Light, regardless of its source, consists of photons — elementary particles, quanta of the electromagnetic field. The simplest light detector, such as a photometric illuminance meter, operates in a binary mode: it merely registers whether photons hit it or not, giving a 'yes/no' answer. For a long time, it was believed that with such a primitive device, one could not distinguish a laser beam (classical light) from light with specific quantum properties: their output signals from such a detector appear identical.

The key was found in using an adjustable attenuator, an analog of a dimmer, placed before the detector. When the degree of dimming is varied, classical and quantum light exhibit fundamentally different behavior. Classical light behaves predictably: the stronger the attenuation (that is, the fewer photons the 'dimmer' lets through), the less frequently the detector registers flashes — the dependence here is linear. Quantum light, however, surprised researchers: under strong dimming, the detector starts clicking noticeably more often than one would expect from a simple reduction in flux. It is as if the particles overcome the barrier not one by one, but together — a manifestation of purely quantum correlations, where the probability of joint photon detection is higher than for independent particles.

This anomaly, an increase in click rates under deep attenuation, serves as a clear signature of the nonclassical, quantum nature of light. Thus, a simple adjustable attenuator turns a crude indicator into a sensitive spectroscopic probe, capable of revealing subtle quantum features of radiation that were previously inaccessible to primitive detectors.