1. Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Strasse 1, 85748 Garching, Germany
2. National Institute of Information and Communications Technology, 588-2 Iwaoka, Nishi-ku, Kobe 651-2492, Japan
3. Sektion Physik der Universität München, Am Coulombwall 1, 85748 Garching, Germany

Correspondence to: Wolfgang Lange¹ Email: Wolfgang.Lange@mpq.mpg.de

Abstract

The controlled production of single photons is of fundamental and practical interest; they represent the lowest excited quantum states of the radiation field, and have applications in quantum cryptography¹ and quantum information processing². Common approaches use the fluorescence of single ions³, single molecules^{4, 5}, colour centres^{6, 7} and semiconductor quantum dots^{8, 9, 10, 11, 12}. However, the lack of control over such irreversible emission processes precludes the use of these sources in applications (such as quantum networks¹³) that require coherent exchange of quantum states between atoms and photons. The necessary control may be achieved in principle in cavity quantum electrodynamics. Although this approach has been used for the production of single photons from atoms^{14, 15, 16}, such experiments are compromised by limited trapping times, fluctuating atom–field coupling and multi-atom effects. Here we demonstrate a single-photon source based on a strongly localized single ion in an optical cavity. The ion is optimally coupled to a well-defined field mode, resulting in the generation of single-photon pulses with precisely defined shape and timing. We have confirmed the suppression of two-photon events up to the limit imposed by fluctuations in the rate of detector dark counts. The stream of emitted photons is uninterrupted over the storage time of the ion, as demonstrated by a measurement of photon correlations over 90 min.

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