Cooperative coupling of ultracold atoms and surface plasmons

Cooperative coupling between optical emitters and light fields is one of the outstanding goals in quantum technology. It is both fundamentally interesting for the extraordinary radiation

properties of the participating emitters and has many potential applications in photonics. Although this goal has been achieved using high-finesse optical cavities, attention has turned to

broadband, easy to build cavity-free approaches. Here we demonstrate cooperative coupling of ultracold atoms with surface plasmons propagating on a plane gold surface. While the atoms are

moving towards the surface they are excited by an external laser pulse. The interaction between the excited atom fluorescence and surface plasmons is probed by detecting the photons emitted

into the substrate when the plasmon excitations decay. A maximum Purcell factor of ηP = 4.9 is reached at an optimum distance of z = 250 nm from the surface. The coupling leads to the

observation of a Fano-like resonance in the spectrum.

C.Z. gratefully acknowledges financial support by the DFG. C.S. was supported by Carl-Zeiss Stiftung Baden-Württemberg. S.S. is indebted to the Baden-Württemberg Stiftung for the financial

support of this research project by the Eliteprogramm for Postdocs.

Physikalisches Institut and Center for Collective Quantum Phenomena in LISA+, Universität Tübingen, Auf der Morgenstelle 14, D-72076 Tübingen, Germany

C.Z. provided the laboratory and experimental facilities, C.S. made the measurements, and S.S. analysed the data and wrote the paper.

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