Wei-Wen Cheng

BIDIRECTIONAL QUANTUM SECURE DIRECT COMMUNICATION IN DRIVEN CAVITY QED

A theoretical scheme of bidirectional quantum secure direct communication is proposed in the context of driven cavity QED. We first present an entanglement swapping scheme in cavities where two atoms without previous interaction can be entangled with a success probability of unity. Then, based on a novel property of entanglement swapping, we propose a bidirectional quantum secure direct communication protocol, in which two legitimate users can exchange their different secret messages simultaneously in a direct way. The probability of success in our scheme is 1.0. This scheme does not involve apparent (or direct) Bell-state measurements and is insensitive to the cavity decay and the thermal field.

Scaling of geometric quantum discord close to a topological phase transition.

Quantum phase transition is one of the most interesting aspects in quantum many-body systems. Recently, geometric quantum discord has been introduced to signature the critical behavior of various quantum systems. However, it is well-known that topological quantum phase transition can not be described by the conventional Landaus symmetry breaking theory, and thus it is unknown that whether previous study can be applicable in this case. Here, we study the topological quantum phase transition in Kitaevs 1D p-wave spinless quantum wire model in terms of its ground state geometric quantum discord. The derivative of geometric quantum discord is nonanalytic at the critical point, in both zero temperature and finite temperature cases. The scaling behavior and the universality are verified numerically. Therefore, our results clearly show that all the key ingredients of the topological phase transition can be captured by the nearest neighbor and long-range geometric quantum discord.

Signature of topological quantum phase transitions via Wigner-Yanase skew information

We apply the Wigner-Yanase skew information approach to analyze two typical models that exhibit a topological quantum phase transition. Based on the exact solutions of the ground states, the Wigner-Yanase skew information between two nearest sites for each of the two models is obtained. For the one-dimensional Kitaev chain model, the first-order derivative of the Wigner-Yanase skew information is non-analytical around the critical point. The scaling behavior and the universality are verified numerically. In particular, the skew information can also detect the factorization transition in such a model. For the two-dimensional Kitaev honeycomb model, the first-order derivative of the Wigner-Yanase skew information shows some singularities at the critical points where the system transits from the gapless phase to the gapped one. Our results suggest that the Wigner-Yanase skew information can serve as a good indicator of the topological phase transitions in these models and shed considerable light on the relationships between topological quantum phase transition and information theory.

Renormalization-group approach to quantum Fisher information in an XY model with staggered Dzyaloshinskii-Moriya interaction.

We investigate the quantum Fisher information and quantum phase transitions of an XY spin chain with staggered Dzyaloshinskii-Moriya interaction using the quantum renormalization-group method. The quantum Fisher information, its first-derivatives, and the finite-size scaling behaviors are rigorously calculated respectively. The singularity of the derivatives at the phase transition point as a function of lattice size is carefully discussed and it is revealed that the scaling exponent for quantum Fisher information at the critical point can be used to describe the correlation length of this model, addressing the substantial role of staggered Dzyaloshinskii-Moriya interaction in modulating quantum phase transitions.

Measurement-induced disturbance near Anderson localization in one-dimensional systems

We study the localization transition in several typical one-dimensional single-particle systems by means of measurement-induced disturbance (MID). The results show that the MID presents a rapid drop around the boundary between the localized state and extended ones, and the corresponding first-order derivative exhibits a behavior of divergence around the critical point for deterministic on-site potential systems (e.g. the quasi-periodic model). These characteristics can capture a phase diagram as well as the traditional method. For the non-deterministic on-site systems (e.g. the random dimer model), the states around the resonant energies possess relatively large values for MID, which means that they are extended. In addition, as the random potential ϵ b exceeds the critical value, the states possessing a large MID vanish completely. These results show that MID can be useful in detecting localization transition in these typical one-dimensional systems.

Finite-temperature scaling of trace distance discord near criticality in spin diamond structure

In this work we explore the quantum correlation quantified by trace distance discord as a measure to analyze the quantum critical behaviors in the Ising-XXZ diamond structure at finite temperatures. It is found that the first-order derivative of the trace distance discord exhibits a maximum around the critical point at finite temperatures. By analyzing the finite-temperature scaling behavior, we show that such a quantum correlation can detect exactly the quantum phase transitions from the entan-gled state in ferrimagnetic phase to an unentangled state in ferrimagnetic phase or to an unentangled state in ferromagnetic phase. The results also indicate that the above two kinds of transitions can be distinguished by the different finite-temperature scaling behaviors. Moreover, we find that the trace distance discord, in contrast to other typical quantum correlations (e.g., concurrence, quantum discord and Hellinger distance), may be more reliable to exactly spotlight the critical points of this model at finite temperatures under certain situations.

Scaling of geometric phases close to the topological quantum phase transition in Kitaev's quantum wire model

We present a unified study of the topological quantum phase transition in Kitaevs 1D p-wave spinless quantum wire model in terms of its ground state geometric phase (GP). We also analyze zero-temperature and finite-temperature scaling parameters, extracted from the GP near the critical point. The derivative of the ground-state GP is nonanalytic at the phase boundary. A finite-size scaling and finite-temperature scaling analysis are carried out and, furthermore, the scaling behavior and universality are verified numerically.

Simple method to detect topological Bloch bands in one-dimensional optical lattice

We propose a simple method to detect the Zak phase in one-dimensional optical lattice. This phase is the Berry phase acquired during an adiabatic motion of a particle along closed trajectories in the one-dimensional Brillouin zone, which now has been used to characterize the topological properties of Bloch bands. We show that this phase can be manifested by the shift of the Wannier center in the optical lattice. Through in situ detection of atomic density, the topological properties of the Bloch bands are directly revealed. This method is simple but also general, which could be generalized to detect two-dimensional topological invariants for identification of topological insulators, including the first Chern number and the Z 2 topological index.

Anomalous decoherence effects in driven coupled quantum spin systems

We discuss anomalous decoherence effects at zero and finite temperatures in driven coupled quantum spin systems. By numerical simulations of the quantum master equation, it is found that entanglement dynamics of two coupled spin qubits is qualitatively affected by the noise strength. The effects of noise strength, the detuning and finite temperature of independent boson reservoirs on the steady state entanglement are addressed in detail. Pumped by an external field drive, non-trivial steady states can be found, the steady state entanglement increases monotonically up to a maximum at certain optimal noise strength and decreases steadily for higher values. Furthermore, increasing the detuning can not only induce but also suppress steady state entanglement, which depends on the value of noise strength. At last, we delimit the border between presence or absence of steady state entanglement in the related finite temperatures.

ENTANGLEMENT DYNAMICS OF TWO QUBITS COUPLED TO SQUEEZED DISSIPATIVE ENVIRONMENTS

By analytically solving the Lindblad form of the master equation, we investigate entanglement dynamics of two qubits coupled via the XY interaction, where each qubit is interacting with an independent reservoir with the squeezing parameters and squeezing angles. In the weak-squeezed reservoir, we show that the entanglement sudden death and entanglement sudden birth will happen for various entangled states. Some initial product states evolve into entangled ones, initially entangled states lose completely or partially their entanglement. The effects of varying the degree of entanglement of the initial states, the spin chain system parameters and different values of the degree of squeezing on the sudden death, revival and birth times are analyzed in detail. We also see that the steady state concurrence appears in the squeezed dissipative environments, which is affected by both the system parameters and the degree of squeezing.