We introduce a phase-contrast ghost-imaging scheme for the characterization of temporal phase objects in terms of intensity correlations at two photodetectors. The technique is analogous to Zernike's phase-contrast imaging method and is based on utilizing a suitable filter function which renders the small-amplitude phase variations visible in the intensity correlation function. The approach is insensitive to temporal distortions and offers a promising method to analyze the phases of optical pulses.

Entanglement entropy is an essential metric for characterizing quantum many-body systems, but its numerical evaluation for neural network representations of quantum states has so far been inefficient and demonstrated only for the restricted Boltzmann machine architecture. Here we estimate generalized Rényi entropies of autoregressive neural quantum states with up to $N=256$ spins using quantum Monte Carlo methods. A naive “direct sampling” approach performs well for low-order Rényi entropies but fails for larger orders when benchmarked on a one-dimensional Heisenberg model. We therefore propose an improved “conditional sampling” method exploiting the autoregressive structure of the network ansatz, which outperforms direct sampling and facilitates calculations of higher-order Rényi entropies in both one- and two-dimensional Heisenberg models. Access to higher-order Rényi entropies allows for an approximation of the von Neumann entropy as well as extraction of the single-copy entanglement. Both methods elucidate the potential of neural network quantum states in quantum Monte Carlo studies of entanglement entropy for many-body systems.

We address a phase estimation scheme using Gaussian states in the presence of non-Gaussian phase noise. At variance with previous analysis, we analyze situations in which the noise occurs before encoding phase information. In particular, we study how squeezing may be profitably used before or after phase diffusion. Our results show that squeezing the probe after the noise greatly enhances the sensitivity of the estimation scheme, as witnessed by the increase of the quantum Fisher information. We then consider a realistic setup in which homodyne detection is employed at the measurement stage, and we address its optimality as well as its performance in the two different scenarios.

We study Rydberg antihydrogen interaction with a metallic or dielectric surface by calculating the adiabatic potential curves. For a metallic or pure dielectric surface with permittivity $\epsilon =\infty$, the image charges create an attractive potential that pulls the positron away from the antiproton when the atom approaches the surface. This is no longer the case with low-dielectric-constant materials ($\epsilon \approx 1$). Furthermore, using a negative positron work function, meaning that positrons are repelled by these surfaces, only repulsive potential curves exists. This suggests that Rydberg antihydrogen can probably be repelled by such ultralow-$k$ materials, opening the way for simple manipulation and guiding of antimatter (antihydrogen or positronium) systems.

We study the triple-differential cross section (TDCS) for the electron-impact ionization of the highest occupied molecular orbital of tetrahydrofuran at a projectile energy $E_0=91\mathrm{eV}$. The experimental data were measured using a reaction microscope, which covers a large part of the full solid angle for the secondary electron emission with energies ranging from 6 to 15 eV, and projectile scattering angles ranging from $-{10}^{\circ }$ to $-{20}^{\circ }$. The experimental TDCSs are internormalized across all measured scattering angles and ejected energies. They are compared with predictions from the multicenter distorted-wave (MCDW) approximation and a modified MCDW-Nee method which includes the postcollision interaction (PCI) using the Ward-Macek factor. Additional calculations were obtained using a molecular three-body distorted-wave (M3DW) approach which accounts for PCI in an exact treatment. Generally, the MCDW-Nee and M3DW models show better agreement with experiment than the MCDW calculations. This shows the importance of accounting for PCI for low-energy outgoing electrons in electron-impact ionization processes.

Knots and links are fundamental topological objects that play a key role in both classical and quantum fluids. In this paper, we propose a scheme to generate torus vortex knots and links through the reconnections of vortex rings perturbed by Kelvin waves in trapped Bose-Einstein condensates, which makes it possible to generate topologically complex structures from topologically trivial objects controllably. The transfer of helicity between knots, links, and coils occur with different pathways which can be controlled through designing specific initial states. The generation of a knot or link can be achieved by setting the parity of the Kelvin wave number. The stability of knots and links can be greatly improved with tunable parameters, including the ideal relative angle and the minimal distance between the initial vortex rings.

We study an optomechanical (OM) system with optical and mechanical modes interacting through a dispersive linear and quadratic optomechanical coupling (QOC) in an unresolved sideband limit. The presence of a QOC plays a crucial role in altering the effective OM interaction and the spring constant of the mechanical resonator. The choice of a positive QOC in the system increases the spring constant, inducing a ‘stiffer mode.’ The existence of this stiffer mode leads to unconventional behavior in the total displacement noise consisting of imprecision, backaction, and thermal noises. We further show that unlike the conventional OM system wherein the optimal total displacement noise is limited by the imprecision and backaction noises, it is due to the competing imprecision and thermal noise, making it feasible to observe displacement sensing below the conventional standard quantum limit.

In this paper, we investigate the nonlinear optical properties and photon correlations in a cavity containing quantum wells and interacting with a third-order nonlinear medium. More precisely, we study the interaction of the photonic Kerr nonlinearity with polariton and dipolariton quasiparticles in thermal environments. The optical bistability, the intensity spectrum, and the squeezing spectrum of the transmitted radiation are analyzed. It is shown that the regime of the bistability is reached in the dipolariton cavity faster than attained by the polariton system. Furthermore, the increase of the coupling with the wells reduces the squeezing. More interestingly, it turns out that the additional interactions arising in the dipolariton cavity, between direct and indirect excitons, slow down the variation of the squeezing. As a consequence, dipolaritons offer a greater margin of squeezed light than polaritons, and also than the ${\chi }^{\left(3\right)}$ medium in the absence of quantum wells. Moreover, the dipolariton system is found to be much more robust against the bath temperature in the weak-coupling regime.

We revisit the polaron-molecule transition in three-dimensional (3D) fermion systems using the well-established variational approach. The molecule is found to be intrinsically unstable against lowest-order particle-hole excitations, and it can only approximate the ground state of impurity systems with a finite total momentum in the strong-coupling regime. The polaron-molecule transition can therefore be reinterpreted as a first-order transition between single impurity systems with different total momenta. Within a certain interaction window near their transition, both states appear as local minima in the dispersion curve, indicating they can coexist in a realistic system. We have further confirmed the polaron-molecule coexistence in the presence of a finite impurity concentration and at low temperature, which directly leads to a smooth polaron-molecule transition as observed in recent experiments of 3D ultracold Fermi gases. Our results have provided an unambiguous physical picture for the competition and conversion between a polaron and molecule, and also shed light on Fermi polaron properties in low dimensions.

The quantum channels with memory, known as non-Markovian channels, are of crucial importance for a realistic description of a variety of physical systems, and pave ways for new methods of decoherence control by manipulating the properties of an environment such as its frequency spectrum. In this work, the reduced dynamics of a coin in a discrete-time quantum walk is characterized as a non-Markovian quantum channel. A general formalism is sketched to extract the Kraus operators for a $t$-step quantum walk. Non-Markovianity, in the sense of P indivisibility of the reduced coin dynamics, is inferred from the nonmonotonous behavior of distinguishability of two orthogonal states subjected to it. Furthermore, we study various quantum information-theoretic quantities of a qubit under the action of this channel, putting in perspective the role such channels can play in various quantum information processing tasks.

We consider in what sense quantum tunneling is associated with nonclassical probabilistic behavior. We use the Wigner function quasiprobability picture. We give a definition of tunneling that allows us to say whether in a given scenario there is tunneling or not. We prove that this can only happen if the Wigner function is negative and/or a certain measurement operator which we call the tunneling rate operator has a negative Wigner function. We also investigate tunneling in postquantum theories.

Quantum simulation in experiments of many-body systems may bring new phenomena which are not well studied theoretically. Motivated by a recent work of quantum simulation on a superconducting ladder circuit, we investigate the rung-pair localization of the Bose-Hubbard ladder model without quenched disorder. Our results show that, in the hard-core limit, there exists a rung-pair localization both at the edges and in the bulk. Using the center-of-mass frame, the two-particle system can be mapped to an effective single-particle system with an approximate sublattice symmetry. Under the condition of the hard-core limit, the effective system is forced to have a defect at the left edge leading to a zero-energy flatband, which is the origin of the rung-pair localization. We also study the multiparticle dynamics of the Bose-Hubbard ladder model, which is beyond the single-particle picture. In this case, we find that the localization can still survive despite the existence of interaction between the pairs. Moreover, the numerical results show that the entanglement entropy exhibits a long-time logarithmic growth and the saturated values satisfy a volume law. This phenomenon implies that the interaction plays an important role during the dynamics, although it cannot break the localization. Our results reveal another interesting type of disorder-free localization related to a zero-energy flatband, which is induced by on-site interaction and specific lattice symmetry.

Practical application of quantum information requires distinguishing multipartite entanglement in a complete, nondestructive, and robust way. Here we explore the possibility to accomplish such a task using Rydberg atoms based on blockade effect and user-defined-passage-based geometric quantum logic gates. Our proposal focuses on distinguishing groups of many-body Greenberger-Horne-Zeilinger (GHZ) states and groups of cluster states, and for bipartite cases distinguishing Bell states, which are basic ingredients of quantum information processing. We validate our proposal by numerical results, showing complete and nondestructive accomplishment of distinguishing above multipartite entangled states and also robustness against decoherence. We exemplify two practical applications regarding quantum teleportation and quantum dense coding to demonstrate the value of our proposal. This idea is scalable and experimentally relevant and can be readily generalized to other quantum information candidates, such as superconducting circuits, NV centers, as well as trapped-ion systems with different-level structures.

We examine continuous-variable gate teleportation using entangled states made from pure product states sent through a beam splitter. We show that such states are Choi states for a (typically) nonunitary gate, and we derive the associated Kraus operator for teleportation, which can be used to realize non-Gaussian, nonunitary quantum operations on an input state. With this result, we show how gate teleportation is used to perform error correction on bosonic qubits encoded using the Gottesman-Kitaev-Preskill (GKP) code. This result is presented in the context of deterministically produced macronode cluster states, generated by constant-depth linear optical networks, supplemented with a probabilistic supply of GKP states. The upshot of our technique is that state injection for both gate teleportation and error correction can be achieved without active squeezing operations—an experimental bottleneck for quantum optical implementations.

In present-day quantum communications, one of the main problems is the lack of a quantum repeater design that can simultaneously secure high rates and long distances. Recent literature has established the end-to-end capacities that are achievable by the most general protocols for quantum and private communication within a quantum network, encompassing the case of a quantum repeater chain. However, whether or not a physical design exists to approach such end-to-end capacities remains a challenging objective. Driven by this motivation, in this work we put forward a design for continuous-variable quantum repeaters and show that it can actually achieve the feat. We also show that even in a noisy regime, our rates surpass the Pirandola-Laurenza-Ottaviani-Banchi bound. Our repeater setup is developed upon using noiseless linear amplifiers, quantum memories, and continuous-variable Bell measurements. Furthermore, we propose a nonideal model for continuous-variable quantum memories that we make use of in our design. We then show that potential quantum communication rates would deviate from the theoretical capacities, as one would expect, if the quantum link is too noisy, and/or if low-quality quantum memories and amplifiers are employed.

The Morris-Shore (MS) transformation is a powerful tool for decomposition of the dynamics of multistate quantum systems to a set of two-state systems and uncoupled single states. It assumes two sets of states wherein any state in the first set can be coupled to any state in the second set, but the states within each set are not coupled between themselves. Another important condition is the degeneracy of the states in each set, although all couplings between the states from different sets can be detuned from resonance by the same detuning. The degeneracy condition limits the application of the MS transformation in various physically interesting situations, e.g., in the presence of electric and/or magnetic fields or light shifts, which lift the degeneracy in each set of states, e.g., when these sets comprise the magnetic sublevels of levels with nonzero angular momentum. This paper extends the MS transformation to such situations, in which the states in each of the two sets are nondegenerate. To this end, we develop an alternative way for the derivation of Morris-Shore transformation, which can be applied to nondegenerate sets of states. We present an approximated eigenvalue approach, by which, in the limit of small detunings from degeneracy, we are able to generate an effective Hamiltonian that is dynamically equivalent to the nondegenerate Hamiltonian. The effective Hamiltonian can be mapped to the Morris-Shore basis with a two-step similarity transformation. After the derivation of the general framework, we demonstrate the application of this technique to the popular $\mathrm{\Lambda }$ three-state system, and the four-state tripod, double-$\mathrm{\Lambda }$, and diamond systems. In all of these systems, our formalism allows us to reduce their quantum dynamics to simpler two-state systems even in the presence of various detunings, e.g., generated by external fields of frequency drifts.

Measurements of the loss rates for strontium $n{}^{3}S_1$ Rydberg atoms excited in a dense Bose-Einstein condensate are presented for values of principal quantum number $n$ in the range $49\lesssim n\lesssim 150$ and local atom densities of $\sim 1$ to $3\times {10}^{14}{\mathrm{cm}}^{-3}$. Two main processes contribute to loss, associative ionization, and state changing. The relative importance of these two loss channels is investigated, and their $n$ and density dependences are discussed using a model in which the Rydberg atom loss is presumed to involve a close collision between the Rydberg core ion and ground-state atoms. The present measurements are compared to earlier results obtained using rubidium Rydberg atoms. For both species the observed loss rates are sizable, $\sim {10}^5-{10}^6{\mathrm{s}}^{-1}$, and limit the timescales over which measurements involving Rydberg atoms immersed in quantum degenerate gases can be conducted.

A single-nanoparticle plasmon laser with ultratight optical confinement in all three dimensions is a long-cherished but yet to be realized goal. Here we propose an experimentally realizable plasmon laser using a single nanorod. We show that, by harnessing a Au-nanorod cavity with a large gain from a Purcell-enhanced stimulated emission rate and greatly suppressed high-order mode dissipation, it is possible to implement a dipole-mode plasmon laser with accessible material parameters. Our approach may lead to a real single-nanoparticle plasmon laser with three-dimensional ultraconfinement and offers an opportunity to explore and utilize the enhanced light-matter interaction at a lower dimension.

We present a discrete implementation of generalized Harvey-Shack scalar scattering theory to calculate angular intensity distributions from height profiles of select randomly textured surfaces proposed for use in solar cells and covering a wide range of surface characteristics. We compare these calculations to high-resolution angular intensity distribution measurements. These comparisons suggest that the pupil function does benefit from an additional correction factor for rough surfaces containing lateral feature sizes on the order of the wavelength, which can be attributed to effective medium effects. Moreover, secondary interactions within the surface topography are shown to be a mechanism that partly redistributes scattered power, affecting angular intensity distribution results. These mechanisms emerge as the two main limitations of the Harvey-Shack scalar scattering theory in the far field, which nonetheless produces remarkable quantitative predictions.

We report on the coherent control of boosted ultrashort terahertz pulses emitted from air plasma pumped by a femtosecond sawtooth pulse composed of three optical harmonic fields. With an inline optical setup, the relative phases between the three optical components of the pump pulse can be varied independently with attosecond precision without the complexity of external phase stabilization. We observe that the amplitude of the terahertz emission pumped by the phase-optimized sawtooth wave is enhanced by 1.8 times compared to the widely used two-color scheme. Moreover, by manipulating the relative phases between the three colors at attosecond precision, coherent control of the azimuthal angle, ellipticity, and polarity of terahertz radiation is achieved. A local current model reproduces the coherent control observations well.