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Europhysics Letters (EPL)

Nonlinear parametric interaction and higher-order squeezing in coherent population trapping

Fei Wang, Xiangming Hu

Published Dec 1, 2014• 5 citations
Publisher

Abstract

In this paper, we show that coherent population trapping (CPT) system can act as a nonlinear parametric interaction process, in which higher-order nonlinear effects are included, giving rise to the occurrence of nonclassical quantum correlation. Furthermore, our numerical results confirm that single- and two-mode higher-order squeezing are possible to obtain under different conditions because of the higher-order nonlinear effects, which are strongly determined by the atomic coherence. The higher-order squeezing obtained in CPT is robust against dissipation since the atoms are nearly trapped into the ground state, which may be useful for high-precision measurement.

DOI
10.1209/0295-5075/108/54007

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In this paper, we show that coherent population trapping (CPT) system can act as a nonlinear parametric interaction process, in which higher-order nonlinear effects are included, giving rise to the occurrence of nonclassical quantum correlation. Furthermore, our numerical results confirm that single- and two-mode higher-order squeezing are possible to obtain under different conditions because of the higher-order nonlinear effects, which are strongly determined by the atomic coherence. The higher-order squeezing obtained in CPT is robust against dissipation since the atoms are nearly trapped into the ground state, which may be useful for high-precision measurement.
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Coherent population trapping (CPT) is a technique in ultrahigh-resolution spectroscopy, developed in recent years. It relies on the nonlinear coherent interaction between an atomic system and incident electromagnetic radiation and is followed by an analysis of the fine structure of the response to this interaction, which contains information about the spectral characteristics of the quantum system. The basic physics of CPT are reviewed, and the effects of level relaxation and structure, and of external factors, on CPT dynamics and characteristics are examined. Experimental results confirming the validity of theoretical ideas on CPT are presented. Possible applications of CPT to frequency stabilization, laser cooling of atoms, and so on are discussed.
To achieve a high degree of quantum noise squeezing, an optical cavity is often employed to enhance the interaction time between light and matter. Here, we propose to utilize the effect of coherent population trapping (CPT) to directly generate squeezed light without any optical cavity. Combined with the slow propagation speed of light in a CPT medium, a coherent state passing through an atomic ensemble with a high optical density (OD) can evolve into a highly squeezed state even in a single passage. Our study reveals that noise squeezing of more than 10 dB can be achieved with an OD of 1000, which is currently available in experiments. A larger OD can further increase the degree of squeezing. As the light intensity and two-photon detuning are key factors in the CPT interaction, we also demonstrate that the minimum variance at a given OD can be reached for a wide range of these two factors, showing the proposed scheme is flexible and robust. Furthermore, there is no need to consider the phase-matching condition in the CPT scheme. Our introduction of high OD in atomic media not only brings a long light-matter interaction time comparable to optical cavities, but also opens an alternative avenue in the generation of squeezed light for quantum interface.
We present a scheme to generate unitarily two-mode and four-mode field squeezing in optical cavities with the near-resonantly dressed three-level atoms in the $\mathrm{\ensuremath{\Lambda}}$ configuration. The dressing fields open two coherent population trapping (CPT) windows, within which the atoms stay predominantly in the dark state and yield strong nonlinearities for both the dressing fields themselves and the other applied fields. The two-mode squeeze operator is separated from degrees of freedom of atoms in a common CPT window, and the multimode squeeze operator is segregated in different CPT windows. The resulting two-mode and multimode interaction strengths are at least one order higher than in the previous dispersive schemes, where all fields are far off resonant with the atoms. The present near-resonant scheme is robust to spontaneous emission since atoms are nearly trapped in a long-lived superposition state.
We show that it is possible to obtain perfect higher-order squeezing via strong nonlinearities in the microwave-modified electromagnetically induced transparency (EIT). In a typical $\mathrm{\ensuremath{\Lambda}}$ three-level system coupled to a control field and a microwave field, the strong nonlinearity is existent under the full-resonant conditions and can be effectively controlled by the relative intensity of the optical and microwave fields. Via dressed-state and Bogoliubov mode transformation, we explore that the internal nonlinearity is closely related to the squeezing parameter and dissipative rate for two cavity modes. As a result, it is found that the two-mode higher-order squeezing is nearly close to 100100%100 under ideal conditions, which is verified by our numerical and analytical results. In addition, we reveal that the higher-order squeezing in the present scheme is robust against the dephasing rate between two lower levels. This may find potential applications in high-precision measurement and provide a convenient way for experimental implementation.