We provide a method for estimating spectral gaps in low-dimensional systems. Unlike traditional phase estimation, our approach does not require ancillary qubits nor does it require well-characterized gates. Instead, it only requires the ability to perform approximate Haar random unitary operations, applying the unitary whose eigenspectrum is sought and performing measurements in the computational basis. We discuss application of these ideas to in-place amplitude estimation and quantum device calibration.

We describe a scheme for entangling mechanical resonators which is efficient beyond the resolved sideband regime. It employs the radiation pressure force of the squeezed light produced by a degenerate optical parametric oscillator, which acts as a reservoir of quantum correlations (squeezed reservoir), and it is effective when the spectral bandwidth of the reservoir and the field frequencies are appropriately selected. It allows for the steady-state preparation of mechanical resonators in entangled Einstein-Podolsky-Rosen states and can be extended to the preparation of many entangled pairs of resonators which interact with the same light field, in a situation in which the optomechanical system realizes a starlike harmonic network.

Scalar-pseudoscalar $\mathcal{P},\mathcal{T}$-odd interaction between the electron and the nucleus in atomic systems is constructed within the standard model as an exchange by Higgs boson. The necessary $\mathcal{P}$- and $\mathcal{T}$-violating contribution is obtained at the three-loop level on the basis of Cabibbo-Kobayashi-Maskawa matrix. This contribution, unlike the corresponding contribution to the electron electric dipole moment (EDM), does not vanish since the “Higgs charges” of quarks, contrary to their electric charges, are flavor dependent. Order-of-magnitude estimates of the effect expressed as an “equivalent” electron EDM give the values within the range $d_e^{\mathrm{eqv}}\sim {10}^{-40}\text{–}{10}^{-45}e\text{cm}$, depending on the known different estimates for the electron EDM. This can be compared with the known “benchmark” two-photon $\mathcal{P},\mathcal{T}$-odd electron-nucleus interaction effect, which provides $d_e^{\mathrm{eqv}}\sim {10}^{-38}e\text{cm}$.

In this work, we introduce a semi-implicit second-order correction scheme to the kinetic Kohn-Sham lattice model. This approach is validated by performing realistic exchange-correlation energy calculations of atoms and dimers of the first two rows of the Periodic Table, finding good agreement with the expected values. Additionally, we simulate the ethane molecule, where we recover the bond lengths and compare the results with standard methods. Finally, we discuss the current applicability of pseudopotentials within the lattice kinetic Kohn-Sham approach.

The intensities of photoelectron satellite lines, corresponding to the double core hole (DCH) states of Ar $1s$ ionization by hard x rays, are calculated using the many-body perturbation theory. Calculations support the interpretation of the most intense lines as the shake-up excitations $2p\to 4p$. It is demonstrated that the intensities of the spectrum lines corresponding to $4s$ (and $3d$) excited states in the DCH field can be explained only taking into account the knock-up process $2p\to 3d$ along with the shake-up process $1s\to 4s$ that accompanies $2p$ photoionization.

We propose a method to study the rotational relaxation of polar molecules [here taking fluoromethane $\left({\mathrm{CH}}_3\mathrm{F}\right)$ as an example] in collisions with 3.5 K buffer-gas helium (He) atoms by using an electrostatic guiding technique. The dependence of the guiding signal of ${\mathrm{CH}}_3\mathrm{F}$ on the injected He flux and the dependence of the guiding efficiency of ${\mathrm{CH}}_3\mathrm{F}$ on its rotational temperature are investigated both theoretically and experimentally. By comparing the experimental and simulated results, we find that the translational and rotational temperatures of the buffer-gas cooled ${\mathrm{CH}}_3\mathrm{F}$ molecules can reach to about 5.48 and 0.60 K, respectively, and the ratio between the translational and average rotational collisional cross sections of ${\mathrm{CH}}_3\mathrm{F}-\mathrm{He}$ is $\gamma ={\sigma }_t/{\sigma }_r=36.49\pm 6.15$. In addition, the slowing, cooling, and boosting effects of the molecular beam with different injected He fluxes are also observed and their forming conditions are investigated in some detail. Our study shows that our proposed method can not only be used to measure the translational and rotational temperatures of the buffer-gas cooled molecules, but also to measure the ratio of the translational collisional cross section to the average rotational collisional cross section, and even to measure the average rotational collisional cross section when the translational collisional cross section is measured by fitting the lifetime of molecule signal to get a numerical solution from the diffusion equation of buffer-gas He atoms in the cell.

Based on mean-field Gross-Pitaevskii and Bogoliubov–de Gennes approaches, we investigate excitations of a one-dimensional soft-core interacting ultracold Bose gas under the effect of an optical lattice. It is found that no matter how deep the lattice is, at $q\to 0$ the lowest mode corresponds to a gapless phonon, ${\omega }_1^2=v_1^2{q}^2$, whereas the second lowest mode corresponds to a gapped optical phonon, ${\omega }_2^2={\mathrm{\Delta }}^2\pm v_2^2{q}^2$. Determination of the velocities $v_1,v_2$, the gap $\mathrm{\Delta }$, and the possible sign change in ${\omega }_2$ upon the change of lattice depth can give decisive measures to the transitions across various supersolid and solid states. The power law $v_1\sim {\left(f_s\right)}^{1/2}$ with $f_s$ the superfluid fraction is identified in the present system at the tight-binding regime.

The influence of the laser-pulse temporal shape on the nonlinear Thomson scattering on-axis photon spectrum is analyzed in detail. Using the classical description, analytical expressions for the temporal and spectral structure of the scattered radiation are obtained for the case of symmetric laser-pulse shapes. The possibility of reconstructing the incident laser pulse from the scattered spectrum averaged over interference fringes in the case of high peak intensity and symmetric laser-pulse shape is discussed.

TE-polarized electromagnetic waves, guided by a three-layer slab structure consisting of a central film with quartic permittivity placed between two half spaces with Kerr permittivity, are studied. Traveling-wave solutions of Maxwell's equations are expressed in terms of Weierstrass's elliptic function $\wp$. A general dispersion relation is derived and evaluated by using a phase diagram analysis. Emphasis is placed on the conditions of existence and solvability of the dispersion relation. Numerical results are presented.

We derive equations for the strongly coupled system of light and dense atomic ensembles. The formalism includes an arbitrary internal-level structure for the atoms and is not restricted to weak excitation of atoms by light. In the low-light-intensity limit for atoms with a single electronic ground state, the full quantum field-theoretical representation of the model can be solved exactly by means of classical stochastic electrodynamics simulations for stationary atoms that represent cold atomic ensembles. Simulations for the optical response of atoms in a quantum degenerate regime require one to synthesize a stochastic ensemble of atomic positions that generates the corresponding quantum statistical position correlations between the atoms. In the case of multiple ground levels or at light intensities where saturation becomes important, the classical simulations require approximations that neglect quantum fluctuations between the levels. We show how the model is extended to incorporate corrections due to quantum fluctuations that result from virtual scattering processes. In the low-light-intensity limit, we illustrate the simulations in a system of atoms in a Mott-insulator state in a two-dimensional optical lattice, where recurrent scattering of light induces strong interatomic correlations. These correlations result in collective many-atom subradiant and superradiant states and a strong dependence of the response on the spatial confinement within the lattice sites.

Single, double, and triple Auger decay rates of ${\mathrm{C}}^{+}(1s2s^{2}2p^2{}^{2}D,{}^{2}P)$ resonances were calculated in the framework of perturbation theory. The direct double Auger decay probabilities were calculated by using the approximate formulas according to the knockout and shakeoff mechanisms, in which the knockout mechanism was found to be dominant. Then the knockout mechanism was employed to investigate the complex triple Auger decay process, and the calculated rates have good agreement with the available experimental values.

We develop a model that reduces quantum electrodynamics (QED) in time plus three spatial dimensions to time plus a single spatial dimension, making it is possible to numerically calculate the dynamic behavior of simple QED systems. The dimensionality is restricted in such a way as to preserve the influence of spin and angular momentum. In contrast to the $S$-matrix scattering approach, these calculations are not perturbative within the zero- and one-photon sector of the relevant Hilbert space. The model restricts the electron occupation number to one and the photon occupation number to zero or one. We use this model to calculate the dynamics of a so-called bare electron that dresses itself by a photon field.

We present a bidirectional modification of the standard one-qubit teleportation protocol, where both Alice and Bob transfer noisy versions of their qubit states to each other by using single Bell state and auxiliary (trigger) qubits. Three schemes are considered: the first where the actions of parties are governed by two independent quantum random triggers, the second with single random trigger, and the third as a mixture of the first two. We calculate the fidelities of teleportation for all schemes and find a condition on correlation between trigger qubits in the mixed scheme which allows us to overcome the classical fidelity boundary of 2/3. We apply the Choi–Jamiolkowski isomorphism to the quantum channels obtained in order to investigate an interplay between their ability to transfer the information, entanglement-breaking property, and auxiliary classical communication needed to form correlations between trigger qubits. The suggested scheme for bidirectional teleportation can be realized by using current experimental tools.

Two-, three-, and four-boson systems are studied close to the unitary limit using potential models constructed to reproduce the minimal information given by the two-body scattering length $a$ and the two-body binding energy or virtual-state energy $E_2$. The particular path used to reach the unitary limit is given by varying the potential strength. In this way the energy spectrum in the three- and four-boson systems is computed. The lowest-energy states show finite-range effects absorbed in the construction of level functions that can be used to study real systems. Higher-energy levels are free from finite-range effects, therefore the corresponding level functions tend to the zero-range universal function. Using this property, a zero-range equation for the four-boson system is proposed and the four-boson universal function is computed.

Electron multiple recollision dynamics under intense midinfrared laser fields is studied by means of the de Broglie–Bohm framework of Bohmian mechanics. Bohmian trajectories contain all the information embedded in the time-dependent wave function. This makes the method suitable to investigate the coherent dynamic processes for which the phase information is crucial. In this study, the appearance of the subpeaks in the high-harmonic-generation time-frequency profiles and the asymmetric fine structures in the above-threshold ionization spectrum are analyzed by the comprehensive and intuitive picture provided by Bohmian mechanics. The time evolution of the individual electron trajectories is closely studied to address some of the major structural features of the photoelectron angular distributions.

Aubry-André-Harper lattice models, characterized by a reflection-asymmetric sinusoidally varying nearest-neighbor tunneling profile, are well known for their topological properties. We consider the fate of such models in the presence of balanced gain and loss potentials $\pm i\gamma$ located at reflection-symmetric sites. We predict that these models have a finite $\mathcal{PT}$-breaking threshold only for specific locations of the gain-loss potential and uncover a hidden symmetry that is instrumental to the finite threshold strength. We also show that the topological edge states remain robust in the $\mathcal{PT}$-symmetry-broken phase. Our predictions substantially broaden the possible experimental realizations of a $\mathcal{PT}$-symmetric system.

Recently, several proposals have been made to test the quantum superposition principle in the mesoscopic regime. Most of these tests consist of a careful measurement of the loss of interference due to decoherence. Here we consider, instead, the spread in position of a freely falling nanosphere. We study in depth the dependence of this spread on self-gravity in the presence of decoherence (exotic and nonexotic). We show that the influence of self-gravity is robust in the presence of weak decoherence, and quantify this robustness by introducing a parameter, the critical decoherence, aimed at estimating the critical value above which self-gravity is overwhelmed by decoherence. We also emphasize the crucial role played by the spread of the initial wave packet for the sensitivity of free-fall experiments to decoherence.