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Frontiers in Optics

Four-Wave Mixing in Potassium Vapor

Ryan T. Glasser, Jon D. Swaim

Published Jan 1, 2017• 0 citations
Publisher

Abstract

In this paper we demonstrate the generation of bright, squeezed twin beams via a four-wave mixing process in warm atomic potassium vapor. The atomic transitions surrounding the probe beam frequency are completely overlapped at room temperature and above, due to Doppler broadening. This results in strong asymmetric absorption of the probe beam relative to the generated conjugate. Despite this significant absorption, quantum squeezing is shown to survive.

DOI
10.1364/fio.2017.jw3a.31

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In this paper we demonstrate the generation of bright, squeezed twin beams via a four-wave mixing process in warm atomic potassium vapor. The atomic transitions surrounding the probe beam frequency are completely overlapped at room temperature and above, due to Doppler broadening. This results in strong asymmetric absorption of the probe beam relative to the generated conjugate. Despite this significant absorption, quantum squeezing is shown to survive.
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.
We will present results that demonstrate squeezed light source based on Four Wave Mixing (FWM) in hot Potassium vapor. Squeezed light with the noise level of -6 dB below standard quantum limit is observed in the difference signal between correlated probe and conjugate beams, generated via FWM in potassium vapor. The vapor is contained in the vacuum K cell. For generating FWM we use nearly co-propagating pump and probe beam in a double Λ atomic scheme, which is similar to schemes used to demonstrate sqeezing in KD and in K. Obtained level of squeezing is improvement from the previous results for squeezing in potassium.
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.
We have investigated experimentally and theoretically four-wave mixing (FWM) in hot potassium vapor as a medium for producing slow and store light. Two pump and a single probe photon were mixed in double-Λ system, characterized by parametric amplification of coupled co-propagating probe and conjugate beams. We have observed very high gains of probe and conjugate, up to 60 and 80 respectively, at two photon Raman resonance, under large one photon detuning, and high K-cell temperature. Using an amplifying medium to alleviate the absorption and distortion of propagating pulses are important for slow-light propagation. We observed ultraslow propagation of probe and conjugate pulses under conditions when parametric amplification of twin beams is well below their maximum values. A fractional delays of close to 5 at a gain ∼1.5, for the probe pulse of 20 ns, have been achieved simultaneously for both probe and accompanying conjugate pulse. FWM mixing process under conditions of the experiment results in broadening of twin beams, close to observed delays.
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.