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arXiv:quant-ph/0512226 · Physical Review A

Efficient quantum memory and entanglement between light and an atomic ensemble using magnetic fields

Christine A. Muschik, Klemens Hammerer, Eugene Simon Polzik, J. I. Cirac

Published Jun 21, 2006• 65 citations
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Abstract

We present two protocols, one for the storage of light in an atomic ensemble and the subsequent retrieval, and another one for the generation of entanglement between light and atoms. They rely on two passes of a single pulse through the ensemble, Larmor precessing in an external field. Both protocols work deterministically and the relevant figures of merit---such as the fidelity or the EPR variance---scale exponentially in the coupling strength. We solve the corresponding Maxwell-Bloch equations describing the scattering process and determine the resulting input-output relations which only involve one relevant light mode that, in turn, can be easily accessed experimentally.

DOI
10.1103/physreva.73.062329

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We present two protocols, one for the storage of light in an atomic ensemble and the subsequent retrieval, and another one for the generation of entanglement between light and atoms. They rely on two passes of a single pulse through the ensemble, Larmor precessing in an external field. Both protocols work deterministically and the relevant figures of merit---such as the fidelity or the EPR variance---scale exponentially in the coupling strength. We solve the corresponding Maxwell-Bloch equations describing the scattering process and determine the resulting input-output relations which only involve one relevant light mode that, in turn, can be easily accessed experimentally.
We discuss theoretically quantum interface between light and a spin polarized ensemble of atoms with the spin $\ensuremath{\geqslant}1$ based on an off-resonant Raman scattering. We present the spectral theory of the light--atom interaction and show how particular spectral modes of quantum light couple to spatial modes of the extended atomic ensemble. We show how this interaction can be used for quantum memory storage and retrieval and for deterministic entanglement protocols. The proposed protocols are attractive due to their simplicity since they involve just a single pass of light through atoms without the need for elaborate pulse shaping or quantum feedback. As a practically relevant example we consider the interaction of a light pulse with hyperfine components of D1{D}_{1}D1​ line of 87Rb^{87}\mathrm{Rb}87Rb. The quality of the proposed protocols is verified via analytical and numerical analysis.
We present an experimentally feasible protocol for the complete storage and retrieval of arbitrary light states in an atomic quantum memory using the Faraday interaction between light and matter. Our protocol relies on multiple passages of a single light pulse through the atomic ensemble without the impractical requirement of kilometer-long delay lines between the passages. A time-dependent interaction strength enables the storage and retrieval of states with arbitrary pulse shapes. The fidelity approaches unity exponentially without squeezed or entangled initial states, as illustrated by calculations for a photonic qubit.
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.
We present a protocol for the teleportation of the quantum state of a pulse of light onto the collective spin state of an atomic ensemble. The entangled state of light and atoms employed as a resource in this protocol is created by probing the collective atomic spin, Larmor precessing in an external magnetic field, off resonantly with a coherent pulse of light. We take here full account of the effects of Larmor precession and show that it gives rise to a qualitatively different type of multimode entangled state of light and atoms. The protocol is shown to be robust against the dominating sources of noise and can be implemented with an atomic ensemble at room temperature interacting with free-space light. We also provide a scheme to perform the readout of the Larmor precessing spin state enabling the verification of successful teleportation as well as the creation of spin squeezing.
In this thesis, we explore the quantum dynamics of light interactions with optically thick collections of atoms. We provide a theoretical description of several recent experiments in which some key operations necessary for the implementation of quantum communication networks are demonstrated. Collective Raman scattering from an atomic ensemble is shown to produce probabilistic entanglement between the polarization of a scattered photon and an associated collective atomic excitation. The predicted correlations agree with experimental observations. We also propose a method to use cascade transitions to produce entanglement between a photon with a frequency in the telecom range (ideal for transmission over optical fibers) and a near infrared photon (ideal for storage in an atomic ensemble), and a description of the experimental demonstration is provided. We also propose and describe the implementation of a deterministic source of single photons.\n\nIn addition, we generalize the theory of dark-state polaritons in ensembles of three level Lambda atoms to account for the nuclear spin degeneracy of alkali atoms. This generalized theory provides a description of the first demonstration of single photon storage and retrieval from atomic ensembles. Additionally, in the presence of a uniform magnetic field, we predict the occurrence of collapses and revivals of the photon retrieval efficiency as a function of storage time within the ensemble. These predictions are in very good agreement with subsequent experimental observations. We also exploit the ability of photon storage to entangle remote atomic qubits.
We present a protocol for deterministically generating an optimal two-mode-squeezed state of distant free-space atomic ensembles. The protocol relies on two passes of a single coherent pulse through two separate atomic ensembles placed in homogeneous magnetic fields. In contrast to previous single-pass schemes, our approach significantly enhances the degree of entanglement due to the exponential behavior of the Einstein-Podolsky-Rosen variance. We also provide an efficient scheme to perform Bell measurement of atomic states, enabling the implementation of high-fidelity atomic quantum teleportation.