Wei-Wei Zhang

A watermark strategy for quantum images based on quantum fourier transform

We present a robust watermark strategy for quantum images. The watermark image is embedded into the fourier coefficients of the quantum carrier image, which will not affect the carrier image’s visual effect. Before being embedded into the carrier image, the watermark image is preprocessed to be seemingly meaningless using quantum circuit, which further ensures the security of the watermark image. The properties of fourier transform ensure that the watermark embedded in the carrier image resists the unavoidable noise and cropping.

Efficient quantum private comparison employing single photons and collective detection

Two efficient quantum private comparison (QPC) protocols are proposed, employing single photons and collective detection. In the proposed protocols, two distrustful parties (Alice and Bob) compare the equivalence of information with the help of a semi-honest third party (TP). Utilizing collective detection, the cost of practical realization is reduced greatly. In the first protocol, TP gains the result of the comparison. While in the second protocol, TP cannot get the comparison result. In both of our protocols, Alice and Bob only need be equipped with unitary operation machines, such as phase plates. So Alice and Bob need not to have the expensive quantum devices, such as qubit generating machine, quantum memory machine and quantum measuring machine. Security of the protocols is ensured by theorems on quantum operation discrimination.

Cryptanalysis and improvement of the quantum private comparison protocol with semi-honest third party

Recently, a quantum private comparison (QPC) protocol with a dishonest third party (TP) (Yang et al. in Quantum Inf Process, 2012. doi:10.1007/s11128-012-0433-4) was proposed, which pointed out that the assumption of semi-honest third party (TP) is unreasonable. Here we find this protocol is not so secure as it was expected, and then we give some improvement strategies, which ensure that both players’ secrets will not be leaked to anyone. We also discuss the assumption for TP in QPC protocls, which gives a constructive suggestions for the design of a new QPC protocol.

Improving the security of arbitrated quantum signature against the forgery attack

As a feasible model for signing quantum messages, some cryptanalysis and improvement of arbitrated quantum signature (AQS) have received a great deal of attentions in recent years. However, in this paper we find the previous improvement is not suitable implemented in some typical AQS protocols in the sense that the receiver, Bob, can forge a valid signature under known message attack. We describe the forgery strategy and present some corresponding improved strategies to stand against the forgery attack by modifying the encryption algorithm, an important part of AQS. These works preserve the merits of AQS and lead some potential improvements of the security in quantum signature or other cryptography problems.

Exciton polarization, fine-structure splitting, and the asymmetry of quantum dots under uniaxial stress.

We derive a general relation between the fine-structure splitting (FSS) and the exciton polarization angle of self-assembled quantum dots under uniaxial stress. We show that the FSS lower bound under external stress can be predicted by the exciton polarization angle and FSS under zero stress. The critical stress can also be determined by monitoring the change in exciton polarization angle. We confirm the theory by performing atomistic pseudopotential calculations for the InAs/GaAs quantum dots. The work provides deep insight into the dot asymmetry and their optical properties and a useful guide in selecting quantum dots with the smallest FSS, which are crucial in entangled photon source applications.

A quantum protocol for millionaire problem with Bell states

We propose a quantum protocol for the millionaire problem with Bell states, where two distrustful parties can compare the values of their fortune with the help of a semi-dishonest third party. The efficiency of our protocol is higher than that of previous protocols for millionaire problem. In our protocol, any information about the values of their fortune will not be leaked out. The security of our protocol is also discussed.

Atomistic pseudopotential theory of optical properties of exciton complexes in InAs/InP quantum dots

The photoluminescence (PL) spectra of exciton complexes in quantum dots (QDs) are of primary importance to understand their optical properties, which are well understood in InAs/GaAs QDs both theoretically and experimentally. However the knowledge can not be directly applied to InAs/InP QDs. In this work, we investigate the differences of the PL spectra of the two dots using an empirical pseudopotential method. The alignment of exciton complex transition lines of the two QDs are very different and the reasons are explained. We show that the “hidden” correlation energies in InAs/InP QDs are smaller than those in InAs/GaAs QDs and the lifetime of exciton complexes in InAs/InP QDs are about twice longer than those in InAs/GaAs QDs.We present a comprehensive study of the optical properties of InAs/InP self-assembled quantum dots (QDs) using an empirical pseudopotential method and configuration interaction treatment of the many-particle effects. The results are compared to those of InAs/GaAs QDs. The main results are: (i) The alignment of emission lines of neutral exciton, charged exciton and biexciton in InAs/InP QDs is quite different from that in InAs/GaAs QDs. (ii) The hidden correlation in InAs/InP QDs is 0.7 0.9 meV, smaller than that in InAs/GaAs QDs. (iii) The radiative lifetimes of neutral exciton, charged exciton and biexciton in InAs/InP QDs are about twice longer than those in InAs/GaAs QDs. (v) The phase diagrams of few electrons and holes in InAs/InP QDs differ greatly from those in InAs/GaAs QDs. The filling orders of electrons and holes are shown to obey the Hund’s rule and Aufbau principle, and therefore the photoluminescence spectra of highly charged excitons are very different from those of InAs/GaAs QDs.

Analysis of the two-particle controlled interacting quantum walks

We have recently proposed the two-particle controlled interacting quantum walks for building quantum Hash schemes (Li et al. Quantum Inf Proc, 2012. doi:10.1007/s11128-012-0421-8). In this paper, we adopt the mutual information, the measurement-induced disturbance and the quantum mutual information to measure the classical correlation, the quantum correlation and the total correlation between two particles respectively. Our conclusion is that the correlation between the particles of the two-particle controlled interacting quantum walks is similar to that of the two-particle interacting quantum walks. It is superb for keeping the quantum Hash scheme safe.

Creating cat states in one-dimensional quantum walks using delocalized initial states

Cat states are coherent quantum superpositions of macroscopically distinct states and are useful for understanding the boundary between the classical and the quantum world. Due to their macroscopic nature, cat states are difficult to prepare in physical systems. We propose a method to create cat states in one-dimensional quantum walks using delocalized initial states of the walker. Since the quantum walks can be performed on any quantum system, our proposal enables a platform-independent realization of the cat states. We further show that the linear dispersion relation of the effective quantum walk Hamiltonian, which governs the dynamics of the delocalized states, is responsible for the formation of the cat states. We analyze the robustness of these states against environmental interactions and present methods to control and manipulate the cat states in the photonic implementation of quantum walks.

Quantum Walks on Two Kinds of Two-Dimensional Models

In this paper, we numerically study quantum walks on two kinds of two-dimensional graphs: cylindrical strip and Mobius strip. The two kinds of graphs are typical two-dimensional topological graph. We study the crossing property of quantum walks on these two models. Also, we study its dependence on the initial state, size of the model. At the same time, we compare the quantum walk and classical walk on these two models to discuss the difference of quantum walk and classical walk.