We report integral, differential, and momentum transfer cross sections for elastic scattering of electrons by tetramethylmethane [$\mathrm{C}{\left({\mathrm{CH}}_3\right)}_4$], tetramethylsilane [$\mathrm{Si}{\left({\mathrm{CH}}_3\right)}_4$], and tetramethylgermane [$\mathrm{Ge}{\left({\mathrm{CH}}_3\right)}_4$]. The scattering cross sections were calculated with the Schwinger multichannel method implemented with pseudopotentials, in the static-exchange and static-exchange plus polarization levels of approximation, for impact energies up to 30 eV. All three molecules present in their integral elastic cross section a broad structure between 5 and 15 eV, which is a superposition of two shape resonances assigned to the $E$ and $T_2$ symmetries of $T_d$. The low-energy behavior of the $s$-wave integral cross section and the $s$-wave eigenphase support the presence of a Ramsauer-Townsend minimum for $\mathrm{C}{\left({\mathrm{CH}}_3\right)}_4$ at 0.16 eV, for $\mathrm{Si}{\left({\mathrm{CH}}_3\right)}_4$ at 0.32 eV, and for $\mathrm{Ge}{\left({\mathrm{CH}}_3\right)}_4$ at 0.40 eV. In general, the present elastic integral cross sections show a good qualitative agreement with the experimental total cross sections of Stefanowska-Tur et al. [J. Chem. Phys. 150, 094303 (2019)].

A system of three-species fermions in one spatial dimension (1D) with a contact three-body interaction is known to display a scale anomaly. This anomaly is identical to that of a two-dimensional (2D) system of two-species fermions. The exact relation between those two systems, however, is limited to the two-particle sector of the 2D case and the three-particle sector of the 1D case. Here, we implement a nonperturbative Monte Carlo approach, based on the worldline representation, to calculate the thermodynamics and static response of three-species fermions in 1D, thus tackling the many-body sector of the problem. We determine the energy, density, and pressure equations of state and the compressibility and magnetic susceptibility for a wide range of temperatures and coupling strengths. We compare our results with the third-order virial expansion and interpret the classical-quantum crossover as the onset of trimer degrees of freedom.

In the late 1980's, Ou and Mandel experimentally observed signal beatings by performing a non-time-resolved coincidence detection of two photons having interfered in a balanced beam splitter [Phys. Rev. Lett 61, 54 (1988)]. In this work, we provide an alternative interpretation of the fringe pattern observed in this experiment as the direct measurement of the chronocyclic Wigner distribution of a frequency Schrödinger-cat-like state produced by local spectral filtering. Based on this analysis, we also study the time-resolved HOM experiment to measure such frequency state.

We show that the mean time, which a quantum particle needs to escape from a system to the environment, is quantized and independent from most dynamical details of the system. In particular, we consider a quantum system with a general Hermitian Hamiltonian $\widehat{H}$ and one decay channel, through which probability dissipates to the environment with rate $\mathrm{\Gamma }$. When the system is initially prepared exactly in the decay state, the mean decay time $〈T〉$ is quantized and equal to $w/\left(2\mathrm{\Gamma }\right)$. $w$ is the number of distinct energy levels, i.e., eigenvalues of $\widehat{H}$, that have overlap with the decay state, and is also the winding number of a transform of the resolvent in the complex plane. Apart from the integer $w, 〈T〉$ is completely independent of the system's dynamics. The complete decay time distribution can be obtained from an electrostatic analogy and features rare events of very large dissipation times for parameter choices close to critical points, where $w$ changes, e.g., when a degeneracy is lifted. Experiments of insufficient observation time may thus measure a too small value of $w$. We discuss our findings in a disordered tight-binding model and in the two-level atom in a continuous-wave field.

The topological degeneracy of ground states in transverse field Ising chain cannot be removed by local perturbation and allows it to be a promising candidate for topological computation. We study the dynamic processes of crystallization and dissolution for the gapped ground states in an Ising chain. For this purpose, the real-space renormalization method is employed to build an effective Hamiltonian that captures the low-energy physics of a given system. We show that the ground state and the first-excited state of an $\left(N+1\right)$-site chain can be generated from that of the $N$-site one by adding a spin adiabatically and vice versa. Numerical simulation shows that the robust quasidegenerate ground states of finite-size chain can be prepared with high fidelity from a set of noninteracting spins by a quasiadiabatic process. As an application, we propose a scheme for entanglement transfer between a pair of spins and two separable Ising chains as macroscopic topological qubits.

When measuring quantum spins at two or more different times, the later measurements are affected by measurement backaction occurring due to the earlier measurements. This makes the measurement of temporal quantum correlation functions challenging. In this paper, I propose a measurement protocol that mitigates the effect of measurement backaction by exploiting spin selection rules. I show that, under suitable conditions, the effect of measurement backaction on two-time quantum correlations becomes negligible when probing a system consisting of spins with large spin quantum numbers $l\gg s$ by coupling it to a spin-$s$ ancilla degree of freedom. A potential application of such a measurement protocol is the probing of an array of Bose-Einstein condensates by light.

A quantum system may be purified, i.e., projected into a pure state, faster if one applies feedback operations during the measurement process. However, the existing results suggest that such an enhancement is only possible when the measurement efficiency exceeds 0.5, which is difficult to achieve experimentally. We address the task of finding the global optimal feedback control for purifying a single qubit in the presence of measurement inefficiency. We use the Bloch vector length, a more physical and practical quantity than purity, to assess the quality of the state, and employ a backward-iteration algorithm to find the globally optimal strategy. Our results show that a speedup is available for quantum efficiencies well below 0.5, which opens the possibility of experimental implementation in existing systems.

We investigate the phenomenology of nonlinear Compton scattering of polarized photons by unpolarized electrons in plane-wave backgrounds. The energy and angular spectra of polarized photons are calculated for linearly and circularly polarized pulses, monochromatic fields and constant crossed field backgrounds. When the field intensity is in the weakly nonlinear regime, photons in different polarization states are predicted to possess very different energies and angular distributions. We explain this difference by calculating the spectrum for nonlinear Thomson scattering in a linearly polarized monochromatic background and projecting the electron's trajectory onto the different polarization directions. Finally, we calculate a scenario for multi-GeV electrons and, by considering the energy and angular-resolved photon polarization purity, find one can achieve a GeV-photon beam with polarization purities of over $90\%$.

We study the system of multibody interacting bosons on a two-dimensional optical lattice and analyze the formation of bound bosonic pairs in the context of the Bose-Hubbard model. Assuming a repulsive two-body interaction we obtain the signatures of pair formation in the regions between the Mott insulator lobes of the phase diagram for different choices of higher-order local interactions. Considering the most general Bose-Hubbard model involving local multibody interactions we investigate the ground state properties utilizing the cluster mean-field theory approach and further confirm the results by means of sophisticated infinite projected entangled pair states calculations. By using various order parameters, we show that the choice of higher-order interaction can lead to pair superfluid phase in the system between two different Mott lobes. We also analyze the effect of temperature and density-dependent tunneling to establish the stability of the PSF phase.

Recent advances in the study of synthetic dimensions revealed a possibility to employ the frequency space as an additional degree of freedom which allows for investigating and exploiting higher-dimensional phenomena in a priori low-dimensional systems. However, the influence of nonlinear effects on the synthetic frequency dimensions was studied only under significant restrictions. In the present paper, we develop a generalized mean-field model for the optical field envelope inside a single driven-dissipative resonator with quadratic and cubic nonlinearities, whose frequencies are coupled via an electro-optical resonant temporal modulation. The leading-order equation takes the form of a driven Gross-Pitaevskii equation with a cosine potential. We numerically investigate the nonlinear dynamics in such a microring resonator with a synthetic frequency dimension in the regime where parametric frequency conversion occurs. We observe that the modulation brings additional control to the system, enabling one to readily create and manipulate bright and dark dissipative solitons inside the cavity. In the case of anomalous dispersion, we find that the presence of electro-optical mode coupling confines and stabilizes the chaotic modulation instability region. This leads to the appearance of an unconventional type of stable coherent structure which emerges in the synthetic space with restored translational symmetry, in a region of parameters where conventionally only chaotic modulation instability states exist. This structure appears in the center of the synthetic band and, therefore, is referred to as the band soliton. Finally, we extend our results to the case of multiple modulation frequencies with controllable relative phases creating synthetic lattices with nontrivial geometry. We show that an asymmetric synthetic band leads to the coexistence of chaotic and coherent states of the electromagnetic field inside the cavity, i.e., dynamics that can be interpreted as chimeralike states. Recently developed ${\chi }^{\left(2\right)}$ microresonators can open the way to experimentally exploring our findings.

Biomolecular light-harvesting antennas operate as nanoscale devices in a regime where the coherent interactions of individual light, matter, and vibrational quanta are nonperturbatively strong. The complex behavior arising from this could, if fully understood, be exploited for myriad energy applications. However, nonperturbative dynamics are computationally challenging to simulate, and experiments on biomaterials explore very limited regions of the nonperturbative parameter space. So-called quantum simulators of light-harvesting models could provide a solution to this problem, and here we employ the hierarchical equations-of-motion technique to investigate the recent superconducting experiments of Potočnik et al. [A. Potočnik et al., Nat. Commun. 9, 904 (2018)] used to explore excitonic energy capture. By explicitly including the role of optical driving fields, nonperturbative dephasing noise, and the full multiexcitation Hilbert space of a three-qubit quantum circuit, we predict the measurable impact of these factors on transfer efficiency. By analysis of the eigenspectrum of the network, we uncover a structure of energy levels that allows the network to exploit optical “dark” states and excited-state absorption for energy transfer. We also confirm that time-resolvable coherent oscillations could be experimentally observed, even under the strong, nonadditive action of the driving and optical fields.

Electronic stopping cross sections of different transition metals (Nb, Pd, Ta, and Pt) for light ions have been experimentally determined in a wide energy range. We performed relative measurements using different backscattering geometries for protons (from 50 to 5000 keV) and helium (from 80 to 10 000 keV). Data are compared to values from the literature, as well as to the widely used semiempirical (srim) and modeling (dpass) approaches. The magnitude and energy dependence of the deduced stopping power at energies around the Bragg peak, as well as the different trends observed within individual periods, are analyzed with respect to target atomic number and electronic structure. We also compare the observed magnitude of electronic stopping to several different theoretical approaches.

We present a theoretical investigation of high-order-harmonic generation (HHG) from impurity-doped materials. Based on the analysis of the exact numerical solutions of the time-dependent Schrödinger equation (TDSE) for one-dimensional models, we demonstrate the factorization of HHG yields as a product of an electron wave packet and the recombination cross section, in analogy to HHG from atoms and molecules in the gas phase. Furthermore, we show that the quantitative rescattering model based on this factorization accurately reproduces the TDSE results. This opens up new possibilities to study impurities in materials using the available techniques from strong-field physics.

In the general case, upon free-space propagation of a paraxial optical vortex (OV) its topological charge (TC) is not conserved, unlike the orbital angular momentum (OAM), which remains unchanged. In this work, we discuss a Gaussian beam with fractional TC in the source plane, showing theoretically and numerically in which way the TC is changing (but remains integer) upon free-space propagation. There are four evolution scenarios for an original OV with fractional TC that depend on how close the original TC is to an integer even or odd number. If the TC in the initial plane has an arbitrary fractional value, it becomes an odd integer in the Fourier plane. If the initial TC is close to an odd integer number, then, on propagation, an OV with a TC of +1 is born in the Fresnel diffraction zone, while in the far field an OV with a TC of –1 appears. This optical vortex with a TC of –1 is located in the beam periphery where the intensity is almost zero and thus it is difficult to detect it experimentally. Additional vortices with a TC of +1 and −1 are generated in the beam with the initial fractional TC due to the interference between a linear source appearing in the area of phase jump and an ordinary optical vortex with an integer TC. An experiment on registering the optical vortices by using the interferograms matches the theory and simulation. For simple OVs (such as Laguerre-Gaussian or Bessel-Gaussian modes), TC is conserved both upon propagation and following weak phase distortions. When scattered from a random phase screen, the integer TC of an OV is experimentally shown to conserve until random path-difference distortion reaches a half wavelength. Because of this, under weak-turbulence conditions, it makes sense to measure a discretely changing TC, rather than measuring the continuously varying OAM.

Quantum tomography is an essential experimental tool for testing any quantum technology implementation. Transverse spatial quantum states of light play a key role in many experiments in the field of quantum information as well as in free-space optical communications. In this paper we propose and experimentally demonstrate the tomography of spatial quantum states with a deformable mirror. It may be used to significantly outperform the conventional method with a spatial light modulator in terms of speed and efficiency, at the same time providing polarization insensitive operation.

We study tunable optomechanically induced transparency by controlling the dark-mode effect induced by two mechanical modes coupled to a common cavity field. This is realized by introducing a phase-dependent phonon-exchange interaction, which is used to form a loop-coupled configuration. Combining this phase-dependent coupling with the optomechanical interactions, the dark-mode effect can be controlled by the quantum interference effect. In particular, the dark-mode effect in this two-mechanical-mode optomechanical system can lead to a double-amplified optomechanically induced transparency (OMIT) window and a higher efficiency of the second-order sideband in comparison with the standard optomechanical system. This is because the effective mechanical decay rate related to the linewidth of the OMIT window becomes a twofold increase in the weak-coupling limit. When the dark-mode effect is broken, controllable double transparency windows appear and the second-order sideband, as well as the light delay or advance, is significantly enhanced. For an $N$-mechanical-mode optomechanical system, we find that in the presence of the dark-mode effect, the amplification multiple of the linewidth of the OMIT window is nearly proportional to the number of mechanical modes, and that the OMIT with a single window becomes the one with $N$ tunable windows by breaking the dark-mode effect. The study will be useful in optical information storage within a large-frequency bandwidth and multichannel optical communication based on optomechanical systems.

The most unstable quantum states and elementary particles possess more than a single decay channel. At the same time, it is well known that typically the decay law is not simply exponential. Therefore, it is natural to ask how to spot the nonexponential decay when (at least) two decay channels are opened. In this work, we study the tunneling phenomenon of an initially localized particle in two spatially opposite directions through two different barriers, mimicking two decay channels. In this framework, through specific quantum mechanical examples which can be accurately solved, we study the general properties of a two-channel decay that apply for various unstable quantum states (including unstable particles). Apart from small deviations at early times, the survival probability and the partial tunneling probability along the chosen direction are very well described by the exponential-decay model. In contrast, the ratios of the decay probabilities and probability currents are evidently not a simple constant (as they would be in the exponential limit), but display time-persisting oscillations. Hence, these ratios are optimal witnesses of deviations from the exponential-decay law.

We present a formalism to study many-particle quantum transport across a lattice locally connected to two finite, nonstationary (bosonic or fermionic) reservoirs, both of which are in a thermal state. We show that, for conserved total particle number, a system of nonlinear quantum-classical master equations describes the concurrent many-particle time evolution on the lattice and in the reservoirs. The finiteness of the reservoirs makes a macroscopic current emerge, which decreases exponentially in time and asymptotically drives the many-particle configuration into an equilibrium state where the particle flow ceases. We analytically derive the timescale of this equilibration process, and, furthermore, investigate the imprint of many-particle interferences on the transport process.

The dynamics of a closed quantum system, under a unitary time evolution $U$, is obviously linear. But, the reduced dynamics of an open quantum system $S$, interacting with an environment $E$, is not linear, in general. Dominy et al. [Quantum Inf. Process. 15, 465 (2016)] considered the case that the set $\mathcal{S}=\left\{{\rho }_{SE}\right\}$ of possible initial states of the system-environment is convex, and also possesses another property, which they called $U$-consistency. They have shown that, under such circumstances, the reduced dynamics of the system $S$ is linear. Whether the Dominy-Shabani-Lidar framework is the most general one is the subject of this paper. We assume that the reduced dynamics is linear and show that this leads us to their framework. In other words, the reduced dynamics of the system is linear if and only if it can be formulated within the Dominy-Shabani-Lidar framework.