We discuss reservoir-induced phase transitions of lattice fermions in the nonequilibrium steady state of an open system with local reservoirs. These systems may become critical in the sense of a diverging correlation length on changing the reservoir coupling. We here show that the transition to a critical state is associated with a vanishing gap in the damping spectrum. It is shown that, although in linear systems there can be a transition to a critical state, there is no reservoir-induced quantum phase transition between distinct phases with a nonvanishing damping gap. We derive the static and dynamical critical exponents corresponding to the transition to a critical state and show that their possible values, defining universality classes of reservoir-induced phase transitions, are determined by the coupling range of the independent local reservoirs. If a reservoir couples to $N$ neighboring lattice sites, the critical exponent can assume all fractions from 1 to $1/(N-1)$.

We show that cloaking of isolated objects using transformation-based cloaks is subject to a diameter-bandwidth product limitation: as the size of the object increases, the bandwidth of good (small-cross-section) cloaking decreases inversely with the diameter, as a consequence of causality constraints even for perfect fabrication and materials with negligible absorption. This generalizes a previous result that perfect cloaking of isolated objects over a nonzero bandwidth violates causality. Furthermore, we demonstrate broader causality-based scaling limitations on any bandwidth-averaged cloaking cross section, using complex analysis and the optical theorem to transform the frequency-averaged problem into a single-scattering problem with transformed materials.

Lenses play a key role in quantum imaging but inevitably constrain the spatial resolution and working wavelength. In this work we develop and demonstrate a lensless quantum ghost imaging by engineering quadratic nonlinear photonic crystals. With a transverse parabolic domain modulation introduced into the lithium tantalate crystal, the entangled photon pairs generated from parametric down-conversion will self-focus. Therefore we can dispense with additional lenses to construct imaging in a nonlocal way. The lensless imaging is found to follow a specific imaging formula where the effective focal length is determined by the domain modulation and pump wavelength. Additionally, two nonlocal images can be retrieved when the entangled photon pair is generated under two concurrent noncollinear phase-matching geometries. Our work provides a principle and method to realize lensless ghost imaging, which may be extended to other wavelengths and stimulate new types of practical quantum technologies.

We have theoretically demonstrated an enhanced Fizeau effect due to dragging the light that occurs when the group velocity of light is ultraslow. The proposed experiment can be done in a cell of atomic Rb vapor under conditions such that the group velocity of light is of the order of a few hundred meters per second. We show theoretically that higher-order dispersion can influence the Fizeau effect and can be observed experimentally. It has been shown that the change of phase is sensitive to the motion of the cell with the speed of the order of ${10}^{-3}$ cm/s and for possible displacements as small as 10 Å. The enhanced dragging effect can be applied for position control, detection of slow mechanical motion, and efficient modulators of light.

We have carried out calculations to search Borromean windows for the bound $F$ and $G$ states of the H${}_2^{+}$ system, where the nuclei and the electrons interact through a screened Coulomb (Yukawa-type) potential. The bound $F$ and G state energies for different values of the screening parameters are obtained using correlated exponential wave functions in the framework of the Ritz variational principle. The critical values of the screening parameters for the bound $F$ and $G$ states are reported for which the H${}_2^{+}$ system is stable, while all the possible fragments are unbound; that is, it shows windows for Borromean binding.

We study the photon generation from vacuum via the dynamical Casimir effect in a cavity containing a $N$-level detector in equally spaced, resonant ladder configuration or $N-1$ identical resonant two-level atoms. If the modulation frequency equals exactly twice the unperturbed cavity frequency, the photon growth goes on steadily for odd $N$, while for even $N$ at most $N-2$ photons can be generated. This finding is corroborated by numerical calculations. In the limit $N\to \infty$ we obtain the harmonic oscillator model of the detector, which admits analytical results.

We show that coherence vortices, namely screw-type topological defects in the phase of the cross-spectral density associated with a partially coherent scalar electromagnetic field, can be obtained for paraxial fields even in the absence of both scatterers and spatial vortices. The results are especially interesting at x-ray wavelengths, for which the paraxial (small-angle) approximation is usually met. A formal analogy is drawn between the cross-spectral density of a translation-invariant partially coherent system and a focused coherent wave field. This correspondence leads to a natural explanation of the coherence vortices as analogous to physical vortices that may appear around the focus of aberrated lenses. In addition, the Gouy phase shift, typical of any focused complex scalar wave field, is also derived for the one-dimensional cross section of cross-spectral density.

We demonstrate Einstein-Podolsky-Rosen (EPR) entanglement by detecting purely spatial quantum correlations in the near and far fields of spontaneous parametric down-conversion generated in a type-2 beta barium borate crystal. Full-field imaging is performed in the photon-counting regime with an electron-multiplying CCD camera. The data are used without any postselection, and we obtain a violation of Heisenberg inequalities with inferred quantities taking into account all the biphoton pairs in both the near and far fields by integration on the entire two-dimensional transverse planes. This ensures a rigorous demonstration of the EPR paradox in its original position-momentum form.

Motivated by studies of typical properties of quantum states in statistical mechanics, we introduce phase-random states, an ensemble of pure states with fixed amplitudes and uniformly distributed phases in a fixed basis. We first give a sufficient condition for canonical states to typically appear in subsystems of phase-random states, which reveals a trade-off relation between the initial state in the bounded energy subspace and the energy eigenstates that define that subspace. We then investigate the simulatability of phase-random states, which is directly related to that of time evolution in closed systems, by studying their entanglement properties. We find that, starting from a separable state, time evolutions under Hamiltonians composed of only separable eigenstates generate extremely high entanglement and are difficult to simulate with matrix-product states. We also show that random quantum circuits consisting of only two-qubit diagonal unitaries can generate an ensemble with the same average entanglement as phase-random states.

We have studied the Ba $6p_{1/2}nk$, $n=17$, $5⩽k⩽n-1$ Stark autoionizing states by isolated core excitation (ICE) of the Ba $6snk$ Stark bound states in very weak electric fields, an unexplored regime. In zero field the broad $\ell =5$ state overlaps all the higher $\ell$ states, and in fields less than $\sim$12 V/cm the Stark states composed of the $\ell >5$ states are within the zero-field width of the $nh$ state. In the ICE spectra the strongest transition from the $6snk$ Stark state is to the $6p_{1/2}nk$ Stark state, and shake-up satellite transitions are observed to higher-lying Stark states but not to lower-lying Stark states, a phenomenon which we attribute to the fact that the two Stark manifolds are both incomplete and different. The observed spectra can be described by treating the $6p_{1/2}nk$ Stark states, composed of the $\ell >5$ states, as discrete states embedded in a continuum, the broad $6p_{1/2}nh$ state.

The “fast-forward”approach by Masuda and Nakamura generates driving potentials to accelerate slow quantum adiabatic dynamics. First we present a streamlined version of the formalism that produces the main results in a few steps. Then we show the connection between this approach and inverse engineering based on Lewis-Riesenfeld invariants. We identify in this manner applications in which the engineered potential does not depend on the initial state. Finally we discuss more general applications exemplified by wave splitting processes.

We present a master equation describing the interaction of light with dielectric objects of arbitrary sizes and shapes. The quantum motion of the object, the quantum nature of light, as well as scattering processes to all orders in perturbation theory are taken into account. This formalism extends the standard master-equation approach to the case where interactions among different modes of the environment are considered. It yields a genuine quantum description, including a renormalization of the couplings and decoherence terms. We apply this approach to analyze cavity cooling of the center-of-mass mode of large spheres. Furthermore, we derive an expression for the steady-state phonon numbers without relying on resolved-sideband or bad-cavity approximations.

I theoretically demonstrate all-optical modulation and amplification in a one-dimensional photonic-crystal (PC) cavity with a large number of two-level quantum dots (QD's). Populations in the ground and excited states of two-level QD's with large transition dipole moments are greatly changed by an external laser, and this affects the electrical susceptibility of the QD's. In other words, all-optical modulation, in which light (the signal light) is modulated by light (the external laser), becomes possible. Moreover, a large contrast of the electromagnetic local densities of states in the PC cavity enables steady-state population inversion in two-level systems, which is generally considered impossible. The output signal power is more than twice as large as the input power at certain frequencies.

In this Brief Report we present an analysis deriving physical results in the entanglement amplification of fermonic systems in the relativistic regime, that is, beyond the single-mode approximation. This leads a recent work by Montero and Martín-Martínez [J. High Energy Phys. 07 (2011) 006] to a physical result, and confirms that the phenomenon of entanglement amplification can actually happen in the relativistic regime.

We have searched for $S$-wave shape resonances in the Ps${}^{-}$ system. The calculations are carried out in the framework of the complex-coordinate rotation method using both Hylleraas-type wave functions involving powers of interparticle coordinates, and the exponential correlated wave functions. We have located an $S$-wave shape resonance lying above the Ps ($N$ $=$ 2) threshold. By changing the mass of the positively charged particle from one unit of the electron mass to infinitely heavy, we have traced this resonance pole from Ps${}^{-}$ to H${}^{-}$. Results for the shape resonance in H${}^{-}$ are comparable with the available results in the literature.

The resonance interaction that takes place in planar nanochannels between pairs of excited-state atoms is explored. We consider interactions in channels of silica, zinc oxide, and gold. The nanosized channels induce a dramatically different interaction from that in free space. Illustrative calculations for two lithium and cesium atoms demonstrate that there is a short-range repulsion followed by long-range attraction. The binding energy is strongest near the surfaces. The size of the enlarged molecule is biggest at the center of the cavity and increases with channel width. Since the interaction is generic, we predict that enlarged molecules are formed in porous structures, and that the molecule size depends on the size of the nanochannels.

We study the propagation of electromagnetic waves in the limit of geometrical optics for a class of nearly transparent nonlinear uniaxial metamaterials for which their permittivity tensors present a negative principal component. Their permeabilities are assumed positive and dependent on the electric field. We show that light waves experience triple refraction—trirefringence. In addition to the ordinary wave, two extraordinary waves propagate in such media.

We propose a multiparty quantum clock synchronization protocol that makes optimal use of the maximal multipartite entanglement of GHZ-type states. To realize the protocol, different versions of maximally entangled eigenstates of collective energy are generated by local transformations that distinguish among different groupings of the parties. The maximal sensitivity of the entangled states to time differences between the local clocks can then be accessed if all parties share the results of their local time-dependent measurements. The efficiency of the protocol is evaluated in terms of the statistical errors in the estimation of time differences and the performance of the protocol is compared to alternative protocols proposed previously.

It is shown that the observability of a large class of operations on mixed states is fundamentally limited. We consider trace-preserving, unital operations. This class includes unitary and perfect premeasurement operations. An upper bound on the trace distance between an untransformed state and a state transformed by one of these operations is derived. The bound is dependent only on the purity of the state. In the case of maximal mixedness, the bound implies all operations of this class are unobservable.

We present a protocol for generating entanglement over long distances in a two-dimensional quantum network based on the surface-error-correction code. This protocol requires a fixed number of quantum memories at each node of the network and tolerates error rates of up to $1.67\%$ in the quantum channels. If local operations are of an error rate of ${10}^{-3}$, error rates of about $1.27\%$ in the quantum channels are tolerable.