Entanglement temperature in molecular magnets composed of S−spin dimers

In the present work, we investigate the quantum thermal entanglement in molecular magnets composed of dimers of spin S, using an Entanglement Witness built from measurements of magnetic susceptibility. An entanglement temperature, Te, is then obtained for some values of spin S. From this, it is shown that Te is proportional to the intradimer exchange interaction J and that entanglement appears only for antiferromagnetic coupling. The results are compared to experiments carried on three isostructural materials: KNaMSi4O10 (M=Mn, Fe or Cu). Introduction. – For about a decade it has been realized that quantum entanglement is a valuable resource for quantum information processing, since it allows forms of communication that are classically impossible [1,2]. However, until recently it was believed that the phenomenon could not exist beyond atomic scale, due to the interaction between the system and the environment. Such interaction would lead to decoherence of the quantum state, destroying entanglement. However, some theoretical works raised the possibility that solid state systems could also exhibit quantum entanglement at finite temperatures [3, 4]. This “thermal entanglement” might be experimentally detected with the help of some observables, or “witnesses”, that are related to thermodynamical quantities, which could be directly measured [5–10]. An Entanglement Witnesses (EW), by definition, has a negative expectation value for certain types of entangled states [11–14]. The demonstration that quantum entanglement can influence the behavior of thermodynamical properties of solids, such as magnetic susceptibility [8–10, 15–18], shows that quantum effects can be related to important macroscopic quantities. These features have established that the study of entanglement in solid state systems [19] based on the observation of such EWs are helpful tools to quantum information and quantum computation, since many proposals of quantum chips are solid state based [20–25]. The class of materials known as molecular magnets [26] are among those which can exhibit thermal entanglement. In this class of materials, the intermolecular magnetic interactions are extremely weak compared to those within individual molecules. Thus a bulk sample, comprised by a set of non-interacting molecular clusters, is completely described in terms of independent clusters. The small number of coupled spins is very convenient from the point of view of the theoretical description, which can be made through analytical functions on a low dimensional Hilbert space. From a physical point of view, a molecular magnet can combine classical properties found in a macroscopic magnet [26] and quantum properties, such as quantum interference [27] and entanglement [5–9]. Recently, molecular magnets have been pointed out as good systems to be used in high-density information memories and also, due to their long coherence times [28], in quantum computing devices [20–25]. In this work, we investigate the quantum entanglement