The three-body recombination rate coefficients of the $\mathrm{H}+\mathrm{H}+\mathrm{H}\to {\mathrm{H}}_2+\mathrm{H}$ process for different final rovibrational levels of ${\mathrm{H}}_2$ are determined using a fully quantum-mechanical approach at zero total angular momentum. The Jahn-Teller coupling between the lowest electronic states of the ${\mathrm{H}}_3$ system is accounted for. It is found that the Jahn-Teller effect substantially enhances the recombination rates for deeply bound dimers at room temperature but only leads to a 12% increase in the total three-body recombination rate. It is also found that the nascent population of the ${\mathrm{H}}_2$ molecules, formed in the recombination process, is dominated by highly excited rovibrational levels.

Terahertz (THz) generation in a gas medium is simulated by quantum calculations with multicolor intense laser pulses of various frequency ratios. By correlating THz radiation with the above-threshold-ionization photoelectron spectrum, we confirm that our previously proposed free-free transition model for the generation of THz radiation is also applicable to multicolor pulses. THz photon emissions can proceed through transitions between continuous states with similar energies that have opposite parities. This mechanism predicts that THz waves can be efficiently generated at special two-color laser frequency ratios when the multiphoton mixing condition is satisfied. Applying this model to multicolor fields (sawtooth wave shape), we provide a quantum-mechanical interpretation for the reason for THz radiation enhancement. A scheme combining multiple lasers to raise THz radiation satisfying multiphoton mixing conditions is also proposed.

We obtain a striking spin soliton in a two-component Bose-Einstein condensate and investigate its motions in the presence of a constant force. The initially static spin soliton first moves in a direction opposite to the force and then changes direction, showing an extraordinary ac oscillation. The underlying mechanism is uncovered: the spin soliton can exhibit a periodic transition between negative and positive inertial mass because of a particular relation between its energy and moving velocity. We then develop a quasiparticle model that can account for this extraordinary oscillation. Important implications and possible applications are discussed.

We report on the study of the impact of the finite resolution of the chirp rate applied on the frequency difference between the Raman lasers beamsplitters onto the phase of a free fall atom gravimeter. This chirp induces a phase shift that compensates the one due to gravity acceleration, allowing for its precise determination in terms of frequencies. In practice, it is most often generated by a direct digital synthesizer (DDS). Besides the effect of eventual rounding errors, we evaluate here the bias on the $g$ measurement due to the finite time and frequency resolution of the chirp generated by the DDS, and show that it can compromise the measurement accuracy. However, this effect can be mitigated by an adequate choice of the DDS chirp parameters resulting from a trade-off between interferometer phase resolution and induced bias.

We consider the interaction of two-component bright-bright solitons with a narrow potential barrier (splitter) in the framework of a system of two Gross-Pitaevskii (nonlinear-Schrödinger) equations modeling a binary Bose-Einstein condensate, with self-attraction in each component and cross-attraction between them. The objective is to study splitting of composite solitons, which may be used in the design of two-component soliton-interferometer schemes. We produce approximate analytic results, assuming a weak barrier and applying the perturbation theory in the limit in which the system is integrable and the solitary waves may be considered as exact solitons. We do this in the case of negligible interspecies interactions, and also when the nonlinearities are strongly asymmetric, allowing one to neglect the self-interaction in one of the species. Then, we use systematic simulations to study the transmissions of both components in regions outside these approximations and, in particular, to compare numerical results with their analytical counterparts. We conclude that there is an appreciable parameter range where one component is almost entirely transmitted through the barrier, while the other one is reflected. The excitation of internal vibrations in the passing and rebounding solitons is explored as well, with a conclusion that it is weak in the regime of high-quality splitting.

Macroscopic entangled cat states not only are significant in the demonstration of the fundamentals of quantum physics, but also have wide applications in modern quantum technologies such as continuous-variable quantum information processing and quantum metrology. Here we propose a scheme for generation of macroscopic entangled cat states in a longitudinal-coupled cavity-QED model. In particular, we introduce the scheme by using the terms of a molecular cavity-QED system, which is composed of an organic molecule (including electronic and vibrational states) coupled to a single-mode cavity field. In this system, the resonance frequencies of the molecular vibration and the cavity field are modulated simultaneously, then the molecular vibrational displacement is enhanced significantly and hence macroscopic entangled cat states between the molecular vibrational mode and the cavity mode can be created. We also study quantum coherence effects in the generated states by calculating the joint Wigner function and the degree of entanglement. The dissipation effects are included by considering the state generation in the open-system case. Our results will pave the way to the study of quantum physics and quantum chemistry in molecular cavity-QED systems. The physical mechanism behind this scheme is general and can be extended to other longitudinal-coupled cavity-QED systems.

We present a generic theoretical framework to describe nonreciprocal microwave circulation in a multimode cavity magnonic system and assess the optimal performance of practical circulator devices. We show that high isolation ($>56$ dB), extremely low insertion loss ($<0.05$ dB), and flexible bandwidth control can be potentially realized in high-quality-factor superconducting cavity based magnonic platforms. These circulation characteristics are analyzed with materials of different spin densities. For high-spin-density materials such as yttrium iron garnet, the strong-coupling operation regime can be harnessed to obtain a broader circulation bandwidth. We also provide practical design principles for a highly integratable low-spin-density material (vanadium tetracyanoethylene) for narrow-band circulator operation, which could benefit noise-sensitive quantum microwave measurements. This theory can be extended to other coupled systems and provide design guidelines for achieving tunable microwave nonreciprocity for both classical and quantum applications.

We study experimentally and theoretically a dual-polarization fiber laser submitted to time-delayed frequency-shifted optical feedback. In addition to the usual frequency-locking regime that is expected when the frequency shift is close to the polarization beat frequency, we observe at high pumping rates a dynamical pulsing regime inside the locking range. This regime is experimentally evidenced in a full-fibered experiment based on an erbium-doped distributed-feedback fiber laser in which polarization beat frequency is about 1 GHz. A rate-equation model including the frequency-shifted feedback term reproduces well the experimental bifurcation maps, provided that both the time delay and a phase-amplitude coupling parameter ($\alpha$ factor) are taken into account. The impact on microwave-photonics applications is discussed.

Based on a Liouville-space formulation of open systems, we present two methods to solve the quantum dynamics of coupled harmonic oscillators experiencing Markovian loss. The starting point is the quantum master equation in Liouville space which is generated by a Liouvillian that induces a Lie algebra. We show how this Lie algebra allows to define ladder operators that construct Fock-like eigenstates of the Liouvillian. These eigenstates are used to decompose the time-evolved density matrix and, together with the accompanying eigenvalues, provide insight into the transport properties of the lossy system. Additionally, a Wei-Norman expansion of the generated time evolution can be found by a structure analysis of the algebra. This structure analysis yields a construction principle to implement effective non-Hermitian Hamiltonians in lossy systems.

The phenomenon of quantum steering in bipartite quantum systems can be reduced to the question of whether or not the first party can perform measurements such that the conditional states on the second party can be explained by a local hidden-state model. Clearly, the answer to this depends on the measurements which the first party is able to perform. We introduce a local hidden-state model explaining the conditional states for all generalized measurements on Bell-diagonal states of two qubits. More precisely, it is known for the restricted case of projective measurements and Bell-diagonal states characterized by the correlation matrix $T$ that a local hidden-state model exists if and only if $R_T=2\pi N_T\left|det\left(T\right)\right|\ge 1$, where $N_T$ is defined by an integral over the Bloch sphere. For generalized measurements described by positive operator-valued measures we construct a model if $R_T\ge 6/5$. Our work paves the way for a systematic study of steerability of quantum states with generalized measurements beyond the highly symmetric Werner states.

Complexity of dynamics is at the core of quantum many-body chaos and exhibits a hierarchical feature: higher-order complexity implies more chaotic dynamics. Conventional ergodicity in thermalization processes is a manifestation of the lowest-order complexity, which is represented by the eigenstate thermalization hypothesis (ETH) stating that individual energy eigenstates are thermal. Here, we propose a higher-order generalization of the ETH, named the $k$-ETH ($k=1,2,\cdots$), to quantify higher-order complexity of quantum many-body dynamics at the level of individual energy eigenstates, where the lowest-order ETH (1-ETH) is the conventional ETH. The explicit condition of the $k$-ETH is obtained by comparing Hamiltonian dynamics with the Haar random unitary of the $k$-fold channel. As a nontrivial contribution of the higher-order ETH, we show that the $k$-ETH with $k\ge 2$ implies a universal behavior of the $k\mathrm{th}$ Rényi entanglement entropy of individual energy eigenstates. In particular, the Page correction of the entanglement entropy originates from the higher-order ETH, while, as is well known, the volume law can be accounted for by the 1-ETH. We numerically verify that the 2-ETH approximately holds for a nonintegrable system, but does not hold in the integrable case. To further investigate the information-theoretic feature behind the $k$-ETH, we introduce a concept named a partial unitary $k$-design (PU $k$-design), which is an approximation of the Haar random unitary up to the $k\mathrm{th}$ moment, where “partial” means that only a limited number of observables are accessible. The $k$-ETH is a special case of a PU $k$-design for the ensemble of Hamiltonian dynamics with random-time sampling. In addition, we discuss the relationship between the higher-order ETH and information scrambling quantified by out-of-time-ordered correlators. Our framework provides a unified view on thermalization, entanglement entropy, and unitary $k$-designs, leading to deeper characterization of higher-order quantum complexity.

Quantum process tomography (QPT) methods aim at identifying, i.e., estimating, a given quantum process. QPT is a major quantum information processing tool, since it especially allows one to characterize the actual behavior of quantum gates, which are the building blocks of quantum computers. However, usual QPT procedures are complicated, since they set several constraints on the quantum states used as inputs of the process to be characterized. In this paper, we extend QPT so as to avoid two such constraints. On the one hand, usual QPT methods require one to know, hence to very precisely control (i.e., prepare), the specific quantum states used as inputs of the considered quantum process, which is cumbersome. We therefore propose a blind, or unsupervised, extension of QPT (i.e., BQPT), which means that this approach uses input quantum states whose values are unknown and arbitrary, except that they are requested to meet some general known properties (and this approach exploits the output states of the considered quantum process). On the other hand, usual QPT methods require one to be able to prepare many copies of the same (known) input state, which is constraining. In contrast, we propose “single-preparation BQPT methods” (SBQPT) , i.e., methods which can operate with only one instance of each considered input state. These two concepts are here illustrated with practical (S)BQPT methods which are numerically validated, in the case when (i) random pure states are used as inputs and their required properties are especially related to the statistical independence of the random variables that define them and (ii) the considered quantum process is based on cylindrical-symmetry Heisenberg spin coupling. As a benchmark, we moreover introduce nonblind QPT methods dedicated to the considered Heisenberg process, we analyze their theoretical behavior (this requires the tools developed in this paper for random input states), and we numerically test their sensitivity to systematic and nonsystematic errors, which are most likely to occur in practice. This shows that, even for very low preparation errors (especially systematic ones), these nonblind QPT methods yield much lower performance than our SBQPT methods. Our blind and single-preparation QPT concepts may be extended, e.g., to a much wider class of processes and to SBQPT methods based on other quantum state properties, as outlined in this paper.

A challenge to upcoming experiments that plan to collide a particle beam with laser pulses of moderate intensity is how to correctly incorporate quantum effects into simulation frameworks. Using a uniform approach, we extend the widely used locally constant field approximation (LCFA) to derive an improved rate of photon-seeded pair creation (the nonlinear Breit-Wheeler process). By benchmarking our “ULCFA” expressions with the lightfront spectrum of (i) exact analytical results and (ii) numerical integration QED probability for short pulses, we show that our extended approach remains accurate at smaller values of the intensity parameter than the standard LCFA.

The wavelength and rate of the $5p-5s$ transition of W xiv–W xvi ions have been calculated by the relativistic configuration interaction method with the implementation of the flexible atomic code. A reasonable collisional-radiative model has been constructed to simulate the $5p-5s$ transition spectrum of W xiv–W xvi ions, which had been observed in the electron beam ion trap device. The results are in reasonable agreement with the available experimental and theoretical data, which might be applied to identify the controversial spectra. The confusion on the assignment of the ionization stage is solved in the present work.

We measure the lifetime of the cesium $5{}^{2}D_{5/2}$ state using a time-resolved single-photon-counting method. We excite atoms in a hot vapor cell via an electric quadrupole transition at a wavelength of 685 nm and record the fluorescence of a cascade decay at a wavelength of 852 nm. We extract a lifetime of $1353\left(5\right)$ ns for the $5{}^2{D}_{5/2}$ state, in agreement with a recent theoretical prediction. In particular, the observed lifetime is consistent with the literature values of the polarizabilities of the cesium $6P$ states. Our measurement contributes to resolving a long-standing disagreement between a number of experimental and theoretical results.

We consider nonradiative annihilation of a positron on electrons bound by the field of a heavy nucleus. This process proceeds via the interaction of the annihilating positron-electron pair with another bound electron which is emitted carrying away the energy release. We present a fully relativistic treatment of this process which employs exact Dirac wave functions to describe the motion of the leptons in the nuclear field and regards the interaction between the leptons as a perturbation. We calculate the total cross section for this process for a number of elements ranging from silver to fermium. Our results substantially deviate from those previously reported in the literature, both for the absolute values of the cross section and its dependence on the energy of the incident positron. However, when we combine our results and the principle of detailed balance to get the cross section for the inverse process (so-called negative-continuum dielectronic recombination), a very good agreement is found with the existing results obtained by direct calculations. We also compare nonradiative annihilation with radiative annihilation proceeding with emission of a single photon and show that the cross sections for the former are much smaller.

A recent investigation of nitrogen $1s$ ionization in ${\mathrm{N}}_2$ by 40-keV photons has shown that only 70% of the recoil-induced internal energy appears as kinetic energy of the two nitrogen ions that are produced following the Auger decay of the core-ionized molecule. It has been concluded that the remaining 30% appears as part of the Auger transition energy. Presented here is an investigation using both classical and quantum mechanical models of how this unexpected result might come about. With these models, and the assumption of an isotropic distribution of the recoil momentum with respect to the molecular axis, it is possible to account for as much as 16% of the internal excitation contributing to the Auger transition energy, but within this framework there does not seem to be an obvious mechanism for producing a contribution as high as 30%. Consideration of refinements to the simple models does not improve this disagreement between prediction and observation. Assumption of a sufficiently anisotropic angular distribution can bring prediction and experiment into agreement, but such anisotropy is not consistent with other experimental results or theoretical expectations.

In this work, we propose a method to characterize the temporal structure of arbitrary optical laser pulses with low pulse energies. Our theory is based on strong-field photoelectron holography with the glory rescattering effect as the underlying mechanism in the near-forward direction. Utilizing the subfemtosecond glory rescattering process as a fast temporal gate to sample the unknown light pulse, the time-dependent vectorial electric field can be retrieved from the streaking photoelectron momentum spectra. Our method avoids the challenging task of generation or manipulation of attosecond pulses and signifies important progress in arbitrary optical waveform characterization.

We experimentally and theoretically investigate the ultrafast collisional dynamics of a symmetric-top molecule (${\mathrm{C}}_2{\mathrm{H}}_6$) in pure gas and mixtures with He at high density by employing the rotational alignment echo created by a pair of time-delayed intense laser kicks. The decrease of the amplitude of the echo when increasing the delay between the two laser pulses, reflecting the collisional relaxation of the system, is measured by probing the transient birefringence induced in the medium. The theoretical predictions, carried using purely classical molecular dynamics simulations, reproduce well the observed features, as demonstrated previously for a linear molecule. The analysis shows that the dissipation of the ethane alignment, despite the fact that this species has an extra rotational degree of freedom as compared to a linear molecule, barely involves more complex collisional relaxation channels due to characteristics of the ${\mathrm{C}}_2{\mathrm{H}}_6-{\mathrm{C}}_2{\mathrm{H}}_6$ and ${\mathrm{C}}_2{\mathrm{H}}_6-\mathrm{He}$ interactions. However, our findings reveal that the dissipative dynamics of a symmetric-top molecule can be properly approached using the recently discovered rotational alignment echoes, which, so far, have been only tested for probing rotational decoherence of simpler (linear) molecules.