1. Institut Néel, CNRS, 25 Ave. des Martyrs, BP166, 38042 Grenoble Cedex 9, France
2. Laboratoire de Chimie Inorganique et Biologique (UMR-E3 CEA-UJF), INAC, CEA-Grenoble, 17 Ave. des Martyrs, 38054 Grenoble Cedex 9, France
3. Fakultät für Chemie, Universität Bielefeld, Postfach 100131, D-33501 Bielefeld, Germany
4. Department of Chemistry, Ben-Gurion University of the Negev, PO Box 653, 84105 Beer-Sheva, Israel
5. Present address: National High Magnetic Field Laboratory, Florida State University, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310, USA.

Correspondence to: A. Müller³B. Barbara^1,2 Correspondence and requests for materials should be addressed to B.B. (Email: bernard.barbara@grenoble.cnrs.fr) or A.M. (Email: a.mueller@uni-bielefeld.de).

The term 'molecular magnet' generally refers to a molecular entity containing several magnetic ions whose coupled spins generate a collective spin, S (ref. 1). Such complex multi-spin systems provide attractive targets for the study of quantum effects at the mesoscopic scale. In these molecules, the large energy barriers between collective spin states can be crossed by thermal activation or quantum tunnelling, depending on the temperature or an applied magnetic field^{2, 3, 4}. There is the hope that these mesoscopic spin states can be harnessed for the realization of quantum bits—'qubits', the basic building blocks of a quantum computer—based on molecular magnets^{5, 6, 7, 8}. But strong decoherence⁹ must be overcome if the envisaged applications are to become practical. Here we report the observation and analysis of Rabi oscillations (quantum oscillations resulting from the coherent absorption and emission of photons driven by an electromagnetic wave¹⁰) of a molecular magnet in a hybrid system, in which discrete and well-separated magnetic clusters are embedded in a self-organized non-magnetic environment. Each cluster contains 15 antiferromagnetically coupled S = 1/2 spins, leading to an S = 1/2 collective ground state^{11, 12, 13}. When this system is placed into a resonant cavity, the microwave field induces oscillatory transitions between the ground and excited collective spin states, indicative of long-lived quantum coherence. The present observation of quantum oscillations suggests that low-dimension self-organized qubit networks having coherence times of the order of 100 s (at liquid helium temperatures) are a realistic prospect.

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