A new scheme for generating an amplitude-squeezed state is proposed. The photon-flux fluctuation of a semiconductor-laser output wave is measured with a quantum nondemolition (QND) detector, and is negatively fed back to the laser pumping current. The operator Langevin equations are derived by combining the quantum-mechanical analyses on the laser internal-external field fluctuations, the quantum nondemolition detector based on an optical Kerr effect, and a negative-feedback circuit. The output wave features a reduced photon number noise below the standard quantum limit, 〈(\ensuremath{\Delta}n^
)2〉^〉, and an enhanced phase noise above that, 〈(\ensuremath{\Delta}\ensuremath{\psi}^
)2〉>(1/4)〈n^${〉}^{\mathrm{\ensuremath{-}}1}$, while the minimum uncertainty product is still preserved. The observed photoelectron statistics in a negative-feedback GaAs laser diode using a conventional p-i-n photodiode (not a QND detector) are shown to exhibit sub-Poissonian statistics with the variance 〈(\ensuremath{\Delta}n^
)2〉apeq20.26〈n^〉. The measured photocurrent fluctuation spectral density is also indicated to be below the standard quantum limit by a factor of 0.2 (=-7 dB). The experimental results also confirm that such amplitude squeezing is only observed inside the feedback loop and cannot be extracted from the loop unless a quantum nondemolition detector is used.
We have investigated possible implementations of optical quantum nondemolition measurements, using rubidium atoms in a magneto-optical trap as a nonlinear medium. Using a $\ensuremath{\Lambda}$-type three-level system in the D1 line of
87Rb, the observed performances are quantitatively the best obtained so far for a single back action evading measurement. Moreover, the magneto-optical trap and the quantum nondemolition effect are both running continuously at the same time and mutual perturbations have been avoided by using a ``dark spot'' technique. This experiment demonstrates clearly the interest of using cold atoms for controlling the quantum fluctuations of light.