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arXiv:1803.08911 · New Journal of Physics

Darkness of two-mode squeezed light in Λ-type atomic system

E. S. Moiseev, Arina Tashchilina, Sergey A. Moiseev, Alexander I. Lvovsky

Published Dec 6, 2019• 8 citations
arXivHTMLPDF

Abstract

Abstract We show that, under certain circumstances, an optical field in a two-mode squeezed vacuum (TMSV) state can propagate through a lossy atomic medium without degradation or evolution. Moreover, the losses give rise to that state when a different state is initially injected into the medium. Such a situation emerges in a Λ-type atomic system, in which both optical transitions are driven by strong laser fields that are two-photon resonant with the respective signal modes. Then the interactions of the two signal modes with the ground-state atomic coherence interfere destructively, thereby ensuring the preservation of the TMSV with a particular squeezing parameter. This mechanism permits unified interpretation of recent experimental results and predicts new phenomena.

DOI
10.1088/1367-2630/ab5fac

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•Nov 1, 2019•1 citation
Abstract We show that, under certain circumstances, an optical field in a two-mode squeezed vacuum (TMSV) state can propagate through a lossy atomic medium without degradation or evolution. Moreover, the losses give rise to that state when a different state is initially injected into the medium. Such a situation emerges in a Λ-type atomic system, in which both optical transitions are driven by strong laser fields that are two-photon resonant with the respective signal modes. Then the interactions of the two signal modes with the ground-state atomic coherence interfere destructively, thereby ensuring the preservation of the TMSV with a particular squeezing parameter. This mechanism permits unified interpretation of recent experimental results and predicts new phenomena.
The interaction of classical and quantized electromagnetic fields with an ensemble of atoms in an optical cavity is considered. Four fields drive a double-lambda level scheme in the atoms, consisting of a pair of lambda systems sharing the same set of lower levels. Two of the fields produce maximum coherence, rho12 = -1/2, between the ground state sublevels 1 and 2. This pumping scheme involves equal intensity fields that are resonant with both the one and two-photon transitions of the lambda system. There is no steady-state absorption of these fields, implying that the fields induce a type electromagnetically-induced transparency (EIT) in the medium. An additional pair of fields interacting with the second lambda system, combined with the EIT fields, leads to squeezing of the atom spin associated with the ground state sublevels. Our method involves a new mechanism for creating steady-state spin squeezing using an optical cavity. As the cooperativity parameter C is increased, the optimal squeezing varies as C^{-1/3}. For experimentally accessible values of C, squeezing as large as 90% can be achieved.
The effect of a dark state on the entanglement properties of the two-mode field from a laser-driven Λ-type three-level atomic system is investigated by taking into account the spontaneously generated coherence (SGC). When the dark state is present, it is found that the entanglement between the two cavity modes can be significantly enhanced compared to the case without the dark state. When the value of SGC is maximal, the dark state population depending on the initial values of the system can affect the entanglement dramatically. Moreover, the effects of a squeezed parameter and Rabi frequency on entanglement evolution are also presented in the presence of a dark state.
The influences of detuning δ and initial squeezed light_field factor r on the light squeezing and Cauchy_Schwartz inequality in the system of two_mode squeezing vacuum field interacting non_resonantly with a Λ_type three_level atom is investigated.The results show that the light field is squeezed regularly. The light squeezing level is evidently relative to r and δ. With the increase r, the nonclassical correlation properties between the two mode cavities get steady. The detuning makes the nonclassical correlation degree steady.
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.