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Physica Scripta

An approximate effective beamsplitter interaction between light and atomic ensembles

Richard Tatham, David Menzies, Natalia Korolkova

Published Feb 1, 2011• 1 citation
Publisher

Abstract

We present a method for designing a lossless beamsplitter interaction between an incident field mode and an atomic spin ensemble. This interaction is only approximately true for weak quantum non-demolition coupling between light and atomic ensemble and for a subset of particular fields and atomic states. We explore the accuracy of this approximation for various Gaussian states in an ideal pure state description. We also discuss possible applications of this interaction including entanglement concentration of entangled atomic ensembles.

DOI
10.1088/0031-8949/2011/t143/014023

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The interaction between an atomic ensemble and a light mode in a high-finesse optical cavity can easily reach the strong-coupling regime, where quantum effects dominate.In this regime, the interaction can be used to generate both atom-light and atom-atom entanglement.We analyze the dominant effects on the collective atomic state and the light field, and derive a unified approach that can account for atomic entanglement induced both by measurements on the light field, and by ignoring the state of the light field altogether.We present analytical expressions for the entanglement induced by the interaction, and determine the conditions that maximize the entanglement-induced gain over the standard quantum limit in quantum sensors and atomic clocks.
This thesis contributes to the theory of a light-matter quantum interface based on the Kerr-effect arising in the dispersive interaction of pulsed laser light with a spin-polarized atomic ensemble. The scattering process can be described on the basis of a simple, yet fully quantum mechanical model involving only a small number of bosonic modes referring to transverse spin components and quadratures of forward scattered light. Based on this model we derive protocols for the creation of entangled states of light and atoms and the teleportation of quantum states of light onto atoms employing this entanglement. Furthermore we present a protocol, which allows one to exchange the state of light and atoms and thus provides a quantum memory for states of the light field. For both, the storage and the teleportation of coherent states we prove a benchmark on the average fidelity, which is achievable by purely classical protocols. Both protocols allow one to significantly surpass this threshold and to demonstrate thereby a gain in employing quantum strategies. Under common experimental conditions the initial state of light and atoms is Gaussian and the given interaction preserves this property. Motivated by this observation, we examine in this context, how a given interaction can be used to simulate others and how it can be employed to create entanglement at optimal rates. The results are applied to construct protocols based on several passes of light through an atomic ensemble.
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.