Cause of the emitter-count discrepancy between free-space and fiber-coupled measurements

Determine the exact cause of the discrepancy between detecting approximately 140 SnV$^-$ emitters by widefield photoluminescence excitation at 4 K and distinguishing only approximately 70 emitters after cooling in a dilution refrigerator and switching to fiber-array coupling, including whether it arises from limited pump power, reduced photonic-integrated-circuit collection efficiency, overlapping emitter peaks, or zero-phonon lines outside the fiber-measured frequency range.

Background

The paper reports approximately 140 emitters across four diamond waveguides under free-space widefield photoluminescence excitation at 4 K. After the device is cooled in a dilution refrigerator and interrogated through a commercial fiber array, only approximately 70 emitters can be distinguished. This unresolved discrepancy affects interpretation of the device’s fiber-coupled emitter yield and may limit the scalability of the quantum repeater photonic integrated circuit.

The authors identify several possible explanations—limited pump power, reduced collection efficiency through the photonic integrated circuit, overlapping emitter peaks, and emitter zero-phonon lines lying outside the narrower fiber-measured frequency range—but do not determine which explanation, or combination of explanations, is responsible.