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Quantum-Atom Optics Downunder (QAO)

Relative intensity squeezing using a novel four-wave mixing system in rubidium

M. Jasperse, RE Scholten

Published Jan 1, 2007• 1 citation
Publisher

Abstract

It has been recently demonstrated (Opt. Lett. 32 p178, 2007) that relative intensity squeezed light beams can be produced by non-degenerate four-wave mixing (4WM) in rubidium vapour. Using a system based on the lambda-structure of the 5S ground state of Rb, they obtained noise reduction of −7.1 dB below the standard quantum limit. We present a new scheme using the cascade 5S-5P-5D levels of Rb, which should achieve strong relative intensity squeezing at low Fourier frequency and low (~10mW) laser power.

DOI
10.1364/qao.2007.qwe35

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It has been recently demonstrated (Opt. Lett. 32 p178, 2007) that relative intensity squeezed light beams can be produced by non-degenerate four-wave mixing (4WM) in rubidium vapour. Using a system based on the lambda-structure of the 5S ground state of Rb, they obtained noise reduction of −7.1 dB below the standard quantum limit. We present a new scheme using the cascade 5S-5P-5D levels of Rb, which should achieve strong relative intensity squeezing at low Fourier frequency and low (~10mW) laser power.
We have measured -3.5 dB (-8.1 dB corrected for losses) relative intensity squeezing between probe and conjugate beams generated by stimulated, nondegenerate four-wave mixing in hot rubidium vapor. Unlike early observations of squeezing in atomic vapors based on saturation of a two-level system, our scheme uses a resonant nonlinearity based on ground-state coherences in a three-level system. Since this scheme produces narrowband, squeezed light near an atomic resonance, it is of interest for experiments involving cold atoms or atomic ensembles.
We study a variety of mechanisms that introduce noise into squeezed light generated by a four-wave mixing (4WM) process in Rb vapor. The noise from the seeding beam itself is a general noise that appears in any squeezed light generated from a seeding process. This noise dominates in the squeezed light at acoustic and lower measurement frequencies. A second excess noise source is observed in the twin beams pumped by either a diode laser system or a Ti:sapphire laser system. This excess noise is much stronger in the diode laser systems. It is present in the twin beams at measurement frequencies when the 4WM gain is reduced toward unity. Most of this excess noise can be removed with a dual-seeded 4WM scheme. A third noise source we examine is from a two-beam coupling that degrades the squeezing of the dual-seeded 4WM process at low frequencies of the order of the atomic transition linewidth. This noise can be avoided by seeding skew rays in the 4WM gain region. This gives us an insight to solve this "cross talk" problem by imaging the source in the 4WM gain region. In addition to studying noise sources, we propose a gain-independent calibration scheme that relies on higher order correlation function for the absolute calibration of photodiodes. Having low frequency squeezing is really important if we record quantum images with a CCD camera, which has a slow shutter speed. Also, it's been very difficult for people to get low-frequency squeezing. We obtain a record level of low-frequency squeezing using a simple dual-seeding technique. With this study of noise sources we are closer to having a portable quantum light source using diode lasers.
We experimentally investigate four-wave mixing (FWM) in a diamond interaction scheme using ⁸⁵Rb vapor, and identify the optimal conditions for joint amplification and relative intensity squeezing of two optical fields: one near the ⁸⁵Rb D1 optical transition (λ = 794.6 nm) and the other in the telecom O-band (λ = 1324 nm). We achieved a reduction of relative intensity noise by up to 2.6 ± 0.4 dB compared to the shot noise level, signi- fying the non-classical quantum correlations. The ob- served level of intensity squeezing is primarily limited by the available pump laser power, which constrains the achievable FWM gain. Numerical simulations show good agreement with the experimental results.
We report the measurement of the intensity difference squeezing via the non-degenerate four-wave mixing process in a rubidium atomic vapor medium. Two pairs of balanced detection systems are employed to measure the probe and the conjugate beams, respectively. It is convenient to get the quantum shot noise limit, the squeezed and the amplified noise power spectra. We also investigate the influence of the input extra quadrature amplitude noise of the probe beam. The influence of the extra noise can be minimized and the squeezing can be optimized under the proper parameter condition. We measure the −3.7-dB intensity difference squeezing when the probe beam has a 3-dB extra quadrature amplitude noise. This result is slightly smaller than −4.1 dB when the ideal coherent light (no extra noise) for the probe beam is used.
In this talk, we demonstrate a system capable of producing pulsed squeezed light via four-wave mixing in a rubidium vapor. By employing a pulsed input, we produce nanosecond relative-intensity squeezed pulses and employ time-resolved detection to measure the degree of squeezing obtained. With respect to recent noise-spectrum squeezing experiments in atomic vapors, the present work is based on time-domain detection. The basic idea behind the generation of relative-intensity squeezed light as presented in this work relies on off-resonant four-wave mixing in a double lambda-system.
We have built a compact light source for bright squeezed twin-beams at 795\,nm based on four-wave-mixing in atomic 85^{85}85Rb vapor. With a total optical power of 400\,mW derived from a free running diode laser and a tapered amplifier to pump the four-wave-mixing process, we achieve 2.1\,dB intensity difference squeezing of the twin beams below the standard quantum limit, without accounting for losses. Squeezed twin beams generated by the type of source presented here could be used as reference for the precise calibration of photodetectors. Transferring the quantum correlations from the light to atoms in order to generate correlated atom beams is another interesting prospect. In this work we investigate the dispersion that is generated by the employed four-wave-mixing process with respect to bandwidth and dependence on probe detuning. We are currently using this squeezed light source to test the transfer of spatial information and quantum correlations through media of anomalous dispersion.