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arXiv:quant-ph/0510141 · Physical Review A

Possibility of inhomogeneous coupling leading to decoherence in an electromagnetically-induced-transparency quantum-memory process

Xiong-Jun Liu, Zheng-Xin Liu, Xin Liu, Mo‐Lin Ge

Published Jan 30, 2006• 9 citations
arXivHTMLPDF

Abstract

We study the effect of inhomogeneous coupling between atoms and external light fields on quantum memories that employ electromagnetically induced transparency (EIT). By introducing atomic subensembles to describe the present inhomogeneous coupling case we find that there is a nonsymmetric dark-state subspace that allows the EIT quantum-memory technique to function perfectly. This shows that such a memory scheme can work ideally even if the atomic state is very far from being a symmetric one.

DOI
10.1103/physreva.73.013825

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We study the effect of inhomogeneous coupling between atoms and external light fields on quantum memories that employ electromagnetically induced transparency (EIT). By introducing atomic subensembles to describe the present inhomogeneous coupling case we find that there is a nonsymmetric dark-state subspace that allows the EIT quantum-memory technique to function perfectly. This shows that such a memory scheme can work ideally even if the atomic state is very far from being a symmetric one.
We theoretically studied the implementation of a quantum memory and an optical transistor in a system composed by a single atom trapped inside a high finesse cavity. In order to store and map the quantum state of an input pulse onto internal states of the single atom (quantum memory) we employ the electromagnetically induced transparency (EIT) phenomenon (which can work out as an optical transistor) where the information can be transferred to the dark state of the atom modelled by a three-level system in a Λ-type configuration. In our model we consider a suitable temporal shape for the control field that ensures the adiabaticity of the storage process and retrieval of the probe pulse. The dynamic of the field inside the cavity was obtained by master equation approach, while the outside field was calculated by input-output formalism. We have analysed two different setups: i) two-sided and ii) single-sided cavities. While the first setup is the most appropriate and commonly used to observe cavity-EIT in the transmission spectrum, thus the best configuration for the optical transistor, the maximum quantum memory efficiency can not reach reasonable values, being limited to 50% for symmetric cavities. On the other hand, with single-sided cavity the quantum memory efficiency increases considerably and can reach values close to 100% in the strong atom-field coupling regime. However this specific setup is not favourable to observe the cavity-EIT effect in the transmission spectrum and then it is not appropriate to control the transmission of light pulses.
Interaction of a control and a signal field with an ensemble of three-level atoms allows direct mapping of the quantum state of the signal field into long lived coherences of an atomic ground state. For a vapor of cesium atoms, using Electromagnetically Induced Transparency (EIT) and Zeeman coherences, we compare the case where a tunable single-sideband is stored independently of the other one to the case where the two symmetrical sidebands are stored using the same transparency window. We study the conditions in which simultaneous storage of two non-commuting variables carried by light and subsequent read-out is possible. We show that excess noise associated with spontaneous emission and spin relaxation is small, and we evaluate the quantum performance of our memory by measuring the signal transfer coefficient T and the conditional variance V and using the T-V criterion as a state independent benchmark.
In this thesis, after a brief review of some concepts of Quantum Optics, we analyze a three-level atomic system in the conditions of electromagnetically induced transparency (EIT), and we investigate the propagation of a gaussian pulse along a cigar-shaped cloud of both cold and hot atoms in EIT regime. In particular, we show that it is possible to amplify a slow propagating pulse without population inversion. We also analyze the regime of anomalous light propagation showing that it is possible to observe superluminal energy propagation. In these conditions, it is possible to imprint reversibly ('write') the information carried by the photons onto the atoms, specifically as a coherent pattern of atomic spins, and later the information stored in the atomic spins can be transferred back ('read') to the light field, implementing in this way a quantum memory. Besides, we analyze the propagation of a quantum field in an EIT medium sustaining dark state polaritons (DSP) in a quasi-particle picture. Here, the decoherence effects in this quantum memory for photons, by analyzing the fidelity of the quantum state transfer, and the emergence of parastatistics in the quasi-particle picture in gain medium are discussed. Finally, we introduce a polarization quantum memory for photons by using a tripod atomic configuration in which two ideal EIT windows appear and the two DSPs, scattering each other, show a solitonic behavior.
We study the influence of a lossless material medium on the coherent storage and quantum-state transfer of a quantized probe light in an ensemble of $\ensuremath{\Lambda}$-type atoms. The medium is modeled as uniformly distributed two-level atoms with the same energy level spacing, coupling to a probe light. This coupled system can be simplified to a collection of two-mode polaritons which couple to one transition of the $\ensuremath{\Lambda}$-type atoms. We show that, when the other transition of $\ensuremath{\Lambda}$-type atoms is controlled by a classical light, electromagnetically induced transparency can also occur for the polaritons. In this case the coherent storage and quantum transfer for photon states are achievable through novel dark states with respect to the polaritons. By calculating the corresponding dispersion relation, we find that the ensemble of three-level atoms with $\ensuremath{\Lambda}$-type transitions may serve as quantum memory for it slows or even stops light propagation through the mechanism of electromagnetically induced transparency.
We present a quantum multimodal treatment describing electromagnetically induced transparency (EIT) as a mechanism for storing continuous-variable quantum information in light fields. Taking into account the atomic noise and decoherences of realistic experiments, we numerically model the propagation, storage, and readout of signals contained in the sideband amplitude and phase quadratures of a light pulse using phase space methods. An analytical treatment of the effects predicted by this model is then presented. Finally, we use quantum information benchmarks to examine the properties of the EIT-based memory and show the parameters needed to operate beyond the quantum limit.