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arXiv:0811.2315 · Physical Review A

Non-Gaussian two-mode squeezing and continuous-variable entanglement of linearly and circularly polarized light beams interacting with cold atoms

Ricardo José Missori, M. C. de Oliveira, K. Furuya

Published Feb 2, 2009• 5 citations
arXivHTMLPDF

Abstract

We investigate how entangled coherent states and superpositions of low intensity coherent states of non-Gaussian nature can be generated via nonresonant interaction between either two linearly or circularly polarized field modes and an ensemble of X-like four-level atoms placed in an optical cavity. We compare our results to recent experimental observations and argue that the non-Gaussian structure of the field states may be present in those systems.

DOI
10.1103/physreva.79.023801

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We investigate how entangled coherent states and superpositions of low intensity coherent states of non-Gaussian nature can be generated via nonresonant interaction between either two linearly or circularly polarized field modes and an ensemble of X-like four-level atoms placed in an optical cavity. We compare our results to recent experimental observations and argue that the non-Gaussian structure of the field states may be present in those systems.
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We report on the generation of non-separable beams produced via the interaction of a linearly polarized beam with a cloud of cold caesium atoms placed in an optical cavity. We convert the squeezing of the two linear polarization modes into quadrature entanglement and show how to find the best entanglement generated in a two-mode system using the inseparability criterion for continuous variables (Duan et al 2000 Phys. Rev. Lett. 84 2722). We verify this method experimentally with a direct measurement of the inseparability using two homodyne detectors. We then map this entanglement into a polarization basis and achieve polarization entanglement .
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We propose a conditional implementation of superpositions of Gaussian operations on different modes of light. These operations are conditional entanglers generating non-Gaussian entanglement between two modes of propagating light beams. The all-optical operations emulate weak effects of highly nonlinear operations only available when light interacts with individual atoms. In particular, we propose schemes to implement superposition of coherent displacements and squeezings on two beams of light and discuss physical properties of generated non-Gaussian entanglement. These methods together with high quality of optical tomography of highly nonclassical states allow one to understand non-Gaussian entangling processes.
The generation of two-mode squeezed and entangled light in a nonlinear coupler inside an optical cavity was studied.The master equation of the two-mode field was solved by the method of characteristic function.It shows that the state of the two-mode field is a two-mode Gaussian one,and under some conditions,the two-mode cavity field exhibits stable two-mode squeezing and continuous variable entanglement.The degree of the squeezing and entanglement is dependent on the damping rate of the cavity field and the coupling constants between the two modes.
We investigate experimentally and theoretically the generation of a continuous variable entanglement using laser cooled atomic gas in free space and cavity mode. This is of a fundamental interest due to the nonclassical correlation of quadrature observables of the two-mode squeezed (TMS) state. The TMS state is the basis for complete quantum teleportation and continuous variable quantum repeaters. It is also used in quantum metrology and quantum key distribution. We employ a novel scheme for generating a TMS vacuum based on a non-degenerate four-wave mixing process in a cold atomic cloud coupled to a cavity. We send two pumps collinear to the cavity mode and ensure that the two generated quantum fields are simultaneously resonant with the cavity. The cavity enhancement enables us to demonstrate results close to a free space in hot atomic gases (-5.2 dB) where optical densities are much higher. Our system is versatile because, together with continuous variables, it is suitable for the discrete variable domain using the Duan-Lukin-Cirac-Zoller protocol. The design is oriented to make the delay between two modes possible and could be used for hybrid continuous repeaters. The system is devised and built fully as a part of my PhD, wherein we characterize our system and demonstrate the squeezing of -3.7 dB. Since the quantum dynamics of the system is not intuitive, we provide simple models and consolidate segmental information on generation of TMS states. To make sure that the dynamics is not oversimplified, we construct a novel theory where we treat both fields and atoms as quantum objects. The theory supports and enriches our understanding and confirms our experimental data. Based on the experimental and theoretical results we spotlight the detrimental effects and propose improvements to our system. We theoretically investigate that the certain class of TMSV states are optically dark. These states propagate through a polarized to one hyperfine level Λ atoms without degradation or evolution. Based on this we propose a new way of generation of TMSV through the dissipation or dynamically controlled broadening.