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Decoherence in crystals of quantum molecular magnets


Quantum decoherence is a central concept in physics. Applications such as quantum information processing depend on understanding it; there are even fundamental theories proposed that go beyond quantum mechanics1,2,3, in which the breakdown of quantum theory would appear as an ‘intrinsic’ decoherence, mimicking the more familiar environmental decoherence processes4. Such applications cannot be optimized, and such theories cannot be tested, until we have a firm handle on ordinary environmental decoherence processes. Here we show that the theory for insulating electronic spin systems can make accurate and testable predictions for environmental decoherence in molecular-based quantum magnets5. Experiments on molecular magnets have successfully demonstrated quantum-coherent phenomena6,7,8 but the decoherence processes that ultimately limit such behaviour were not well constrained. For molecular magnets, theory predicts three principal contributions to environmental decoherence: from phonons, from nuclear spins and from intermolecular dipolar interactions. We use high magnetic fields on single crystals of Fe8 molecular magnets (in which the Fe ions are surrounded by organic ligands) to suppress dipolar and nuclear-spin decoherence. In these high-field experiments, we find that the decoherence time varies strongly as a function of temperature and magnetic field. The theoretical predictions are fully verified experimentally, and there are no other visible decoherence sources. In these high fields, we obtain a maximum decoherence quality-factor of 1.49 × 106; our investigation suggests that the environmental decoherence time can be extended up to about 500 microseconds, with a decoherence quality factor of 6 × 107, by optimizing the temperature, magnetic field and nuclear isotopic concentrations.

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Figure 1: Typical ESR spectra, showing echo intensity as a function of transverse magnetic field, H.
Figure 2: Calculated contributions to the decoherence coming from the coupling to nuclear spins, phonons and magnons.
Figure 3: Measured and calculated decoherence times T 2 in samples 1 and 2, as a function of temperature.


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This work was supported by the NSF (DMR-0520481, DMR-0703925), the Keck Foundation (S.T. and J.v.T.), NSERC, CIFAR, PITP, the John E. Fetzer Memorial Trust (grant D21-C62) and the Center for Philosophy and the Natural Sciences, California State University, Sacramento (I.S.T. and P.C.E.S.). The National High Magnetic Field Laboratory is supported by NSF Cooperative Agreement DMR-0654118, by the State of Florida, and by the DOE.

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S.T., I.S.T. and P.C.E.S. contributed to the writing of the manuscript. S.T., I.S.T. and P.C.E.S. conceived the ESR experiments. The ESR measurements were carried out by S.T. and J.v.T. The theoretical work was done by I.S.T. and P.C.E.S. C.C.B and D.N.H. synthesized Fe8 crystals and performed X-ray diffraction measurements.

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Correspondence to S. Takahashi or P. C. E. Stamp.

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The authors declare no competing financial interests.

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Takahashi, S., Tupitsyn, I., van Tol, J. et al. Decoherence in crystals of quantum molecular magnets. Nature 476, 76–79 (2011).

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