Precision Zeeman-Stark Spectroscopy of the Metastable $a(1)[{}^3\Sigma ^{+}]$ State of PbO

The metastable $a(1)[{}^3\Sigma ^{+}]$ state of PbO has been suggested as a suitable system in which to search for the electric dipole moment of the electron. We report here the development of experimental techniques allowing high-sensitivity measurements of Zeeman and Stark effects in this system, similar to those required for such a search. We observe Zeeman quantum beats in fluorescence from a vapor cell of PbO, with shot-noise limited extraction of the quantum beat frequencies, high counting rates, and long coherence times. We argue that improvement in sensitivity to the electron electric dipole moment by at least 2 orders of magnitude appears possible using these techniques.

Figure 1

The energies of the low-lying $X(0)$ and $a(1)$ states in a magnetic field $\mathbf{B}$ are shown. The coherent $R0$ ($Q1$) transition leading to quantum beats at ${\nu }_b\propto g_{e}B(g_{f}B)$ is indicated by solid (dashed) arrows. The $\Omega$-doublet splitting ${\Delta }_{\Omega }$ (which is independent of $\mathbf{B}$ and $M$) is shown greatly exaggerated. Transitions between the doublet levels can be driven by a rf electric field at ${\nu }_{\mathrm{rf}}={\Delta }_{\Omega }$ (dotted arrows).

Figure 2

Zeeman quantum beat data representing 0.5 s of integration are shown with a fit to Eq. (1).

Figure 3

Change in quantum beat frequency $\Delta {\nu }_b$ versus the frequency ${\nu }_{\mathrm{rf}}$ of a rf pulse. A $\pi$ pulse at the frequency of the $\Omega$-doublet splitting transfers population from the $e$ to $f$ levels and changes the beat frequency from ${\nu }_b^e\propto g_{e}B$ to ${\nu }_b^f\propto g_{f}B$.