Observation of Quantum Jumps. T. Sauter, W. Neuhauser, R. Blatt and P. Toschek in Phys. Rev. Lett. 57:1696 (1986).  What the paper says!?

This is another seminal paper on the shelving mechanism that leads to the alleged observation of quantum jumps by shutting off strong resonance fluorescence with a small, rare, one-quantum transition to the shelving state. It follows (in print) Nagourney et al. but comes otherwise essentially simultaneously. It precedes the—this time, exactly simultaneously—same report by Bergquist. et al. They have a more detailed and better version of this paper in Opt. Commun.

This one comes with its share of obfuscation. Their pumping scheme depicts the shelf (D$_{5/2}$) being excited directly from the dipole-forbidden transition state P$_{1/2}$, while one could expect it to come from P$_{3/2}$ instead, especially following earlier works since P$_{3/2}$ is excited with a lamp by Nagourney. Here, it is not and serves as a virtual state for intermediate electronic Raman-Stokes scattering, and the wiggly line actually corresponds to «"small even-parity contributions may be mixed into the P₁/₂ level by the strong light fields and allow some E2 decay to the D₅/₂ level."» This could have been clarified better, in particular in the caption.

This prompts them to claim that:

A related concept even closer to Bohr's original idea makes use of weak spontaneous emission, coupled to the upper level of the strong resonance line. Quantum jumps of the atomic particle into an intermediate third level of long enough life will show up as random extinction of the strong resonance scattering. This concept has also been discussed recently.^9,10

where their [9, 10] are . They have various, not quite well identified incoherent feeding mechanisms for the shelf (D$_{5/2}$) and this comes from spontaneous emission, rather than incoherent excitation, what they regard as "closer to Bohr".

They do have a hollow cathode (in Fig. 2), but as a frequency reference for the scanned red laser, not for shelf loading.

In their description of the quantum jump itself, they also seem to challenge the mechanism proper, making it "less abrupt" in presence of driving to the shelf (as opposed to their spontaneous emission):

Occasional excitation of the weak transition, and the concomitant presence of the atomic system in the corresponding upper state, will initiate immediate quenching of the strong resonance fluorescence, if the absorption on the weak line is indeed accompanied by a quantum jump of the atom. Since the absorption of classical radiation involves the excitation and synchronization of the induced atomic dipole, the response may be slightly less abrupt.

but they know the—by then published—Nagournay work which shows it is also neatly telegraphic and abrupt. Theirs is also less symmetric than the one of Nagourney, probably because quantum jumps happen spontaneously and cannot be tuned to optimum shelving:

The main highlight—if one excepts the co-discovery as essentially simultaneous and carried-out independently—seems to be the spontaneous emission aspect of the shelving.