We present a physical (gedanken) implementation of a generalized remote state preparation of relativistic quantum field states for an arbitrary set of observers. The prepared states are created in regions that are outside the future light cone of the generating region. The mechanism, which is based on utilizing the vacuum state of a relativistic quantum field as a resource, sheds light on the well known Reeh-Schlieder theorem, indicating its strong connection with the mathematical phenomenon of superoscillations.

Recent progress in optical lattice clocks requires unprecedented precision in controlling systematic uncertainties at the ${10}^{-18}$ level. Tuning of nonlinear light shifts is shown to reduce lattice-induced clock shift for a wide range of lattice intensity. Based on theoretical multipolar, nonlinear, anharmonic, and higher-order light shifts, we numerically demonstrate possible strategies for Sr, Yb, and Hg clocks to achieve lattice-induced systematic uncertainty below $1\times {10}^{-18}$.

The third-order susceptibility is investigated in a five-level atomic system in which the laser beams couple the ground state to a four-level closed-loop system. It is found that under the condition of the multiphoton resonance, one can enhance the Kerr nonlinearity of such a medium by properly adjusting the amplitudes and phases of the applied fields. In this case, the linear and nonlinear absorption reduce considerably in a region with a positive group velocity. It is demonstrated that the third-order susceptibility is very sensitive to the relative phase of the applied fields. An analytical model is presented to elucidate such phase control of the Kerr nonlinearity. A comparison is also made between the Kerr-nonlinear indices for the five-, four-, and three-level systems. It is realized that the magnitude of the Kerr nonlinearity for the five-level system is larger than that of the three- and four-level counterparts. Finally, it is shown that effect of Doppler broadening can lead to an enhanced Kerr nonlinearity while maintaining linear and nonlinear absorption.

The laser-induced intermolecular force that exists between two or more particles in the presence of an electromagnetic field is commonly termed “optical binding.” Distinct from the single-particle forces that are at play in optical trapping at the molecular level, the phenomenon of optical binding is a manifestation of the coupling between optically induced dipole moments in neutral particles. In other, more widely known areas of optics, there are many examples of chiral discrimination—signifying the different response a chiral material has to the handedness of an optical input. In the present analysis, extending previous work on chiral discrimination in optical binding, a mechanism is identified using a quantum electrodynamical approach. It is shown that the optical binding force between a pair of chiral molecules can be significantly discriminatory in nature, depending upon both the handedness of the interacting particles and the polarization of the incident light, and it is typically several orders of magnitude larger than previously reported.

In a recent paper [Phys. Rev. A 89, 022513 (2014)], Wu et al. reported a theoretical study of the effects of hyperfine interaction on the angular distribution of the $K{\alpha }_1$ x-ray emission following radiative electron capture into initially H-like ions. In the present Comment, we prove that their results are incorrect by showing that their assumption that hyperfine-structure splitting of the $1s2p_{3/2}{}^1{P}_1$ level is large compared to its natural width is wrong. In fact, the hyperfine interaction does not affect at all the angular distribution of $1s2p_{3/2}{}^{1}P_1\to 1s^2{}^{1}S_0$ fine-structure component of the $K{\alpha }_1$ line due to the very short lifetime of the upper level. In addition to this correction, we point out an inaccuracy in their treatment of the $E1\text{−}M2$ multipole mixing occuring in the $1s2p_{3/2}{}^{3}P_2\to 1s^2{}^{1}S_0$ component of $K{\alpha }_1$. Indeed, the theory developed by Wu et al. did not consider the contribution from the hyperfine-induced $E1$ channel in its pure form. We found that the ratio of the $E1$ and $M2$ $1s2p_{3/2}{}^{3}P_{2}F=3/2\to 1s^2{}^{1}S_{0}F=1/2$ transition probabilities is of the order of 7% in the case of ${}^{119}\mathrm{Sn}^{48+}$ ion and goes up to 38% for ${}^{207}\mathrm{Tl}^{79+}$.

In a recent work [Phys. Rev. A 89, 022513 (2014)], the $K{\alpha }_1$ ($1s2p_{3/2}{}^{1,3}P{}_{1,2}\to 1s^2{}^1{S}_0$) x-ray emission following the radiative electron capture into initially hydrogen-like ions has been explored for ions with nonzero nuclear spin $(I\ne 0)$. A rather strong influence upon the angular distribution of the (hyperfine- and fine-structure averaged) $K{\alpha }_1$ radiation was found, especially for isotopes with nuclear spin $I=1/2$, while this effect are less important for isotopes with nuclear spin $I>1/2$. Two comments were made by Inal and Benmouna about this work with regard to (i) the incoherent summation of the individual hyperfine components of the $1s2p_{3/2}{}^1{P}_1\to 1s^2{}^1{S}_0$ transition and (ii) the treatment of the hyperfine-induced $E1\text{−}M2$ multipole mixing in the $1s2p_{3/2}{}^3{P}_2\to 1s^2,{}^1{S}_0$ fine-structure component. While we agree with the first comment and here provide updated anisotropy parameters, the hyperfine-induced modification of the $K{\alpha }_1$ emission remains valid and may help in the future to determine the nuclear parameters of radioactive isotopes. We also explain that the hyperfine-induced $E1\text{−}M2$ mixing has already been fully taken into account in our previous work.

We provide experimental evidence of quantum features in bipartite states classified as entirely classical according to a conventional criterion based on the Glauber $\mathcal{P}$ function but possessing nonzero Gaussian quantum discord. Their quantum nature is experimentally revealed by acting locally on one part of the discordant state. We experimentally verify and investigate the effect of discord increase under the action of local loss and link it to the entanglement with the environment. Adding an environmental system purifying the state, we unveil the flow of quantum correlations within a global pure system using the Koashi-Winter inequality. For a discordant state generated by splitting a state in which the initial squeezing is destroyed by random displacements, we demonstrate the recovery of entanglement highlighting the role of system-environment correlations.

We study the phase diagram of mass- and spin-imbalanced unitary Fermi gases, in search for the emergence of spatially inhomogeneous phases. To account for fluctuation effects beyond the mean-field approximation, we employ renormalization group techniques. We thus obtain estimates for critical values of the temperature, mass and spin imbalance, above which the system is in the normal phase. In the unpolarized, equal-mass limit, our result for the critical temperature is in accordance with state-of-the-art Monte Carlo calculations. In addition, we estimate the location of regions in the phase diagram where inhomogeneous phases are likely to exist. We show that an intriguing relation exists between the general structure of the many-body phase diagram and the binding energies of the underlying two-body bound-state problem, which further supports our findings. Our results suggest that inhomogeneous condensates form for mass imbalances $m_{\downarrow }/m_{\uparrow }\gtrsim 3$. The extent of the inhomogeneous phase in parameter space increases with increasing mass imbalance.

A disordered mean-field model for multimode laser in open and irregular cavities is proposed and discussed within the replica analysis. The model includes the dynamics of the mode intensity and accounts also for the possible presence of a linear coupling between the modes, due, e.g., to the leakages from an open cavity. The complete phase diagram, in terms of disorder strength, source pumping, and nonlinearity, consists of four different optical regimes: incoherent fluorescence, standard mode locking, random lasing, and the spontaneous phase locking. A replica symmetry-breaking phase transition is predicted at the random lasing threshold. For a high enough strength of nonlinearity, a whole region with nonvanishing complexity anticipates the transition, and the light modes in the disordered medium display typical discontinuous glassy behavior, i.e., the photonic glass has a multitude of metastable states that correspond to different mode-locking processes in random lasers. The lasing regime is still present for very open cavities, although the transition becomes continuous at the lasing threshold.

A spectrograph is an optical instrument that disperses photons of different energies into distinct directions and space locations and that images photon spectra on a position-sensitive detector. Spectrographs consist of collimating, angular dispersive, and focusing optical elements. Bragg reflecting crystals arranged in an asymmetric scattering geometry can be used as the dispersing elements in the hard-x-ray regime. A ray-transfer matrix technique is applied to propagate x-rays through the optical elements. Several optical designs of hard-x-ray spectrographs are proposed and their performance is analyzed. Spectrographs with an energy resolution of 0.1 meV and a spectral window of imaging up to a few tens of meVs are shown to be feasible for inelastic x-ray scattering (IXS) spectroscopy applications. In another example, a spectrograph with a 1-meV spectral resolution and 85-meV spectral window of imaging is considered for Cu $K$-edge resonant IXS.

We report numerical and experimental results on a finite-size, weakly disordered, amplifying multilayer with underlying periodicity. Computed transmission spectra evidence the simultaneous existence of perturbed band-edge modes and random lasing modes in exclusive configurations of disorder. A larger energy is seen to outcouple through the random lasing modes as compared to the band-edge lasing modes. With the gain being uniform across the band gap, this phenomenon originates from the increased quality factor of modes by the addition of disorder. Experiments carried out on a linear array of weakly disordered microresonators exhibit excellent agreement with the numerical observations, demonstrating threshold reduction by the addition of disorder in a periodic system.

Enhancement of the diffusive albedo from a random amplifying medium is discussed. A pump-probe setup is used to study the amplification of the reflected probe pulse light outside the backscattering cone. Strong super-reflection is measured and its dependence on the disorder strength is studied. An optimal disorder strength is observed at high excitation energy wherein the super-reflection is maximal. The origins of this behavior are traced to the interplay between the out-coupling and amplification from a disordered amplifying medium with an inhomogeneous distribution of gain.

Besides the conventional transverse couplings between superconducting qubits (SQs) and electromagnetic fields, there are additional longitudinal couplings when the inversion symmetry of the potential energies of the SQs is broken. We study nonclassical-state generation in a SQ which is driven by a classical field and is coupled to a single-mode microwave field. We find that the classical field can induce transitions between two energy levels of the SQs, which either generate or annihilate, in a controllable way, different photon numbers of the cavity field. The effective Hamiltonians of these classical-field-assisted multiphoton processes of the single-mode cavity field are very similar to those for cold ions, confined to a coaxial rf-ion trap and driven by a classical field. We show that arbitrary superpositions of Fock states can be more efficiently generated using these controllable multiphoton transitions, in contrast to the single-photon resonant transition when there is only a SQ-field transverse coupling. The experimental feasibility for different SQs is also discussed.

We construct families of discrete solitons (DSs) in an array of self-defocusing waveguides with an embedded parity-time- ($\mathcal{PT}$-) symmetric dimer, which is represented by a pair of waveguides carrying mutually balanced gain and loss. Four types of states attached to the embedded defect are found, namely, staggered and unstaggered bright localized modes and gray or antigray DSs. Their existence and stability regions expand with the increase of the strength of the coupling between the dimer-forming sites. The existence of the gray and staggered bright DSs is qualitatively explained by dint of the continuum limit. All the gray and antigray DSs are stable (some of them are unstable if the dimer carries the nonlinear $\mathcal{PT}$ symmetry, represented by balanced nonlinear gain and loss; in that case, the instability does not lead to a blowup, but rather creates oscillatory dynamical states). The boundary between the gray and antigray DSs is predicted in an approximate analytical form.

On the basis of general theoretical results developed previously in [JETP 112, 246 (2011)], we analyze the reflection of quasiresonant light from a plane surface of dense and disordered ensemble of motionless point scatters. Angle distribution of the scattered light is calculated both for $s$ and $p$ polarizations of the probe radiation. The ratio between coherent and incoherent (diffuse) components of scattered light is calculated. We analyze the contributions of scatters located at different distances from the surface and determine on this background the thickness of surface layer responsible for reflected beam generation. The inhomogeneity of dipole-dipole interaction near the surface is discussed. We study also dependence of total reflected light power on the incidence angle and compare the results of the microscopic approach with predictions of the Fresnel reflection theory. The calculations are performed for different densities of scatters and different frequencies of a probe radiation.

We demonstrate the formation of a narrow beam from a long ($L\gg \lambda$) laser with subwavelength transverse dimensions (wire laser) as an image of the subwavelength laser waveguide formed by a spherical lens. The beam is linearly diverging with the angle determined by the ratio of the wavelength to the lens radius, while the minimum beam spot size is the same as that of the image of a point source. We realize such a beam experimentally using a terahertz quantum cascade wire laser.

We exploit the theory of quantum estimation to investigate quantum state estimation in the presence of noise. The quantum Fisher information is used to assess the estimation performance. For the qubit in Bloch representation, general expressions are derived for the quantum score and then for the quantum Fisher information. From this latter expression, it is proved that the Fisher information always increases with the purity of the measured qubit state. An arbitrary quantum noise affecting the qubit is taken into account for its impact on the Fisher information. The task is then specified to estimating the phase of a qubit in a rotation around an arbitrary axis, equivalent to estimating the phase of an arbitrary single-qubit quantum gate. The analysis enables determination of the optimal probing states best resistant to the noise, and proves that they always are pure states but need to be specifically matched to the noise. This optimization is worked out for several noise models important to the qubit. An adaptive scheme and a Bayesian approach are presented to handle phase-dependent solutions.

We demonstrate that the Casimir-Polder force for a molecule near the surface of a real dielectric substrate out of thermal equilibrium displays distinctive behaviors as compared to that at thermal equilibrium. In particular, when the molecule-substrate separation is much less than the molecular transition wavelength, the CP force in the high-temperature limit can be dramatically manipulated by varying the relative magnitude of the temperatures of the substrate and the environment so that the attractive-to-repulsive transition can occur beyond a certain threshold temperature of either the substrate or the environment depending on which one is higher for molecules both in the ground and excited states. More remarkably, when the separation is comparable to the wavelength, such transitions which are impossible at thermal equilibrium may happen for longitudinally polarizable molecules with a small permittivity, while for isotropically polarizable ones the transitions can even occur at room temperature for some dielectric substrates such as sapphire and graphite which is much lower than the temperature for the transition to take place in the thermal equilibrium case, thus making the experimental demonstration of such force transitions easier.

In open-shell atoms and ions, processes such as photoionization, combination (Raman) scattering, electron scattering, and recombination are often mediated by many-electron compound resonances. We show that their interference (neglected in the independent-resonance approximation) leads to a coherent contribution, which determines the energy-averaged total cross sections of electron- and photon-induced reactions obtained using the optical theorem. In contrast, the partial cross sections (e.g., electron recombination or photon Raman scattering) are dominated by the stochastic contributions. Thus, the optical theorem provides a link between the stochastic and coherent contributions of the compound resonances. Similar conclusions are valid for reactions via compound states in molecules and nuclei.