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Quantum correlated light beams from non-degenerate four-wave mixing in an atomic vapor: the D1 and D2 lines of ^85Rb and ^87Rb

Raphael C. Pooser, Alberto M. Marino, Vincent Boyer, K. M. Jones, Paul D. Lett

Published Sep 3, 2009• 65 citations

Abstract

We present experimental results showing that quantum correlated light can be produced using non-degenerate, off-resonant, four-wave mixing (4WM) on both the D1 (795 nm) and D2 (780 nm) lines of (85)Rb and (87)Rb, extending earlier work on the D1 line of (85)Rb. Using this 4WM process in a hot vapor cell to produce bright twin beams, we characterize the degree of intensity-difference noise reduction below the standard quantum limit for each of the four systems. Although each system approximates a double-lambda configuration, differences in details of the actual level structure lead to varying degrees of noise reduction. The observation of quantum correlations on light produced using all four of these systems, regardless of their substructure, suggests that it should be possible to use other systems with similar level structures in order to produce narrow frequency, non-classical beams at a particular wavelength.

DOI: 10.1364/oe.17.016722

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We present experimental results showing that quantum correlated light can be produced using non-degenerate, off-resonant, four-wave mixing (4WM) on both the D1 (795 nm) and D2 (780 nm) lines of (85)Rb and (87)Rb, extending earlier work on the D1 line of (85)Rb. Using this 4WM process in a hot vapor cell to produce bright twin beams, we characterize the degree of intensity-difference noise reduction below the standard quantum limit for each of the four systems. Although each system approximates a double-lambda configuration, differences in details of the actual level structure lead to varying degrees of noise reduction. The observation of quantum correlations on light produced using all four of these systems, regardless of their substructure, suggests that it should be possible to use other systems with similar level structures in order to produce narrow frequency, non-classical beams at a particular wavelength.

We experimentally investigate the quantum-correlated twin beams generated through stimulated nondegenerate four-wave mixing in the double-lambda atomic system. A 2.5-dB noise reduction of intensity difference with 18.4-GHz frequency difference at the cesium

{D}_{1}

line is observed in a Cs vapor cell. The quantitative theoretical analysis reveals the experimental difficulty in getting high quantum correlation in Cs atoms because of the large hyperfine splitting of the ground states. However, it is favorable for obtaining quantum correlation in a wide range of pump detunings and relative long lengths of vapor cells. This quantum correlation provides a potential resource for possible coherent interfaces between atomic and solid-state systems due to its wavelength at the Cs

{D}_{1}

line which lies well within the wavelength regime of the exciton emission from InAs quantum dots.

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

{D}_{1}

line, it is of interest for experiments involving quantum imaging and coherent interfaces between atomic and solid-state systems.

Squeezed states of light have received renewed attention due to their applicability to quantum-enhanced sensing. To take full advantage of their reduced noise properties to enhance atomic-based sensors, it is necessary to generate narrowband near or on atomic resonance single-mode squeezed states of light. We have previously generated bright two-mode squeezed states of light, or twin beams, that can be tuned to resonance with the D1 line of Rb87 with a non-degenerate four-wave-mixing process in a double-lambda configuration in a Rb85 vapor cell. Here, we report on the use of feedforward to transfer the amplitude quantum correlations present in the twin beams to a single beam for the generation of single-mode amplitude squeezed light. With this technique, we obtain a single-mode squeezed state with −2.9±0.1 dB of squeezing when tuned off resonance and −2.0±0.1 dB when tuned on resonance with the D1 F=2 to F′=2 transition of Rb87.

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

8\phantom{\rule{0.3em}{0ex}}\mathrm{dB}

. The absence of a cavity makes the system immune to external perturbations, and strong quantum noise reduction is observed at frequencies as low as

4.5\phantom{\rule{0.3em}{0ex}}\mathrm{kHz}

and over a large frequency range.

We demonstrate a new phase-matching geometry for four-wave mixing processes in hot Rb85 vapor, in which all four fields propagate in different directions but two of them are degenerate in frequency. When used as a parametric amplifier with an injected seed, two types of quantum mechanically correlated twin-beam states, either frequency degenerate or nondegenerate, can be generated. The quantum noise reduction in the intensity difference is almost 7 dB for the nondegenerate type and nearly 5 dB for the degenerate type. The spatial nondegeneracy of the four waves allows a variety of configurations of parametric processes, leading to flexible control for both phase insensitive and sensitive parametric amplification. The spatially nondegenerate but frequency degenerate four-wave mixing process will find wide applications in quantum metrology, quantum communication, and quantum information of continuous variables.

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.