Zhi-Yong Wang

Hierarchical quantum-information splitting

Abstract We present a scheme for asymmetric quantum-information splitting, where a sender distributes asymmetrically a quantum secret (quantum state) to distant partners in a network. The asymmetric distribution leads to that the partners have different powers to recover the sender’s secret. In other words, their authorities for getting the secret are hierarchized. In the scheme, the partners do not need to make any nonlocal operation. The scheme can also be modified to implement threshold-controlled teleportation.

Photonic two-qubit parity gate with tiny cross–Kerr nonlinearity

The cross-Kerr nonlinearity (XKNL) effect can induce efficient photon interactions in principle with which photonic multiqubit gates can be performed using far fewer physical resources than linear optical schemes. Unfortunately, it is extremely challenging to generate giant cross-Kerr nonlinearities. In recent years much effort has been made to perform multiqubit gates via weak XKNLs. However, the required nonlinearity strengths are still difficult to achieve in an experiment. We here propose an XKNL-based scheme for realizing a two-photon polarization-parity gate, a universal two-qubit gate, in which the required strength of the nonlinearity could be orders of magnitude weaker than those required for previous schemes. The scheme utilizes a ring cavity fed by a coherent state as a quantum information bus which interacts with a path mode of the two polarized photons (qubits). The XKNL effect makes the bus pick up a phase shift dependent on the photon number of the path mode. Even when the potential phase shifts are very small they can be effectively measured using photon-number resolving detectors, which accounts for the fact that our scheme can work in the regime of tiny XKNL. The measurement outcome reveals the parity (even parity or odd parity) of the two polarization qubits.

Insights from Monte Carlo simulations on charge inversion of planar electric double layers in mixtures of asymmetric electrolytes

Monte Carlo simulations of a planar negatively charged dielectric interface in contact with a mixture of 1:1 and 3:1 electrolytes are carried out using the unrestricted primitive model under more realistic hydrated ion sizes. Two typical surface charge densities are chosen to represent the systems from the weak to strong coupling regimes. Our goal is to determine the dependence of the degree of charge inversion on increasing concentration of both mono- and trivalent salts and to provide a systematic study on this peculiar effect between short-range and electrostatic correlations. The numerical results show that addition of monovalent salt diminishes the condensation of trivalent counterions due to either the favorable solvation energy or the available space constraints. As the concentration of trivalent salt increases, on the other hand, the inclusion of the ionic size and size asymmetry results in a damped oscillatory charge inversion at low enough surface charge and another counterintuitive surface charge amplification. It is proposed that both of the anomalous events in the weak coupling regime are thought to be entropic in origin which is completely different from the electrostatic driven charge inversion in the strong coupling regime. In addition, the electrostatic images arising from the dielectric mismatch lead to a decaying depletion effect on the structure of double layer with growing salt concentration in the case of low charged interface but have no effect at high surface charge values. The microscopic information obtained here points to the need for a more quantitative theoretical treatment in describing the charge inversion phenomenon of real colloidal systems.

Monte Carlo determination of mixed electrolytes next to a planar dielectric interface with different surface charge distributions.

Employing canonical ensemble Monte Carlo simulations, we report a calculation of the distribution of small ions next to a planar negatively charged surface in the presence of mixed electrolytes of monovalent and trivalent salt ions within the framework of the primitive model under more realistic hydrated ion size conditions. The effects of surface charge discreteness and dielectric breakdown on charge inversion are discussed based on increasing concentration of both monovalent and trivalent salt. Moreover, a comparison of the simulation results for different discretization models is made along with the case of uniformly distributed charge in terms of the ionic density profiles as well as the integrated charge distribution function. For finite size charged groups located inside the lower dielectric region, a complete equivalence with the case of uniform distribution is observed if the quantities of interest are exclusively analyzed as a function of the distance to the charged interface. With protruding head groups into the aqueous solution, the excluded volume dominates over the correlation effect, therefore the ions are less accumulated in the vicinity of the charged surface, inducing that the onset position of charge inversion experiences an evident shift toward the aqueous environment. Overall, the effect of repulsive image forces on the diffuse double layer structure can be significant at low surface charge density irrespectively of surface charge distributions.

Charge reversal at a planar boundary between two dielectrics.

Despite the ubiquitous character and relevance of the electric double layer in the entire realm of interface and colloid science, very little is known of the effect that surface heterogeneity exerts on the underlying mechanisms of ion adsorption. Herein, computer simulations offer a perspective that, in sharp contrast to the homogeneously charged surface, discrete groups promote multivalent counterion binding, leading to charge reversal but possibly having not a sign change of the electrophoretic mobility. Counterintuitively, the introduction of dielectric images yields a significantly greater accumulation of counterions, which further facilitates the magnitude of charge reversal. The reported results are very sensitive to both the degree of ion hydration and the representation of surface charges. Our findings shed light on the mechanism for charge reversal over a broad range of coupling regimes operating the adsorption of counterions through surface group bridging attraction with their own images and provide opportunities for experimental studies and theoretical development.

Examining the Contributions of Image-Charge Forces to Charge Reversal: Discrete Versus Continuum Modeling of Surface Charges

The effects of both repulsive and attractive image-charge forces on the structure of electric double layers are addressed by Monte Carlo determination, based on a primitive model of electrolytes in contact with two types of identically charged surfaces: one with a homogeneously smeared-out charge density and the other with discrete interfacial groups. It is shown that the behavior of ions is closely related to surface charge distributions. Moreover, charge reversal in the absence of image charges witnesses an initial enhancement and then follows a fast suppression with increasing valence of the interfacial groups. The situation is quite similar to what are observed in the presence of repulsive image charges, which can significantly facilitate counterion condensation by overcoming the electrostatic barrier presented by the low dielectric substrate. With transition to attractive image-charge interactions, however, charge reversal remains widely unaffected in different surface charge representations, which even becomes much weaker when compared to the corresponding cases of both no images and repulsive images, provided that the interfacial groups have adequate valences. The overall scenario is found to be independent of the surface charge density values under study. These findings clearly illustrate the enormous improvement in our quantitative understanding of the electric double layer structure and the associated charge reversal phenomenon at the interface of various substrates.

Ion association at discretely-charged dielectric interfaces: Giant charge inversion

Giant charge reversal has been identified for the first time by Monte Carlo simulation for a discretely charged surface in contact with a trivalent electrolyte solution. It takes place regardless of the surface charge density under study and the monovalent salt. In stark contrast to earlier predictions based on the 2-dimensional Wigner crystal model to describe strong correlation of counterions at the macroion surface, we find that giant charge reversal reflects an intricate interplay of ionic volume effects, electrostatic correlations, surface charge heterogeneity, and the dielectric response of the confined fluids. While the novel phenomenon is yet to be confirmed with experiment, the simulation results appear in excellent agreement with a wide range of existing observations in the subregime of charge inversion. Our findings may have far-reaching implications to understanding complex electrochemical phenomena entailing ionic fluids under dielectric confinements.

Image-induced overcharging in the weakly charged surfaces

Electrostatic interactions at dielectric boundaries can bring about novel properties which are vital not only to understand the complex behavior at biological interfaces but also to design desirable interfacial processes for a broad variety of practical applications. Here, we aim to provide comprehensive information by Monte Carlo simulations about the annihilation and formation of the image-induced overcharging effect, a new class of charge accumulation phenomena resulting from an excess of adsorbed ions on the like-charged surfaces. It is observed that whether the image-charge interactions are repulsive or attractive, the buildup of overcharging depends critically on both the concentration and the charge-asymmetry of electrolytes. On the other hand, overcharging is gradually cancelled with an increase in the size of ions and the concentration of monovalent salt as well as in the magnitude of surface charge density. Also, our simulations show that the image-induced overcharging effect cannot promote charge reversal. These findings are especially relevant to colloidal stability and dispersibility, and will hopefully serve as a foundation for the development of adequate theories.

Effect of hydrophobic mismatch on domain formation and peptide sorting in the multicomponent lipid bilayers in the presence of immobilized peptides

In the plasma membranes, many transmembrane (TM) proteins/peptides are anchored to the underlying cytoskeleton and/or the extracellular matrix. The lateral diffusion and the tilt of these proteins/peptides may be greatly restricted by the anchoring. Here, using the coarse-grained molecular dynamics simulation, we investigated the domain formation and peptide sorting in the ternary lipid bilayers in the presence of the immobilized peptide-grid and peptide-cluster. We mainly focused on examining the combining effect of the peptide immobilization and hydrophobic mismatch on the domain formation and peptide sorting in the lipid bilayers. Compared to the lipid bilayers inserted with free TM peptides, our results showed that, because of the tilt restriction imposed on the peptides, the hydrophobic mismatch effect more significantly influences the domain size, the dynamics of domain formation, and the peptide sorting in our systems. Our results provide some theoretical insights into understanding the formation of nanosized lipid rafts, the protein sorting in the lipid rafts and the interaction between the cytoskeleton, the extracellular matrix, and the plasma membranes.

Looking deeper into the structure of mixed electric double layers near the point of zero charge

Molecular simulations have been carried out using the Metropolis Monte Carlo approach to investigate the structure of planar electric double layers containing counterion mixture within the framework of the unrestricted primitive model. The results reveal that near the point of zero charge, the rise of monovalent salt drastically elevates the collapse of ions regardless of their polarity. In particular, we fail to observe the formation of a strongly correlated liquid in the first counterion layer due to favorable entropic effects, in contrast to the early data from molecular dynamics simulations [corrected] for a spherical electric double layer [R. Messina, E. González-Tovar, M. Lozada-Cassou, and C. Holm, Europhys. Lett. 60, 383 (2002)]. Moreover, the large size of coions is found to be a pivotal factor in determining the reversal of electrophoretic mobility. On the other hand, the repulsive image charge forces thoroughly annihilate this peculiar reversal of mobility within the investigated scope of concentrations, but exert no effect on the emergence of charge reversal. These findings highlight potential applications of coions characteristics to control gene delivery and colloidal stability as well as to design viral packing and polyelectrolyte self-assembly.