Interaction Between Single Photons and Atomic Ensembles

Pieter Kok, Brendon W. Lovett

In Chapter 7 we discussed the interaction between atoms and photons. We found that the coupling between a single atom and a single photon is rather weak, unless the atom and the photon are contained within a small cavity. The only way to significantly affect a free atom with light is to use a state of light with many photons, and in particular we showed how to use classical laser fields to prepare particular atomic states. However, there is another way of enhancing the interaction in a useful way. We can use many atoms rather than a single one. In this chapter, we will show that the interaction between a single photon and an ensemble of atoms can be greatly increased through the phenomenon of collective enhancement. We will show that this gives an atomic ensemble a relatively large susceptibility, and that multilevel atoms can be used to generate large optical nonlinearities. These offer the possibility of slowing, and even stopping, a beam of light; a phenomenon that goes hand in hand with electromagnetically induced transparency. We will discuss how a quantum state of light can be stored in and retrieved from an ensemble of atoms, and how the states of different atomic ensembles can be entangled with one another. We will show that collective states of atoms can even be used as a single qubit or several qubits, and finally, we will show that atomic ensembles can be use to mediate a photon–photon interaction.

L. Lanco

U. Shin Kim, Yong Sup Ihn, Chung Hyun Lee +1 more

Photons can be trapped into an atomic ensemble while maintaining its photonic part, resulting a strong confinement of photons in the ensemble. It leads potential applications about strong photon-photon interactions without a strong coupling between light and atoms based on cavity [1]. Recently, the observation for single stationary dark-state polariton has been reported [2], yet the trapping of single photon into the ensemble has not been reported so far. Here, we present the first experimental demonstration for the trapping of free-propagating single photon into an atomic ensemble.

L. Lanco

Alessandro Ceré, Florian Wolfgramm, Marta Abad +3 more

We discuss an interaction free measurement of the optical density of a hot sample of 87Rb atoms. This system can be used as narrowband filter for light, with the filtered radiations presenting a spectrum suitable for efficient interaction with an atomic ensemble. This is useful especially together with broadband single photon sources for matching the spectral requirements of interaction with cold atoms ensembles. Experimental progress in the realization of narrowband single photon state will be presented.