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A quantum switch for light

Arno Rauschenbeutel
Jürgen Volz

This project employs single neutral rubidium atoms which are coupled to a novel type of whispering-gallery-mode (WGM) microresonator—a so-called bottle microresonator (BMR). BMRs are conceptually similar to traditional WGM microresonators and share many advantageous properties with the latter. They are true four-port devices and enable a resonator–mediated nonlinear interaction between two distinct optical signals. We propose to realize a BMR-based quantum switch for light in which the internal state of a single atom, strongly coupled to the BMR mode, defines the output port for the incident light field. In contrast to its classical counterpart, the switch can also be initiated in a superposition state, leading to entanglement between the atomic state and the output mode. Our system will allow us to implement advanced devices and protocols at the level of real applications such as, e.g., single-photon optical transistors. In addition, it permits the exchange of quantum information as well as the generation of entanglement between light and matter. In particular, the low intrinsic losses of the system should enable us to prepare mesoscopic entangled states—so-called nonlocal Schrödinger cat states. We plan to systematically study the properties of these states and to change the mean photon number in a continuous fashion. We will thereby investigate the transition from the microscopic world, where the fundamental rules of quantum mechanics are obeyed, to the classical world, where the quantum mechanical features are lost due to decoherence.

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