Strong Low-Frequency Quantum Correlations From a Four-Wave Mixing Amplifier
C F. McCormick, Alberto M. Marino, V Boyer, Paul D. Lett
Quantum-correlated light displays sub-classical noise fluctuations between frequency sidebands on a single beam (quadrature squeezing) or between the intensities or phases of different beams (two-mode squeezing). Exploiting this noise reduction can improve the accuracy of optical measurements or enable quantum information processing. We have developed a simple and robust source for generating quantum-correlated light based on single-pass four-wave-mixing amplification in an atomic vapor. Pairs of light beams from the source are narrowband, tuned near an atomic resonance, and display more than 8 dB of noise reduction in their intensity difference. In addition, the source's immunity to environmental noise permits the noise reduction to extend deep into the audio range, making it useful for a number of applications. Most notably, it is suitable for use with atom-based quantum memories. A distributed gain/loss model quantitatively fits our measurements and allows us to predict the optimum conditions for the source's operation. The present phase-insensitive amplifier should be easily adaptable to phase-sensitive amplification and the generation of bright-beam and vacuum quadrature squeezing.