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arXiv:1703.03438 · Physical Review A

Squeezed-twin-beam generation in strongly absorbing media

Jon D. Swaim, Ryan T. Glasser

Published Sep 11, 2017• 26 citations
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Abstract

We experimentally demonstrate the generation of squeezed, bright twin beams that arise due to competing gain and absorption, in a medium that is overall transparent. To accomplish this, we make use of a nondegenerate four-wave-mixing process in warm potassium vapor such that one of the twin beams experiences strong absorption. At room temperature and above, due to Doppler broadening and smaller frequency detunings compared to other schemes, the ground-state hyperfine splittings used in the present double-$\mathrm{\ensuremath{\Lambda}}$ setup are completely overlapped. We show that despite the resulting significant asymmetric absorption of the twin beams, quantum correlations may still be generated. Our results in this regime demonstrate that the simplified model of gain, followed by loss, is insufficient to describe the amount of quantum correlation resulting from the process.

DOI
10.1103/physreva.96.033818

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We experimentally demonstrate the generation of squeezed, bright twin beams that arise due to competing gain and absorption, in a medium that is overall transparent. To accomplish this, we make use of a nondegenerate four-wave-mixing process in warm potassium vapor such that one of the twin beams experiences strong absorption. At room temperature and above, due to Doppler broadening and smaller frequency detunings compared to other schemes, the ground-state hyperfine splittings used in the present double-\ensuremathΛ\mathrm{\ensuremath{\Lambda}}\ensuremathΛ setup are completely overlapped. We show that despite the resulting significant asymmetric absorption of the twin beams, quantum correlations may still be generated. Our results in this regime demonstrate that the simplified model of gain, followed by loss, is insufficient to describe the amount of quantum correlation resulting from the process.
Nondegenerate forward four-wave mixing in hot atomic vapors has been shown to produce strong quantum correlations between twin beams of light [McCormick et al., Opt. Lett. 32, 178 (2007)], in a configuration which minimizes losses by absorption. In this paper, we look at the role of the phase-matching condition in the trade-off that occurs between the efficiency of the nonlinear process and the absorption of the twin beams. To this effect, we develop a semiclassical model by deriving the atomic susceptibilities in the relevant double-$\ensuremath{\Lambda}$ configuration and by solving the classical propagation of the twin-beam fields for parameters close to those found in typical experiments. These theoretical results are confirmed by a simple experimental study of the nonlinear gain experienced by the twin beams as a function of the phase mismatch. The model shows that the amount of phase mismatch is key to the realization of the physical conditions in which the absorption of the twin beams is minimized while the cross coupling between the twin beams is maintained at the level required for the generation of strong quantum correlations. The optimum is reached when the four-wave mixing process is not phase matched for fully resonant four-wave mixing.
We investigate both theoretically and experimentally four-wave mixing (FWM) in hot potassium vapor, generated by a copropagating pump and probe in an off-resonant double-$\mathrm{\ensuremath{\Lambda}}$ system, and present conditions when this atomic system is (1) a strong phase-insensitive parametric amplifier and (2) a source of large-amplitude squeezing. Theoretically, nonperturbative numerical calculations of optical Bloch-Maxwell equations have been solved for a four-level atomic system of K in order to derive the atomic polarization and then amplitudes of propagating optical waves, pump, probe, and conjugate. For potassium, to our knowledge, there are no such comparisons of theoretical and experimental results of gains of twin beams under the large range of FWM parameters as presented here. Results have shown that one-photon detuning has to be slightly larger than the Doppler broadened transition for large gains and strong squeezing. The gain is particularly large for small red two-photon detuning ($\ensuremath{-}2--6\phantom{\rule{0.16em}{0ex}}\mathrm{MHz}$) and high K density ($5.5--10\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$). Following experimentally and theoretically determined relation between gains, probe transmissions, and squeezing in Rb and Cs, we have found parameters of FWM when maximum squeezing in hot K vapor is expected.
Using a nondegenerate four-wave mixing process based on a double-$\mathrm{\ensuremath{\Lambda}}$ scheme in hot cesium vapor, we generate quantum correlated twin beams with a maximum intensity-difference squeezing of 6.5 dB. The substantially improved squeezing can be mainly attributed to very good frequency and phase-difference stability between the pump and probe beams in our experiment. Intensity-difference squeezing can be observed within a wide experimental parameter range, which guarantees its robust generation. Since this scheme produces multi-spatial-mode twin beams at the Cs D1{D}_{1}D1​ line, it is of interest for experiments involving quantum imaging and coherent interfaces between atomic and solid-state systems.
Using a simple scheme based on nondegenerate four-wave mixing in a hot vapor, we generate bright twin beams which display a quantum noise reduction in the intensity difference of more than 8dB8\phantom{\rule{0.3em}{0ex}}\mathrm{dB}8dB. The absence of a cavity makes the system immune to external perturbations, and strong quantum noise reduction is observed at frequencies as low as 4.5kHz4.5\phantom{\rule{0.3em}{0ex}}\mathrm{kHz}4.5kHz and over a large frequency range.
We use the quantum correlations of twin-beams of light to probe the added noise when one of the beams propagates through a medium with anomalous dispersion. The experiment is based on two successive four-wave mixing processes in rubidium vapor, which allow for the generation of bright two-mode-squeezed twin-beams followed by a controlled advancement while maintaining the shared quantum-correlations between the beams. The demonstrated effect allows the study of irreversible decoherence in a medium exhibiting anomalous dispersion, and for the first time shows the advancement of a bright nonclassical state of light. The advancement and corresponding degradation of the quantum correlations are found to be operating near the fundamental quantum limit imposed by using a phase-insensitive amplifier.