Influence of Multi-Photon Process on Entanglement of Interacting System of the Qubit and Thermal Field

We study the system of a single qubit couples to a single mode thermal field according to a multi-photon Jaynes-Cummings-type interaction with phase decoherence. Both the time evolving entanglement and the stationary state entanglement are calculated by adopting the log-negativity as a measure. It is found that the multi-photonprocess can enhance the stationary state entanglement of this system and can enlarge the range of the parameter Δ/g and the mean photon number of initial thermal field in which the stationary state is distillable.     

Implementing remote controlled-NOT gates and entanglement swapping via geometric phase gates in ion-trap systems

We propose a scheme for the implementation of remote controlled-NOT gates and entanglement swapping via geometric phase gates in ion-trap systems. The proposed scheme uses the two ground states of the A-type ions as memory instead of the vibrational mode. And the system is robust against the spontaneous radiation and the dephasing.     

Implementation of a Quantum Conditional Phase Gate for the Quantum Fourier Transform in Circuit QED

It is well known that multiple superconducting charge qubits coupled to a transmission line resonator can be controlled to achieve quantum logic gates between two arbitrary qubits. We propose a scheme to realize a quantum conditional phase gate with a geometric property by circuit electrodynamics, and it is applied naturally to reaJize the quantum Fourier transform with high fidelity. It is also demonstrated that the application is feasible and considerable under the present experimental technology.     

Quantum logic networks for probabilistic teleportation

By eans of the primitive operations consisting of single-qubit gates.two-qubit controlled-not gates,Von Neuman measurement and classically controlled operations.,we construct efficient quantum logic networks for implementing probabilistic teleportation of a single qubit,a two-particle entangled state,and an N-particle entanglement.Based on the quantum networks,we show that after the partially entangled states are concentrated into maximal entanglement,the above three kinds of probabilistic teleportation are the same as the standard teleportation using the corresponding maximally entangled states as the quantum channels.     

Entanglement Manipulation for a Two-Mode Squeezed Vacuum State

By numerically analysing the entropy of entanglement of the output state from a Mach-Zehnder interferometer for the two-mode squeezed vacuum state input, it is found that if the internal phase shift of the interferometer is adjusted to the value of 0 or 7r, the entangling characteristic of the input state is efficiently preserved at the output. If the internal phase shift is tuned to the value of Tr/2, the two-mode squeezed vacuum state is completely disentangled at the output of the setup. If the internal phase shift deviates from the above values, the input state is degraded into a partially entangled output state. Based on these results, a method for optically realizing the entanglement preservation, entanglement degradation, and disentanglement via the interferometer is obtained.     

Multipartite Entanglement Transfer from Multi-Mode Coherent State to Spin Qubits

The multipartite entanglement transfer from continuous variable system to spin qubits is investigated. We select multi-mode coherent field as continuous variable field. It is found that the qubits can not gain tripartite entanglement for states of close to GHZ state from the multi-mode coherent field. Moreover, the ability of the qubits gain the tripartite entanglement for states close to W state and bipartite entanglement from the continuous variable system is depended on the phase of multi-mode coherent field.     

Entanglement Preserving in Quantum Copying of Three－Qubit Entangled State

We study the degree to which quantum entanglement survives when a three-qubit entangled state is copied by using local and non-local processes,respectively,and investigate iterating quantum copying for the three-qubit system.There may exist inter-three-qubit entanglement and inter-two-qubit entanglement for the three-qubit system.We show that both local and non-local copying processes degrade quantum entanglement in the three-particle system due to a residual correlation between the copied output and the copying machine.we also show that the inter-two-qubit entanglement is preserved better than the inter-three-qubit entanglement in the local cloning process.We find that non-local cloning is much more efficient than the local copying for broadcasting entanglement,and output state via non-local cloning exhiits the fidelity better than local cloning.     

Secure deterministic communication scheme based on quantum remote state preparation

Based on the techniques of the quantum remote state preparation via a deterministic way, this paper proposes a quantum communication scheme to distribute the secret messages in two phases, i.e., the carrier state checking phase and the message state transmitting phase. In the first phase, the secret messages are encoded by the sender using a stabilizer quantum code and then transmitted to the receiver by implementing three CNOT gates. In the second phase, the communicators check the perfectness of the entanglement of the transmitted states. The messages can be distributed to the receiver even if some of the transmitted qubits are destroyed.     

Noise-Induced Entanglement Transition in One-Dimensional Random Quantum Circuits

A random quantum circuit is a minimally structured model to study entanglement dynamics of many-body quantum systems.We consider a one-dimensional quantum circuit with noisy Haar-random unitary gates using density matrix operator and tensor contraction methods.It is shown that the entanglement evolution of the random quantum circuits is properly characterized by the logarithmic entanglement negativity.By performing exact numerical calculations,we find that,as the physical error rate is decreased...     

Quantum correlation of a three-particle W-class state under quantum decoherence

We investigate the quantum characteristics of a three-particle W-class state and reveal the relationship between quan- tum discord and quantum entanglement under decoherence. We can also identify the state for which discord takes a maximal value for a given decoherence factor, and present a strong bound on quantum entanglement-quantum discord. In contrast, a striking result will be obtained that the quantum discord is not always stronger than the entanglement of formation in the case of decoherence. Furthermore, we also theoretically study the variation trend of the monogamy of quantum correlations for the three-particle W-class state under the phase flip channel, and find that the three-particle W-class state could transform from polygamous into monogamous, owing to the decoherence.     

Remote Implementation of Multi-qubit Quantum Phase Gates

Based on Wu et al.’s original idea (Phys. Lett. A 372:2802, 2008), we propose a scheme to remotely implement multi-qubit quantum phase gates. With the assistance of entanglement swapping, classical communication and quantum repeater, multi-qubit quantum phase gates can be realized perfectly nearly. It is emphasized that our proposal can overcome the limitation that error probability scales exponentially with the length of the channel during the realization of the gates.     

Implementing remote controlled-NOT gates and entanglement swapping via geometric phase gates in ion-trap systems

We propose a scheme for the implementation of remote controlled-NOT gates and entanglement swapping via geometric phase gates in ion-trap systems. The proposed scheme uses the two ground states of the $\Lambda$-type ions as memory instead of the vibrational mode. And the system is robust against the spontaneous radiation and the dephasing.     

Generic entanglement can be generated efficiently

We find that generic entanglement is physical, in the sense that it can be generated in polynomial time from two-qubit gates picked at random. We prove as the main result that such a process generates the average entanglement of the uniform (unitarily invariant) measure in at most O(N3) steps for N qubits. This is despite an exponentially growing number of such gates being necessary for generating that measure fully on the state space. Numerics furthermore show a variation cutoff allowing one to associate a specific time with the achievement of the uniform measure entanglement distribution. Various extensions of this work are discussed. The results are relevant to entanglement theory and to protocols that assume generic entanglement can be achieved efficiently.     

Time-dependent exactly solvable models for quantum computing

A time-dependent periodic Hamiltonian admitting exact solutions is applied to construct a set of universal gates for a quantum computer. The time evolution matrices are obtained in an explicit form and used to construct logic gates for computation. A way of obtaining an entanglement operator is discussed, too. The method is based on transformation of soluble time-independent equations into time-dependent ones by employing a set of special time-dependent transformation operators. The text was submitted by the authors in English.     

Quantum entanglement and quantum operation

It is a simple introduction to quantum entanglement and quantum operations. The authors focus on some applications of quantum entanglement and relations between two-qubit entangled states and unitary operations. It includes remote state preparation by using any pure entangled states, nonlocal operation implementation using entangled states, entanglement capacity of two-qubit gates and two-qubit gates construction. Supported by the National Fundamental Research Program of China (Grant No. 2001CB309306), the National Natural Science Foundation of China (Grant Nos. 60621064 and 10674127) and the Innovation Funds from Chinese Academy of Sciences     

Progress of quantum entanglement in a trapped-ion based quantum computer

In recent years, there have been significant progress toward building a practical quantum computer, demonstrating key ingredients such as single-qubit gates and a two-qubit entangling gate. Among various physical platforms for a potential quantum computing processor, a trapped-ion system has been one of the most promising platforms due to long coherence times, high-fidelity quantum gates, and qubit connectivity. However, scaling up the number of qubits for a practical quantum computing faces a core challenge in operating high-fidelity quantum gates under influence from neighboring qubits. In particular, for the trapped-ion system, unwanted quantum crosstalk between qubits and ions’ quantum motional states hinder performing high-fidelity entanglement as the number of ions increases. In this review, we introduce a trapped-ion system and explain how to perform single-qubit gates and a two-qubit entanglement. Moreover, we mainly address theoretical and experimental approaches to achieve high-fidelity and scalable entanglement toward a trapped-ion based quantum computer.     

The influence of laser fluctuations on entanglement generation in a non-degenerate parametric amplifier

We consider the effects of the phase and the amplitude fluctuations of the pump field upon the entanglement generation in a non-degenerate optical parametric amplifier. We show that the entanglement between the signal and idler modes in a NOPA system are suppressed by these fluctuations. Our results also show that entanglement is more sensitive to phase fluctuations than to amplitude fluctuations.     

Proposal for Remotely Realizing Multi-qubit Controlled-Phase Gates

A feasible proposal is explored to remotely perform a family of multi-qubit controlled-phase gates (MQCPG), with the aid of entanglement transfer and quantum repeater. To remotely realize the gates, some local transformations, including single-qubit rotating operation, controlled-NOT gate and quantum phase gate, are employed during the realization. It turns out that MQCPG can be implemented among separate spatially agents with nearly unit fidelity and success probability. Besides, required classical information consumption of the proposed scenario is worked out. Furthermore, some attractive issues are discussed including the feature of present proposal and the experimental feasibility based on current technologies. Remarkably, it is revealed that our proposal essentially accesses to several nontrivial features lying in breaking through the limitation that error probability scales exponentially with the length of the channel in the course of the realization of the gates, and well compatibility with today’s experimental technologies.     

Entanglement distribution and entangling power of quantum gates

Quantum gates, which play a fundamental role in quantum computation and other quantum information processes, are unitary evolution operators Û that act on a composite system, changing its entanglement. In the present contribution, we study some aspects of these entanglement changes. By recourse to a Monte Carlo procedure, we compute the so-called “entangling power” for several paradigmatic quantum gates and discuss results concerning the action of the CNOT gate. We pay special attention to the distribution of entanglement among the several parties involved.