We use the molecular convergent close-coupling (MCCC) method to perform calculations of 10–1000 eV electron scattering on the ground state of ${\mathrm{HeH}}^{+}$. Cross sections are presented for excitation of the $n=2-3$ singlet and triplet states (where $n$ is the united-atoms-limit principle quantum number), as well as ionization. We also present cross sections for ${\mathrm{He}}^{+}$ and ${\mathrm{H}}^{+}$ ion production following dissociative excitation and ionization. The ${\mathrm{He}}^{+}$ production cross section is compared with the measurements of Lecointre, Jureta, Urbain, and Defrance [J. Lecointre, J. J. Jureta, X. Urbain, and P. Defrance, J. Phys. B At. Mol. Opt. Phys. 47, 015203 (2014)]. We find that the MCCC results are up to 30% higher than experiment.

Two-dimensional infrared (2DIR) spectroscopy is widely used to study molecular dynamics, but it is typically restricted to solid and liquid phase samples and modest spectral resolution. Only recently has its potential to study gas-phase dynamics begun to be realized. Moreover, the recently proposed technique of cavity-enhanced 2D spectroscopy using frequency combs and developments in multicomb spectroscopy is expected to dramatically advance capabilities for acquisition of rotationally resolved 2DIR spectra. This demonstrates the need for rigorous and quantitative treatment of rotationally resolved, polarization-dependent third-order response of gas-phase samples. In this article, we provide a rigorous and quantitative description of rotationally resolved 2DIR spectroscopy using density-matrix, time-dependent perturbation theory and angular momentum algebra techniques. We describe the band and branch structure of 2D spectra, decompose the molecular response into polarization-dependence classes, use this decomposition to derive and explain special polarization conditions, and relate the liquid-phase polarization conditions to gas-phase ones. Furthermore, we discuss the rotational coherence dynamics during the waiting time.

We show through simulation that the improved quantitative rescattering model (QRS) can successfully predict the nonsequential double ionization (NSDI) process by intense elliptically polarized laser pulses. Using the QRS model, we calculate the correlated two-electron and ion momentum distributions of NSDI in Ne exposed to intense elliptically polarized laser pulses with a wavelength of 788 nm at a peak intensity of $5.0\times {10}^{14}$ W/${\mathrm{cm}}^2$. We analyze the asymmetry in the doubly charged ion momentum spectra observed by Kang et al. [Phys. Rev. Lett. 120, 223204 (2018)] in going from linearly to elliptically polarized laser pulses. Our model reproduces the experimental data well. Furthermore, we find that the ellipticity-dependent asymmetry arises from the drift velocity along the minor axis of the elliptic polarization. We explain how the correlated electron momentum distributions along the minor axis provide access to the subcycle dynamics of recollision. From these findings, we expect that we can extend the QRS model for NSDI toward more complicated laser fields in the future.

The ability to prepare a specific superposition of electronic excited states leading to a transitory symmetry breaking of the electronic density in complex systems remains a challenging concern. We investigate how an initial coherence can be controlled by laser fields. The selected molecular system is a symmetric dimer of phenylene ethynylene presenting different interesting properties: Two bright nearly degenerate excited states coupled through a conical intersection are addressable by orthogonal transition dipole moments and are well separated from neighboring states. Creating a superposed state with equal weights corresponds to a right or left electronic localization as in the double-well system followed by a transitory oscillation between the two wells. To ensure a spectral bandwidth typically smaller than 0.25 eV, the pulse duration is in the tens of femtoseconds range, so nuclear motion cannot be neglected. Optimal control theory (OCT) is applied with guess fields that effectively create the target coherence in the absence of dephasing due to the vibrational baths. We analyze the field reshaping proposed by the control and we further fit a sequence of pulses on the optimal field. The overall result is efficient and robust disymmetry control over reasonable timescales of few tens of femtoseconds, exceeding the pulse duration. The monotonically convergent algorithm is combined with the hierarchical equations of motion (HEOM) able to treat strongly coupled non-Markovian dynamics. We also check the implementation of the combined OCT-HEOM approach in the tensor-train representation with propagation using the time-dependent variational method.

The propagation direction momentum component of direct photoelectrons emitted by ionization of an atomic target in an intense, linearly polarized, midinfrared laser field is analyzed. Within the dipole approximation, the average value of this component is zero. However, when nondipole corrections are included, it becomes nonzero. Applying the saddle-point approximation to compute the integral over the ionization times in the expression for the nondipole strong-field approximation differential ionization rate, we surprisingly find a negative momentum shift, corresponding to a shift against the radiation pressure force. Our analysis shows that there is a positive contribution originating from individual ionization pathways within one optical cycle. The interference of contributions from ionization pathways arising within the same optical cycle of the field (intracycle interference) causes an oscillatory behavior, which, crossing to negative values, induces the shift against the radiation pressure force.

A scheme is proposed to dynamically excite distinct eigenstate superpositions in three-body Bose-Fermi mixtures confined in a two-dimensional harmonic trap. The system is initialized in a noninteracting state with a variable spatial extent, and the scattering lengths are subsequently quenched spanning the regime from weak to strong interactions. For spatial widths smaller than the three-body harmonic oscillator length, a superposition of trimers and atom-dimers is dynamically attained, otherwise trap states are predominantly populated, as inferred from the frequency spectrum of the fidelity. Accordingly, the Tan contacts evince the buildup of short-range two- and three-body correlations in the course of the evolution. A larger spatial extent of the initial state leads to a reduction of few-body correlations, endowed, however, with characteristic peaks at the positions of the avoided crossings in the energy spectra, thereby signaling the participation of atom-dimers. Our results expose ways to dynamically excite selectively trimers, atom-dimers, and trapped few-body states characterized by substantial correlations, and they are likely to be accessible within current experiments.

We theoretically investigate the properties of two interacting ultracold highly magnetic atoms trapped in a one-dimensional harmonic potential. The atoms interact via an anisotropic long-range dipole-dipole interaction, which in one dimension effectively can be modeled by the contact interaction. We investigate the interplay of the external magnetic field, the spin-spin interaction, and the trapping potential and how they affect the magnetization of the system. We show the role of indistinguishability and symmetries in the dynamics by studying the time evolution of the observables that could be measured experimentally. The presented model may depict the on-site interaction of the extended Hubbard models, therefore giving a better understanding of the fundamental building block of the respective many-body quantum simulators.

By periodically driving a single bosonic Josephson junction (BJJ) with an impurity, a synthetic gauge field is generated in the Fock space of the system. At a critical synthetic gauge flux the ground state undergoes a quantum phase transition which is analogous to the Meissner-Abrikosov-vortex transition found in type-II superconductors with an applied magnetic field. A second quantum phase transition involving attractive interactions between the bosons of the BJJ is shown to enhance the sensitivity of the system to the Meissner-Abrikosov-vortex transition.

The ideal measurement-device-independent quantum key distribution (MDI-QKD), which is immune to all detector side-channel attacks, is built on the Bell state measurement of two single-photon states. However, in practical MDI-QKD where phase-randomized weak coherent pulses (PR-WCPs) are used, the mismatch between states preparation and measurement in X basis leads to the bit error rate of more than 25%, which poses a challenge to the security of QKD. In this paper, we provide a security analysis of practical MDI-QKD based on polarization coding. We analyze the Hong-Ou-Mandel interference of PR-WCPs and Poisson-distributed photon number states (PNs), and find that these two sources are equivalent in MDI-QKD. The security analysis based on entanglement distillation is carried out on PNs, and the measurement results in Z basis and X basis are averaged to overcome the noncorrelation of the error rates in these two bases. Compared with GLLP scheme, a tighter key rate is obtained in this work, and the key rate deviates from the linear key rate bound in a short distance due to the finite proportion of multiphoton terms.

The quantum approximate optimization algorithm was proposed as a heuristic method for solving combinatorial optimization problems on near-term quantum computers and may be among the first algorithms to perform useful computations in the postsupremacy, noisy, intermediate-scale era of quantum computing. In this work we exploit the recently proposed digital-analog quantum computation paradigm, in which the versatility of programmable universal quantum computers and the error resilience of quantum simulators are combined to improve platforms for quantum computation. We show that the digital-analog paradigm is suited to the quantum approximate optimization algorithm due to the algorithm's variational resilience against the coherent errors introduced by the scheme. By performing large-scale simulations and providing analytical bounds for its performance in devices with finite single-qubit operation time we observe regimes of single-qubit operation speed in which the considered variational algorithm provides a significant improvement over nonvariational counterparts in the digital-analog scheme.

Quantum chemistry calculations constitute an important application of quantum annealing (QA). For practical applications in quantum chemistry, it is essential to estimate the ground-state energy of the Hamiltonian with chemical accuracy. However, there is no known method for guaranteeing the accuracy of the energy estimated via QA. Here, we propose a method for guaranteeing the accuracy of the energy estimated via QA using the Weinstein and Temple bounds. In our scheme, before QA is performed, the energies of the ground state and first excited state must be preestimated with some error bars (corresponding to the possible estimation error) via classical computation with some approximations. We show that, if the expectation value and standard deviation of the energy of the state after QA are lower than certain threshold values (that we can calculate from the preestimation), the ground-state energy estimated by QA is closer to the ground-state energy than the preestimation. As the expectation value and standard deviation of the energy can be experimentally measured via QA, our results pave the way for accurate estimation of the ground-state energy using QA.

We calculate the Abraham force and the Aharonov-Casher force exerted by the electromagnetic quantum vacuum on alkali-metal atoms in highly excited, long-living Rydberg states. Because of their high polarizability and long lifetime such atoms are good candidates to observe these forces.

We present here a theoretical investigation on the Goos-Hänchen (GH) lateral shift of a probe field as it is reflected or transmitted from a three-layer system with a Rydberg atomic gas sandwiched between two dielectric slabs. Driven by this weak probe field and a strong-coupling field, Rydberg atoms with dipole-dipole interactions are capable of producing a nonlocal Kerr effect whose strength could far exceed the corresponding local Kerr effect at a rather low atomic density. The resultant GH shift is distinct from that observed in an extremely diluted atomic gas with negligible Rydberg-Rydberg interactions and has been examined in two particular cases specified by different coupling frequencies for a fixed probe frequency. In both cases, the nonlocal Kerr effect is found to result in an obvious enhancement of the GH shift and more importantly provide an alternative way for controlling the GH shift by varying the atomic density in an appropriate range. Finally, we present a possible realization of a highly sensitive displacement sensor by exploiting an approximately linear relation between the displacement of one dielectric slab and the GH shift of the probe field.

The electric-field spatial gradient is a very important property of an optical near-field (ONF). In this study, its effect on the single-photon ionization is theoretically investigated via the three-dimensional time-dependent Schrödinger equation and perturbation theory. The results show that the field gradient can lead to dipole-forbidden transitions, which competes with the usual dipole transition and results in changes of the photoelectron momentum distribution. Additionally, we establish a relationship between the field gradient with the angular distribution of the photoelectrons. Through this mapping, the field gradient can be accurately retrieved. Furthermore, a way for extracting the phase difference between the partial waves with odd and even parity from the photoelectron momentum distribution is proved to be feasible. We find that the Cooper minimum can enhance the effect of the field gradient and thus improves the accuracy of our proposed retrieval procedure. Our results pave the way to exploit atomic structural properties as a probe of the gradient of the ONF.

Helicity $H$, chirality $C$, and spin angular momentum $\mathbf{S}$ are three physical observables that play an important role in the study of optical fields. These quantities are closely related, but their connection is hidden by the use of four different vector fields for their representation, namely, the electric and magnetic fields $\mathbf{E}$ and $\mathbf{B}$, and the two transverse potential vectors ${\mathbf{C}}^{\perp }$ and ${\mathbf{A}}^{\perp }$. Helmholtz's decomposition theorem restricted to solenoidal vector fields entails the introduction of a bona fide inverse curl operator, which permits one to express the above three quantities in terms of the observable electric and magnetic fields only. This yields clear expressions for $H,C$, and $\mathbf{S}$, which are automatically gauge invariant and display electric-magnetic democracy.

Both correlated-mode coupling efficiency and photon pair flux (brightness) of spontaneous parametric down-conversion (SPDC) coupled into single-mode fibers are key parameters for various quantum information science applications. We present an experimental investigation on the optimization of the correlated-mode coupling efficiency and the brightness of frequency-degenerated SPDC photon pairs into single-mode optical fibers. Using photon pairs generated from a type-I phase-matched bulk $\beta$-barium borate crystal pumped by a continuous-wave laser, we carefully evaluated both the correlated-mode coupling efficiency and the brightness of SPDC light coupled to a single-mode optical fiber, while continuously changing the focusing of the pump and collection modes, experimenting with Rayleigh lengths from 3 to 280 (for the pump) and 0.2 to 20 (for the collection) times the crystal length. The measured data suggest that the correlated-mode coupling efficiency can be close to unity for any pump focus, however, the optimal Rayleigh length of the collection mode depends on the pump focus parameter, and the brightness can be increased by tightening the pump focus, provided that the Rayleigh length of the collection mode is precisely adjusted. A maximal correlated-mode coupling efficiency of $99.0\pm 1.3\%$ was experimentally achieved. These results will be useful for photonic quantum science and technology using SPDC sources with high heralding efficiencies.

We compare the global properties of different types of $N$-mode states and discuss their distinguishability in the macroscopic limit $N\gg 1$ by using the collective Wigner function, which is obtained by averaging over all the individual modes. It is shown that factorized $N$-mode states with a concentration of photons in a few modes are asymptotically indistinguishable from their symmetrized (i.e., highly correlated) counterparts in the case of collective measurements. In contrast, the collective properties of states with intermode distributed excitations are very different when compared to their symmetrized versions. We also discuss a class of scalable states for which the functional form of the collective Wigner function does not depend on the number of field modes, and we present results for the negativity of the collective Wigner function, for which in the macroscopic limit some states lose negativity and some do not.

Quantum states are important resources and their preparations are essential prerequisites to all quantum technologies. However, they are extremely fragile due to the inevitable dissipations. Here the all-optical generation of a deterministic squeezed Schrödinger-cat state based on dissipation is proposed. Our system is based on the Fredkin-type interaction between three optical modes, one of which is subject to coherent two-photon driving and the others are coherent driving. We show that an effective degenerate three-wave-mixing process can be engineered in our system, which can cause the simultaneous loss of two photons, resulting in the generation of a deterministic squeezed Schrödinger-cat state. More importantly, by controlling the driving fields in our system, the two-photon loss can be adjustable, which can accelerate the generation of squeezed Schrödinger-cat states. In addition, we exploit the squeezed Schrödinger-cat states to estimate the phase in the optical interferometer and show that the quantum Fisher information about the phase can reach the Heisenberg limit in the limit of a large photon number. Meanwhile, it can have an order of magnitude factor improvement over the Heisenberg limit in the low-photon-number regime, which is very valuable for fragile systems that cannot withstand large photon fluxes. This work proposes an all-optical scheme to deterministically prepare the squeezed Schrödinger-cat state with high speed and can also be generalized to other physical platforms.

We investigate the generation of nonreciprocal single photons in the chiral waveguide-cavity-emitter system. Due to the chirality of light-emitter interactions, photons in the left direction couple to a cavity which induces only linear optical effects, while photons in the right direction couple to a cavity-emitter system which can generate optical nonlinearity. In the regime of single-photon transmission, photons in the left direction are absorbed by the cavity at the resonance frequency, while photons in the right direction are transmitted due to the Rabi splitting. In the regime of two-photon transmission, optical linearity in the left direction cannot produce photon-photon interactions and does not change the statistics of photons. However, the effective repulsive interaction between photons can be produced by the optical nonlinearity in the right direction, suppressing two-photon transmission (photon blockade). Physically, the photon-photon bound state, which emerges due to the strong-photon-photon correlation mediated by the cavity-emitter system, plays a key role in generation of photon blockade at the resonance frequency. As a result, nonreciprocal single photons can be realized in the combination of single-photon transmission and photon blockade at the resonance frequency. Additionally, photon-photon interactions are sensitive to the width of the wave packet since photons should coincide at the interaction site.

We study theoretically the competition between directional asymmetric coupling and disorder in a one-dimensional array of quantum emitters chirally coupled through a waveguide mode. Our calculation reveals a highly nontrivial phase diagram for the eigenstates' spatial profile, nonmonotonously depending on the disorder and directionality strength. The increase of the coupling asymmetry drives the transition from Anderson localization in the bulk through delocalized states to chirality-induced localization at the array edge. Counterintuitively, this transition is not smeared by strong disorder but becomes sharper instead. Our findings could be important for the rapidly developing field of waveguide quantum electrodynamics, where chiral interactions and disorder play crucial roles.