Decoherence effects on quantum Fisher information of multi-qubit W states

Quantum Nonlocality of N-qubit W States

An experimental setup for testing quantum nonlocality of N qubits is proposed. This method is a generalization of the optical setup proposed by Banaszek and Wódkiewicz [1]. The quantum nonlocality of N qubits can be obtained through its violation of N-qubit Bell inequalities. The correlation function measured in the experiment is described by the Wigner function. The effect of inefficient detec...

Decoherence of an n-qubit quantum memory.

We analyze decoherence of a quantum register in the absence of nonlocal operations, i.e., n noninteracting qubits coupled to an environment. The problem is solved in terms of a sum rule which implies linear scaling in the number of qubits. Each term involves a single qubit and its entanglement with the remaining ones. Two conditions are essential: first, decoherence must be small, and second, t...

Quantum Fisher information as a predictor of decoherence in the preparation of spin-cat states for quantum metrology

Qubit-loss-free fusion of W states

With the assistance of weak cross-Kerr nonlinearities, we introduce an optical scheme to fuse two small-size polarization entangled photonic W states into a large-scale W state without qubit loss, i.e.,Wn+m state can be generated from an n-qubit W state and a m-qubit W state. To complete the fusion task, two polarization entanglement processes and one spatial entanglement process are applied. T...

Fisher information under decoherence in Bloch representation

Wei Zhong,1,2 Zhe Sun,1,3 Jian Ma,1,2 Xiaoguang Wang,1,2,* and Franco Nori1,4 1Advanced Science Institute, RIKEN, Wako-shi, Saitama 351-0198, Japan 2Zhejiang Institute of Modern Physics, Department of Physics, Zhejiang University, Hangzhou 310027, China 3Department of Physics, Hangzhou Normal University, Hangzhou 310036, China 4Physics Department, The University of Michigan, Ann Arbor, Michigan...

Quantum Fisher information for superpositions of spin states

In terms of quantum Fisher information, a quantity χ was introduced by Pezzé and Smerzi [Phys. Rev. Lett. 102, 100401 (2009)], which is a multiparticle entanglement measure, and provides a necessary and sufficient condition for sub-shot-noise phase estimation sensitivity. We derive a general expression of χ for arbitrary symmetric multiqubit states with nonzero mean spins. It is shown that the ...