We demonstrate that in-bulk vortex localized modes and their surface half-vortex (“horseshoe”) counterparts self-trap in two-dimensional nonlinear optical systems with $\mathcal{PT}$-symmetric photonic lattices (PLs). The respective stability regions are identified in the underlying parameter space. The in-bulk states are related to truncated nonlinear Bloch waves in gaps of the PL-induced spectrum. The basic vortex and horseshoe modes are built, severally, of four and three beams with appropriate phase shifts between them. Their stable complex counterparts, built of up to 12 beams, are also reported.

Quantum mechanics allows entanglement enhanced measurements to be performed, but loss remains an obstacle in constructing realistic quantum metrology schemes. However, recent work has revealed that entangled coherent states (ECSs) have the potential to perform robust subclassical measurements [J. Joo et al., Phys. Rev. Lett. 107, 083601 (2011)]. Up to now no read-out scheme has been devised that exploits this robust nature of ECSs, but we present here an experimentally accessible method of achieving precision close to the theoretical bound, even with loss. We show substantial improvements over unentangled classical states and highly entangled NOON states for a wide range of loss values, elevating quantum metrology to a realizable technology in the near future.

We propose a single-photon frequency converter via a one-dimensional waveguide coupled to a three-level $V$-type atom. An on-demand classical driving field is used to couple to the atom, allowing it to absorb a photon with a given frequency and then emit a photon with a different carried frequency. We study such a single-photon frequency conversion mechanism in two kinds of realistic physical systems: the system of coupled-resonator waveguide with cosine dispersion relation and the one of waveguide with linear dispersion relation. To demonstrate the single-photon transfer efficiency, we introduce the concept of scattering flows via the calculation of group velocities and find that the driving field prefers to be weak in the coupled-resonator waveguide but arbitrarily strong in the linear waveguide to achieve an optimal transfer efficiency. Furthermore, we demonstrate that our theoretical model is experimentally feasible with currently available technologies.

We show that the Beliaev damping of elementary excitations in a homogeneous Bose-Einstein condensate can undergo resonant enhancement by several orders of magnitude when the superfluid is interacting with a far-detuned radiation field of an optical resonator. The photonic tuning of the quasiparticle damping can be controlled by an external laser drive.

We study the Casimir-Lifshitz force and the radiative heat transfer in a system consisting of three bodies held at three independent temperatures and immersed in a thermal environment, the whole system being in a stationary configuration out of thermal equilibrium. The theory we develop is valid for arbitrary bodies, i.e., for any set of temperatures, dielectric, and geometrical properties, and describes each body by means of its scattering operators. For the three-body system we provide a closed-form unified expression of the radiative heat transfer and of the Casimir-Lifshitz force (both in and out of thermal equilibrium). This expression is thus first applied to the case of three planar parallel slabs. In this context we discuss the nonadditivity of the force at thermal equilibrium, as well as the equilibrium temperature of the intermediate slab as a function of its position between two external slabs having different temperatures. Finally, we consider the force acting on an atom inside a planar cavity. We show that, differently from the equilibrium configuration, the absence of thermal equilibrium admits one or more positions of minima for the atomic potential. While the corresponding atomic potential depths are very small for typical ground-state atoms, they may become particularly relevant for Rydberg atoms, becoming a promising tool to produce an atomic trap.

Weak quantum measurement (WM) is unique in measuring noncommuting operators and other peculiar, otherwise-undetectable phenomena predicted by the two-state-vector-formalism (TSVF). The aim of this article is threefold: (i) introducing the foundations of WM and TSVF, (ii) studying temporal peculiarities predicted by TSVF and manifested by WM, and (iii) presenting applications of WM to single particles.

We show that correlations between the detector positions at the two-photon Young interference plane exhibit contextual behavior. Contextuality is demonstrated by showing the violation of the $n$-cycle noncontextuality inequalities [M. Araújo et al., Phys. Rev. A 88, 022118 (2013)] for any even number $n$ of observables ranging from 4 to 14. These violations exclude noncontextual hidden-variable theories as an explanation of the conditional two-photon Young pattern. Unlike recent contextuality experiments, ours is free of the compatibility loophole.

Absolutely separable states form a special subset of the set of all separable states, as they remain separable under any global unitary transformation unlike other separable states. We consider the set of absolutely separable bipartite states and show that it is convex and compact in any arbitrary dimensional Hilbert space. Through a generic approach of construction of suitable Hermitian operators we prove the completeness of the separation axiom for identifying any separable state that is not absolutely separable. We demonstrate the action of such witness operators in different qudit systems. Examples of mixed separable systems are provided, pointing out the utility of the witness in entanglement creation using quantum gates. Decomposition of witnesses in terms of spin operators or photon polarizations facilitates their measurability for qubit states.

The scaling of entanglement entropy is computationally studied in several ($1\le d\le 2$)-dimensional free-fermion systems that are connected by one or more point contacts (PCs). For both the $k$-leg Bethe lattice $(d=1)$ and $d=2$ rectangular lattices with a subsystem of $L^d$ sites, the entanglement entropy associated with a single PC is found to be generically $S\sim L$. We argue that the $O\left(L\right)$ entropy is an expression of the subdominant $O\left(L\right)$ entropy of the bulk entropy-area law. For $d=2$ (square) lattices connected by $m$ PCs, the area law is found to be $S\sim a{L}^{d-1}+bmlogL$ and is thus consistent with the anomalous area law for free fermions ($S\sim LlogL$) as $m\to L$. For the Bethe lattice, the relevance of this result to density-matrix renormalization-group schemes for interacting fermions is discussed.

Time and energy of quantum processes are a tradeoff against each other. We propose to ascribe to any given quantum process a time-energy cost to quantify how much computation it performs. Here, we analyze the time-energy costs for general quantum measurements, along a similar line as our previous work for quantum channels, and prove exact and lower bound formulas for the costs. We use these formulas to evaluate the efficiencies of actual measurement implementations. We find that one implementation for a Bell measurement is optimal in time energy. We also analyze the time-energy cost for unambiguous state discrimination and find evidence that only a finite time-energy cost is needed to distinguish any number of states.

We present a multipartite protocol in a counterfactual paradigm. In counterfactual quantum cryptography, secure information is transmitted between two spatially separated parties even when there is no physical travel of particles transferring the information between them. We propose here a tripartite counterfactual quantum protocol for the task of certificate authorization. Here a trusted third party, Alice, authenticates an entity Bob (e.g., a bank) that a client Charlie wishes to securely transact with. The protocol is counterfactual with respect to either Bob or Charlie. We prove its security against a general incoherent attack, where Eve attacks single particles.

We report on a kinematically complete experiment on ionization of $H_2$ by proton impact. While a significant impact of the projectile coherence properties on the scattering-angle dependence of double-differential cross sections (DDCSs), reported earlier, is confirmed by the present data, only weak coherence effects are found in the electron and recoil-ion momentum dependence of the DDCSs. This suggests that the phase angle in the interference term is determined primarily by the projectile momentum transfer rather than by the recoil-ion momentum. We therefore cannot rule out the possibility that the interference observed in our data is not primarily due to a two-center effect.

We analyze asymptotically and numerically the motion around a single impurity and a network of impurities inserted in a two-dimensional superfluid. The criticality for the breakdown of superfluidity is shown to occur when it becomes energetically favorable to create a doublet—the limiting case between a vortex pair and a rarefaction pulse on the surface of the impurity. Depending on the characteristics of the potential representing the impurity, different excitation scenarios are shown to exist for a single impurity as well as for a lattice of impurities. Depending on the lattice characteristics it is shown that several regimes are possible: dissipationless flow, excitations emitted by the lattice boundary, excitations created in the bulk, and the formation of large-scale structures.

We put forward an experimental configuration to observe transverse Anderson localization of partially coherent light beams with a tunable degree of first-order coherence. The scheme makes use of entangled photons propagating in disordered waveguide arrays and is based on the unique relationship between the degree of entanglement of a pair of photons and the coherence properties of the individual photons constituting the pair. The scheme can be readily implemented with current waveguide-on-a-chip technology, and surprisingly the tunability of the coherence properties of the individual photons is done at the measurement stage, without resorting to changes of the light source itself.

In this paper we review the conditions needed for the localization and stationarity of a wave packet during propagation in a linear transparent medium. The requirements on the dimensionality and spatiotemporal frequency content for wave packets with such characteristics are discussed. In particular, we demonstrate how the localization of the stationary two-dimensional solution of the propagation equation depends on the features and shape of its spatiotemporal spectral bandwidth. The spatiotemporal properties of the one-dimensional (1D) spatial and 1D temporal beams stationarily propagating in dispersive materials are illustrated both in the normal and in the anomalous dispersion regime.

Spatial rocking is a kind of resonant forcing able to convert a self-oscillatory system into a phase-bistable, pattern forming system, whereby the phase of the spatially averaged oscillation field locks to one of two values differing by $\pi$. We propose the spatial rocking in an experimentally relevant system—the vertical-cavity surface-emitting laser (VCSEL)—and demonstrate its feasibility through analytical and numerical tools applied to a VCSEL model. We show phase bistability, spatial patterns, such as roll patterns, domain walls, and phase (dark-ring) solitons, which could be useful for optical information storage and processing purposes.

We examine Green's functions and densities of states for bosons which move in materials with interfaces. Motivated by the interest in materials with quasirelativistic dispersion relations, we demonstrate that modification of Klein-Gordon-type contributions to the Hamiltonian in an interface yields Green's functions and densities of states which exhibit two-dimensional behavior at high energies. Three-dimensional behavior in a low-energy range is recovered if the shift of the mass parameter in the interface is small.

We propose a continuous-variable measurement-device-independent quantum key distribution (CV-MDI QKD) protocol, in which detection is conducted by an untrusted third party. Our protocol can defend all detector side channels, which seriously threaten the security of a practical CV QKD system. Its security analysis against arbitrary collective attacks is derived based on the fact that the entanglement-based scheme of CV-MDI QKD is equivalent to the conventional CV QKD with coherent states and heterodyne detection. We find that the maximal total transmission distance is achieved by setting the untrusted third party close to one of the legitimate users. Furthermore, an alternate detection scheme, a special application of CV-MDI QKD, is proposed to enhance the security of the standard CV QKD system.