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International Conference on Intelligence Science and Information Engineering

Entanglement of Graph Qutrit States

Haitao Li, Xiaoyu Chen

Published Aug 1, 2011• 3 citations

Abstract

The number of inequivalent classes of up to 8-qutrit graph states is 1002, with 239 decomposable graphs and 763 indecomposable graphs. Apparently, the former can be decomposed to several indecomposable parts. For qubit graph states, the upper and lower bounds of entanglement have been given. If the two bounds don't coincide, the entanglement can be calculated by iterative method. For qutrit graph states, their lower bounds can be found through the local complementation operations. But its hard to find the upper bound because the qutrit graph states have 2 basis states which are GHZ state and W state. Fortunately, the iterative method can also be used here. In this paper, we calculated the entanglement of all the 1002 graph states. Through researching the results, we found the similar feature with graph qubit states that the decimal part of entanglement is stable.

DOI: 10.1109/isie.2011.10

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The number of inequivalent classes of up to 8-qutrit graph states is 1002, with 239 decomposable graphs and 763 indecomposable graphs. Apparently, the former can be decomposed to several indecomposable parts. For qubit graph states, the upper and lower bounds of entanglement have been given. If the two bounds don't coincide, the entanglement can be calculated by iterative method. For qutrit graph states, their lower bounds can be found through the local complementation operations. But its hard to find the upper bound because the qutrit graph states have 2 basis states which are GHZ state and W state. Fortunately, the iterative method can also be used here. In this paper, we calculated the entanglement of all the 1002 graph states. Through researching the results, we found the similar feature with graph qubit states that the decimal part of entanglement is stable.

The entanglement of graph states up to eight qubits is calculated in the regime of iteration calculation. The entanglement measures could be the relative entropy of entanglement, the logarithmic robustness or the geometric measure. All 146 local inequivalent graphs are classified as two categories: graphs with identical upper LOCC entanglement bound and lower bipartite entanglement bound, graphs with unequal bounds. The late may displays non-integer entanglement. The precision of iteration calculation of the entanglement is less than

10^{-14}

.

We propose a new classification for the entanglement in graph states based on generalized con- currence. The numerical results indicate that the eight different three-qubit graph states in three categories, 64 four-qubit graph states in five categories and 1024 five-qubit graph states are in ten classes. We also compare this classification with equivalence classes of these graph states under local complementation (LC) operator, and the obtained result suggests that classification by generalized concurrence is not in contradiction with the LC-rule.

The set of entanglement measures proposed by Hein, Eisert, and Briegel for

n

-qubit graph states [Phys. Rev. A 69, 062311 (2004)] fails to distinguish between inequivalent classes under local Clifford operations if $n\ensuremath{\ge}7$. On the other hand, the set of invariants proposed by van den Nest, Dehaene, and De Moor (VDD) [Phys. Rev. A 72, 014307 (2005)] distinguishes between inequivalent classes, but contains too many invariants (more than $2\ifmmode\times\else\texttimes\fi{}{10}^{36}$ for

n=7

) to be practical. Here we solve the problem of deciding which entanglement class a graph state of $n\ensuremath{\le}8$ qubits belongs to by calculating some of the state's intrinsic properties. We show that four invariants related to those proposed by VDD are enough for distinguishing between all inequivalent classes with $n\ensuremath{\le}8$ qubits.