Decoherence in Nature has become one of the most pressing problems inphysics. Many applications, including quantum information processing, depend onunderstanding it; and fundamental theories going beyond quantum mechanics havebeen suggested [1-3], where the breakdown of quantum theory appears as an'intrinsic decoherence', mimicking environmental decoherence [4]. Such theoriescannot be tested until we have a handle on ordinary environmental decoherenceprocesses. Here we show that the theory for insulating electronic spin systemscan make accurate predictions for environmental decoherence in molecular-basedquantum magnets [5]. Experimental understanding of decoherence in molecularmagnets has been limited by short decoherence times, which make coherent spinmanipulation extremely difficult [6-9]. Here we reduce the decoherence byapplying a strong magnetic field. The theory predicts the contributions to thedecoherence from phonons, nuclear spins, and intermolecular dipolarinteractions, for a single crystal of the Fe8 molecular magnet. In experimentswe find that the decoherence time varies strongly as a function of temperatureand magnetic field. The theoretical predictions are fully verifiedexperimentally - there are no other visible decoherence sources. Ourinvestigation suggests that the decoherence time is ultimately limited bynuclear spins, and can be extended up to about 500 microseconds, by optimizingthe temperature, magnetic field, and nuclear isotopic concentrations.
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