Observation of coherent population trapping in a V-type two-electron system

TL;DR

This paper reports the first observation of coherent population trapping (CPT) in a two-electron atom, specifically ytterbium-174, demonstrating a transition from a V-type to a Λ-type system enabling dark state formation.

Contribution

It reveals that electron interactions in a two-electron atom can transform level structures, allowing CPT in a transition that one-electron theory predicts should not support it.

Findings

CPT observed in $^{174}$Yb using a specific transition.

The CPT resonance linewidth is approximately 300 kHz.

Magnetic field lifts degeneracy, revealing five fluorescence dips.

Abstract

We observe coherent population trapping (CPT) in a two-electron atom---$^{174}$Yb---using the $\rm {^1S_0,F=0} \rightarrow {^3P_1,F'=1}$ transition. CPT is not possible for such a transition according to one-electron theory because the magnetic sublevels form a V-type system, but in a two-electron atom like Yb, the interaction of the electrons transforms the level structure into a $ \Lambda $-type system, which allows the formation of a dark state and hence the observation of CPT. Since the two levels involved are degenerate, we use a magnetic field to lift the degeneracy. The single fluorescence dip then splits into five dips---the central unshifted one corresponds to coherent population oscillation, while the outer four are due to CPT. The linewidth of the CPT resonance is about 300 kHz and is limited by the natural linewidth of the excited state, which is to be expected because the excited state is involved in the formation of the dark state.