Entanglement-Assisted Classical Capacity of a Generalized Amplitude Damping Channel

The entanglement-assisted capacity of a generalized amplitude damping channel is investigated by using the properties of partial symmetry and concavity of mutual information. The numerical and analytical results of the entanglement-assisted capacity are obtained under certain conditions. It is shown that the entanglement-assisted capacity depends on the channel parameters representing the ambient temperature and dissipation, and the prior entanglement between sender and receiver can approximately double the classical capacity of the generalized amplitude damping channel.     

Dynamics of entanglement under decoherence in noninertial frames

In this paper,we investigate the entanglement dynamics of a two-qubit entangled state coupled with its noisy environment,and plan to utilize weak measurement and quantum reversal measurement to study the entanglement dynamics under different decoherence channels in noninertial frames.Through the calculations and analyses,it is shown that the weak measurement can prevent entanglement from coupling to the amplitude damping channel,while the system is under the phase damping and flip channels.This protection protocol cannot prevent entanglement but will accelerate the death of entanglement.In addition,if the system is in the noninertial reference frame,then the effect of weak measurement will be weakened for the amplitude damping channel.Nevertheless,for other decoherence channels,the Unruh effect does not affect the quantum weak measurement,the only exception is that the maximum value of entanglement is reduced to√2/2of the original value in the inertial frames.     

Three methods to distill multipartite entanglement over bipartite noisy channels

We first assume that there are only bipartite noisy qubit channels in a given multipartite system, and present three methods to distill the general Greenberger-Horne-Zeilinger state. By investigating the methods, we show that multipartite entanglement distillation by bipartite entanglement distillation has higher yield than ones in the previous multipartite entanglement distillations.     

Criterion for Genuine Multipartite Entanglement Quantum Channels

We introduce a character matrix for the N-qubit subsystem of a 2N-qubit state and show the criterion for genuine entanglement channel existing between two N-qubit subsystems in the state. The criterion allows us to check conveniently whether genuine quantum channels exist or not in the 2N-qubit state without calculating its N-qubit reduced density matrices.     

Transmissions of quantum entangled states in anisotropic depolarizing channels

We investigate the effect of an anisotropic depolarizing channel on the classical transmission of quantum entangled states. We calculate numerically mutual information between honest parts as a function of the degree of anisotropy of the depolarizing channel. In contrast to the case of isotropic channels, we found that the mutual information depends on both the degree of anisotropy and the degree of entanglement. It increases with increase in the degree of entanglement for a quantum channel with sufficiently large depolarizing anisotropy and decreases with increase in the entanglement.     

Entanglement Swapping of Pair Coherent States with Phase Decoherence

We investigate the entanglement swapping of continuous state and the two-mode squeezed vacuum which is exposed variable using the pair coherent state as the input in a phase decoherence environment as the quantum channel. By adopting the log-negativity as the measure of entanglement, we analyze how entanglement of the two initial states and the phase decoherence environment affect the entanglement swapping quality.     

Universal behaviors of mutual information in one-dimensional ${\sf J}_1-{\sf J}_2$ model

The critical behaviors of the entropy correlation effects in the one dimensional J₁-J₂ Heisenberg model are studied. It is shown that the mutual information or the correlation entropy captures the key features of information about critical fluctuations and can be used to quantify the quantum and finite-temperature phase transitions. At the critical point, the mutual information is power-law decay and the entanglement correlation length is infinite. While far away from the critical point, the mutual information is exponential decay and the entanglement correlation length is finite. A universal property of the mutual information is also found. Based on the critical behaviors of the mutual information, a new method to quantify the infinite order phase transition in the system is proposed.     

On the discrimination of qubit channels

We discuss the discrimination of arbitrary qubit channels and the discrimination of qubit channels with the aid of entanglement. Nevertheless, the maximally entangled states might decrease the distinguishability, even if the channel is not entanglement-breaking one.     

Entanglement purification with noisy apparatus can be used to factor out an eavesdropper

We give a proof that entanglement purification, even with noisy apparatus, is sufficient to disentangle an eavesdropper (Eve) from the communication channel. Our proof applies to all possible attacks (individual and coherent). Due to the quantum nature of the entanglement purification protocol, it is also possible to use the obtained quantum channel for secure transmission of quantum information. Received 10 August 2001 and Received in final form 26 October 2001     

Entanglement-assisted quantum logic gates for two remote qubits

We propose a protocol to realize quantum logic gates for two remote qubits via entanglement swapping. According to the scheme of quantum repeater presented by H.-J. Briegel et al., we can complete long-distance communication and computation. Compared with previous schemes through noisy channels, our protocol can overcome the limitation that error probability scales exponentially with the length of the channel. We illustrate this protocol in cavity QED system, but the idea can also be realized in other physical systems.     

Deterministic secure quantum communication using four-particle genuine entangled state and entanglement swapping

A deterministic secure quantum communication scheme using entanglement swapping is proposed. The sender prepares four-particle genuine entangled states and sends two particles in each state to the receiver and remains the rest particles. If the quantum channel is secure, they begin to communicate. After their four-particle projective measurements, the receiver can obtain the secret information according to his measurement outcomes and classical information from the sender. Using entanglement swapping, there are no particles carrying secret information to be transmitted.     

Protecting Quantum Fisher Information in Correlated Quantum Channels

Quantum Fisher information (QFI) has potential applications in quantum metrology tasks. QFI is investigated when the consecutive actions of a quantum channel on the sequence of qubits have partial classical correlations. The results showed that while the decoherence effect is detrimental to QFI, effects of such classical correlations on QFI are channel-dependent. For the Bell-type probe states, the classical correlations on consecutive actions of the depolarizing and phase flip channels can be harnessed to improve QFI, while the classical correlations in the bit flip and bit-phase flip channels induce a slight decrease of QFI. For a more general parameterization form of the probe states, the advantage of using the initial correlated system on improving QFI can also remain in a wide regime of the correlated quantum channels.     

Entanglement and Its Dynamics with Atomic Spontaneous Decay

The atomic decay for a two-level atom interacting with a single mode of electromagnetic tield is considered. For a chosen initial state, the exact solution of the master equation is found. Therefore, effect of the atomic damping on entanglement （purity loss）, degree of entanglement by the negativity, mutual information and atomic coherence through the master equation are studied.     

Entanglement Dynamics of the Mixed Two-qubit System in Different Noisy Channels

We investigate the entanglement dynamics of the two-qubit entanglement quantum system when they transmitted through the Pauli channels and the depolarizing channel both independently or collectively. Making use of the concurrence we found that the entanglement of a kind of mixed two-qubit system defined in this paper can be preserved in the collective Pauli σ _y noise channel, but the entanglement of the other kind of mixed two-qubit system can be preserved in the collective Pauli σ _z noise channel. Meanwhile, our quantum systems will undergoing the entanglement sudden death (ESD) in collective depolarizing channel when they return to the maximally entangled Bell states. The reason is the Landblad operators in depolarizing channel are non-commuting operators and this finding is in accord with the previous study.     

Effects of single-qubit quantum noise on entanglement purification

We study the stability under quantum noise effects of the quantum privacy amplification protocol for the purification of entanglement in quantum cryptography. We assume that the E91 protocol is used by two communicating parties (Alice and Bob) and that the eavesdropper Eve uses the isotropic Bužek-Hillery quantum copying machine to extract information. Entanglement purification is then operated by Alice and Bob by means of the quantum privacy amplification protocol and we present a systematic numerical study of the impact of all possible single-qubit noise channels on this protocol. We find that both the qualitative behavior of the fidelity of the purified state as a function of the number of purification steps and the maximum level of noise that can be tolerated by the protocol strongly depend on the specific noise channel. These results provide valuable information for experimental implementations of the quantum privacy amplification protocol.     

Interacting spin pairs in rotational magnetic fields and geometric phase

In virtue of the quantum invariant theory, we obtain the rigorous solution of the isotropic bipartite system in rotational magnetic fields, based on which the general expression of the noncyclic geometric phase is worked out and the entanglement dependence of the noncyclic geometric phase in this model is investigated. We show that the influence of the coupling on noncyclic geometric phase depends on the initial condition of the system. We also show that when the magnetic fields are stationary, there is a more general class of states existed of which the noncyclic geometric phase could be interpreted solely in terms of the solid angle enclosed by the geodesically closed curve on a two-sphere parameterized by the evolving Schmidt coefficients.     

Detecting entanglement of quantum channels

Entanglement is the crucial resource for different quantum information processing tasks. While conventional studies focus on the entanglement of bipartite or multipartite quantum states, recent works have extended the scenario to the entanglement of quantum channels, an operational quantification of the channel entanglement manipulation capability. Based on the recently proposed channel entanglement resource framework, here we study a further task of resource detection—witnessing entanglement of quantum channels. We first introduce the general framework and show how channel entanglement detection is related to the Choi state of the channel, enabling channel entanglement detection via conventional state entanglement detection methods. We also consider entanglement of multipartite quantum channels and use the stabilizer formalism to construct entanglement witnesses for circuits consisting of controlled-Z gates. We study the effectiveness of the proposed detection methods and compare their performance for several typical channels. Our work paves the way for systematic theoretical studies of channel entanglement and practical benchmarking of noisy intermediate scaled quantum devices.     

Polarization entanglement purification using the temporal degree of freedom

Polarization entangled photon pairs are easily perturbed in noisy channels. We propose a polarization entanglement purification method using temporal degree of freedom, followed by the conventional iterative purification. The entanglement fidelity can be improved to any degree, and the steps needed are less than those using conventional iteration method.     

Entanglement dynamics of three-qubit states in noisy channels

We study entanglement dynamics of the three-qubit system which is initially prepared in pure Greenberger-Horne-Zeilinger (GHZ) or W state and transmitted through one of the Pauli channels s_z\sigma_z, s_x\sigma_x, s_y\sigma_y or the depolarizing channel. With the help of the lower bound for three-qubit concurrence we show that the W state preserves more entanglement than the GHZ state in transmission through the Pauli channel s_z\sigma_z. For the Pauli channels s_x\sigma_x, s_y\sigma_y and the depolarizing channel, however, the entanglement of the GHZ state is more resistant against decoherence than the W-type entanglement. We also briefly discuss how the accuracy of the lower bound approximation depends on the rank of the density matrix under consideration.     

Repetition versus noiseless quantum codes for correlated errors

We study the performance of simple quantum error correcting codes with respect to correlated noise errors characterized by a finite correlation strength μ. Specifically, we consider bit flip (phase flip) noisy quantum memory channels and use repetition and noiseless quantum codes. We characterize the performance of the codes by means of the entanglement fidelity F(μ,p) as function of the error probability p and degree of memory μ. Finally, comparing the entanglement fidelities of repetition and noiseless quantum codes, we find a threshold μ^*(p) for the correlation strength that allows to select the code with better performance.