mathrm{Gal}/\sqrt{\mathrm{Hz}}$, the overall noise level is suppressed by 80% compared to the no-vibration-compensation method, and 95% of the vibration noise is effectively suppressed. The design of the atom gravimeter in the electronics and laser optics, especially the laser frequency and phase auto-relock technology, guarantees long-term continuous running. The long-term precision of the atom gravimeters is better than $2\mu \mathrm{Gal}$, which is comparable to the best classical gravimeter FG-5(X). Our work provides an application for the high-precision atomic gravimeter based on modern quantum sensing technology in the field of geophysics and geodesy survey.

In this study, molecular-dynamics simulations were conducted to investigate the relaxation of the internal energy in nanosized particles and its impact on the nucleation of atomic clusters. Accurate binary potentials were utilized to analyze the growth and collisional relaxation of the internal energy of ${\mathrm{Ar}}_n{\mathrm{H}}^{+}$ clusters in a metastable Ar gas. The results revealed that small nanoclusters are formed in highly excited rotational-vibrational states and that the relaxation of internal energy and growth of these nascent clusters are concurrent processes with a strong mutual influence. Under nonequilibrium growth conditions, the relaxation of internal energy can delay the cluster growth process. The rates of cluster growth and internal energy relaxation were found to be influenced by the energy-transfer collisions between cluster particles and free Ar atoms of the bath gas. Furthermore, the nonequilibrium growth and internal energy relaxation of small nanoclusters were found to depend on the structure of the cluster's atomic shells. An ensemble of molecular-dynamics simulations was conducted to investigate the growth and time evolution of the kinetic and total energies of ${\mathrm{Ar}}_n{\mathrm{H}}^{+}$ clusters with specified $n\le 11$, and the results were explained by collisional relaxation processes described by the Boltzmann equation. Finally, the general relationship between the rates of internal energy relaxation and nonequilibrium growth of nanoparticles is discussed.

We consider theoretically the mechanisms to realize antibunching between photons scattered on an array of two-level atoms in a general electromagnetic environment. Our goal is antibunching that persists for times much longer than the spontaneous emission lifetime of an individual atom. We identify two mechanisms for such persistent antibunching. The first one is based on subradiant states of the atomic array and the second one does not require any subradiant states. We provide two specific examples of array parameters with optimized antibunching, based on an array in a free space and an array coupled to a waveguide.

A definitive theory for the optical or electromagnetic force density in condensed matter has remained elusive. Integral in such a theory is how material interfaces are treated. To build collective understanding, a general planar interface situation between vacuum and a semi-infinite, nonmagnetizable material medium is studied analytically using two electromagnetic force density formulations and an obliquely incident, $p$-polarized plane wave impinging on the material surface. A form of the Lorentz electromagnetic force density formulation and the Einstein-Laub electromagnetic force density are shown to both predict a total pressure that agrees with that predicted by the so-called “Maxwell-Bartoli” expression modified for oblique incidence. However, the two formulations present different distributions of the total force between the surface and bulk of the material. The quantification of this difference and of the implied outward surface pressure for this broadly applicable field geometry offers opportunities for the experimental determination of a force density theory that accurately predicts experiment.