Observation of sub-Poisson Photon Statistics in the Cavity-QED Microlaser

We have measured the second-order correlation function of the cavity-QED microlaser output and observed a transition from photon bunching to antibunching with increasing average number of intracavity atoms. The observed correlation times and the transition from super- to sub-Poisson photon statistics can be well described by gain-loss feedback or enhanced-reduced restoring action against fluctuations in photon number in the context of a quantum microlaser theory and a photon rate equation picture. However, the theory predicts a degree of antibunching several times larger than that observed, which may indicate the inadequacy of its treatment of atomic velocity distributions.

Figure 1

Schematic of experimental setup. M: mirror, C: cavity mode, P: pump beam, $\theta$: tilt angle, B: Ba atomic beam, D1, D2: start and stop detectors, Counter 1, 2: counter or timing boards.

Figure 2

Photon number stabilization. Solid line: gain $G(n)$; dashed line: loss $L(n)$. The restoring rate of the cavity-QED microlaser, shown in (a), for a momentary deviation $\Delta n$ from a steady-state value $n_0$ is $\partial (L-G)/\partial n{|}_{n_0}$. (a) Cavity-QED microlaser has oscillatory $G(n)$. (b) Conventional laser: $G(n)$ approaches a constant value for large photon number and the restoring rate is $\partial L/\partial n={\Gamma }_c$.

Figure 3

(a) Observed $⟨n⟩$-versus-$⟨N⟩$ curve. The solid curve is a fit based on the quantum microlaser theory employing the averaged gain function in (b). (b) Solid line: normalized gain function per atom averaged over the velocity distribution corresponding to the present experiment. Dot-dashed line: gain function for a monovelocity distribution. (c) Measured second-order correlation function $g^{(2)}(\tau )$ for $⟨N⟩=12$, 26, 68, and 158. Each result is well fit by an exponentially decaying function.

Figure 4

Microlaser correlation times (a) and $Q$ values (b) versus $⟨N⟩$, compared with theory. In (a), cavity decay time is represented by a horizontal dotted line. Solid line, quantum theory. Dashed line, semiclassical theory. In (b), solid line is the quantum theory.