We report observation of intense nonclassical optical field in the cavity-QED microlaser with Mandel Q of −0.6. The photon number variance of the field is only 40% of that of a coherent state with the same mean photon number of about 500. The cavity-QED microlaser consists of a Fabry-Perot cavity with a fitness of one million (cavity decay rate of 170 kHz) and a supersonic beam of barium atoms traversing the cavity mode with a small tilt angle for a uniform atom-cavity coupling constant g of 190 kHz. Before atoms enter the cavity, they are excited by a pump laser using adiabatic following on the 1S0-3P1transition at 791 nm. Sub-Poisson light generation is achieved by operating the microlaser in a photon-number stabilization region where the laser gain exhibits a negative slope as a function of the photon number.
The second-order correlation function observed for the emitted photons from the microlaser exhibits anti bunching, i.e., reduction at time zero.From the amount of reduction, the Mandel Q of the cavity field is obtained to be −0.5. After correction for the detector dead-time (about 24 ns), we obtain Q = −0.6, which well agrees with the theoretical prediction based on the master equation and the quantum trajectory simulations. One immediate application of nonclassical radiation observed in our experiment with more than 3 dB reduction below a shot noise is low-power optical communication. We can achieve the same signal-to-noise ratio used in the current optical communication with only 40% of the current optical power if nonclassical radiation with Mandel Q of −0.6 is employed.