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Entanglement in a Dimerized Antiferromagnetic Heisenberg Chain Xiang Hao and Shiqun Zhu? School of Physical Science and Technology, Suzhou University, Suzhou, Jiangsu 215006, People’s Republic of China Abstract The entanglement properties in an antiferromagnetic dimerized Heisenberg spin-1/2 chain are investigated. The entanglement gap, which is the difference between the ground-state energy and the minimal energy that any separable state can attain, is calculated to detect the entanglement. It is found that the entanglement gap can be increased by varying the alternation parameter. Through thermal energy, the witness of the entanglement can determine a characteristic temperature below that an entangled state can be obtained. The entanglement detected by the energy can provide a lower bound for that determined by the concurrence. If the alternation parameter is smaller than a critical value, there is always no inter-dimer entanglement in the chain. PACS numbers: 03.67.Mn, 03.65.Ud, 75.10.Jm ? Corresponding author, E-mail: szhu@ 1 I. INTRODUCTION The quantum entanglement is considered as key resources of quantum information pro- cessing [1, 2]. The property of entanglement plays an essential role in understanding and quantifying the physical systems [3-9]. In recent years, some useful measures of entangle- ment have been proposed. The entropy of entanglement is used to qualify the entanglement of pure states [3] while the entanglement of formation is one measure for mixed states [4]. The entanglement in optical systems has been theoretically analyzed and observed [5-9]. For further developing the experimental detection, the separability criterion [10, 11] was suggested. On this basis, the thermal energy [12, 13], the magnetization [14] and the sus- ceptibility [15] have been used as entanglement witness [16] to detect the entanglement. If the expected value of the witness is negative, an entangled state could be obtained. These measurements can provide an intuitiv