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Detecting entanglement of n qubits by observing fluctuations of Bell operators

Zeqian Chen, Quanhua Xu

Published May 23, 2005• 0 citations
OpenAlex

Abstract

It is presented a criterion of efficiently detecting all pure entangled states of n qubits by observing fluctuation of Bell's operators in place of expectation in Bell's inequalities. The entanglement witnesses are some of MK Bell operators corresponding to Mermin-Klyshko inequalities, which are easily constructed by resolving a maximum problem and using a computable method, respectively. We illustrate generalized GHZ states, generalized W and pure three-qubit states, respectively, for seeing how good our criterion is to detect the entanglement of $n$ qubits. Therefore, we eventually resolve the problem of efficiently detecting pure entangled states of n qubits.

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It is presented a criterion of efficiently detecting all pure entangled states of n qubits by observing fluctuation of Bell's operators in place of expectation in Bell's inequalities. The entanglement witnesses are some of MK Bell operators corresponding to Mermin-Klyshko inequalities, which are easily constructed by resolving a maximum problem and using a computable method, respectively. We illustrate generalized GHZ states, generalized W and pure three-qubit states, respectively, for seeing how good our criterion is to detect the entanglement of nnn qubits. Therefore, we eventually resolve the problem of efficiently detecting pure entangled states of n qubits.
A possibility to reveal the entanglement in generalized n-qubit two-parameter GHZ states, as well as in any n-qubit states, with the help of the Mermin and Ardehali inequalities from the collection generally called the Mermin–Ardehali–Belinskii–Klyshko inequalities has been studied. Formulas for the calculation of the Mermin and Ardehali correlation functions in any quantum n-qubit states are derived, and criteria of the violation of corresponding inequalities by specific states are obtained. A set of states that are absolutely insensitive to the Mermin and Ardehali operators is revealed. Modified Mermin and Ardehali operators are proposed, the set of which makes it possible to extend the class of n-qubit states, in which quantum correlations can be revealed.
We consider 3‐qubit entangled pure states of the form α|000〉+β|111〉 and explore nonlocal correlations between the qubits by analyzing a Bell inequality belonging to the Mermin‐Klyshko (MK) set of N‐qubit inequalities. We find an expression for the maximum expectation value of the MK Bell operator and show that it does not increase smoothly with the amount of entanglement between the qubits. A subset of these tripartite entangled states does not violate the bound of 2 imposed by local hidden variable theories.
We provide a novel criterion for identifying quantum correlation, which allows us to find connections between Bell type inequalities, entanglement detection, and correlation. We utilize the criterion to construct witness operators that can detect genuine multi-qubit entanglement with fewer local measurements. The connection between identifications of quantum correlation and Mermin's inequality is discussed. Detection of genuine four-level tripartite entanglement with two local measurement settings is shown in the same manner. Further, through the criterion of quantum correlation, we derive a new Bell inequality for arbitrary high-dimensional bipartite systems, which requires fewer analyses of the measured outcomes.
We develop a necessary condition of quantum correlation. It is utilized to construct a ddd-level bipartite Bell-type inequality which is strongly resistant to noise and requires only analyses of O(d)O(d)O(d) measurement outcomes compared to the previous result O(d2)O({d}^{2})O(d2). Remarkably, a connection between the arbitrary high-dimensional bipartite Bell-type inequality and entanglement witnesses is found. Through the necessary condition of quantum correlation, we propose that the witness operators to detect truly multipartite entanglement for a generalized Greenberger-Horne-Zeilinger (GHZ) state with two local measurement settings and a four-qubit singlet state with three settings. Moreover, we also propose a robust entanglement witness to detect a four-level tripartite GHZ state with only two local measurement settings.