The interaction of quantum systems with themselves has been the subject of extensive theoretical and experimental investigations [2,3]. Here, we present new results on the interaction of an ensemble of cold atoms with a time-delayed version of its own spontaneous emission light.
Experimentally, we form a magneto-optical trap (MOT) of caesium atoms around the waist of an optical nanofibre (diameter ~400 nm). One end of the nanofibre terminates in a single-photon detector module (SPCM) and the other end is connected to a 250 m length of normal optical fibre and terminated with a Fibre Bragg Grating (FBG). After accumulating the desired number of atoms, the MOT trapping beams are turned off, and the ensemble is illuminated with a train of short pulses (pulse duration $\sim$200 ns) of an external pump beam, close to resonance with the $D_2$ transition in caesium and intersecting the optical nanofibre at right angles. We detect the arrival times of the spontaneous emission photons into the optical nanofibre.
We observe photons emitted directly towards the SPCM, but also photons that have been emitted towards the FBG, are reflected back, interact with the atoms again and arrive at the detector after a time delay. The delayed photons are partially absorbed by the MOT atoms, thereby creating a system of atoms interacting with a distant mirror image .
We present an investigation into this interaction, changing the pump laser power, polarisation and detuning, as well as the atom number in the MOT. We also investigate the interaction of the spontaneous emission photons with excited atoms by matching the repetition time of the short pulses to the flight time of the photons going to the FBG and back.
 Hannes Pichler, Soonwon Choi, Peter Zoller, and Mikhail D. Lukin. Universal photonic quantum computation via time-delayed feedback. Proceedings of the National Academy of Sciences, 114(43):11362–11367, (2017).
 Solano, Pablo, et al. "Super-radiance reveals infinite-range dipole interactions through a nanofiber." Nature communications 8.1 (2017): 1-7.
 Light interference from single atoms and their mirror images, J. Eschner, Ch. Raab, F. Schmidt-Kaler and R. Blatt, Nature 413, 495 (2001).
|Presenter name||Maarten Hoogerland|
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