Research Of Entanglement Concentration For Quantum Logic Qubit

Quantum Information has been attracting much attention and greatly developing for its advantages and foreground compared to classical information.Generally,quantum information included quantum computing and quantum communication.Quantum entanglement plays an important role in the flied of quantum communication,such as quantum teleportation,quantum key distribution,quantum secure direct communication and other important quantum information protocols.In such applications,the parties should set up a secure quantum channel with the maximally entangled states.Noise is one of the main obstacles in entanglement distribution over fiber or free space.It will degrade the entanglement,which will make the maximally entangled states become the partially entangled state or mixed states.Entanglement concentration is an efficient approach to distill the maximally entangled state from the partially entangled state with local operation and classical communication.The concatenated Greenberger-Horne-Zeilinger(C-GHZ)state is a new type of logic-qubit entanglement,which shows its potential application in both large-scale quantum networks and faulttolerant quantum computation.Therefore,the research about the C-GHZ state is very necessary.Firstly,the entanglement concentration protocols(ECPs)based on the cross-Kerr nonlinearities for the C-GHZ state are proposed.Subsequently,the ECPs for C-GHZ state assisted with single logic qubit are also proposed.These ECPs are firstly discussed in simple cases and then extended to arbitrary C-GHZ state.Moreover,the ECPs can be repeated to obtain a higher success probability.The above two ECPs offer a promising candidate for the realization of long distance quantum communication.The W state is also an important quantum resource in the quantum communication and quantum computing.There were a few schemes about arbitrary N-particle W state entanglement concentration.In the last,we present a universal way to concentrate an arbitrary N-particle partially entangled W state into a maximally entangled W state with different parity check gates.It comprises two protocols.The first protocol is based on the linear optical elements,say the partial parity check gate and the second protocol uses the quantum nondemolition measurement based on the cross-Kerr nonlinearities to construct the complete parity check gate.Both protocols can achieve the concentration task.