Observation of twin-beam-type quantum correlation in optical fiber.

We report generation of pulsed twin beams of light through optical parametric amplification in a fiber Sagnac loop. By pumping the Sagnac loop with picosecond pulses at a wavelength near the zero-dispersion wavelength of the fiber, we achieve phase-matched nondegenerate four-wave mixing with gain. For a gain of 2.2, the intensity noises of the amplified signal and the generated idler (conjugate) pulses are found to be correlated by 5.0 dB, and the subtracted noise drops below the shot-noise limit by 1.1 dB (2.6 dB when corrected for losses). We have investigated the gain dependence of the quantum-noise reduction as well as of the intensity noises of the amplified signal and idler pulses. As the gain increases, we observe the onset of excess noise on the idler pulses.