Intensity-Field Correlation of Single-Atom Resonance Fluorescence

We report measurements of an intensity-field correlation function of the resonance fluorescence of a single trapped ${}^{138}\mathrm{Ba}^{+}$ ion. Detection of a photon prepares the atom in its ground state, and we observe its subsequent evolution under interaction with a laser field of well-defined phase. We record the regression of the resonance fluorescence source field. This provides a direct measurement of the field of the radiating dipole of a single atom and exhibits its strong nonclassical behavior. In the setup, an interference measurement is conditioned on the detection of a fluorescence photon.

Figure 1

Sketch of the experimental setup: A single ${}^{138}\mathrm{Ba}^{+}$ ion in a linear Paul trap is continuously laser excited. Two detection channels, left and right, allow for visual observation of the ion (CCD), or for recording correlations in the emitted light. The LO is coupled out by $M_1$ in front of the trap, attenuated (Att.), and its polarization is adjusted with a $\lambda /2$ plate to match the polarization of the fluorescence beam. The inset shows the relevant electronic levels of ${}^{138}\mathrm{Ba}^{+}$.

Figure 2

Obtained correlation function with the LO path blocked. The solid line shows the theoretical prediction using experimental parameters and 8-level optical Bloch equations.

Figure 3

Measured normalized correlation function $g_{\Phi }^{\mathrm{total}}(\tau )$ obtained (a) at $\Phi =0$, i.e., the fluorescence being in phase with the LO, (c) at $\Phi =\pi$, i.e., the fluorescence being out of phase with the LO and (b) at $\Phi =\pi /2$. The solid curves show the theoretical predictions using Eq. (10).

Figure 4

The intensity-field correlation function $g_{\Phi }^{(1.5)}(\tau )$ with the LO phase adjusted to (a) $\Phi =0$ and (b) $\Phi =\pi /2$, obtained from the data in Fig. 3 using Eq. (11, 12), respectively. The solid lines represent the theoretical model using Eq. (6).