Quantum Optics Seminar: Atoms and optical nanofibers: from unravelling motional decoherence to probing surface physics

Daniel Hümmer:
Ensembles of individual atoms can be trapped in the optical near-field of nanoscale photonic structures such as optical nanofibers and photonic crystal cavities.

Daniel Hümmer
Group Seminar via Zoom
Fri, 28. May 2021, 10:00 am (MEZ)

Abstract:

Ensembles of individual atoms can be trapped in the optical near-field of nanoscale photonic structures such as optical nanofibers and photonic crystal cavities. These nanophotonic atom-photon interfaces are attractive because they promise strong light-matter interaction and great flexibility in designing their photonic properties. Moreover, they allow to trap atoms at distances of only a few hundred nanometers from a surface. A significant disadvantage is that the heating rates of the atomic motion are currently orders of magnitude larger than in comparable free-space traps. This strong heating hampers the use of nanophotonic traps in more involved  experiments or for quantum information processing. Despite its relevance, the origin of this heating had previously remained elusive.

We have now identified a set of thermally excited mechanical modes of the waveguide as the main source of the strong heating in nanofiber-based traps [1]. We were able to predict atom heating rates in excellent quantitative agreement with experimental observations by carefully analyzing the influence of vibrational modes on the guided light fields. This understanding enabled us to propose ways to minimize the heating and overcome a main limitation of current nanophotonic cold-atom systems.

Furthermore, advances like the success in trapping atoms at a distances of a few hundred nanometers from a surface have led to speculations whether it is possible to control atoms even smaller separations. At such small distances, dispersion forces dominate over optical forces and can lead to adsorption of atoms on the surface of the photonic structure. We have recently explored the possibility of performing spectroscopy of the motional states of atoms bound directly to the surface of a nanofiber through dispersion and optical forces [2]. We find that the motional decoherence due to waveguide phonons and the resulting broadening of motional resonances is significant. However, a sufficient reduction of the phonon-induced decoherence is within reach and should result in a clearly quantized motional spectrum. The detection of these states would constitute an important step towards using techniques from quantum optics to investigate and control the dynamics of atoms on surfaces.

[1] PRX 9, 041034 (2019)
[2] PRL 126, 163601 (2021)

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