One successful interference test leaves questions open
Two sources can have similar spectra or first-order coherence yet different photon-counting statistics. Glauber’s 1963 paper introduced a hierarchy of field correlations suitable for discussing these distinctions. Its abstract explicitly notes that traditional optical coherence describes only the first order.
The 2005 recognition concerns a theoretical framework tied to observable detection statistics. It is not a claim that Glauber performed every photon-correlation experiment, nor that every narrow laser beam occupies an ideal coherent quantum state.
Sources: [2]
A coincidence is conditioned on a delay
Split a stationary field and compare detection events in two channels at a specified delay. Normalize the joint rate by the product of single-channel rates. That comparison asks whether events arrive more or less often together than independent arrivals would predict.
For ideal single-mode counting, the zero-delay normalized correlation can be written using photon number. The subtraction of one counts distinct photon pairs in the same mode. The mean photon number must be nonzero; a vacuum record supplies no such normalized estimate.
Sources: [2]
The ideal reference values need their assumptions
An ideal coherent state gives a value of one; a single thermal mode gives two; an ideal single-photon state gives zero. These are model limits. Combining many independent thermal modes reduces the bunching seen by a detector that does not resolve them.
Detector timing, dead time, background and afterpulsing also alter a measured histogram. A dip or peak should therefore be compared with the instrument response and acquisition window. The formula alone is not a certification procedure for a photon source.
Sources: [2]
Coherence is not the same as monochromaticity
Glauber’s abstract states that coherent fields need not be monochromatic. A phase-related pulse contains a spectrum of frequencies. Conversely, spectral filtering can narrow a thermal source without turning its photon statistics into those of a coherent state.
This distinction connects the Michelson article’s visibility measurement with photon counting. First-order and higher-order experiments ask compatible but different questions about the same field.
Inspect the histogram before its fitted label
In the linked correlation Lab, compare the same counting duration across source models. Look for finite-count uncertainty and the effect of delay bins. Existing single-photon evidence articles then connect these statistics to particular historical source experiments; this article supplies the coherence framework.
Sources: [2]