Spectroscopy of gases under high-pressure conditions is of interest in various fields such as plasma physics and astrophysics. Recently, it has also been proposed to utilize a high-pressure noble gas environment as a thermalization medium to extend the wavelength range of photon Bose-Einstein condensates to the vacuum-ultraviolet regime, from the presently accessible visible and near-infrared spectral regimes. In this work, we report on experimental results of two-photon spectroscopy of gaseous and supercritical xenon for pressures as high as 95 bar, probing the transitions from the $5p^6$ electronic ground-state to the $5p^{5}6p$ and $5p^{5}6p^{\prime }$ excited-state configurations. Aiming at the exploration of possible pumping schemes for future vacuum-ultraviolet photon condensates, we have recorded degenerate two-photon excitation spectra of such dense xenon samples. In further measurements, we have investigated whether irradiation of an auxiliary light field can enhance the reabsorption of the emission on the second excimer continuum of xenon, which is subject to a large Stokes shift. To this end, absorption measurements have been conducted, driving the $5p^6\to 5p^{5}6p$ two-photon transitions nondegenerately.

Three-component Bose-Einstein condensates (BECs) offer a rich platform for exploring inquisitive quantum phenomena. We investigate the Rayleigh-Taylor instability at the two interfaces in a phase-separated three-component BEC in the mean-field framework. The instability in the system can be instigated in various ways owing to the versatility of the three-component system and the subsequent dynamics for certain cases has been studied in detail. We also study the impact of immiscibility of the intermediate component on the onset of instability in the system. The rotational symmetry of the system breaks when the atom-atom interaction is tuned in such a way that the interface between the components becomes unstable giving rise to nonlinear patterns of mushroom shapes which grow exponentially with time. We identify these nonlinear patterns as the solutions of the angular Mathieu equation, representing the normal modes.

In this paper, we study the behaviors of spatiotemporal optical vortex (STOV) beams propagating in linear dispersive media. Starting with the Fresnel diffraction integral, we derive a closed-form expression for the STOV field at any propagation distance $z$ in a general second-order dispersive medium. We compare our general result to special cases published in the literature and examine the characteristics of higher-order STOV beams propagating in dispersive materials by varying parameters of the medium and source-plane STOV field. We validate our analysis by comparing theoretical predictions to numerical computations of a higher-order STOV beam propagating through fused silica, where we model the index of refraction with the corresponding Sellmeier equation.

Dissipative solitons in optical microcavities have attracted significant attention in recent years due to their direct association with the generation of optical frequency combs. Here, we address the problem of dissipative soliton breathers in a microresonator with second-order nonlinearity, operating at the exact phase matching for efficient second-harmonic generation. We elucidate the vital role played by the group-velocity difference between the first- and second-harmonic pulses for the breather existence. We report the dissipative-breather-gas phenomenon, in which multiple breathers propagate randomly in the resonator and collide nearly elastically. Finally, when the breather gas reaches an out-of-equilibrium statistical stationarity, we show how the velocity locking between first and second harmonics is still preserved, naming such phenomena turbulence locking.

We present and analyze a protocol for driven-dissipatively preparing and stabilizing a manifold of quantum many-body entangled states with symmetry-protected topological order. Specifically, we consider the experimental platform consisting of superconducting transmon circuits and linear microwave resonators. We perform theoretical modeling of this platform via pulse-level simulations based on physical features of real devices. In our protocol, transmon qutrits are mapped onto spin-1 systems. The qutrits' sharing of nearest-neighbor dispersive coupling to a dissipative microwave resonator enables elimination of state population in the $S^{\mathrm{total}}=2$ subspace for each adjacent pair, and thus, the stabilization of the many-body system into the Affleck, Kennedy, Lieb, and Tasaki state up to the edge mode configuration. We also analyze the performance of our protocol as the system size scales up to four qutrits, in terms of its fidelity as well as the stabilization time. Our work shows the capacity of driven-dissipative superconducting cQED systems to host robust and self-corrected quantum many-body states that are topologically nontrivial.

We present the time dynamics of twisted quantum states. We find an explicit connection between the well-known stationary Landau state and an evolving twisted state, even when the Hamiltonian accounts for linear energy dissipation. Utilizing this unitary connection, we analyze nonstationary Landau states and unveil some of their properties. The proposed transformation enables simple evaluation of different operator mean values for the evolving twisted state based on the solution to the classical Ermakov equation and matrix elements calculated on the stationary Landau states. The suggested formalism may significantly simplify analysis and become a convenient tool for further theoretical development on the dissipative evolution of the twisted quantum wave packet.

We propose a method to implement absorbing layers for the numerical solution of the time-dependent Schrödinger equation (TDSE) in the open domain. The method is based on introducing an imaginary second-order differential operator which effectively absorbs waves propagating both toward the boundary and away from it. The survival probability is 0.01 for de Broglie wavelength equal to the layer width and decreases exponentially as the wavelength decreases. The proposed method can be used in both length and velocity gauges for the electric field when solving strong-field physics problems. We also propose the propagator for the absorbing operator for convenient use in the Fourier split-step method of the TDSE solution. We demonstrate the high accuracy of using the proposed method in simulations of the high-order harmonics generation in atomic hydrogen by femtosecond laser pulses and calculations of photoelectron momentum distributions using projections of the absorbed wave packets onto Volkov states.

The spectrum of a bound state plays a crucial role in the field-induced tunneling ionization, as ionization is essentially a dressing dissipative process. Based on the Fano-resonance theory, a single Lorentzian spectrum can be approximately obtained for each bound state. The coupling transition between bound states enriches the spectrum and complicates the tunneling dynamics. Previous attempts to extract the spectrum characteristics of the tunneling system from the ionization rate required an artificial filter function, and only the spectrum characteristics of the ground state can be obtained. In this study, we achieve a two-level model by parametrizing the main peaks of the spectrum with a Lorentzian function. This model successfully reproduces the ionization rate. By matching the central positions and widths of the Fano-type peaks with the behaviors of the ionization rate, we establish the relations between the model parameters and the details of the spectral peaks, namely, the Fano-type peaks. Our work introduces a systematic method to obtain key information of the spectrum, which can be generalized to multiple-level systems.

We report the observation of stationary turbulence in antiferromagnetic spin-1 Bose-Einstein condensates driven by a radio-frequency magnetic field. The magnetic driving injects energy into the system by spin rotation and the energy is dissipated via dynamic instability, resulting in the emergence of an irregular spin texture in the condensate. Under continuous driving, the spinor condensate evolves into a nonequilibrium steady state with characteristic spin turbulence, while the low-energy scale of spin excitations ensures that the sample's lifetime is minimally affected. When the driving strength is on par with the system's spin interaction energy and the quadratic Zeeman energy, remarkably, the stationary turbulent state exhibits spin-isotropic features in spin composition and spatial spin texture. We numerically show that ambient field fluctuations play a crucial role in sustaining the turbulent state within the system. These results open up different avenues for exploring quantum turbulence in spinor superfluid systems.

Bose gas on a two-leg ladder exhibits an interesting topological phase. We show the presence of a bosonic symmetry-protected-topological (SPT) phase protected by $Z_2\times Z_2$ symmetry. This symmetry leads to $Z_4$ fractional quantization of the $Z_4$ Berry phase, which is a topological order parameter to identify the bulk. Using the $Z_4$ Berry phase, we show that the interacting bosonic system possesses rich topological phases depending on the particle density and strength of interaction. Based on the bulk-edge correspondence, each edge state of the SPT phases is discussed in relation to the $Z_4$ Berry phases. In particular, we have found an intermediate phase that is not adiabatically connected to a simple adiabatic limit that possesses unconventional edge states, which we numerically demonstrate by employing the density-matrix renormalization-group algorithm.

Quantum squeezing is an important resource in modern quantum technologies, such as quantum precision measurement and continuous-variable quantum information processing. The generation of squeezed states of mechanical modes is a significant task in cavity optomechanics. Motivated by recent interest in multimode optomechanics, it becomes an interesting topic to create quadrature squeezing in multiple mechanical resonators. However, in the multiple-degenerate-mechanical-mode optomechanical systems, the dark-mode effect strongly suppresses the quantum effects in mechanical modes. Here we study the generation of mechanical squeezing in a two-mechanical-mode optomechanical system by breaking the dark-mode effect with the synthetic-gauge-field method. We find that, when the mechanical modes work at a finite temperature, the mechanical squeezing is weak or even disappears due to the dark-mode effect, while the strong mechanical squeezing can be generated once the dark-mode effect is broken. In particular, the thermal-phonon-occupation tolerance of the mechanical squeezing is approximately three orders of magnitude larger than that without breaking the dark-mode effect. We also generalize this method to break the dark modes and to create the mechanical squeezing in multiple-mechanical-mode optomechanical systems. Our results describe a general physical mechanism and pave the way towards the generation of noise-resistant quantum resources.

Unresolved spatially random microstructure, in an illuminated sample, can lead to position-dependent blur when an image of that sample is formed. For a small propagation distance, between the exit surface of the sample and the entrance surface of a position-sensitive detector, the paraxial approximation implies that the blurring influence of the sample may be modeled using an anomalous-diffusion field. This diffusion field may have a scalar or tensor character, depending on whether the random microstructure has an autocorrelation function that is rotationally isotropic or anisotropic, respectively. Partial differential equations are written and then solved, in a closed-form manner, for several variants of the inverse problem of diffusion-field retrieval given suitable intensity images. Both uniform-illumination and structured-illumination schemes are considered. Links are made between the recovered diffusion field and certain statistical properties of the unresolved microstructure. The developed theory—which may be viewed as a crudely parallel form of small-angle scattering under the Guinier approximation—is applicable to a range of paraxial radiation and matter fields, such as visible light, x rays, neutrons, and electrons.

Exceptional points in an optical dimer of spheres, which have the same size and operate in the spectral region of the dipolar resonance, are considered. By choosing different materials of these spheres, we can offset the radiative loss and create a gain-loss contrast to achieve a parity-time ($\mathcal{PT}$)-symmetric dimer. In this case, an exceptional point corresponds to the point where the $\mathcal{PT}$ symmetry is broken. At the same time, if we consider a symmetric dimer, where both spheres are made of the same material (which may have a purely real dielectric constant), exceptional points occurring due to the radiative loss non-Hermiticity can also be observed. We study the transition between the two regimes and demonstrate that the exceptional point emerges due to the retardative nature of the coupling between the spheres, which makes the equation for eigenfrequencies nonlinear and allows it to have nontrivial solutions even when there is no contrast between the spheres.

Quantum steering is the phenomenon whereby one party (Alice) proves entanglement by “steering” the system of another party (Bob) into distinct ensembles of states, by performing different measurements on her subsystem. Here, we investigate steering in a network scenario involving $n$ parties, who each perform local measurements on part of a global quantum state, that is produced using only two-party entangled states, and mixing with ancillary separable states. We introduce three scenarios which can be straightforwardly implemented in standard quantum optics architecture, which we call random $\frac{n}{2}$-pair entanglement, random pair entanglement, and semirandom pair entanglement. We study steerability of the states across two-party marginals which arise in the three scenarios, and analytically derive the necessary and sufficient steering criteria for different sets of measurement settings. Strikingly, using the semirandom pair entanglement construction, one party can steer every one of the $n-1$ other parties, for arbitrarily large $n$, using only two measurements. Finally, exploiting symmetry, we study various small network configurations (three or four parties) in the three scenarios, under different measurements and produced by different two-party entangled states.

We formulate a model in which quantum mechanics emerges from classical mechanics. Given a local Hamiltonian $H$ acting on $n$ qubits, we define a local classical model with an additional spatial dimension whose boundary dynamics is approximately—but to arbitrary precision—described by Schrödinger's equation and $H$. The bulk consists of a lattice of classical bits that propagate towards the boundary through a circuit of stochastic matrices. The bits reaching the boundary are governed by a probability distribution whose deviation from the uniform distribution can be interpreted as the quantum-mechanical wave function. Bell nonlocality is achieved because information can move through the bulk much faster than the boundary speed of light. We analytically estimate how much the model deviates from quantum mechanics, and we validate these estimates using computer simulations.

Starting from the stochastic Liouville equation, we obtain an integral functional equation for describing the finite-temperature dynamics of the asymmetric spin-boson model. Like the zero-temperature case, this equation displays a hierarchical structure characterizing the multiple-timescale nature of quantum dissipative dynamics. We are thereby able to establish a perturbation series and illustrate the contributions with the conventional concepts of blips and sojourns. We clarify the speciality of the Toulouse limit by presenting the analytical properties of intrablip and blip-sojourn interactions. This speciality leads to a closed-form equation for the blip, from which the exact result of the dissipative dynamics can be obtained. We also demonstrate the exact solution by summing the perturbative series. Moreover, we calculate the exact equilibrium distribution at the Toulouse limit and compare it with the Boltzmann distribution valid for weak dissipation.

Precision measurements with atoms and molecules can search for subtle violations of time-reversal symmetry $T$ in nuclei and thereby probe a variety of new-physics models. We present a detailed scheme for a nuclear $T$-violation search experiment using ${}^{153}\mathrm{Eu}^{3+}$ ions doped in noncentrosymmetric sites within a ${\mathrm{Y}}_2{\mathrm{SiO}}_5$ crystal. The ions in this solid contain nuclei that are highly sensitive to $T$ violation, and the ions are strongly polarized by the fields from neighboring ions. But in particular, the system and methods that we discuss here enable the use of vast numbers of nuclei trapped in crystals while also offering a number of stringent tests to ward off systematic errors. Our approach maps out a path to probe new physics at the PeV energy scale.

Originally proposed for the temporal characterization of train of attosecond pulses, the reconstruction of attosecond beating by interference of two-photon transitions (RABBITT), has become a wide-spread, powerful technique, capable of capturing ultrafast electron dynamics through an interferometric approach. Starting from the well-known strong-field approximation (SFA) description of a two-color photoelectron spectrum, here we develop a model that interprets a RABBITT trace as the interference of different Floquet ladder states generated in the continuum by a femtosecond infrared (IR) pulse after ionization by the attosecond radiation. In turn, this allowed us to develop an analytical model capable of predicting the amplitude and phase of the oscillating sidebands and main bands while including the effect of nonstandard interference paths and, in first approximation, of the finite IR pulse envelope. Our results thus suggest a way to extend the RABBITT model to higher intensities and beating frequencies, and disentangle different oscillating signals in a congested photoelectron spectrogram as the one associated with molecular targets.

We theoretically and computationally investigate the behavior of infinite atom arrays when illuminated by nearly resonant light. We use higher-order mean-field equations to investigate the coherent reflection and transmission and incoherent scattering of photons from a single array and from a pair of arrays as a function of detuning for different values of the Rabi frequency. For the single array case, we show how increasing the light intensity changes the probabilities for these different processes. For example, the incoherent scattering probability initially increases with light intensity before decreasing at higher values. For a pair of parallel arrays at near resonant separation, the effects from increasing light intensity can become apparent with incredibly low-intensity light. In addition, we derive the higher-order mean-field equations for these infinite arrays, giving a representation that can be evaluated with a finite number of equations.