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A Quantum Switch for Light

Final Report Summary - QUSWITCH (A Quantum Switch for Light)

The goal of the QuSwitch project is the generation of a fibre-based quantum switch for light. In this quantum mechanical pendent of a classical optical switch, the internal state of a single laser-cooled atom routes incoming light into one out of two possible output ports. The central element of the proposal is a whispering-gallery-mode bottle microresonator in which light is guided by total internal reflection. The resonator is interfaced with two optical nanofibres. Thanks to its high optical quality factor, a single atom coupled to the evanescent field of the resonator, suffices to significantly modify its transmission properties.

In the first step of the project, Dr. Jürgen Volz together with his team established a controlled way to deliver single Rubidium 85 atoms to a distance of about 100 nm to the resonator surface. For this purpose, a real-time experimental control system was designed and optimized. It allows one to precisely determine when a single atom - delivered by an atomic fountain - is present in the evanescent field and to react accordingly.

Using this control system, the researcher and his team studied the atom-resonator coupling using spectroscopic techniques.They observed that the experimental results were clearly at odds with the predictions from the established theory. The researchers identified the longitudinal polarization components of the light in the evanescent field of the resonator as the physical origin of this deviation from theory. In essence, due to the strong confinement of light, longitudinal polarization components occur which render counter-propagating resonator modes distinguishable and change the spontaneous emission properties of single particles in the evanescent field. Under these circumstances, atoms can emit light only into one direction. This has dramatic consequences on the light-matter interaction in such resonators. The researcher and his team then discovered that this effect occurs in many physical system. They experimentally demonstrated that this effect allows one to realize novel ways for the routing and control of the flow of light in nanoscale optical waveguides. This result opens up a broad range of novel integrated micro-scale photonics devices as well as novel optical detection schemes.

Based on the improved understanding of light-matter interaction in strongly confined fields, the researcher and his team demonstrated new ways for the control of light. First, they demonstrated a fibre-integrated optical switch, where a single atom controls into which output port of the whispering-gallery-mode resonator the light is routed. The experimental results show switching of light with simultaneously high contrast and high survival probability of the guided photons. Furthermore, the researchers demonstrated that the switch can be exploited for photon number dependent routing of light. Thanks to the nonlinear nature of the atom-resonator coupling, single photons are routed to a different output port than two simultaneously arriving photons.

Second, they made use of the fact that the atom in the resonator can only interact with a single photon per time in order to realize a maximally strong optical Kerr-nonlinearity. Depending on the number of simultaneously arriving photons, the resonator-atom system imprints either a phase shift of zero (one photon) or pi (two photons) on the light field that propagates along the coupling fibre. This nonlinear phase shift generates an effective interaction between the fibre guided photons. This was exploited to generate entanglement between two incident photons.

The demonstration of a maximally strong photon-photon interaction is the key ingredient for the realization of deterministic quantum gates between optical photons. Therefore, the results obtained by the fellow, open up the way towards deterministic photonic quantum information processing. Furthermore, the performed experiments demonstrated a novel paradigm for light-matter interaction in micro- and nanophotonic systems. This opens up completely novel ways for the realization of miniaturized devices for the control of light propagation in integrated waveguide structures.