We investigate the nonlinear optical response of suspended one-dimensional (1D) photonic crystal nanocavities fabricated on a silicon nitride chip. Strong thermo-optical nonlinearities are demonstrated for input powers as low as $2\mu \text{W}$ and a self-sustained pulsing regime is shown to emerge with periodicity of several seconds. As the input power and laser wavelength are varied the temporal patterns change in period, duty cycle, and shape. This dynamics is attributed to the multiple timescale competition between thermo-optical and thermo-optomechanical effects and closely resembles the relaxation oscillations states found in mathematical models of neuronal activity. We introduce a simplified model that reproduces all the experimental observations and allows us to explain them in terms of the properties of a 1D critical manifold which governs the slow evolution of the system.

Detection and quantification of chiral enantiomers of biomolecules are of considerable importance for biomedical diagnostics and pathogen analyses. However, the chiral signals from natural molecules are typically small and weak; a long measuring time and a large amount of samples are needed to fulfill the detection limit for the present technique. How to improve the detection efficiency for realizing ultrasensitive probing of chiral molecules has become a key problem. Here we propose a scheme to perform ultrasensitive detection of chiral molecules based on degeneracy states of orthogonal polarization modes in the achiral dielectric nanostructure. The degeneracy states are realized by designing dielectric photonic crystal slabs with air holes. Based on such degeneracy states, homogeneous superchiral fields of both handednesses over arbitrarily large areas can be obtained in a wide spectral range, resulting in the enhancement of the circular dichroism signal by more than three orders of magnitude, thus paving the road toward ultrasensitive characterization and quantification of molecular chirality.

Optomechanical systems are suitable for elucidating quantum phenomena at the macroscopic scale in the sense of the mass scale. The systems should be well isolated from the environment to avoid classical noises, which conceal quantum signals. Optical levitation is a promising way to isolate optomechanical systems from the environment. To realize optical levitation, all degrees of freedom need to be trapped. Until now, longitudinal trapping and rotational trapping of a mirror with optical radiation pressure have been studied in detail and validated with various experiments. However, less attention has been paid to the transversal trapping of a mirror. Herein, we report a pioneering result where we experimentally confirmed transversal trapping of a mirror of a Fabry-Pérot cavity using a torsional pendulum. Through this demonstration, we experimentally proved that optical levitation is realizable with only two Fabry-Pérot cavities that are aligned vertically. This work paves the way toward optical levitation and realizing a macroscopic quantum system.

We experimentally demonstrate a dissipative phase transition in a mirrorless optical parametric oscillator (MOPO) using a double-$\mathrm{\Lambda }$ system in thermal atomic vapor. Bistable behavior is observed in the output power of the beams generated via the MOPO with the variation of the input power of the driving fields. We show that the long-lived hyperfine ground-state coherence induced by the Gaussian pump and the generated fields leads to the observation of optical bistability in the MOPO threshold. We further study the critical slowing-down of nonequilibrium dynamics in the vicinity of the forward threshold where the switching time of the generated fields diverges with the critical exponent, $\alpha =-0.86\pm 0.02$. Our observations pave the way to understanding and characterizing the phase transitions in driven-dissipative systems using a simple nonlinear optical system like the MOPO.

We propose a method for finding two-dimensional spatial modes of thermal field through a direct measurement of the field intensity and an offline analysis of its spatial fluctuations. Using this method, in a simple and efficient way we reconstruct the modes of a multimode fiber and the spatial Schmidt modes of squeezed vacuum generated via high-gain parametric down-conversion. The reconstructed shapes agree with the theoretical results.

The anisotropy parameters $\left(\beta \right)$ of photofragments near a Fano resonance vary with the excitation energies. We call the curve describing such a relationship the $\beta$ profile. In this Rapid Communication, we report measurements of the $\beta$ profiles for the $\mathrm{H}\left(2l\right)$ and $\mathrm{D}\left(2l\right)$ fragments along six Fano resonances in the predissociation of ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ near the second dissociation threshold. In contrast to a previous theoretical prediction, the measured $\beta$ profiles are found to be asymmetric. An analytical expression of the $\beta$ profile was derived based on the Fano formula and was used to fit the measured $\beta$ profiles. The $\beta$ profiles are found to be more sensitive than the Fano profiles to the Fano $q$ parameters, as well as to the intensity ratios between the interacting and noninteracting continuum states. The $\beta$ values due to the resonance state, the continuum state, and the interference between the resonance and the continuum states in the Fano resonance were also determined. The $\beta$ profile should not only provide us with a tool to measure the parameters characterizing the Fano resonance, but also help gain further physical insight into the Fano resonances.

We study the charging process of open quantum batteries mediated by a common dissipative environment in two different scenarios. In the first case, we consider a quantum charger-battery model in the presence of a non-Markovian environment, where the battery can be properly charged in a strong coupling regime, without any external power and any direct interaction with the charger, i.e., a wireless-like charging happens. The environment plays a major role in the charging of the battery, while this does not happen in a weak coupling regime. In the second scenario, we show the effect of individual and collective spontaneous emission rates on the charging process of quantum batteries by considering a two-qubit system in the presence of Markovian dynamics. Our results demonstrate that open batteries can be satisfactorily charged in Markovian dynamics by employing an underdamped regime and/or strong external fields. We also present a robust battery by taking into account subradiant states and an intermediate regime. Moreover, we propose an experimental setup to explore the ergotropy in the first scenario.

In the context of generalized measurement theory, the Gleason-Busch theorem assures the unique form of the associated probability function. Recently, in Flatt et al. [Phys. Rev. A 96, 062125 (2017)] the case of subsequent measurements has been treated with the derivation of the Lüders rule and its generalization (Kraus update rule). Here we investigate the special case of subsequent measurements where an intermediate measurement is a composition of two measurements ($a\text{or}b$) and the case where the causal order is not defined ($a\text{and}b\text{or}b\text{and}a$). In both cases interference effects can arise. We show that the associated probability cannot be written univocally, and the distributive property on its arguments cannot be taken for granted. The two probability expressions correspond to the Born rule and the classical probability; they are related to the intrinsic possibility of obtaining definite results for the intermediate measurement. For indefinite causal order, a causal inequality is also deduced. The frontier between the two cases is investigated in the framework of generalized measurements with a toy model, a Mach-Zehnder interferometer with a movable beam splitter.

Quantum computing enables quantum neural networks (QNNs) to have great potential to surpass artificial neural networks. The powerful generalization of neural networks is attributed to nonlinear activation functions. Although various models related to QNNs have been developed, they are facing the challenge of merging the nonlinear, dissipative dynamics of neural computing into the linear, unitary quantum system. In this paper, we establish different quantum circuits to approximate nonlinear functions and then propose a generalizable framework to realize any nonlinear quantum neuron. We present two quantum neuron examples based on the proposed framework. The quantum resources required to construct a single quantum neuron are polynomial in function of the input size. Finally, both IBM Quantum Experience results and numerical simulations illustrate the effectiveness of the proposed framework.

On-top conditional correlation functions of many-electron theory are rearranged into a set of correlon quasiparticles representing the local effect of electron correlation in the ground and excited states. An individual correlon is characterized with a one-particle wave function, the imaginary part of which (or covalent correlon) gives the amplitude of the on-top depletion of the (conditional electron) charge (ODC) due to (strong) electron correlation. In its turn, the real part (ionic correlon) gives the amplitude of the on-top accumulation of the (electron) charge (OAC) due to the ionic squeezing of electrons. The proposed correlon theory is applied to analyze the local correlation effects in the ground and first excited states of the hydrogen molecule as well as of the equidistant and alternate linear hydrogen chains from ${\mathrm{H}}_4$ to ${\mathrm{H}}_{12}$. The covalent and ionic correlons obtained at the multiconfigurational self-consistent-field level of correlated functions are demonstrated to be the robust descriptors of the covalency of the ground and the ionicity of the excited ${\mathrm{H}}_n$ states.

Positron binding energies and annihilation spectra are calculated using the positron-electron correlation-polarization potential model for a series of six chloroethene molecules. From detailed analysis of the positron densities and positron-molecule interactions, it was found that the positron is attracted by both the potential well around the negative end of the dipole moment and the $\pi$ electrons of the C=C double bond. The positron annihilation spectra were simulated using the Feshbach resonance widths, which can be calculated from the couplings between the positron motion and vibrational motion. The importance of $\pi$ electrons in positron binding is discussed in terms of the calculated resonance widths.

We resonantly excite the $K$ series of ${\mathrm{O}}^{5+}$ and ${\mathrm{O}}^{6+}$ up to principal quantum number $n=11$ with monochromatic x rays, producing $K$-shell holes, and observe their relaxation by soft-x-ray emission. Some photoabsorption resonances of ${\mathrm{O}}^{5+}$ reveal strong two-electron–one-photon (TEOP) transitions. We find that for the ${\left[{\left(1s2s\right)}_{1}5p_{3/2}\right]}_{3/2;1/2}$ states, TEOP relaxation is by far stronger than the radiative decay and competes with the usually much faster Auger decay path. This enhanced TEOP decay arises from a strong correlation with the near-degenerate upper states ${\left[{\left(1s2p_{3/2}\right)}_{1}4s\right]}_{3/2;1/2}$ of a Li-like satellite blend of the He-like $K\alpha$ transition. Even in three-electron systems, TEOP transitions can play a dominant role, and the present results should guide further research on the ubiquitous and abundant many-electron ions where electronic energy degeneracies are far more common and configuration mixing is stronger.

We describe the results of experiments and simulations performed with the aim of extending photoelectron spectroscopy with intense laser pulses to the case of molecular compounds. Dimer frame photoelectron angular distributions generated by double ionization of ${\mathrm{N}}_2-{\mathrm{N}}_2$ and ${\mathrm{N}}_2-{\mathrm{O}}_2$ van der Waals dimers with ultrashort, intense laser pulses are measured using four-body coincidence imaging with a reaction microscope. To study the influence of the first-generated molecular ion on the ionization behavior of the remaining neutral molecule we employ a two-pulse sequence comprising of a linearly polarized and a delayed elliptically polarized laser pulse that allows distinguishing the two ionization steps. By analysis of the obtained electron momentum distributions we show that scattering of the photoelectron on the neighboring molecular potential leads to a deformation and rotation of the photoelectron angular distribution as compared to that measured for an isolated molecule. Based on this result we demonstrate that the electron momentum space in the dimer case can be separated, allowing us to extract information about the ionization pathway from the photoelectron angular distributions. Our work, when implemented with a variable pulse delay, opens up the possibility of investigating light-induced electronic dynamics in molecular dimers using angularly resolved photoelectron spectroscopy with intense laser pulses.

Spin polarization of photoelectrons is considered in the ionization of atoms by a laser field and its second harmonic, concentrating on the effects of interference between the two ionization paths. The possibility for a crucial increase of the spin polarization by manipulating the relative phase of the harmonics is demonstrated. Observation of these effects, exemplified by the ionization of neon atoms, is accessible by modern free-electron lasers working in the extreme-ultraviolet wavelength regime.

The driven-dissipative nature of light-matter interaction inside a multimode, dye-filled microcavity makes it an ideal system to study nonequilibrium phenomena, such as transport. In this work, we investigate how light is efficiently transported inside such a microcavity, mediated by incoherent absorption and emission processes. In particular, we show that there exist two distinct regimes of transport, viz. conductive and localized, arising from the complex interplay between the thermalizing effect of the dye molecules and the nonequilibrium influence of driving and loss. The propagation of light in the conductive regime occurs when several localized cavity modes undergo dynamical phase transitions to a condensed, or lasing, state. Furthermore, we observe that, while such transport is robust for weak disorder in the cavity potential, strong disorder can lead to localization of light even under good thermalizing conditions. Importantly, the exhibited transport and localization of light is a manifestation of the nonequilibrium dynamics rather than any coherent interference in the system.

A dimension witness provides a device-independent certification of the minimal dimension required to reproduce the observed data without imposing assumptions on the functioning of the devices used to generate the experimental statistics. In this paper, we provide a family of Bell expressions where Alice and Bob perform $2^{n-1}$ and $n$ number of dichotomic measurements, respectively, which serve as the device-independent dimension witnesses of Hilbert space of $2^m$ dimensions with $m=1,2,...,2^{\lfloor n/2\rfloor }$. The family of Bell expressions considered here determines the success probability of a communication game known as the $n$-bit parity-oblivious random access code. The parity obliviousness constraint is equivalent to the preparation noncontextuality assumption in an ontological model of an operational theory. For any given $n\ge 3$, if such a constraint is imposed on the encoding scheme of the random access code, then the local bound of the Bell expression reduces to the preparation noncontextual bound. We provide explicit examples for the $n=4\text{and}5$ case to demonstrate that the relevant Bell expressions certify the qubit and two-qubit system, and for the $n=6$ case to demonstrate that the relevant Bell expression certifies the qubit, two-qubit, and three-qubit systems. We further demonstrate the sharing of quantum preparation contextuality by multiple Bobs sequentially to examine whether the number of Bobs sharing the preparation contextuality is dependent on the dimension of the system. We provide explicit examples of $n=5$ and 6 to demonstrate that the number of Bobs sequentially sharing the contextuality remains the same for any of the $2^m$-dimensional systems.

Source-independent quantum networks are considered as a natural generalization to the Bell scenario where we investigate the nonlocal properties of quantum states distributed and measured in a network. Considering the simplest network of entanglement swapping, recently Gisin et al. [Phys. Rev. A 96, 020304(R) (2017)] and Andreoli [New. J. Phys. 19, 113020 (2017)] independently provided a systematic characterization of the set of quantum states leading to violation of the so-called bilocality inequality. In this work, we consider the complexities in quantum networks with an arbitrary number of parties distributed in chain-shaped and star-shaped networks. We derive the maximal violation of the “$n$-local” inequality that can be achieved by arbitrary two-qubit states for such chain- and star-shaped networks. This would further provide a deeper understanding of quantum correlations in complex structures.