It is shown that inhomogeneous broadening of the spectral line of active impurities may sustain simultaneously parity ($\mathcal{P}$) and time ($\mathcal{T})$ symmetries of a medium, in a finite range of field frequencies, which is forbidden by the causality principle in media without broadening. If a spectral width of a propagating pulse is less than the inhomogeneous broadening, the medium for such a pulse becomes quasi-$\mathcal{PT}$-symmetric. The effect of the broadband quasi-$\mathcal{PT}$-symmetry in a finite frequency domain, is illustrated by examples of unidirectional diffraction of pulses in Bragg and Laue geometries, propagating in photonic crystals.

We study the mobility of localized solutions in a nonlinear Lieb photonic lattice. We characterize different families of nonlinear solutions looking for different regions of parameters to observe coherent transport across the system. In particular, we analytically derive a family of compact discrete solitons, which originate at the flat band of this lattice at zero power. For low level of power, we found a transparent region where two well-localized nonlinear modes, which are close in Hamiltonian, show a very good mobility. We numerically observe a perfect transport across the system with negligible radiation, where two compact solutions adiabatically transform one into the other. Although the ring mode stabilizes for larger power, it is not parametrically connected to any other stationary solution and, therefore, it is not allowed to move across the system for high power.

In this work, dissipative effects from a phonon bath on the resonance fluorescence of a solid-state two-level system embedded in a high-quality semiconductor microcavity and driven by an intense laser are investigated. Within the density operator formalism, we derive a variational master equation valid for broader ranges of temperatures, pumping rates, and radiation-matter couplings than previous studies. From the obtained master equation, fluorescence spectra for various thermal and exciting conditions are numerically calculated and compared to those computed from weak coupling and polaronic master equations, respectively. Our results evidence the breakdown of those rougher approaches under increased temperature and strong pumping.

We study noninteracting fermionic systems dissipatively driven at their boundaries, focusing in particular on the case of a non-number-conserving Hamiltonian, which for example describes an $XY$ spin chain. We show that despite the lack of number conservation, it is possible to convert the problem of calculating the normal modes of the master equations and their corresponding rapidities, into diagonalizing simply an $L\times L$ tridiagonal bordered 2-Toeplitz matrix, where $L$ is the size of the system. Such structure of matrix allows us to further reduce the problem into solving a scalar trigonometric nonlinear equation for which we also show, in the case of an Ising chain, exact analytical explicit, and system size independent, solutions.

We study the decoherence properties of a two-level (qubit) system homogeneously coupled to an environmental many-body system at a quantum transition, considering both continuous and first-order quantum transitions. In particular, we consider a $d$-dimensional quantum Ising model as environment system. We study the dynamic of the qubit decoherence along the global quantum evolution starting from pure states of the qubit and the ground state of the environment system. This issue is discussed within dynamic finite-size scaling frameworks. We analyze the dynamic finite-size scaling of appropriate qubit-decoherence functions. At continuous quantum transitions, they develop power laws of the size of the environment system, with a substantial enhancement of the growth rate of the qubit decoherence with respect to the case in which the environment system is in normal noncritical conditions. The enhancement of the qubit decoherence growth rate appears much larger at first-order quantum transitions, leading to exponential laws when increasing the size of the environment system.

We investigate the problem of bounding the quantum process fidelity given bounds on the fidelities between target states and the action of a process on a set of pure input states. We formulate the problem as a semidefinite program and prove convexity of the minimum process fidelity as a function of the errors on the output states. We characterize the conditions required to uniquely determine a process in the case of no errors and derive a lower bound on its fidelity in the limit of small errors for any set of input states satisfying these conditions. We then consider sets of input states whose one-dimensional projectors form a symmetric positive operator-valued measure (POVM). We prove that for such sets the minimum fidelity is bounded by a linear function of the average output state error. A symmetric POVM with minimal number of elements contains $d+1$ states, where $d$ is the Hilbert space dimension. Our bounds applied to such states provide an efficient method for estimating the process fidelity without the use of full process tomography.

A central task in quantum-information processing is to characterize quantum processes. In the realm of optical quantum-information processing, this amounts to characterizing the transformations of the mode creation and annihilation operators. This transformation is unitary for linear optical systems, whereas these yield the well-known Bogoliubov transformations for systems with Hamiltonians that are quadratic in the mode operators. In this paper, we propose a shot-noise limited scheme for characterizing both these kinds of evolutions by employing a modified Mach-Zehnder interferometer. In order to characterize a $N$-mode device, we require $O\left(N^2\right)$ measurements. Although it suffices to use coherent states for the characterization of linear optical systems, we additionally require single photons to characterize quadratically nonlinear optical systems.

Identifying nonclassicality unambiguously and inexpensively is a long-standing open challenge in physics. The no-signaling-in-time protocol was developed as an experimental test for macroscopic realism, and serves as a witness of quantum coherence in isolated quantum systems by comparing the quantum state to its completely dephased counterpart. We show that it provides a lower bound on a certain resource-theoretic coherence monotone. We go on to generalize the protocol to the case where the system of interest is coupled to an environment. Depending on the manner of the generalization, the resulting witness either reports on system coherence alone, or on a disjunction of system coherence with either (i) the existence of nonclassical system-environment correlations or (ii) non-negligible dynamics in the environment. These are distinct failure modes of the Born approximation in nonisolated systems.

We characterize the operational task of environment-assisted distillation of quantum coherence under different sets of free operations when only a finite supply of copies of a given state is available. We first evaluate the one-shot assisted distillable coherence exactly, and introduce a semidefinite programming bound on it in terms of a smooth entropic quantity. We prove the bound to be tight for all systems in dimensions 2 and 3, which allows us to obtain computable expressions for the one-shot rate of distillation, establish an analytical expression for the best achievable fidelity of assisted distillation for any finite number of copies, and fully solve the problem of asymptotic zero-error assisted distillation for qubit and qutrit systems. Our characterization shows that all relevant sets of free operations in the resource theory of coherence have exactly the same power in the task of one-shot assisted coherence distillation, and furthermore resolves a conjecture regarding the additivity of coherence of assistance in dimension 3.

Recently, a connection between quantum coherence and quantum steering was established, and criteria for quantum steering or, in other words, nonlocal advantage of quantum coherence (NAQC) were derived for two-qubit states. Here we derive a set of complementarity relations between the steering or NAQC inequalities achieved by various criteria. We also extend the idea in the multipartite scenario, specifically, in the three-qubit scenario, which can easily be generalized to the multipartite scenario.

We study the electron dynamics at a monocrystalline Pd(111) surface with stepped vicinal nanostructures modeled in a simple Kronig-Penney scheme. The unoccupied bands of the surface are resonantly excited via the resonant charge transfer (RCT) interaction of the surface with a hydrogen anion reflected at grazing angles. The interaction dynamics is simulated numerically in a quantum mechanical wave packet propagation approach. Visualization of the wave packet density shows that, when the electron is transferred to the metal, the surface and image subband states are the most likely locations of the electron as it evolves through the superlattice. The survival probability of the interacting ion exhibits strong modulations as a function of the vicinal-terrace size and shows peaks at those energies that access the image state subband dispersions. A simple square well model producing standing waves between the steps on the surface suggests the application of such ion scattering at shallow angles to map electronic substructures in vicinal surfaces. The work also serves as proof-of-principle in the utility of our computational method to address, via RCT, surfaces with nanometric patterns.

We study the origin of dichroic effects in elastic scattering of high-energy electrons by hydrogen atoms in the presence of a two-color bicircular laser field of commensurate frequencies, in the domain of moderate intensities below $10\mathrm{TW}/{\mathrm{cm}}^2$. We use a semiperturbative approach in which the interaction of the hydrogen atom with the laser field is treated in second-order perturbation theory, while the interaction of the projectile electron with the laser field is described by Gordon-Volkov wave functions. An analytical formula of circular dichroism in the angular distribution of scattered electrons is derived in the weak-field domain for a two-color laser field that is a combination of the fundamental and its third harmonic. A comparison between the two-photon differential cross sections for two-color co- and counter-rotating circularly polarized laser fields is made, and the effect of the intensity ratio of the monochromatic field components on the circular dichroism is investigated. The dichroic effect in the angular distribution of scattered electrons for two-photon absorption is analyzed as a function of the scattering and azimuthal angles. We show that the two-color bicircular laser field can induce a strong circular dichroism in the angular distribution of scattered electrons at small scattering angles where the atomic dressing effects are important, as well at larger scattering angles. At small scattering angles we demonstrate that the dichroic effect for two-photon transitions can be predicted under the following conditions: the scattering process is treated in fist-order Born approximation, and the dressing of the atomic states by the laser field is carried out at least in first-order time-dependent perturbation theory.

We study the formation and the dynamics of a shock wave originating from the collision between two ultracold clouds of strongly interacting fermions as observed at a lower temperature in an experiment by Joseph et al. [Phys. Rev. Lett. 106, 150401 (2011)]. We use the Boltzmann equation within the test-particle method to describe the evolution of the system in the normal phase. We also show a direct comparison with the hydrodynamic approach and insist on the necessity of including a shear-viscosity and a thermal-conductivity term in the equations to prevent unphysical behavior from taking place.

We have investigated self-bound ultradilute bosonic binary mixtures at zero temperature within density functional theory using a local density approximation. We provide the explicit expression of the Lee-Huang-Yang correction in the general case of heteronuclear mixtures, and investigate the general thermodynamic conditions which lead to the formation of self-bound systems. We have determined the conditions for stability against the evaporation of one component, as well as the mechanical and diffusive spinodal lines. We have also calculated the surface tension of the self-bound state as a function of the interspecies interaction strength. We find that relatively modest variations of the latter result in order-of-magnitude changes in the calculated surface tension. We suggest experimental realizations which might display the metastability and phase separation of the mixture when entering regions of the phase diagram characterized by negative pressures. Finally, we show that these droplets may sustain stable vortex and vortex pairs.

Systems with competing attractive and repulsive interactions have a tendency to condense into droplets. This is the case for water in a sink, liquid helium, and dipolar atomic gases. Here we consider a photon fluid which is formed in the transverse plane of a monochromatic laser beam propagating in an attractive (focusing) nonlocal nonlinear medium. In this setting we demonstrate the formation of the optical analog of matter-wave droplets and study their properties. The system we consider admits droplets that carry orbital angular momentum. We find bound states possessing liquidlike properties, such as bulk pressure and compressibility. Interestingly, these droplets of light, as opposed to optical vortices, form due to the competition between long-range $s$-wave (monopole) and $d$-wave (quadrupole) interactions as well as diffraction.

We consider quantum discrimination of bosonic loss based on both symmetric and asymmetric hypothesis testing. In both approaches, an entangled resource is able to outperform any classical strategy based on coherent-state transmitters in the regime of low photon numbers. In the symmetric case, we then consider the low-energy detection of bacterial growth in culture media. Assuming an exponential growth law for the bacterial concentration and the Beer-Lambert law for the optical transmissivity of the sample, we find that the use of entanglement allows one to achieve a much faster detection of growth with respect to the use of coherent states. This performance is also studied by assuming an exponential photo degradable model, where the concentration is reduced by increasing the number of photons irradiated over the sample. This investigation is then extended to the readout of classical information from suitably designed photodegradable optical memories.

It is typical of modern quantum technologies employing nanomechanical oscillators to demand few mechanical quantum excitations, for instance, to prolong coherence times of a particular task or to engineer a specific nonclassical state. For this reason, we devote the present work to exhibiting how to bring an initially thermalized nanomechanical oscillator to near its ground state. Particularly, we focus on extending the novel results of D. D. B. Rao et al. [Phys. Rev. Lett. 117, 077203 (2016)], where a mechanical object can be heated up, squeezed, or cooled down to near its ground state through conditioned single-spin measurements. In our work, we study a similar iterative spin-mechanical system when $N$ spins interact with the mechanical oscillator. Here, we have also found that the postselection procedure acts as a discarding process; i.e., we steer the mechanics to the ground state by dynamically filtering its vibrational modes. We show that when considering symmetric collective spin postselection, the inclusion of $N$ spins in the quantum dynamics is highly beneficial—in particular, decreasing the total number of iterations to achieve the ground state, with a success rate of probability comparable with the one obtained from the single-spin case.

We report on the discovery of a unidirectional continuous orbital motion of nanoparticles which occurs spontaneously in room-temperature air and can be manipulated by light. Trapped nanoparticles exhibit a transition between Brownian particles in two separate lattice sites and orbiting particles in a single lattice site. The orbital motion is sensitive to air pressure and is vanishing at low pressure, suggesting that the orbital motion is supported by air. Our results pave the way for manipulating nanoscale objects on the basis of their cooperative dynamics.

We investigate a network of degenerate optical parametric oscillators (DOPOs) as a model of the coherent Ising machine, an architecture for solving Ising problems. The network represents the interaction in the Ising model, which is a generalization of a previously proposed one for the two-DOPO case. Dynamics of the DOPOs is described by the Fokker-Planck equation in the positive $P$ representation. We obtain approximate distribution of steady states for arbitrary Ising problems under some ansatz. Using the method of statistical mechanics, we analytically demonstrate that the most probable states in a particular range of the parameters correspond to the true optimal states for two rather simple problems, i.e., fully connected ferromagnetic coupling without or with binary random fields. In particular, for the random-field problem, the distribution correctly detects the phase transition that occurs in the target Ising model with varying the magnitude of the fields.

We consider bound states in the continuum (BICs) in low-contrast dielectric gratings. It is demonstrated that the BICs originate from the reduced guided modes on the effective dielectric slab with the permittivity equal to the average permittivity of the dielectric grating. In the case of isolated resonances, the positions of BICs can be found from two-wave dispersion relationships for guided leaky modes. In the case of the degeneracy between the two families of leaky modes, the system exhibits an avoided crossing of resonances. In the spectral vicinity of the avoided crossing the transmittance as well as the emergence of BICs is described in the framework of the generic formalism by Volya and Zelevinsky [Phys. Rev. C 67, 054322 (2003)] with a single fitting parameter.