We report elastic integral and differential cross sections for positron collisions with uracil (${\mathrm{C}}_4{\mathrm{H}}_4{\mathrm{N}}_2{\mathrm{O}}_2$) for impact energies up to 10 eV. The cross sections were calculated employing the Schwinger multichannel (SMC) method in the static plus polarization approximation. The Born-closure procedure was applied to account for the long-range potential due to the permanent dipole moment of uracil. The present results are compared with experimental and theoretical data available in the literature, showing an overall good agreement with previously reported results. We also applied our differential cross sections to provide a correction to the total cross-section measurements by Anderson et al. [J. Chem. Phys. 141, 034306 (2014)] in order to account for the lack of angular resolution in the apparatus used by these authors. Besides, we applied a simple model employed by Franz and Gianturco [Phys. Rev. A 88, 042711 (2013)] to estimate the temperature effects on the uracil cross section. In particular, the level of agreement of the SMC differential cross sections as compared with the experimental findings is encouraging.

Molecular hydrogen ions are of metrological relevance due to the possibility of precise theoretical evaluation of their spectrum and of external-field-induced shifts. In homonuclear molecular ions, the electric dipole $E1$ transitions are strongly suppressed, and of primary laser spectroscopy interest is the electric quadrupole ($E2$) transition spectrum. In continuation of previous work on the ${{\mathrm{H}}_2}^{+}$ ion, we report here the results of the calculations of the hyperfine structure of the laser-induced electric quadrupole transitions between a large set of rovibrational states of ${{\mathrm{D}}_2}^{+}$; the inaccuracies of previous evaluations have been corrected. The effects of the laser polarization are studied in detail. We show that the electric quadrupole moment of the deuteron can in principle be determined with low fractional uncertainty $(\simeq 1\times {10}^{-4})$ by comparing the results presented here with future data from precision spectroscopy of ${{\mathrm{D}}_2}^{+}$.

We theoretically study the ionization enhancement of the diatomic molecular ion ${\mathrm{H}}_2{}^{+}$ at two critical internuclear distances $R$, using orthogonal two-color laser fields. The polarization of the fundamental infrared laser field and a weak second-harmonic field is parallel and perpendicular to the molecular axis, respectively. It is observed that adding the second-harmonic field raises slightly the first ionization peak at the smaller critical $R$, whereas it enhances the second one at the larger critical $R$ significantly. We further analyze the observable evidence which distinguishes two underlying mechanisms responsible for the enhanced ionization of ${\mathrm{H}}_2{}^{+}$: (i) the resonant excitation along with the coherent interference of the ionizing wave packets from the $1s{\sigma }_g$ and $2p{\sigma }_u$ states and (ii) the easier ionization from the up-field site of the molecule.

The use of bichromatic counter-rotating laser field is known to generate high-order harmonics with nonzero ellipticity. By combining such laser field with a plasmonic-enhanced spatially inhomogeneous field, we propose a way to influence the subcycle dynamics of the high-harmonic generation process. Using the numerical solution of the time-dependent Schrödinger equation combined with classical trajectory Monte Carlo simulations, we show that the change of the direction and the strength of the plasmonic field selectively enhances or suppresses certain recombining electron trajectories. This in turn modifies the ellipticity of the emitted attosecond pulses.

Quantum-state preparation has a wide range of applications ranging from quantum optics to quantum metrology to fundamental physics precision measurements. In the context of nuclear $\beta$-decay correlation experiments such as the ones using argon, one needs to prepare a source of highly polarized atoms. Creating such a state-prepared sample amounts to generating a so-called stretched state, i.e., a state with the maximal projection of the total angular momentum along the quantization axis. This is typically achieved through optical pumping. Since this technique inherently depends on cycles of absorption followed by spontaneous emission, it may lead to undesirable heating and population loss. It is therefore crucial to devise polarization methods with both optimized efficiency and minimized drawbacks. We propose and compare various schemes, which we have been investigating numerically in the case of argon-35 atoms. We show that polarization degrees as high as $99.99\%$ can be obtained within less than 140 $\mu \mathrm{s}$.

We propose a method to prepare Majorana pairs at the corners of imprinted defects on a two-dimensional cold-atom optical lattice with $s$-wave superfluid pairing. Different from previous proposals that manipulate the effective Dirac masses, our scheme relies on the sign flip of the spin-orbit coupling at the corners, which can be tuned in experiments by adjusting the angle of incident Raman lasers. The Majorana corner pairs are found to be located at the interface between two regimes with opposite spin-orbit coupling strengths in an anticlockwise direction and are robust against certain symmetry-persevered perturbations. Our work provides a way for implementing and manipulating Majorana pairs with existing cold-atom techniques.

We investigate the role of the squeezing interaction in inducing topological Bogoliubov excitations of a bosonic system. We introduce a squeezing transformation which is capable of reducing the corresponding Bogoliubov–de Gennes Hamiltonian to an effective noninteracting one with the spectra and topology unchanged. In the weak interaction limit, we apply perturbation theory to investigate the squeezing-induced topological gap opening on bosonic Bogoliubov excitations and find that the squeezing interaction plays a role equivalent to that of spin-orbit or Zeeman-like coupling in the effective Hamiltonian. We thus apply this formalism to two existing models to provide a deeper understanding of their topological structures. We also construct minimal models based on the elegant Clifford algebra for realizing bosonic topological Bogoliubov excitations. Our construction is potentially applicable to experiments in bosonic systems.

We elaborate a comprehensive theory of the sharp variations of the four-wave-mixing (FWM) threshold happening with the tuning of the pump frequency along the dispersive tails of the nonlinear resonance in the driven Kerr microresonators with high finesses and high finesse dispersions. Our theory leads to the explicit estimates for the difference in the pump powers required for the excitation of one sideband pair with momenta $\pm \mu$ and for the simultaneous excitation of the two neighboring pairs with momenta $\pm \mu , \pm (\mu +1)$. This power difference also measures the depth of the Arnold tongues in the pump-frequency and pump-power parameter space, associated with the aforementioned power variations. A set of select pump frequencies and powers, where the instabilities of the two sideband pairs come first, forms the threshold of complexity, which is followed by a sequence of conditions specifying critical powers for the simultaneous instabilities of three, four and so on pairs. We formally link finesse dispersion and the density of states notion borrowed from the condensed matter context, and report a value of the finesse dispersion and a critical mode number signaling the transition from the low- to the high-contrast tongue structure. We also demonstrate that the large finesse dispersion makes possible a surprising for the multimode resonators regime of the bistability without FWM.

A simple analytical way of creating superpositions of Bessel-Gaussian light beams with knotted nodal lines is proposed. It is based on the equivalence between the paraxial wave equation and the two-dimensional Schrödinger equation for a free particle. The 2D Schrödinger propagator is expressed in terms of Bessel functions, which allows to obtain directly superpositions of beams with the desired topology of nodal lines. Four types of knots are constructed in an explicit way: the unknot, the Hopf link, the (3,3)-torus knot, and the trefoil. It is also shown, using the example of the figure-eight knot, that more complex structures require a larger number of constituent beams as well as high precision both from the numerical and the experimental sides. A tiny change of beam's intensity can lead to the knot “switching.” The figures refer to structures that can be called nanoknots. However, they equally apply to larger knots of up to several tens of the wavelength, if the scaling possibility provided by the presence of the parameter $\gamma$ is used.

We present a physically motivated variational wave function for the ground state of the asymmetric quantum Rabi model (AQRM). The wave function is a weighted superposition of squeezed coherent states entangled with nonorthogonal qubit states and relies only on three variational parameters in the regimes of interest where the squeezing effect becomes negligible. The variational expansion describes the ground state remarkably well in almost all parameter regimes, especially with arbitrary bias. We use the variational result to calculate various relevant physical observables of the ground state and make a comparison with existing approximations and the exact solution. The results show that the variational expansion is a significant improvement over the existing approximations for the AQRM.

In this article, we theoretically investigate the first- and second-order quantum dissipative phase transitions of a three-mode cavity with a Hubbard interaction. In both types, there is a mean-field (MF) limit cycle phase where the local U(1) symmetry and the time-translational symmetry of the Liouvillian superoperator are spontaneously broken. In MF, this spontaneous symmetry breaking manifests itself through the appearance of an unconditionally and fully squeezed state at the cavity output, connected to the well-known Goldstone mode. By employing the Wigner function formalism, hence, properly including the quantum noise, we show that away from the thermodynamic limit and within the quantum regime, fluctuations notably limit the coherence time of the Goldstone mode due to the phase diffusion. Our theoretical predictions suggest that interacting multimode photonic systems are rich, versatile test beds for investigating the crossovers between the mean-field picture and quantum phase transitions, a problem that can be investigated in various platforms including superconducting circuits, semiconductor microcavities, atomic Rydberg polaritons, and cuprite excitons.

Quantum Zeno and anti-Zeno effects are studied in an asymmetric nonlinear optical coupler composed of a probe waveguide and a system waveguide. The system is a nonlinear waveguide operating under the nondegenerate hyper-Raman process, while both the pump modes in the system are constantly interacting with the probe waveguide. The effect of the presence of the probe on the spatial evolution of the system in terms of the number of photons in Stokes and anti-Stokes modes as well as the phonon number is quantified as the Zeno parameter. The negative (positive) values of the Zeno parameter in the specific mode are considered as the signatures of the quantum Zeno (anti-Zeno) effect in that mode of the system. It is observed that the initial phase difference between all the coherent amplitudes involved in the Stokes and anti-Stokes generation processes can be controlled to induce a transition between quantum Zeno and anti-Zeno effects for the hyper-Raman process in both cases with and without phase matching. However, in the case of the hyper-Raman process in the system waveguide without a phase matching, the phase mismatch parameters, due to difference in the wave vectors of all the modes involved in the Stokes and anti-Stokes generation processes, can also be used analogously to cause the desired crossover. Further, the general nature of the physical system and the perturbative technique used here allow us to analytically study the possibilities of observing quantum Zeno and anti-Zeno effects in a large number of special cases, including situations where the process is spontaneous or partially spontaneous and/or the system is operated under a degenerate hyper-Raman process, or a simple Raman process.

Based on the experiments [S. Putz et al., Nat. Phys. 10, 720 (2014); Nat. Photonics 11, 36 (2017)] on the hybrid quantum system of a superconducting microwave cavity coupled strongly to an inhomogeneous broadening spin ensemble, we use the exact master equation theory to investigate its non-Markovian decoherence dynamics under external driving fields. In the experiments, the spin ensemble is made of negatively charged nitrogen-vacancy defects in diamond. The exact master equation theory generalizes the fluctuation-dissipation relation and depicts in details the transient non-Markovian decoherence, in which the dissipation (relaxation) and fluctuations (noise or dephasing) dynamics is well described and the non-Markovian memory effect can be well characterized. We study the physical picture of the transient non-Markovian decoherence dynamics and explore how the non-Markovian decoherence induced by the inhomogeneous broadening of the spin ensemble is suppressed in the strong-coupling regime. We also show how the spectral hole burning generates localized bound states for the further decoherence suppression. Furthermore, we investigate the two-time correlations to show the relationships between quantum fluctuations and quantum memory.

We explore both pure and mixed state Floquet dynamical quantum phase transitions (FDQFTs) in the one-dimensional $p$-wave superconductor with a time-driven pairing phase. In the Fourier space the model is recast to the noninteracting quasispins subjected to a time-dependent effective magnetic field. We show that FDQFTs occur within a range of driving frequency without resorting to any quenches. Moreover, FDQFTs appear in the region where quasispins are in the resonance regime. In the resonance regime, the population completely cycles the population between the spin down and up states. Additionally, we study the conditions for the appearance of FDQFTs using the entanglement spectrum and purity entanglement measure. Our results imply that the entanglement spectrum can truly capture the resonance regime where FDQFTs occur. Particularly, the dynamical topological region results in the degeneracy of the entanglement spectrum. It is shown that the boundary of the driven frequency range, over which the system reveals FDQFTs, is signaled by the purity entanglement measure.

The main obstacles to the realization of high-fidelity quantum gates are the control errors arising from inaccurate manipulation of a quantum system and the decoherence caused by the interaction between the quantum system and its environment. Nonadiabatic holonomic quantum computation allows for high-speed implementation of whole-geometric quantum gates, making quantum computation robust against control errors. Dynamical decoupling provides an effective method to protect quantum gates against environment-induced decoherence, regardless of collective decoherence or independent decoherence. In this paper, we put forward a protocol of nonadiabatic holonomic quantum computation protected by dynamical decoupling. Due to the combination of nonadiabatic holonomic quantum computation and dynamical decoupling, our protocol not only possesses the intrinsic robustness against control errors but also protects quantum gates against environment-induced decoherence.

We propose a method named as double-scanning method to improve the key rate of measurement-device-independent quantum key distribution (MDI-QKD) drastically. In the method, two parameters are scanned simultaneously to tightly estimate the counts of single-photon pairs and the phase-flip error rate jointly. Numerical results show that the method in this work can improve the key rate by $35\%–280\%$ in a typical experimental setup. Besides, we study the optimization of MDI-QKD protocol with all parameters including the source parameters and failure probability parameters, over symmetric channel or asymmetric channel. Compared with the optimized results with only the source parameters, the all-parameter-optimization method could improve the key rate by about $10\%$.

What could happen if we pinned a single qubit of a system and fixed it in a particular state? First, we show that this leads to difficult static questions about the ground-state properties of local Hamiltonian problems with restricted types of terms. In particular, we show that the pinned commuting and pinned stoquastic Local Hamiltonian problems are quantum-Merlin-Arthur–complete. Second, we investigate pinned dynamics and demonstrate that fixing a single qubit via often repeated measurements results in universal quantum computation with commuting Hamiltonians. Finally, we discuss variants of the ground-state connectivity (GSCON) problem in light of pinning, and show that stoquastic GSCON is quantum-classical Merlin-Arthur–complete.

We propose a high-rate generation method of optical Schrödinger cat states. Thus far, photon subtraction from squeezed vacuum states has been a standard method in cat-state generation, but its constraints on experimental parameters limit the generation rate. In this paper, we consider the state generation by photon number measurement in one mode of two-mode Gaussian states, which is a generalization of conventional photon subtraction, and derive the conditions to generate high-fidelity and large-amplitude cat states. Our method relaxes the constraints on experimental parameters, allowing us to optimize them and attain a high generation rate. Supposing realistic experimental conditions, the generation rate of cat states with large amplitudes ($\left|\alpha \right|\ge 2)$ can exceed megacounts per second, about ${10}^3$ to ${10}^6$ times better than typical rates of conventional photon subtraction. This rate would be improved further by the progress of related technologies. The ability to generate non-Gaussian states at a high rate is important in quantum computing using optical continuous variables, where scalability has been demonstrated but preparation of non-Gaussian states of light remains as a challenging task for universality and fault tolerance. Our proposal reduces the difficulty of the state preparation and opens a way for practical applications in quantum optics.