We present a generalization of the diagrammatic pump-probe approach to coherent backscattering (CBS) of intense laser light for atoms with degenerate energy levels. We employ this approach for a characterization of the double-scattering signal from optically pumped atoms with the transition $J_g\to J_e=J_g+1$ in the helicity-preserving polarization channel. We show that, in the saturation regime, the internal degeneracy becomes manifest for atoms with $J_g\ge 1$, leading to a faster decrease of the CBS enhancement factor with increasing saturation parameter than in the nondegenerate case.

We propose a spectrometric method to reconstruct the motional states of mechanical modes in optomechanics. This is achieved by detecting the single-photon emission and scattering spectra of the optomechanical cavity. Owing to an optomechanical coupling, the a priori phonon-state distributions contribute to the spectral magnitude, and hence we can infer information on the phonon states from the measured spectral data. When the single-photon optomechanical-coupling strength is moderately larger than the mechanical frequency, then our method works well for a wide range of cavity-field decay rates, irrespective of whether or not the system is in the resolved-sideband regime.

The dynamics of truncated Airy beams in optical media with Kerr nonlinearity is studied in the framework of the nonlinear Schrödinger (NLS) model. It is demonstrated that an intense Airy beam with zero total momentum can generate static solitons, as well as moving solitons. The parameters of these solitons are calculated using the Zakharov-Shabat scattering problem associated with the NLS equation. It is found that solitons take the main part of the initial power, while only a small fraction of the power is transformed into a self-accelerating linear packet. The threshold parameters of the Airy beam for the soliton formation are obtained. It is shown that the threshold for the formation of the first static soliton is also a threshold of the solitonless regime.

We demonstrate the existence of tunable bound states in the continuum (BICs) in a one-dimensional quantum wire with two impurities induced by an intense monochromatic radiation field. We find that there is an interesting type of BIC due to the Fano interference between two optical transition channels, in addition to the ordinary BIC due to geometrical interference between electron wave functions emitted by impurities. In both cases the BIC can be achieved by tuning the frequency of the radiation field. Evidence of the BIC can be obtained by observing the absorption rate of a probe photon.

We present a theoretical study of the discrete Green's function for the two-dimensional metamaterial, realized as a transmission line array. We demonstrate that the Green's function possesses distinct ripples when the isofrequency contour is of hyperbolic shape. This effect is a characteristic nonlocal feature of the hyperbolic regime and is beyond the effective medium approximation.

We propose a scheme to generate robust stationary continuous-variable entanglement in optomechanical systems, based on geometrical nonlinearity that occurs for large mechanical displacements. Such nonlinearity is often used to correct the dynamics of the systems in the strong-coupling regime. It appears that geometrical nonlinearity enhances the entanglement and shifts its maximum towards high detuning values. Using the experimental parameters, we find that such a scheme generates a very robust entanglement against thermal decoherence even at room temperature. Our results show that geometrical nonlinearity affects entanglement as the optomechanical quantum interface.

We define a quantum entropy conditioned on postselection which has the von Neumann entropy of pure states as a special case. This conditional entropy can take negative values, which is consistent with part of a quantum system containing less information than the whole, which can be in a pure state. The definition is based on generalized density operators for postselected ensembles. The corresponding density operators are consistent with the quantum generalization of classical conditional probabilities following Dirac's formalism of quasiprobability distributions.

Quantum metrology enhances the sensitivity of parameter estimation using the distinctive resources of quantum mechanics such as entanglement. It has been shown that the precision of estimating an overall multiplicative factor of a Hamiltonian can be increased to exceed the classical limit, yet little is known about estimating a general Hamiltonian parameter. In this paper, we study this problem in detail. We find that the scaling of the estimation precision with the number of systems can always be optimized to the Heisenberg limit, while the time scaling can be quite different from that of estimating an overall multiplicative factor. We derive the generator of local parameter translation on the unitary evolution operator of the Hamiltonian, and use it to evaluate the estimation precision of the parameter and establish a general upper bound on the quantum Fisher information. The results indicate that the quantum Fisher information generally can be divided into two parts: one is quadratic in time, while the other oscillates with time. When the eigenvalues of the Hamiltonian do not depend on the parameter, the quadratic term vanishes, and the quantum Fisher information will be bounded in this case. To illustrate the results, we give an example of estimating a parameter of a magnetic field by measuring a spin-$\frac{1}{2}$ particle and compare the results for estimating the amplitude and the direction of the magnetic field.

We study decoherence of a field-driven qubit in the presence of environmental noises. For a general qubit, we find that driving, whether on-resonance or off-resonance, alters the qubit decoherence rates (including dissipation and pure dephasing), allowing both blue and red sideband contributions from the reservoir. Depending on the noise spectral density, driving field detuning, and driving field phase shift, the qubit decoherence rates could be either accelerated or reduced. We apply our general theory to the system of an electron spin qubit that is confined in a quantum dot and driven by an in-plane electric field. We analyze how spin relaxation induced by the electrical noise due to electron-phonon interaction varies as a function of driving frequency, driving magnitude, driving field phase shift, and spin-orbit coupling strengths.

We study the unitary time evolution of photons interacting with a dielectric resonator using coherent control pulses. We show that non-Markovianity of transient photon dynamics in the resonator subsystem may be controlled to within a photon-resonator transit time. In general, appropriate use of coherent pulses and choice of spatial subregion may be used to create and control a wide range of non-Markovian transient dynamics in photon-resonator systems.

We calculate the Casimir-Lifshitz pressure in a system consisting of two different one-dimensional dielectric lamellar gratings having two different temperatures and immersed in an environment having a third temperature. The calculation of the pressure is based on the knowledge of the scattering operators, deduced using the Fourier modal method. The behavior of the pressure is characterized in detail as a function of the three temperatures of the system as well as the geometrical parameters of the two gratings. We show that the interplay between nonequilibrium effects and geometrical periodicity offers a rich scenario for the manipulation of the force. In particular, we find regimes where the force can be strongly reduced for large ranges of temperatures. Moreover, a repulsive pressure can be obtained, whose features can be tuned by controlling the degrees of freedom of the system. Remarkably, the transition distance between attraction and repulsion can be decreased with respect to the case of two slabs, implying an experimental interest for the observation of repulsion.

Quantum data locking is a uniquely quantum phenomenon that allows a relatively short key of constant size to (un)lock an arbitrarily long message encoded in a quantum state, in such a way that an eavesdropper who measures the state but does not know the key has essentially no information about the message. The application of quantum data locking in cryptography would allow one to overcome the limitations of the one-time pad encryption, which requires the key to have the same length as the message. However, it is known that the strength of quantum data locking is also its Achilles heel, as the leakage of a few bits of the key or the message may in principle allow the eavesdropper to unlock a disproportionate amount of information. In this paper we show that there exist quantum data locking schemes that can be made robust against information leakage by increasing the length of the key by a proportionate amount. This implies that a constant size key can still lock an arbitrarily long message as long as a fraction of it remains secret to the eavesdropper. Moreover, we greatly simplify the structure of the protocol by proving that phase modulation suffices to generate strong locking schemes, paving the way to optical experimental realizations. Also, we show that successful data locking protocols can be constructed using random code words, which very well could be helpful in discovering random codes for data locking over noisy quantum channels.

Since the initial discovery of the Wootters-Zurek no-cloning theorem, a wide variety of quantum cloning machines have been proposed aiming at imperfect but optimal cloning of quantum states within its own context. Remarkably, most previous studies have employed the Bures fidelity or the Hilbert-Schmidt norm as the figure of merit to characterize the quality of the corresponding cloning scenarios. However, in many situations, what we truly care about is the relevant information about certain parameters encoded in quantum states. In this work, we investigate the multiple phase estimation problem in the framework of quantum cloning machines, from the perspective of quantum Fisher information matrix (QFIM). Focusing on the generalized $d$-dimensional equatorial states, we obtain the analytical formulas of QFIM for both universal quantum cloning machine (UQCM) and phase-covariant quantum cloning machine (PQCM), and prove that PQCM indeed performs better than UQCM in terms of QFIM. We highlight that our method can be generalized to arbitrary cloning schemes where the fidelity between the single-copy input and output states is input-state independent. Furthermore, the attainability of the quantum Cramér-Rao bound is also explicitly discussed.

We study general quantum correlations of continuous variable Gaussian states and their interplay with entanglement. Specifically, we investigate the existence of a quantum protocol activating all nonclassical correlations between the subsystems of an input bipartite continuous variable system, into output entanglement between the system and a set of ancillae. For input Gaussian states, we prove that such an activation protocol cannot be accomplished with Gaussian operations, as the latter are unable to create any output entanglement from an initial separable yet nonclassical state in a worst-case scenario. We then construct a faithful non-Gaussian activation protocol, encompassing infinite-dimensional generalizations of controlled-not gates to generate entanglement between system and ancillae, in direct analogy with the finite-dimensional case. We finally calculate the negativity of quantumness, an operational measure of nonclassical correlations defined in terms of the performance of the activation protocol, for relevant classes of two-mode Gaussian states.

The entanglement between the position and the coin state of an $N$-dimensional quantum walker is shown to lead to a thermodynamic theory. The entropy, in this thermodynamics, is associated with the reduced density operator for the evolution of chirality, taking a partial trace over positions. From the asymptotic reduced density matrix it is possible to define thermodynamic quantities, such as the asymptotic entanglement entropy, temperature, and Helmholz free energy. We study in detail the case of a two-dimensional quantum walk, in the case of two initial conditions: a nonseparable coin-position initial state and a separable one. The resulting entanglement temperature is presented as a function of the parameters of the system and those of the initial conditions.

We investigate the onset of photoionization shake-up-induced interatomic Coulombic decay (ICD) in ${\mathrm{He}}_2$ at the ${\mathrm{He}}^{+*}(n=2)$ threshold by detecting two ${\mathrm{He}}^{+}$ ions in coincidence. We find this threshold to be shifted towards higher energies compared to the same threshold in the monomer. The shifted onset of ion pairs created by ICD is attributed to a recapture of the threshold photoelectron after the emission of the faster ICD electron.

We have computed elastic differential, momentum-transfer, excitation, ionization cross sections, collision frequencies, and scattering length for electron impact on the ${\mathrm{O}}_2$ molecule, which are computed using the R-matrix method. The results of the static exchange, correlated one-state, and 22-state close-coupling approximation are presented. We have detected a stable anionic bound state ${}^2{\Pi }_g$ of ${\mathrm{O}}_2{}^{-}$ and two shape resonances of ${}^2{\Pi }_u$ symmetry. The dissociative nature of these shape resonances is explored by performing scattering calculations in which the O-O bond is stretched. These resonances support dissociative attachment yielding O, ${\mathrm{O}}^{-}$ in dissociation of ${\mathrm{O}}_2$. Born correction is applied for higher partial waves neglected in the R-matrix calculations for elastic and dipole allowed transitions.

Longitudinally-polarized-electron-impact excitation cross sections from the ground state to the individual magnetic sublevels of the excited state $1s2s^{2}2p_{3/2}$($J = 2$) of highly charged Be-like ions are calculated using a fully relativistic distorted-wave method. The contributions of the Breit interaction to the cross sections and circular polarizations of the $1s2s^{2}2p_{3/2}$($J = 2$)$\to 1s^{2}2s^2$($J = 0$) magnetic quadrupole (M2) line for selected Be-like ${\mathrm{Ag}}^{43+}$, ${\mathrm{Ho}}^{63+}$, and ${\mathrm{Bi}}^{79+}$ ions are investigated systematically. It is found that the Breit interaction has a large effect and makes the cross sections increase, especially to the $m_f$ = $-1$ and $-2$ sublevels, the Breit interaction can modify the cross sections by several orders of magnitude. These dramatic influences also lead to a remarkable decrease in the circular polarization of subsequent x-ray radiation, the character of which becomes more and more evident with increasing incident energy and atomic number. And all these characteristics are very different from the conclusions for the linear polarization of radiation following the electron-impact process [S. Fritzsche, A. Surzhykov, and T. Stöhlker, Phys. Rev. Lett. 103, 113001 (2009); Z. W. Wu, J. Jiang, and C. Z. Dong, Phys. Rev. A 84, 032713 (2011)].

We study the scattering of matter waves from interacting bosons in a one-dimensional optical lattice, described by the Bose-Hubbard Hamiltonian. We derive analytically a formula for the inelastic cross section as a function of the atomic interaction in the lattice, employing Bogoliubov's formalism for small condensate depletion. A linear decay of the inelastic cross section for weak interaction, independent of the number of particles, condensate depletion, and system size, is found.

Detection of quantum features in mechanical systems at the nanoscale constitutes a challenging task, given the weak interaction with other elements and the available technology. Here we describe the interaction between two monomodal transmission-line resonators (TLRs) mediated by vibrations of a nanoelectromechanical oscillator. This scheme is then employed for quantum nondemolition detection of the number of phonons in the nanoelectromechanical oscillator through a direct current measurement in the output of one of the TLRs. For that to be possible an undepleted field inside one of the TLRs works as an amplifier for the interaction between the mechanical resonator and the remaining TLR. We also show how the nonclassical nature of this system can be used for generation of tripartite entanglement and conditioned mechanical coherent superposition states, which may be further explored for detection processes.