Entanglement Fidelity as a Measure of Preservation of Entanglement in Local Noisy Process

A new formula of entanglement fidelity has been introduced, which can serve as a measure of the preservation of entanglement between two initially entangled subsystems exposed to local noisy environments. For a simple model we derive analytic expressions of concurrence and entanglement fidelity and draw the relationship between them. We find that such entanglement fidelity exhibits the behavior similar to that of the concurrence in quantum evolutions.

Performance of entanglement-assisted quantum codes with noisy ebits over asymmetric and memory channels

Entanglement-assisted quantum error correction codes(EAQECCs)play an important role in quantum communications with noise.Such a scheme can use arbitrary classical linear code to transmit qubits over noisy quantum channels by consuming some ebits between the sender(Alice)and the receiver(Bob).It is usually assumed that the preshared ebits of Bob are error free.However,noise on these ebits is unavoidable in many cases.In this work,we evaluate the performance of EAQECCs with noisy ebits over asymmetric quantum channels and quantum channels with memory by computing the exact entanglement fidelity of several EAQECCs.We consider asymmetric errors in both qubits and ebits and show that the performance of EAQECCs in entanglement fidelity gets improved for qubits and ebits over asymmetric channels.In quantum memory channels,we compute the entanglement fidelity of several EAQECCs over Markovian quantum memory channels and show that the performance of EAQECCs is lowered down by the channel memory.Furthermore,we show that the performance of EAQECCs is diverse when the error probabilities of qubits and ebits are different.In both asymmetric and memory quantum channels,we show that the performance of EAQECCs is improved largely when the error probability of ebits is reasonably smaller than that of qubits.

Fidelity of states in infinite-dimensional quantum systems

We discuss the fidelity of states in the infinite-dimensional systems and give an elementary proof of the infinite-dimensional version of Uhlmann's theorem.This theorem is used to generalize several properties of the fidelity of the finite-dimensional case to the infinite-dimensional case.These are somewhat different from those for the finite-dimensional case.

Experimental simulation of spin-1 states by second-order type-Ⅱ parametric down-conversion

In this paper,by using the second-order parametric down-conversion of the nonlinear crystal,the spin-1 state is simulated by the two-photon polarization entangled modes. Through adjusting the laser pulse power density,the efficiency of second-order parametric down-conversion is enhanced. The intensity of the spin-1 state is 0.5/s. The fidelity of the state is up to F=0.891±0.002,and the contrast is C=17.3. The results provide a new method for Stern-Gerlach measurement on the spin-1 system.

Experimental investigation of the influence of laser intensity on the quality of Einstein-Podolsky-Rosen entangled photons

Using spontaneous parametric down conversion,polarization post selection and coincidence counting technique,the polarization Einstein-Podolsky-Rosen(EPR) entangled states are prepared.Experimental studies on the efficiency,contrast and fidelity with different pump laser intensities are performed systematically.The results show that the pump laser intensity distinctly influences the quality of entangled photons,especially the contrast and the fidelity.On the other hand,the pump efficiency of entangled photons is almost invariable,namely the entangled source brightness increases linearly with the increase of pump laser power.

Quantum information transfer between photonic and quantum-dot spin qubits

We propose schemes for the efficient information transfer between a propagating photon and a quantum-dot（QD） spin qubit in an optical microcavity that have no auxiliary particles required. With these methods, the information transfer between two photons or two QD spins can also be achieved. All of our proposals can work with high fidelity, even with a high leakage rate. What is more, each information transfer process above can also be seen as a controlled-NOT（CNOT） operation. It is found that the information transfer can be equivalent to a CNOT gate. These proposals will promote more efficient quantum information networks and quantum computation.