1. Institut für Experimentalphysik, Universität Wien, Boltzmanngasse 5, A-1090 Wien, Austria
2. Present address: Fachbereich Physik, Universität Konstanz, D-78457 Konstanz, Germany

Correspondence to: Markus Arndt Correspondence and requests for materials should be addressed to M.A. (Email: markus.arndt@univie.ac.at).

Emergent quantum technologies have led to increasing interest in decoherence—the processes that limit the appearance of quantum effects and turn them into classical phenomena. One important cause of decoherence is the interaction of a quantum system with its environment, which 'entangles' the two and distributes the quantum coherence over so many degrees of freedom as to render it unobservable. Decoherence theory^{1, 2, 3, 4} has been complemented by experiments using matter waves coupled to external photons^{5, 6, 7} or molecules⁸, and by investigations using coherent photon states⁹, trapped ions¹⁰ and electron interferometers^{11, 12}. Large molecules are particularly suitable for the investigation of the quantum–classical transition because they can store much energy in numerous internal degrees of freedom; the internal energy can be converted into thermal radiation and thus induce decoherence. Here we report matter wave interferometer experiments in which C₇₀ molecules lose their quantum behaviour by thermal emission of radiation. We find good quantitative agreement between our experimental observations and microscopic decoherence theory. Decoherence by emission of thermal radiation is a general mechanism that should be relevant to all macroscopic bodies.

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