When subject to a nonlocal unitary evolution, qubits in a quantum circuit become increasingly entangled. Conversely, measurements applied to individual qubits lead to their disentanglement from the collective system. The extent of entanglement reduction depends on the frequency of local projective measurements. A delicate balance emerges between unitary evolution, which enhances entanglement, and measurements which diminish it. In the thermodynamic limit, there is a phase transition from volume law entanglement to area law entanglement at a critical value of measurement frequency. This phenomenon, occurring in hybrid quantum circuits with both unitary gates and measurements, is termed as measurement-induced phase transition (MIPT). We study the behavior of MIPT in circuits comprising of two-qubit unitary gates parameterized by Cartan decomposition. We show that the entangling power and gate typicality of the two-qubit local unitaries employed in the circuit can be used to explain the behavior of global bipartite entanglement the circuit can sustain. When the two-qubit gate throughout the circuit is the identity and measurements are the sole driver of the entanglement behavior, we obtain an analytical estimate for the entanglement entropy that shows remarkable agreement with numerical simulations. We also find that the entangling power and gate typicality enable the classification of the two-qubit unitaries by different universality classes of phase transitions that can occur in the hybrid circuit. For all unitaries in a particular universality class, the transition from volume to area law of entanglement occurs with the same exponent that characterizes the phase transition.

Measurement-based quantum computation (MBQC) is a paradigm for quantum computation where computation is driven by local measurements on a suitably entangled resource state. In this work we show that MBQC is related to a model of quantum computation based on Clifford quantum cellular automata (CQCAs). Specifically, we show that certain MBQCs can be directly constructed from CQCAs which yields a simple and intuitive circuit model representation of MBQC in terms of quantum computation based on CQCA. We apply this description to construct various MBQC-based Ansätze for parametrized quantum circuits, demonstrating that the different Ansätze may lead to significantly different performances on different learning tasks. In this way, MBQC yields a family of hardware-efficient Ansätze that may be adapted to specific problem settings and are particularly well suited for architectures with translationally invariant gates such as neutral atoms.

In order to interpret and predict the behavior and properties of fusion plasma, accurate cross sections for electronic processes in collisions between plasma impurities and atomic hydrogen are required. In this work, we investigate the electron capture (or charge exchange), target excitation, and ionization processes occurring in collision of ${\mathrm{N}}^{4+}$ with atomic hydrogen in a broad energy domain ranging from 0.06 to 225 keV/u. We consider ${\mathrm{N}}^{4+}$ ground-state ${\mathrm{N}}^{4+}\left(2s\right)$ and also ${\mathrm{N}}^{4+}\left(2p\right)$ since the impurities in the edge plasma environment may be excited due to collisions with electrons and ions or atoms. Total and partial cross sections in both spin-averaged and spin-resolved cases are calculated using a two-active-electron semiclassical asymptotic-state close-coupling approach. For electron capture cross sections the present results show overall good agreement with available experimental data for both total and partial cross sections, and the origins of observed discrepancies are discussed. Furthermore, we provide data for target excitation and ionization processes, which are essential to improve our understanding of this relevant collision system.

High-harmonic generation (HHG) has recently emerged as a promising method for generating nonclassical states of light with frequencies spanning from the infrared up to the extreme ultraviolet regime. In this work, we theoretically investigate the generation of squeezed states of light through HHG processes in atomic systems that were initially driven to their first excited state. Our study reveals significant single-mode squeezing in both the driving field and low-order harmonic modes. Additionally, we characterize two-mode squeezing features in the generated states, both between fundamental and harmonic modes, and among the harmonic modes themselves. Using these correlations, we demonstrate the generation of optical Schrödinger kitten states through heralding measurements, specifically via photon subtraction in one of the modes influenced by two-mode squeezing.

Intense laser pulses have the capability to couple resonances in the continuum, leading to the formation of a split pair of autoionizing polaritons. These polaritons can exhibit extended lifetimes due to interference between radiative and Auger decay channels. In this work we show how an extension of the Jaynes-Cummings model to autoionizing states quantitatively reproduces the observed phenomenology. This extended model allows us to study how the dressing laser parameters can be tuned to control the ionization rate of the polariton multiplet.

We measure the ac Zeeman force acting on ultracold ${}^{87}\mathrm{Rb}$ atoms due to a radio frequency (rf) field driving intramanifold Zeeman transitions near 8 MHz. We observe about 2.5 times the force of gravity for $\approx 14$ mW of rf power acting on atoms $\approx 100\mu \mathrm{m}$ from the atom chip. Our measurements are consistent with theoretical predictions and demonstrate the spin-state dependence of the force, as well as its resonant and bipolar behavior. We demonstrate the ac Stern-Gerlach effect by spatially separating a spin mixture, using initial detuning to determine which spins are ac Zeeman high- and low-field seekers. With an eye towards using this force for atom trapping, we observe the time dependence of dressed state spin mixing as a function of detuning and power.

We investigate beyond-mean-field corrections to the energy of an elongated homogeneous Bose gas strongly confined in two directions, with dipoles aligned along the long axis of the system. When the dipolar interaction reaches its critical strength, the mean-field approach predicts instability. However, similar to the free-space case, beyond-mean-field effects significantly alter the ground state of the system, leading to the formation of a self-bound atomic cloud known as a quantum droplet. Our analysis demonstrates that the beyond-mean-field contribution to the energy in the quasi-one-dimensional region, in addition to the confinement-induced shift of the mean-field energy, is proportional to the third power of the density approximately equal to $n^3$. Therefore, it can be interpreted as an effective three-body repulsion that stabilizes the gas, preventing collapse and leading to a finite-density solution. We also show that the same effect plays a crucial role in the binding of strongly elongated dipolar droplets under harmonic confinement.

We investigate the statistical properties of electromagnetic field strength in locally disordered and time-varying cavities. Our findings demonstrate that in the majority of cases, temporal disorders in part of the cavity lead to an approximate normal distribution of the electric field strengths at some instants throughout the cavity. However, the stability of this normal distribution significantly deteriorates as time approaches extremely large values. The observation results suggest that this phenomenon is primarily attributed to the exponential increase in energy gain caused by the time interface. By employing the concept of disordered space-time crystals and Lyapunov's theorem, we establish the requisite conditions for achieving a stable normal distribution—the boundedness condition of electric field strength. At long times, the exponential increase in energy resulting from the time interface violates this boundedness condition, leading to an exponential increase in the variance of electric field strength. Consequently, it becomes challenging to observe an approximate normal distribution of electric field strength over extended periods of time. Employing the generalized central limit theorem, we demonstrate that the electric field strengths will ultimately evolve into a non-normal Lévy skew alpha-stable distribution. Furthermore, we discuss potential experimental schemes based on the transmission-line metamaterials that are anticipated to yield future experimental observations of these phenomena. This study elucidates the statistical properties of transient signals in disordered time-varying systems, thereby paving the way for further exploration into the wave dynamics of analogous systems.

Topological pumping is a quantum state transfer phenomena in topological systems, which was studied in one-dimensional and two-dimensional topological systems and is highly valuable in the generation of entangled states by adiabatic management. In this study, we develop a robust, rapid quantum state transfer mechanism based on Su-Schrieffer-Heeger (SSH) chains. This mechanism considers the case where the propagation length determines the interchain coupling. We concentrate on the selection of different slowly varying functions, which is critical for optimizing the quantum pumping. Based on the coupled mode theory, we study the zero-energy state spatial transfer in a one-dimensional SSH chain waveguide and a two-dimensional rhombic lattice waveguide model, extending the zero-energy state transfer from edge to edge to corner to corner. To obtain optical transport with faster transfer speed, higher fidelity, and topological protection, we optimize the tangent function and the length of the optical waveguide so that the zero-energy eigenstates evolve adiabatically following the topological boundary state without exciting other levels. The findings of this work will open up possibilities for the construction of reliable quantum state transfer channels and hold significant promise for information processing and optical manipulation.

We numerically demonstrate a novel regime for generation of coherent pulse trains in a normal-dispersion photonic crystal microresonator. The results show that, if one pumps not a split mode but the neighboring one, it is possible to excite either a single wide triangular-shaped pulse or a periodic sequence of them, depending on the spectral interval between pumped and split modes, with a low pump threshold and high pump-to-comb conversion efficiency. Properties of the generated signals are studied, and excitation conditions are revealed. It is found that nonlinear coupling of counterpropagating waves significantly affects the conditions and dynamics of the generation process.

Heterostructures introduce additional degrees of freedom to two-dimensional materials, providing an alternative way to manipulate solid high-order harmonic generation (HHG). However, the impact of interlayer coupling on the HHG of heterostructures remains unexplored. Taking the phosphorene/graphene heterostructure as an example, we theoretically investigate the interlayer coupling effect in the HHG of heterostructures by solving the semiconductor Bloch equations with the accelerated Bloch-like basis in the Wannier gauge. It is found that there are obvious differences between the high-order harmonics generated by the phosphorene/graphene heterostructure, its monolayers, and the phosphorene/graphene heterostructure without any interlayer couplings. Further study reveals that the lattice strain affects the harmonic yield of the phosphorene/graphene heterostructure, but does not alter the structure of the ellipticity dependence of the harmonic yield. Conversely, the interlayer coupling significantly changes the ellipticity and orientation dependences of the harmonic yield by allowing the transitions between the energy bands of different layers. Moreover, we find that the nearest-neighbor couplings between the specific carbon and phosphorus atoms dominate the overall interlayer coupling. Finally, an all-optical method for retrieving the dephasing time is proposed by means of a one-to-one mapping relationship between the dephasing time and the ellipticity in which the seventh-harmonic yield reaches the maximum. This work provides deeper insight into the HHG mechanism in heterostructures.

Subwavelength resonant metasurfaces empowered by bound states in the continuum (BICs) have recently emerged as promising platforms for boosting optical nonlinearities as they support high-quality ($Q$) quasi-BIC resonances with strong electromagnetic-field confinement. To date, most such studies have focused on optical resonances, termed symmetry-protected quasi-BICs, because they can be readily accessible in broken-symmetry structures. However, for traditional designs of which meta-atoms are placed on low-refractive-index substrates, their smallest achievable geometric symmetries impose a limitation on the realization of ultrahigh-$Q$ resonances and thereby on the harmonic generation efficiency. Here we demonstrate a strategy that allows us to improve the $Q$ factors of quasi-BICs over a broad range of geometrical asymmetry parameter values. Specifically, using asymmetric dielectric metasurfaces placed above a perfectly reflecting mirror, we construct merging BICs in the geometrical parameter space, which pushes the square asymptotic behavior of the $Q$ factors of traditional configurations into the power of 4 or even 6. We demonstrate that merging-BIC designs can remarkably enhance the second-harmonic generation efficiency. This work suggests a route to suppress out-of-plane radiation losses of metasurfaces with potential applications to nonlinear optics, lasing, and sensing.

Topological edge pumping, as an efficient form of energy transfer protected by nontrivial topology, connects the topological characters of a modulated system with its transport behavior. It provides a powerful means to achieve robust wave manipulation between distant nodes and has been intensively investigated in quantum and optical systems. Here we propose to implement multiform energy transfers through the topological defect state in photonic waveguide arrays emulating the extended Fock state lattice (FSL). The FSL supports a topological defect state which enables efficient edge transfer and more complex transmission behavior due to phase transitions induced by on-site energies. By cross-linking two identical FSLs and modulating the linking strength, we construct a fast and robust topological beam splitter with tunable output ratios. When on-site energies are introduced on the occupied odd sites, the cross-linking FSL undergoes a series of phase transitions, after which the original defect channel state becomes a topological channel to transfers excitations from central site to different sites. In addition, we confirm that the multifunctional topological devices based on the defect state in the extended FSL chain manifests strong robustness against mild perturbations in coupling strengths and can be mapped to spatially modulated photonic waveguide arrays. This work verifies a promising route towards robust and multiform photonic excitation transports which may find applications in wave manipulation in integrated optics.

We explore the Majorana stellar representation on the Riemann sphere for optical scalar beams and vector fields, utilizing the spin and orbital angular momentum of light. In this framework, star constellations for a scalar Laguerre-Gaussian beam basis appear at the poles of the sphere, while those for vector fields correspond to well-defined circular polarizations. Leveraging the $\text{su}\left(2\right)$ symmetry of angular momentum, we populate the sphere through generalized unitary rotations of basis elements, producing optical analogs of Bloch and generalized $\text{su}\left(2\right)$ coherent states for both scalar beams and vector fields. These rotations position constellations for scalar Hermite-Gaussian beams and vector Hermite-Gaussian partial Poincaré fields, restricted to linear polarization, along the equator of the sphere. We also address the inverse problem, reconstructing optical scalar beams and vector fields from their stellar constellations. Additionally, we explore scalar and vector analogs of cat codes and kings of quantumness, contributing to the visualization and characterization of complex optical beams and fields. These insights suggest potential applications in optical communications.

Atom-field interactions, induced by the vacuum of the electromagnetic field, exhibit a variety of fundamental phenomena and effects. In this paper, we study the electromagnetically induced atomic grating due to the vacuum state of the radiation field. Using an ensemble of cold atoms, strongly coupled to an optical cavity, we show that a probe field, propagating through the atomic medium, diffracts to zeroth- and first-order diffraction peaks with few photons and even by the electromagnetic vacuum field of the cavity mode. As the number of photons in the cavity increases, the intensity of the first-order diffraction peak initially rises and then exhibits a decreasing trend. Furthermore, we observe that the first-order peak intensity reaches its maximum at resonance for both the vacuum and single-photon cavity state. However, as the number of photons increases further, this peak at resonance transforms into a dip, accompanied by two side peaks at off-resonance positions. This transition from a peak to a dip may potentially be used to distinguish the quantum state of the cavity.

The proof that neutron interference is possible using split-crystal interferometers opens the way to extended arm separation and length and to new experiments exploring quantum mechanics and gravity. Therefore, this paper reexamines the effect of gravitational and Coriolis forces on the Laue diffraction of neutrons by perfect crystals and the operation of crystal interferometers. We give in analytical form the transfer matrices for the propagation of the neutron quantum state, either pure or mixed and subjected to gravity and Coriolis force, in free space and perfect crystals. They are used to study the effect of interferometer aberrations on the quantum-mechanical phase due to the Earth's gravity. We also give an alternative way to understand the impact of gravitational and Coriolis forces in terms of the crystal displacements and tilts perceived by the neutron.

Previous studies have proven that neutron interference using split-crystal interferometers is possible. This proof paves the way for extended arm separation and length, opening the doors to new experiments exploring quantum mechanics and gravity. In a previous publication, we took a closer look at how gravitational and Coriolis forces affect dynamical diffraction in crystal interferometry. This paper uses the formalism developed, which allows for extended capabilities in studying the interferometer operation, to investigate numerically the contributions to the neutron phase of geometrical aberrations and dynamical diffraction. In addition, this work explores an alternative geometry, with the interferometer operated vertically, to determine the gravitationally induced quantum-mechanical phase independently of the dynamical diffraction and self-weight bending of the interferometer.